ON
DIGESTION
AND
DIETETICS.

THE
PHYSIOLOGY OF DIGESTION
CONSIDERED WITH RELATION TO
THE PRINCIPLES OF DIETETICS.

By ANDREW COMBE, M. D.
FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS IN EDINBURGH,

AND

CONSULTING PHYSICIAN TO THEIR MAJESTIES THE KING AND QUEEN
OF THE BELGIANS.

SECOND EDITION
REVISED AND ENLARGED.

“Nor is it left arbitrary, at the will and pleasure of every man, to do as he list; after the dictates of a depraved humour and extravagant phancy, to live at what rate he pleaseth; but every one is bound to observe the Injunctions and Law of Nature, upon the penalty of forfeiting their health, strength, and liberty,—the true and long enjoyment of themselves.”

Mainwayringe.

EDINBURGH:
MACLACHLAN & STEWART;
AND SIMPKIN, MARSHALL & CO. LONDON.
MDCCCXXXVI.

PRINTED BY NEILL & CO. OLD FISHMARKET.

ADVERTISEMENT

TO THE

SECOND EDITION.

The first edition of the present volume consisted of 2000 copies, and has been exhausted in little more than five months. Already, also, it has been twice reprinted in the United States. This success is extremely gratifying, and shews that the desire for information on the subject of the human constitution, is rapidly extending in proportion as it is discovered to be perfectly within the comprehension of every ordinary capacity, and to be directly and easily applicable to the farther improvement of the moral and physical condition of man.

The edition now offered to the public has been carefully revised, and about twenty pages of new matter have been added. Still, I fear, that many imperfections remain, which leisure and more confirmed health might have enabled me to remove, but which, under present circumstances, I feel compelled to leave to the good-natured indulgence of the reader.

It has been suggested by a professional critic for whose judgment I feel the utmost deference, that “the work would really have been more useful if the physiological or introductory part had been more condensed;” as much of it will, he thinks, be neither readily comprehended, nor usefully retained by the general reader.[1] My only reason for not acting on this suggestion is, that I regard the exposition of the laws of digestion of which that part consists, as the foundation on which all the dietetic rules contained in the second part must necessarily rest,—and am therefore extremely anxious that their nature and mode of operation should be thoroughly understood by the ordinary reader, even at the risk of too great minuteness. I am quite aware that the detail into which I have entered must appear tedious to every well educated practitioner; but as the book was intended more for the general than for the medical reader, the latter is evidently a less competent judge in this particular matter than the former. On referring, accordingly, to an unprofessional critic of no small ability and reputation, we find him of an entirely opposite opinion. For—“Of the two divisions of the book,” he thinks, “the FIRST is the most satisfactory and interesting, from the nature of its subject and the popular novelty of much of the information it imparts, or the force and freshness with which obvious truths are presented.”[2] And as other non-medical reviewers concur in this decision, I feel bound to attach more weight to them in what more especially concerns the class of readers to which they belong, and to retain the whole of the part objected to. In a purely medical question, on the other hand, I would as unhesitatingly have yielded to the judgment of the professional critic.

Edinburgh, 8 Alva Street,

November 1, 1836.

CONTENTS

WOOD-CUTS.

• Under-jaw, [41].
• Thoracic and Abdominal Viscera, [66].
• Human Stomach, [67].
• Stomach of a ruminating animal, [69].
• Villous Coat of the Stomach, [75].
• Opening into the stomach of Alexis St Martin, [88].
• Abdominal Viscera, [155].
• Transverse section of the Abdomen, [157].
• Lacteals and Thoracic Duct, [163].
• Thoracic Duct, [166].
• Contents of the Abdomen after removal of the Intestines, [171].
• Mucous Coat of the Duodenum, [172].

PREFACE.

The present volume is essentially a continuation of the work first published about two years ago, under the title of “The Principles of Physiology applied to the Preservation of Health and to the Improvement of Physical and Mental Education;” and its object is the same—namely, to lay before the public a plain and intelligible description of the structure and uses of some of the more important organs of the human body, and to shew how information of this kind may be usefully applied, not only in the prevention of suffering, but in improving the physical, moral, and intellectual condition of man.

In “The Principles of Physiology,” the structure and functions of the skin, muscles, bones, lungs, and nervous system, the laws or conditions of their healthy action, and the unsuspected origin of many of their diseases in infringements of these laws, were explained in succession at considerable length; and the means by which their health and efficiency might best be secured were pointed out. It was stated that, in selecting these organs as subjects for discussion, I had been guided by the desire to notice in preference those functions which are most influential in their operation on the general system, and at the same time least familiarly known; and that, if the attempt to convey the requisite information in a manner suited to the general reader should prove successful, I would afterwards prepare a similar account of others, in the right understanding and management of which our interest is not less deeply involved, but in regard to which much ignorance continues nevertheless to prevail, even among the most liberally educated classes of society.

The numerous proofs which I received of the utility of my former work, not only from professional and literary journals, but also from individuals previously unknown to me,—many of them guardians and instructors of youth, speaking from personal experience,—together with the rapid sale of four large editions in little more than two years, soon completely satisfied me that I had neither been deceived as to the real importance of physiological knowledge to the general public, nor been altogether unsuccessful in the method of conveying it. Thus encouraged, accordingly, I cheerfully resumed my labours, and, from materials which had been long accumulating, began the preparation of the treatise now submitted to the indulgent consideration of the reader.

The matters discussed on the present occasion relate chiefly to the function of Digestion and the principles of Dietetics; and in selecting them I have been guided by the same principle as before. It may, at first sight, be doubted whether I have not exceeded proper bounds in thus dedicating a whole volume to the consideration of a single subject; but the more we consider the real complication of the function of Digestion,—the extensive influence which it exercises at every period of life over the whole of the bodily organization,—the degree to which its morbid derangements undermine health, happiness, and social usefulness,—and especially the share which they have in the production of scrofulous and consumptive as well as of nervous and mental affections,—we shall become more and more convinced of the deep practical interest which attaches to a minute acquaintance with the laws by which it is regulated. In infancy, errors in diet, and derangement of the digestive organs, are admitted to be among the principal causes of the striking mortality which occurs in that period of life. In youth and maturity, the same influence is recognised, not only in the numerous forms of disease directly traceable to that origin, but also in the universal practice of referring every obscure or anomalous disorder to derangement of the stomach or bowels. Hence, too, the interest which has always been felt by the public in the perusal of books on Dietetics and Indigestion; and hence the prevailing custom of using purgatives as remedies for every disorder, very often with good, but not unfrequently with most injurious effects.

Numerous and popular, however, as writings on Dietetics have been, and excellent as are many of the precepts which have been handed down by them from the earliest ages, sanctioned by the warm approval of every successive generation, it is singular how very trifling their influence has been, and continues to be, in altering the habits of those to whom they are addressed. In a general way, we all acknowledge that diet is a powerful agent in modifying the animal economy; yet, from our conduct, it might justly be inferred, that we either regarded it as totally devoid of influence, or remained in utter ignorance of its mode of operation, being left to the guidance of chance alone, or of notions picked up at random, often at variance with reason, and, it may be, in contradiction even with our own daily experience.

It has been alleged by a friendly critic of the first edition, that the author is too sanguine in expecting that the mere communication of knowledge will suffice to alter the habits of the race, and that, although the information conveyed in the present volume may be turned to account by third parties—by mothers and nurses, for example—“yet with respect to the direct effect upon eaters,” slender results must be anticipated. “The world,” it is wittily added, “will read, admire, and applaud Dr Combe on Digestion and Dietetics, and then go on in its usual way eating what it likes, and digesting what it can.” The Author, however, never entertained the hope that his work would immediately produce the slightest perceptible change in the general practices of society, or that many healthy men of mature age and confirmed habits would forsake their accustomed regimen merely because it was shewn to be at variance with the laws of Nature. But as human conduct is in some measure influenced by knowledge, he is still confident enough to believe, that, among valetudinarians and the young of both sexes, whose habits are not formed, and numbers of whom err as much from ignorance as from the force of passion, many may be found who will be glad to obtain the guidance of knowledge and principle in the regulation of their mode of life; and that even many parents, who may not have resolution enough to forsake mischievous indulgences to which they have long been accustomed, may nevertheless be anxious to avail themselves of any assistance on which they can depend for the better bringing up of their children. If in these expectations he is not too sanguine, the future advantage to the race from the present diffusion of dietetic knowledge is as certain, and almost as encouraging, as if its effects were instantaneous on both old and young. In the march of human improvement, months and years count but as moments. The men of to-day will soon have acted their part, and give place to those who are now with youthful energy adding to their knowledge, and throwing off a portion of the prejudices of their fathers. They in their turn will speedily be succeeded by their children, and the discoveries of the one generation will thus become the established and influential truths of the next. Each individual change in the habits of society may be so slow and minute as at the moment to escape our notice, but it is not on that account the less real. Nobody who compares the coarse feeding and riotous convivialities of our forefathers, at the beginning of last century, with the more refined and temperate habits of the present day, will think of denying that a prodigious step has been made in the interval even with respect to eating and drinking, which the critic seems to consider as so much beyond the influence of reason. And yet, if we take any single year of the whole century, we shall be unable to particularize any marked reformation which took place within its limits. This being the case, then, can we, their descendants, maintain that we are arrived so nearly at perfection as to leave no room for corresponding improvement in our day? My conviction is so much the reverse, that it seems to me certain that our onward progress will continue through generations yet unborn, with the same steadiness as it has done through generations long since gathered to their fathers; and that every attempt made to render man better acquainted with the laws of his own constitution, and thereby provide him with fixed and better principles of action, will exert a positive and decided influence on the progress of the race, proportioned in extent to the truth, clearness, and general applicability of the views which are unfolded. On such considerations do I ground my hope that the present volume, notwithstanding its numerous defects, will (in so far as it really embodies truths of practical importance) contribute in its own limited sphere to the general end.

The real cause of the little regard paid to dietetic rules—and it is of consequence to remark it—is not so much indifference to their influence, or even the absolute want of valuable information, as the faulty manner in which the subject is usually considered. In many of our best works, the relation subsisting between the human body on the one hand, and the qualities of alimentary substances on the other, is altogether lost sight of, although it is the only solid principle on which their proper adaptation to each other can be based. In this manner, while the attention is carefully directed to the consideration of the abstract qualities of the different kinds of aliment, little or no regard is paid to the relation in which they stand to the individual constitution, as modified by age, sex, season, and circumstances, or to the observance of the fundamental laws of digestion. And hence, although these conditions are not unfrequently of much greater importance to the general health than even the right selection of food, yet, when indigestion arises from neglecting them, the food alone is blamed, and erroneous conclusions are drawn, by relying on which, upon future occasions, we may easily be led into still more serious mistakes.

It is, indeed, from being left in this way without any guiding principle to direct their experience, and test the accuracy of the precepts laid down to them for the regulation of their conduct, that many persons begin by being bewildered by the numerous discrepancies which they meet with between facts and doctrine, between counsel and experience,—and end by becoming entirely sceptical on the subject of all dietetic rules whatever, and regarding them as mere theoretical effusions, based on fancy, and undeserving of a moment’s consideration.

The true remedy for this state of things is, not to turn away in disgust and despair, but to resort to a more rational mode of inquiry—certain that, in proportion as we advance, some useful result will reward our labours. Such, accordingly, has been my aim in the present publication; and if I shall be found to have been even moderately successful in attaining it, I shall rejoice in the confident conviction that others will be led to still more positive and beneficial results. Utility has been my great object throughout. In following what I conceive to be an improved mode of investigation, I have in some instances placed known facts in a new point of view, and deduced from them practical inferences of considerable value and easy application: but beyond this, I lay no claim to originality; and if I have any where used expressions which may seem either to do injustice to others or to arrogate too much credit to myself, it has been entirely without any such design, and, consequently, I will be prompt to acknowledge my error and rectify the involuntary mistake.

In preparing the present volume for the press, I have derived the utmost advantage from a very valuable work by Dr Beaumont, an American writer, which—though faulty in its arrangement, and necessarily defective in many essential particulars—contains an authentic record of some of the most curious and instructive observations which have ever been made on the process of digestion. That excellent and enlightened physiologist had the rare good fortune to meet with a case where an artificial opening into the stomach existed, through which he could see every thing that took place during the progress of healthy digestion; and, with the most disinterested zeal and admirable perseverance, he proceeded to avail himself of the opportunity thus afforded of advancing human knowledge, by engaging the patient, at a heavy expense, to live with him for several years, and become the subject of numerous and carefully conducted experiments. Of the results thus obtained, I have not scrupled to make the freest and most ample use—not from considering them as positively new (for even Dr Beaumont lays little claim to the merit of a discoverer), but because they come before us so entirely freed from the numerous sources of error and doubt which formerly impaired their value, that they can now, for the first time, be safely trusted as practical guides in the science of dietetics. From Dr Beaumont’s work, also, being still inaccessible to the British reader, it is a bare act of justice towards him, and also the best way of fulfilling the objects he had in view, to make its contents known as widely as possible: for wherever they are known, they will be acknowledged to redound to his credit, not less as a man than as a philosopher.

Objections have been stated to several of the repetitions which occur in the following pages. The only apology I have to offer for them is, that I committed them deliberately, because they seemed to me necessary to ensure clearness, and because the intimate manner in which the different functions are connected with each other, sometimes made it impossible to explain one without again referring to the rest. My prime objects being to render the meaning unequivocally plain, and impress the subject deeply upon the reader’s mind, I thought it better to risk in this way the occasional repetition of an important truth, than to leave it in danger of being vaguely apprehended, or its true value unperceived. For these reasons, it is hoped that the fault—if such it is—will be leniently overlooked.

Those who wish to study more fully the subject of Dietetics, will find much useful information in Dr Hodgkin’s “Lectures on the Means of Promoting and Preserving Health;” Professor Dunglison “On the Influence of Atmosphere and Locality, Change of Air and Climate, Seasons, Food, Clothing, Bathing, Exercise, Sleep, Corporeal and Intellectual Pursuits, &c. &c. &c. on Human Health;” Dr Paris “On Diet;” and Dr Kilgour’s “Lectures on the Ordinary Agents of Life, as applicable to Therapeutics and Hygiène.”

CHAPTER I.
GENERAL VIEW OF THE OBJECTS AND LAWS OF NUTRITION.

Waste or loss of substance always attendant on action.—In the vegetable and animal kingdoms waste is greater than in the physical.—Living bodies are distinguished by possessing the power of repairing waste.—Vegetables, being rooted in one place, are always in connexion with their food.—Animals, being obliged to wander, receive their food at intervals into a stomach.—Nutrition most active when growth and waste are greatest.—In vegetables the same causes which increase these processes also stimulate nutrition.—But animals require a monitor to warn then when food is needed.—The sense of Appetite answers this purpose.—The possession of a stomach implies a sense of Appetite to regulate the supplies of food.

Throughout every department of Nature waste is the invariable result of action. Even the minutest change in the relative position of inanimate objects cannot be effected without some loss of substance. So well is this understood, that it is an important aim in mechanics to discover the best means of reducing to the lowest possible degree the waste consequent upon motion. Entirely to prevent it is admitted to be beyond the power of man; for, however nicely parts may be adjusted to each other, however hard and durable their materials, and however smoothly motion may go on, still in the course of time loss of substance becomes evident, and repair and renewal become indispensable to the continuance of the action.

It is thus a recognised fact, or general law of nature, that nothing can act or move without undergoing some change, however trifling in amount. Not even a breath of wind can pass along the surface of the earth without altering in some degree the proportions of the bodies with which it comes into contact; and not a drop of rain can fall upon a stone without carrying away some portion of its substance. The smoothest and most accurately formed wheel, running along the most level and polished railroad, parts with some portion of its substance at every revolution, and in process of time is worn out and requires to be replaced. The same effect is forcibly, though rather ludicrously, exemplified in the great toe of the bronze statue of St Peter at Rome, which in the course of centuries has been worn down to less than half its original size by the successive kisses of the faithful; and I venture to mention it, because it affords one of the best specimens of the operation of a principle, the existence of which, from the imperceptibly small effect of any single act, might otherwise be plausibly denied.

As regards dead or inanimate matter, the destructive influence of action is constantly forced upon our attention by every thing passing around us; and so much human ingenuity is exercised to counteract its effects, that no reflecting person will dispute the universality of its operation. But when we observe shrubs and trees waving in the wind, and animals undergoing violent exertion, for year after year, and yet both continuing to increase in size, we may be inclined, on a superficial view, to regard living bodies as constituting exceptions to the rule. On more careful examination, however, it will appear that waste goes on in living bodies not only without any intermission, but with a rapidity immeasurably beyond that which occurs in inanimate objects. In the vegetable world, for instance, every leaf of a tree is incessantly pouring out some portion of its fluids, and every flower forming its own fruit and seed, speedily to be separated from and lost to its parent stem; thus causing in a few months an extent of waste many hundred times greater than what occurs in the same lapse of time after the tree is cut down, and all its living operations are at a close. The same thing holds true in the animal kingdom. So long as life continues, a copious exhalation from the skin, the lungs, the bowels, and the kidneys, goes on without a moment’s intermission; and not a movement can be performed which does not at least partially increase the velocity of the circulation, and add something to the general waste. In this way, during violent exertion several ounces of the fluids of the body are sometimes thrown out by perspiration in a very few minutes; whereas, after life is extinguished, all the excretions cease, and waste is limited to that which results from ordinary chemical decomposition.

So far, then, the law that waste is attendant on action, applies to both dead and living bodies; but beyond this point a remarkable difference between them presents itself. In the physical or inanimate world, what is once lost or worn away is lost for ever. There is no power inherent in the piston of the steam-engine by which it can repair its own loss of particles; and consequently in the course of time it must either be laid aside as useless, or be remodelled by the hand of the workman. But living bodies, whether vegetable or animal, possess the distinguishing characteristic of being able to repair their own waste and add to their own substance. The possession of such a power is in fact essential to their very existence. If the sunflower, which in fine weather exhales thirty ounces of fluid between sunrise and sunset, contained no provision within its own structure for replacing this enormous waste, it would necessarily shrivel and die within a few hours, as it actually does when plucked up by the roots; and, in like manner, if man, whose system throws out every day five or six pounds of substance by the ordinary channels of excretion, possessed no means of repairing the loss, his organization would speedily decay and perish. This very result is frequently witnessed in cases of shipwreck and other disasters, where, owing to the impossibility of obtaining food, death ensues from the body wasting away till it becomes incapable of carrying on the operations of life. In some instances this waste has even proceeded so far that three-fourths of the whole weight of the body have been lost before life became extinct.

It is impossible to reflect on these facts, and others of a similar kind, without having the conviction forced upon our minds, that in every department of nature expenditure of material is inseparable from action, and that, in living bodies, waste goes on so rapidly, and by so many different channels, that life could not be maintained for any length of time without an express provision being made for compensating its occurrence.

In surveying the respective modes of existence of vegetables and of animals, with the view of ascertaining by what means this compensation is effected, the first striking difference between them which we perceive, is the fixity of position of the one, and the free locomotive power of the other. The vegetable grows, flourishes, and dies, fixed to the same spot of earth from which it sprang; and, however much external circumstances may change around it, it must remain and submit to their influence. If it be deprived of moisture and solar heat and light, it cannot go in search of them, but must remain to droop and to perish. If the earth to which its roots are attached be removed, and a richer soil be substituted than that which its nature requires, it still has no option: it must grow up in rank and unhealthy luxuriance, in obedience to an impulse which it cannot resist. At all hours and at all seasons it is at home, and in direct communication with the soil from which its nourishment is extracted. And being thus without ceasing in contact with its food, it requires no storehouse in which to lay up provision, but receives immediately from the earth, and at every moment, all that is necessary for its sustenance.

But it is otherwise with animals. These not only enjoy the privilege of locomotion, but are compelled to use it, and often to go to a distance, in search of food and shelter. Consequently, if their vessels of nutrition were like those of vegetables in direct communication with external substances, they would be torn asunder at every movement, and the animals themselves exposed either to die from starvation, or to forego the exercise of the higher functions for which their nature is adapted. But the necessity for a constant change of place being imposed on them, a different arrangement became indispensable for their nutrition: and the method by which the Creator has remedied the inconvenience is not less admirable than simple. To enable the animal to move about and at the same time to maintain a connexion with its food, He has provided it with a receptacle or stomach, where it is able to store up a supply of materials from which sustenance may be gradually elaborated during a period of time proportioned to its necessities and mode of life. It thus carries along with it nourishment adequate to its wants; and the small nutritive vessels imbibe their food from the internal surface of the stomach and bowels, where the nutriment is stored up, just as the roots or nutritive vessels of vegetables do from the soil in which they grow. The possession of a stomach or receptacle for food is accordingly a characteristic of the animal system as contrasted with that of vegetables; it is found even in the lowest orders of zoophytes, which in other respects are so nearly allied to plants.

The sole objects of nutrition being to repair waste and to admit of growth, Nature has so arranged that within certain limits it is always most vigorous when growth or waste proceeds with the greatest rapidity. Even in vegetables this relation is distinctly observable. In spring and summer, when vegetative life is most active, and when leaves, flowers, and fruit, are to be formed, and growth carried on, nourishment is largely drawn from the soil, and the elaboration and circulation of the sap are proportionally vigorous; whereas in winter, when the leaves and flowers have passed away, and vegetable life is in repose, little nourishment is needed, and the circulation of the sap is proportionally slow. In accordance with these facts, every one will recollect how freely a shrub or a tree bleeds, as it is called, when its bark is cut early in the season, and how dry it becomes on the approach of winter. It is the activity of the circulation in summer which renders its temporary suspension by transplanting so generally fatal at that season; whereas, owing to the comparative sluggishness with which it is carried on in winter, its partial interruption is then attended with much less risk.

In vegetables, the quantity of nourishment taken in entirely depends on, and is regulated by, the circumstances in which they are placed. When they are exposed, as in spring and summer, to the stimulus of heat and light, all their functions are excited, waste and growth are accelerated, and a more abundant supply of nourishment becomes indispensable to their health and existence; and hence, in a dry soil incapable of affording a copious supply of sap, they speedily wither and die. Exposed to cold, on the other hand, and shaded from the light, their vitality is impaired, and the demand for nourishment greatly diminished. This is uniformly the case in winter; and many circumstances shew that the change is really owing to the causes mentioned above, and not to any thing inherent in the constitution of the vegetable itself. In tropical climates, for example, where heat, light, and moisture abound, vegetable life is ever active, and the foliage ever thick and abundant; and even in our own northern region, we are able by artificial heat so far to anticipate the natural order of the seasons, as to obtain the ripened fruit of the vine in the very beginning of spring. The whole system of forcing vegetables and fruit, so generally resorted to for the early supply of our markets, is, in truth, founded on the principle we are now discussing; and by the regulated application of heat, light, and moisture, we are able to hasten or to retard, to a very considerable extent, the ordinary stages of vegetable life. But to ensure success in our operations, we must be careful to proportion the supply of nourishment to the state of the plant at the time. If, by the application of heat, we have stimulated it to premature growth and foliage, we must at the same time provide for it an adequate supply of food, otherwise its activity will exhaust itself, and induce premature decay. Hence the regular watering which greenhouse plants require. But if we have retarded its progress and lowered its vitality by excluding heat and light, the same copious nourishment will not only be unnecessary, but will probably do harm by inducing repletion and disease.

In vegetables, the absorption of food is thus regulated chiefly by the circumstances of heat, moisture, and light, under which the plant is placed, and by the consequent necessity which exists at the time for a larger or smaller supply of nourishment to carry on the various processes of vegetable life. According to this arrangement, nutrition is always most active when the greatest expenditure of material is taking place. When growth is going on rapidly, and the leaves are unfolding themselves, sap is sucked up from the earth in immense quantity; but when these processes are completed as summer advances, and almost no fresh materials are required, except for the consolidation of the new growth and the supply of the loss by exhalation, a much smaller amount of nourishment suffices, and the sap no longer circulates in the same profusion. In autumn, again—when the fruit arrives at maturity, the leaves begin to drop off, and the activity of vegetable life suffers abatement—nutrition is reduced to its lowest ebb; and in this state it continues till the return of spring stimulates every organ to new action, and once more excites a demand for an increased supply.

Nor is the same great principle, of supply requiring to be proportioned to demand, less strikingly apparent in animals. Wherever growth is proceeding rapidly, or the animal is undergoing much exertion and expenditure of material, an increased quantity of food is invariably required; and, on the other hand, where no new substance is forming, and where, from bodily inactivity, little loss is sustained, a comparatively small supply will suffice. But as animals are subjected to much more rapid and violent transitions from activity to inactivity than vegetables are—and thus require to pass more immediately from one kind and quantity of nourishment to another, in order to adapt their nutrition to the ever-varying demands made upon the system—they evidently stand in need of some provision to enforce attention when nourishment is necessary, and to enable them always to proportion the supply to the real wants of the body. Not being, like vegetables, in constant connection with their aliment, they might suffer from neglect if they did not possess some contrivance to warn them in time when to seek and in what quantity to consume it. But in endowing animals with the sense of Appetite, or the sensations of Hunger and Thirst generally included under it, the Creator has guarded effectually against the inconvenience, and given to them a guide in every way sufficient for the purpose.

The very possession of a stomach or receptacle, into which food sufficient for a shorter or longer period can be introduced at one time, and which we have already remarked as characterizing all animals from the lowest to the highest, almost necessarily implies the co-existence of some watchful monitor, such as appetite, to enforce attention to the wants of the system, with an earnestness which it shall not be easy to resist. If this were not the case in man, for example—if he had no motive more imperative than reason to oblige him to take food—he would be constantly liable, from indolence and thoughtlessness, or the pressure of other occupations, to incur the penalty of starvation, without being previously aware of his danger. But the Creator, with that beneficence which distinguishes all His works, has not only provided an effectual safeguard in the sensations of hunger and thirst, but moreover, attached to their regulated indulgence a degree of pleasure which never fails to insure attention to their demands, and which, in highly civilized communities, is apt to lead to excessive gratification. Such being the important charge committed to the appetites of hunger and thirst, it will be proper to submit to the reader, before entering upon the consideration of the more complicated process of digestion, a few remarks on their nature and uses.

CHAPTER II.
THE APPETITES OF HUNGER AND THIRST.

Hunger and Thirst, what they are—Generally referred to the stomach and throat, but perceived by the brain—Proofs and illustrations—Exciting causes of hunger—Common theories unsatisfactory—Hunger sympathetic of the state of the body as well as of the stomach—Uses of appetite—Relation between waste and appetite—Its practical importance—Consequences of overlooking it illustrated by analogy of the whole animal kingdom—Disease from acting in opposition to this relation—Effect of exercise on appetite explained—Diseased appetite—Thirst—Seat of Thirst—Circumstances in which it is most felt—Extraordinary effects of injection of water into the veins in cholera—Uses of thirst, and rules for gratifying it.

In the preceding chapter, I endeavoured to shew, first, that nutrition is required only because waste, and a deposition of new particles, are continually going on, so that the body would speedily become exhausted if its constituent materials were not renewed; secondly, that the sense of appetite is given to animals for the express purpose of warning them when a fresh supply of aliment is needed—as, without some such monitor, they would be apt to neglect the demands of nature; and thirdly, that vegetables have no corresponding sensation, simply because, from their being at all times in communication with the soil, their nutrition goes on continuously in proportion as it is necessary, and without requiring any prompter to put it in action at particular times.

If these principles be correct, it follows that, in the healthy state (and let the reader be once for all made aware that in the following pages the state of health is always implied, except where it is otherwise plainly expressed), the dictates of appetite will not be every day the same, but will vary according to the mode of life and wants of the system, and, when fairly consulted, will be sufficient to direct us both at what time and in what quantity we ought to take in either solid or liquid sustenance. But to make this perfectly evident, a few general observations may be required.

It is needless to waste words in attempting to describe what hunger and thirst are: every one has felt them, and no one could understand them without such experience, any more than sweetness or sourness could be understood without tasting sweet or sour objects. Their end is manifestly to proclaim that farther nourishment is required for the support of the system; and our first business is, therefore, to explain their nature and seat, in so far at least as a knowledge of these may be conducive to our welfare.

The sensation of hunger is commonly referred to the stomach, and that of thirst to the upper part of the throat and back of the mouth; and correctly enough to this extent, that a certain condition of the stomach and throat tends to produce them. But, in reality, the sensations themselves, like all other mental affections and emotions, have their seat in the brain, to which a sense of the condition of the stomach is conveyed through the medium of the nerves. In this respect, Appetite resembles the senses of Seeing, Hearing, and Feeling; and no greater difficulty attends the explanation of the one than of the others. Thus, the cause which excites the sensation of colour, is certain rays of light striking upon the nerve of the eye; and the cause which excites the perception of sound, is the atmospherical vibrations striking upon the nerve of the ear; but the sensations themselves take place in the brain, to which, as the organ of the mind, the respective impressions are conveyed. In like manner, the cause which excites appetite is an impression made on the nerves of the stomach; but the feeling itself is experienced in the brain, to which that impression is conveyed. Accordingly, just as in health no sound is ever heard except when the external vibrating atmosphere has actually impressed the ear, and no colour is perceived unless an object be presented to the eye,—so is appetite never felt, except where, from want of food, the stomach is in that state which forms the proper stimulus to its nerves, and where the communication between it and the brain is left free and unobstructed.

But as, in certain morbid states of the brain and nerves, voices and sounds are heard, or colours and objects are seen, when no external cause is present to act upon the ear or the eye,—so, in disease, a craving is often felt when no real want of food exists, and where, consequently, indulgence in eating can be productive of nothing but mischief. Such an aberration is common in nervous and mental diseases, and not unfrequently adds greatly to their severity and obstinacy. In indolent unemployed persons, who spend their days in meditating on their own feelings, this craving is very common, and from being regarded and indulged as if it were healthy appetite, is productive of many dyspeptic affections.[3]

If the correctness of the preceding explanation of the sensation of hunger be thought to stand in need of confirmation, I would refer to the very conclusive experiments by Brachet of Lyons, as setting the question entirely at rest. Brachet starved a dog for twenty-four hours, till it became ravenously hungry, after which he divided the nerves which convey to the brain a sense of the condition of the stomach. He then placed food within its reach, but the animal, which a moment before was impatient to be fed, went and lay quietly down, as if hunger had never been experienced. When meat was brought close to it, it began to eat; and, apparently from having no longer any consciousness of the state of its stomach—whether it was full or empty—it continued to eat till both it and the gullet were inordinately distended. In this, however, the dog was evidently impelled solely by the gratification of the sense of taste; for on removing the food at the beginning of the experiment to the distance of even a few inches, it looked on with indifference, and made no attempt either to follow the dish or to prevent its removal.[4]

Precisely similar results ensued when the nervous sympathy between the stomach and brain was arrested by the administration of narcotics. A dog suffering from hunger turned listlessly from its food when a few grains of opium were introduced into its stomach. It may be said that such a result is owing to the drug being absorbed and carried to the brain through the ordinary medium of the circulation; but Brachet has proved that this is not the case, and that the influence is primarily exerted upon the nerves. To establish this point, two dogs of the same size were selected. In one the nerves of communication were left untouched, and in the other they were divided. Six grains of opium were then given to each at the same moment. The sound dog began immediately to feel the effects of the opium and became stupid, while the other continued lying at the fireside for a long time, without any unusual appearance except a little difficulty of breathing. In like manner, when the experiment was repeated with that powerful poison nux vomica, upon two dogs similarly circumstanced, the sound one fell instantly into convulsions, while the other continued for a long time as if nothing had happened.

These results demonstrate, beyond the possibility of doubt, the necessity of a free nervous communication between the stomach and brain, for enabling us to experience the sensation of hunger. The connexion between the two organs is indeed more widely recognised in practice than it is in theory; for it is a very common custom with the Turks to use opium for abating the pangs of hunger when food is not to be had, and sailors habitually use tobacco for the same purpose. Both substances act exclusively on the nervous system.

The relation thus shewn to subsist between the stomach and the brain, enables us in some measure to understand the influence which strong mental emotions and earnest intellectual occupation exert over the appetite. A man in perfect health, sitting down to table with an excellent appetite, receives a letter announcing an unexpected calamity, and instantly turns away with loathing from the food which, a moment before, he was prepared to eat with relish; while another, who, under the fear of some misfortune, comes to table indifferent about food, will eat with great zest on his “mind being relieved,” as the phrase goes, by the receipt of pleasing intelligence. Excessive and absorbing emotion, even of a joyful kind, has the same effect. Captain Back tells us in the interesting narrative of his last journey, that when he first heard of Captain Ross’s return, “the thought of so wonderful a preservation overpowered, for a time, the common occurrences of life. We had but just sat down to breakfast, but our appetite was gone, and the day passed in a feverish state of excitement,” (P. 245). In such cases, no one will imagine that the external cause destroys appetite otherwise than through the medium of the brain. Occasionally, indeed, the aversion to food amounts to a feeling of loathing and disgust, and even induces sickness and vomiting,—a result which depends so entirely on the state of the brain, that it is often excited by mechanical injuries of that organ.

The analogy between the external senses and the appetite is in various respects very close. If we are wrapt in study, or intent on any scheme, we become insensible to impressions made on the ear or eye. A clock may strike, or a person enter the room, without our being aware of either event. The same is the case with the desire for food. If the mind is deeply engaged, the wants of the system are unperceived and unattended to—as was well exemplified in the instance of Sir Isaac Newton, who, from seeing the bones of a chicken lying before him, fancied that he had already dined, whereas, in reality, he had eaten nothing for many hours. Herodotus ascribes so much efficacy to mental occupation in deadening the sense of hunger, that he speaks of the inhabitants of Lydia having successfully had recourse to gaming as a partial substitute for food, during a famine of many years continuance. In this account there is, of course, gross exaggeration; but it illustrates sufficiently well the principle under discussion.

Many attempts have been made but without much success, to determine what the peculiar condition of the stomach is which excites in the mind the sensation of hunger. For a long time it was imagined that the presence of gastric or stomach juice, irritating the nerves of the mucous membrane, was the exciting cause; but it was at last ascertained, that, after the digestion of a meal is completed, and the chyme has passed into the intestine, the gastric juice ceases to be secreted till after a fresh supply of food has been taken in.[5] It was next supposed that the mere emptiness of the stomach was sufficient to excite hunger, and that the sensation arose partly from the opposite sides rubbing against each other. But this theory is equally untenable; for the stomach generally contains a sufficient quantity of air to prevent the actual contact of its sides, and moreover it may be entirely void of food and yet no appetite be felt. It may be laid down, indeed, as a general rule, that an interval of rest must follow the termination of digestion before the stomach becomes fit to resume its functions, or appetite is experienced in any degree of intensity; and the length of time required for this purpose varies very much, according to the mode of life and to the extent of waste going on in the system. In many diseases, too, the stomach remains empty for days in succession, without any corresponding excitation of hunger. Even in healthy sedentary people, whose expenditure of bodily substance is small, real appetite is not felt till long after the stomach is empty, and hence one of their most common complaints is the want of appetite.

Dr Beaumont suggests a distended state of the vessels which secrete the gastric juice as the exciting cause of hunger, and thinks that this view is strengthened by the rapidity with which the juice is poured out after a short fast—a rapidity, he says, which cannot be accounted for except by supposing the juice to have existed ready made in the vessels or follicles by which it is secreted. But this theory is not more satisfactory than the rest, for in the sudden flow of saliva into the mouth of a hungry man on the unexpected appearance of savoury viands, we have an instance of equally rapid secretion where there was evidently no storing up beforehand, Besides, there is an obvious relation between appetite and the wants of the system, which is not always taken sufficiently into account, and which is nevertheless too important to be overlooked.

If the body be very actively exercised, and a good deal of waste be effected by perspiration and exhalation from the lungs, the appetite becomes keener, and more urgent for immediate gratification; and if it is indulged, we eat with a relish unknown on other occasions, and afterwards experience a sensation of bien-être or internal comfort pervading the frame, as if every individual part of the body were imbued with a feeling of contentment and satisfaction, the very opposite of the restless discomfort and depression which come upon us, and extend over the whole system, when appetite is disappointed. An amusing example of the principle here inculcated is to be found in the Correspondance Inedite de Madame du Deffand,[6] where she describes her friend Madame de Pequigni as an insatiable bustling little woman who consumes two hours every day in devouring her dinner, and “eats like a wolf.” But, then, remarks Madame du Deffand by way of explanation, “Il est vrai qu’elle fait un exercice enragé.”

There is, in short, an obvious and active sympathy between the condition and bearing of the stomach and those of every part of the animal frame—in virtue of which, hunger is felt very keenly when the general system stands in urgent need of repair, and very moderately when no waste has been suffered. This principle is strikingly illustrated during recovery from a severe illness. “In convalescence from an acute disease,” as is well remarked by Brachet, “the stomach digests vigorously, and yet the individual is always hungry. This happens because all the wasted organs and tissues demand the means of repair, and demand them from the stomach, which has the charge of sending them; and, therefore, they keep up in it the continual sensation of want, which, however, is, in this case, only sympathetic of the state of the body.”[7] In alluding to this subject, Blaine observes, that “Hunger and thirst can only be satisfactorily explained by considering them as properties in the stomach by which it sympathizes with the wants of the constitution; and hence it is, that food taken in invigorates, even before it can be digested.”[8] Hence also the prostration of strength that is felt when the stomach has been for some time empty.

This sympathy is sometimes singularly manifest even in disease. In some cases of affection of the mesenteric glands, for example, where stomachic digestion remains for a time pretty healthy, and the general system suffers chiefly from the want of nourishment caused by the passage of the chyle into the blood being obstructed, the appetite continues as keen and often keener than before; because the system, being in want of nourishment, and the stomach healthy, all its natural causes continue to act as before; and accordingly, when food is taken, it is digested there as usual, but the chyle which is formed from it in the intestine can no longer be transmitted through the swollen glands in its usual healthy manner, to be converted into nutritive blood in the lungs; and the system thus failing to receive the required supply, recommences its cravings almost as soon as if no food had been obtained. When the disease has advanced a certain length, however, fever springs up, and destroys both appetite and digestion.

The effects of exercise, also, shew very clearly the connexion between appetite and the state of the system. If we merely saunter out for a given time every day, without being actively enough engaged to quicken the circulation and induce increased exhalation from the skin and lungs, we come in with scarcely any change of feeling or condition; whereas, if we exert ourselves sufficiently to give a general impetus to the circulation, and bring out moderate perspiration, but without inducing fatigue, we feel a lightness and energy of a very pleasurable description, and generally accompanied by a strong desire for food. Hence the keen relish with which the fox-hunter sits down to table after a successful chase.

This intimate communion between the state of the system and that of the stomach is a beautiful provision of Nature, and is one of the causes of the ready sympathy which has often been remarked as existing between the stomach and all the other organs—in other words, of the readiness with which they accompany it in its departure from health, and the corresponding aptitude of their disorders to produce derangement of the digestive function. Apparently for the purpose, among others, of thus intimately connecting the stomach with the rest of the system, it is supplied with a profusion of nervous filaments of every kind, which form a closely-interwoven nervous network in its immediate neighbourhood, and the abundance of which accounts for the severe and often suddenly fatal result of a heavy blow on the pit of the stomach.

Without pretending to determine what the precise condition of the nerves of the stomach is, which, when conveyed to the brain, excites the sensation of appetite, I think it sufficient for every practical purpose if we keep in mind, that the co-operation of the nervous system is necessary for the production of appetite, and that there is a direct sympathy between the stomach and the rest of the body, by means of which the stimulus of hunger becomes unusually urgent where the bodily waste has been great, although a comparatively short time has elapsed since the preceding meal.

Appetite, then, being given for the express purpose of warning as when a supply of food is necessary, it follows that its call will be experienced in the highest intensity when waste and growth—or, in other words, the operations which demand supplies of fresh materials—are most active; and in the lowest intensity when, from indolence and the cessation of growth, the demand is least. In youth, accordingly, when bodily activity is very great, and a liberal supply of nourishment is required both to repair waste and to carry on growth, the appetite is keener and less discriminating than at any other period of life, and, what is worthy of remark, as another admirable instance of adaptation, digestion is proportionally vigorous and rapid, and abstinence is borne with great difficulty; whereas, in mature age, when growth is finished and the mode of life more sedentary, the same abundance of aliment is no longer needed, the appetite becomes less keen and more select in its choice, and digestion loses something of the resistless power which generally distinguishes it in early youth. Articles of food which were once digested with ease are now burdensome to the stomach, and, if not altogether rejected, are disposed of with a degree of labour and difficulty that was formerly unknown. Abstinence also is now more easily supported.

When, however, the mode of life in mature age is active and laborious, and the waste matter thrown out of the system is consequently considerable, the appetite for food and the power of digesting it are correspondingly strong; for in general it is only when the mode of life is indolent and inactive, and the waste consequently small, that the appetite and digestion are weak. So natural, indeed, is the connexion between the two conditions, that exercise is proverbially the first thing we think of recommending to improve the appetite and the tone of the digestive organs, when these are observed to be impaired; and where positive disease does not exist, no other remedy is half so effectual. But, as already noticed, exercise to be beneficial must be of a description calculated to increase the activity of the secretions and excretions; otherwise it cannot place the system in a condition to require an abundant supply or excite vigorous digestion.

It is highly important to notice this natural relation between waste and appetite, and between appetite and digestion; because, if it be real, appetite must be the safest guide we can follow in determining when and how much we ought to eat. It is true that in disease, and amidst the factitious calls and wants of civilized life, its suggestions are often perverted, and that hence we may err in blindly following every thing which assumes its semblance. The conclusion to be drawn from this, however, is, not that the sense of hunger will, if trusted to, generally mislead us, but only that we must learn to distinguish its true dictates before we can implicitly rely on its guidance. If, when fairly consulted, its dictates are found to be erroneous, it will constitute the only known instance where the Creator has failed in the attempt to fulfil his own design—an assumption not only repugnant alike to feeling and to reason, but in fact altogether gratuitous. For the apparent discrepancies which occasionally present themselves between the wants of the system and the dictates of appetite, are easily explicable on the more solid ground of our own ignorance and inattention.

Many practical errors arise from overlooking the relation which nutrition ought to bear to waste and growth. Thus, it is no uncommon thing for young men who have experienced all the pleasures of a keen appetite and easy digestion when growing rapidly or leading an active life, to induce severe and protracted indigestion, by continuing, from mere habit, to eat an equal quantity of food either when growth is finished and the system no longer requires the same extensive supply, or after a complete change from active to sedentary habits has greatly diminished that waste which alone renders food necessary. This is, in fact, one of the chief sources of the troublesome dyspeptic complaints often met with among the youthful inhabitants of our larger cities and colleges, and ought not to be lost sight of in the physical education of the young.

The error, however, is unhappily not confined to the young, but extends generally to all whose pursuits are of a sedentary nature. There are numerous persons, especially in towns and among females, who, having their time and employments entirely at their own disposal, carefully avoid every thing which requires an effort of mind or body, and pass their lives in a state of inaction entirely incompatible with the healthy performance of the various animal functions. Having no bodily exertion to excite waste, promote circulation, or stimulate nutrition, they experience little keenness of appetite, have weak powers of digestion, and require but a limited supply of food. If, while inactive and expending little, such persons could be contented to follow nature so far as not to provoke appetite by stimulants and cookery, and to eat and drink only in proportion to the wants of the system, they would fare comparatively well. But having no imperative occupation and no enjoyment from active and useful exertion, their time hangs heavily on their hands, and they are apt to have recourse to eating as the only avenue to pleasure still open to them; and, forgetful or ignorant of the relation subsisting between waste and nutrition, they endeavour to renew, in the present indulgence of appetite, the real enjoyment which its legitimate gratification afforded under different circumstances. Pursuing the pleasures of the table with the same ardour as before, they eat and drink freely and abundantly, and, instead of trying to acquire a healthy desire for food and increased powers of digestion by exercise, they resort to tonics, spices, wine, and other stimuli, which certainly excite for the moment, but eventually aggravate the mischief by obscuring its progress and extent. The natural result of this mode of proceeding is, that the stomach becomes oppressed by excess of exertion—healthy appetite gives way, and morbid craving takes its place—sickness, head-ach, and bilious attacks, become frequent—the bowels are habitually disordered, the feet cold, and the circulation irregular—and a state of bodily weakness and mental irritability is induced, which constitutes a heavy penalty for the previous indulgence. So far, however, is the true cause of all these phenomena from being perceived even then, that a cure is sought, not in a better regulated diet and regimen, but from bitters to strengthen the stomach, laxatives to carry off the redundant materials from the system, wine to overcome the sense of sinking, and heavy lunches to satisfy the morbid craving which they only silence for a little. Some, of course, suffer in a greater and others in a less degree, according to peculiarities of constitution, mode of life, and extent of indulgence; but daily experience will testify, that, in its main features, the foregoing description is not over-charged, and that victims to such dietetic errors are to be met with in every class of society.

The fact of Nature having meant the inactive and indolent to eat and drink less than the busy and laborious, is established not only by the diminished appetite and impaired digestion of human beings who lead a sedentary life, as contrasted with the keen relish and rapid digestion usually attendant on active exertion in the open air, but on a yet broader scale by the analogy of all other animals. In noticing this relation, Dr Roget remarks that “the greater the energy with which the more peculiarly animal functions of sensation and muscular action are exercised, the greater must be the demand for nourishment, in order to supply the expenditure of vital force created by these exertions. Compared with the torpid and sluggish reptile, the active and vivacious bird or quadruped requires and consumes a much larger quantity of nutriment. The tortoise, the turtle, the toad, the frog, and the chameleon, will indeed live for months without taking any food.” “The rapidity of development,” he continues, “has also great influence on the quantity of food which an animal requires. Thus the caterpillar, which grows very quickly, and must repeatedly throw off its integuments, during its continuance in the larva state, consumes a vast quantity of food compared with the size of its body; and hence we find it provided with a digestive apparatus of considerable size.”[9] Hence, too, the greater demand for food in infancy and youth when growth and activity are both at their height.

In thus insisting on regular bodily and mental activity as indispensable to the enjoyment of a good appetite and sound digestion, the attentive reader will not, I trust, be disposed to accuse me of inconsistency because, when treating of muscular exercise in the former volume,[10] I explained the bad effects, and inculcated the impropriety, of indulging in any considerable exertion immediately before or after a full meal. It is true, as there mentioned, that exercise, either in excess or at an improper time, impairs the tone of the stomach; but it is not on that account the less true that bodily exertion when seasonably and properly practised, is the best promoter of appetite and digestion which we possess; and it is only under the latter conditions that I now speak of it as beneficial and even indispensable to health.

In a work like the present, it is obviously impossible to fence round every general proposition with the numerous limitations which an unusual combination of circumstances, or a departure from the state of health, might demand. And, even if possible, it would not be necessary, as the laws of exercise have been so fully explained in the volume alluded to, that their re-discussion here would unavoidably involve much repetition from its pages. At the same time, some warning remark may be required to prevent any risk of misconception, as it might otherwise be plausibly argued, for example, that there can be no such relation as I have alleged between waste and appetite, because a European perspiring under a tropical sun incurs great waste, and yet loses both appetite and digestive power. To render this a valid exception, it must be shewn that the European is intended by Nature to live in a tropical climate, and that the diet to which he accustoms himself is that sanctioned by experience as the best adapted for his constitution; because, if neither is the case, his condition under such influences must necessarily be more or less closely allied to the state of disease, and therefore beyond the sphere to which alone my remarks are meant to apply. But even in that instance there is less contradiction than might be imagined, for the waste of the system being chiefly fluid, excites—not appetite, but its kindred sensation—thirst, to repair the loss by an unusual demand for refreshing liquids.

So true is it that the Creator has established a relation between action and nutrition, that when we attempt for any length of time to combine a full and nutritious diet with systematic inactivity, the derangement of health which generally ensues gives ample proof of the futility of struggling against His laws. Individuals, indeed, may be met with, who, from some peculiarity of constitution, suffer less than the generality of mankind from making the experiment; but even those among them who escape best, generally owe their safety to the constant use of medicine, or to a natural excess in some of the excretory functions, such as perspiration or the urinary or alvine discharges, by means of which the system is relieved much in the same way as by active exercise; and hence the remark made by Hippocrates, that severe perspirations arising during sleep without any other apparent cause are a sure sign that too much nourishment is made use of. In others, again, the day of reckoning is merely delayed, and there is habitually present a state of repletion, which clogs the bodily functions, and may lead to sudden death by some acute disease when the individual is apparently in the highest health. I am acquainted with several individuals of this description, who, in the absence of all bodily exercise, are accustomed to live very fully,—to eat in the morning a hearty breakfast, with eggs, fish, or flesh,—a good solid luncheon, with wine or malt liquor, in the forenoon,—a most substantial dinner, with dessert and several glasses of wine, and afterwards tea and wine and water, in the evening,—and who nevertheless enjoy tolerably good digestion. But this advantage is generally only temporary, and even when permanent can scarcely be considered as a boon; because it is gained at the direct expense either of a very full habit of body and an unusual liability to abdominal congestion and all its attendant evils, or of frequent and profuse perspirations, and severe attacks of bowel complaint, endangering life; so that strictly considered such cases are no exceptions to the general rule.

It is, then, no idle whim of the physician to insist on active exercise as the best promoter of appetite and digestion. Exercise is, in fact, the condition without which exhalation and excretion cannot go on sufficiently fast to clear the system of materials previously taken in; and where no waste is incurred, no need of a fresh supply, and consequently, in a healthy state of the system, no natural appetite, can exist. It is therefore not less unreasonable than vain for any one to insist on possessing, at the same time, the incompatible enjoyments of luxurious indolence and a vigorous appetite,—sound digestion of a hearty meal, and general health of body; and no one who is aware of the relation subsisting between waste and appetite can fail to perceive the fact, and to wonder at the contrary notion having ever been entertained.

Among the operative part of the community we meet with innumerable examples of an opposite condition of the system, where, from excess of labour, a greater expenditure of energy and substance takes place than what their deficient diet is able to repair. It is true that the disproportion is generally not sufficient to cause that immediate wasting which accompanies actual starvation, but its effects are nevertheless very palpably manifest in the depressed buoyancy, early old age, and shorter lives of the labouring classes. Few, indeed, of those who are habitually subjected to considerable and continued exertion survive their forty-fifth or fiftieth year. Exhausted at length by the constant recurrence of their daily task and imperfect nourishment, they die of premature decay long before attaining the natural limit of human existence.

In those states of the system, again, such as fever, during the continuance of which most of the secretions are vitiated, and that of the gastric juice often entirely suppressed, and where food would consequently be hurtful rather than advantageous, appetite is scarcely felt, and loathing often occupies its place. But the moment that, by the diminution of the disease, the secretions and exhalations begin to return to their healthy state, and nutrition is resumed, appetite begins to be again felt, and by-and-by becomes abundantly vigorous, in order to restore the system to its former state. The utmost caution, however, is still required in its gratification, as a premature indulgence is almost certain again to stop the secretions and to produce a relapse. Ignorance of this principle among the community at large, and the consequent error of giving food when there is no demand for it, and no gastric juice to digest it, often do more to defeat the best laid plan of cure than the severity of the disease itself. The sick man’s friends, in their anxiety to support his strength, too frequently turn a deaf ear to every caution which is suggested, and stealthily administer sustenance when the system does not require it, and when it serves only to aggravate the danger and increase the weakness of the patient. Since the first publication of the preceding passage, I have seen a striking example of its truth. The patient was gradually recovering from inflammation of the chest, for the cure of which low diet was for a time indispensable. By way of supporting the diminished strength, the relations began to give prematurely and clandestinely, about double the quantity of food which was prescribed. For twenty-four hours, an increase of strength was felt accordingly; but very soon it passed into febrile excitement with a quick pulse and increased weakness. A dangerous relapse followed, and its cause was then found out. Abstinence was again enforced and tartar emetic given to excite nausea. To the surprise of the very injudicious friends, the excitement began almost immediately to subside and the strength to improve where they had just seen it fast giving way under a full diet.

Appetite, it ought to be observed, may, like other sensations, be educated or trained to considerable deviations from the ordinary standard of quantity and quality—and this obviously for the purpose of enabling man to live in different climates and under different circumstances, and avoid being fixed down to one spot and to one occupation. In civilized life, however, we are accustomed to take undue advantage of this capability, by training the appetite to desire a greater quantity of food than what the wants of the system require, and stimulating its cravings by a system of cookery little in harmony with the intentions of Nature. But this is evidently an abuse, and no argument whatever against the sufficiency of its natural indications to lead us right.

But the most common source of the errors into which we are apt to fall in taking appetite as our only guide, is unquestionably the confounding of appetite with taste, and continuing to eat for the gratification of the latter long after the former is satisfied; just as the dog already mentioned ate till the œsophagus was distended, although it did not experience the slightest sensation of hunger.[11] In fact, the whole science of a skilful cook is expended in producing this willing mistake on our part; and he is considered decidedly the best artiste whose dishes shall recommend themselves most irresistibly to the callous palate of the gourmand, and excite on it such a sensation as shall at least remind him of the enviable excellence of a natural appetite. If we were willing to limit the office of taste to its proper sphere and to cease eating when appetite expressed content, indigestion would be a much rarer occurrence in civilized communities than it is observed to be.

Viewed, then, in its proper light, appetite is to be regarded as kindly implanted in our nature for the express end of proportioning the supply of nourishment to the wants of the system; and if ever it misleads us, the fault is not in its unfitness for its object, but in the artificial training which it receives at our own hands, and in our habitual neglect of its dictates. When we attend to its real indications, we eat moderately, and at such intervals of time as the previous exercise and other circumstances render necessary; and in so doing, we reap a reward in the daily enjoyment of the pleasure which attends the gratification of healthy appetite. But if we err, either by neglecting the timely warning which it gives, or by eating more than the system requires, mischief is sure to follow. In the former case, waste continues to make progress till the body becomes exhausted; and in almost exact proportion do the cravings of appetite become more and more intense, till they pass into those of uncontrollable hunger, which overthrows all obstacles, and seeks gratification at the risk of life itself. In the latter case, indigestion, gloomy depression, and repletion with its concomitant evils, make their appearance, and either embitter or cut short existence.

Mischief sometimes arises also from people not being sufficiently aware that, in common with other sensations, appetite may be so far deranged by disease as to give very incorrect and unnatural indications. It often happens, for example, that a patient shivers and complains of cold, when we know by the thermometer that the heat of the skin is really above instead of below the natural standard. In like manner, in some morbid states of the nervous system a craving is often felt which impels the patient to eat, but which is not true hunger; and here food, if taken, is digested with great difficulty. Occasionally, on the other hand, no desire for food is experienced when the system really needs it, and when it would be digested with ease if introduced into the stomach. Esquirol alludes to cases of this description, and I have met with similar examples. Voison also mentions, that, in the Hospital of Incurables in Paris, there are some idiots so low in the scale of intelligence, as to make no attempt to take the food which is placed before them, although they eat and digest readily when fed by others. Sometimes, again, appetite is depraved in quality, and the patient desiderates the most nauseous and repulsive kinds of food, such as earth, chalk, coals, or excrement. There are states, too, in which the appetite is prodigiously increased, and the patient consumes incredible quantities of food,—which, however, are very imperfectly digested. Charles Domery, for instance, when a French prisoner at Liverpool, consumed, in one day, four pounds of cow’s udder and ten pounds of raw beef, with two pounds of tallow candles and five bottles of porter; and although allowed the daily rations of ten men he was still not satisfied. Baron Percy speaks of another man, who ate twenty-four pounds of beef in as many hours, and thought nothing of swallowing a dinner prepared for fifteen German boors. I once attended a patient who was afflicted with a similar inordinate craving, and whose only pleasure was in eating. In such cases no restraint except actual coercion is sufficient to prevent indulgence; but the craving itself is as much the product of disease as the shivering in the beginning of fever, and can no more be removed by reasoning than the sensation of cold can be removed by telling a patient that his skin is thermometrically warm. But these, being cases of disease, do not in any degree militate against the accuracy of the exposition above given of the healthy uses of appetite.

The general considerations which I have just submitted to the reader on the subject of appetite for food, apply so closely to the sensation of Thirst, that to enter into any detail concerning the latter would be little else but to be guilty of repetition. I shall therefore limit myself to a very few remarks.

Thirst is generally said to have its seat in the back of the mouth and throat; but the condition of these parts is merely a local accompaniment of a want experienced by the whole frame, and perceived by the nervous system. Local applications, accordingly, go but a short way in giving relief, while the introduction of fluids by any other channel—by immersion in a bath, by injection into the veins, or through an external opening into the stomach—is sufficient to quench thirst without the liquid ever touching the throat. The affection of that part, therefore, is merely a result of the state of the system, and not itself the cause of thirst.

Thirst, or a desire for liquids, is experienced in its greatest intensity when the secretion and exhalation of the animal fluids is most active; and it is consequently most urgent in summer, in warm climates, and in persons engaged in severe exertion, particularly if exposed at the same time to a heated atmosphere. Blacksmiths, glass-blowers, engineers, and others, whose employment exposes them to the heat of furnaces, and in whom perspiration is excessive, are accordingly almost constantly under the influence of thirst; whereas those who are employed in professions requiring only moderate exertion in a temperate atmosphere, and in whom the fluid secretions are very moderate, rarely experience the sensation in an urgent degree. For the same reason great loss of blood induces excessive and intolerable thirst; and hence, in the battle-field, the generous self-denial of him who passed the cup to his wounded neighbour, without stopping even to moisten his own lips, cannot be too highly appreciated.

Thirst varies in intensity also according to the nature of the food. If the diet be hot and stimulating, such as results from a free admixture of spices or salt, the desire for drink is greatly increased. The same thing happens if the food be of a dry and solid nature. The purpose of the increased thirst in the former circumstances is manifestly to dilute and diminish the excess of stimulant, and thereby prevent the injury which it would otherwise inflict. The same principle explains the thirst experienced by those who drink too much wine. In instances of this kind I have heard great thirst in the evening and during the night complained of as habitual, without the person even suspecting that it was owing to the wine; and yet, on abstaining from the latter, the thirst very soon disappeared.

Continued thirst, it is well known, is much more intolerable than continued hunger. The mass of circulating fluid in the body is very great, and, as the various excretions consist chiefly of fluid matter, it necessarily happens that when these have been eliminated for a considerable time without any liquid being received into the system, the proportion of solid matter in the body becomes unduly large. The blood, consequently, becomes thicker and changed in quality, and much more irritating than it is in its natural state. The craving of thirst is thus generally rendered more urgent and overpowering than that of hunger.

In Asiatic Cholera, the watery portion of the blood, on which its fluidity depends, is drained off with frightful rapidity; and the result is, in the first place, an almost complete stoppage of the circulation, and, in the second, a constant craving for drink to supply the place of the lost serum, which consists chiefly of water holding some of the alkaline salts in solution. This circumstance explains in some degree the extraordinary effects which have been produced, even in the worst stages of the disease when life seemed almost extinct, by injecting large quantities of saline solutions into the veins. Patients apparently on the verge of existence, cold, pulseless, and inanimate, have in the course of a few minutes been enabled by this means to sit up in bed, and to exhibit all the signs of restored strength and health. The effect, it is true, was rarely permanent, but for the time it was so wonderful as often to look like restoration from the dead.[12]

Fluids taken into the stomach, it is proper to observe, are not subjected to the slow process of digestion, but are absorbed directly into the system; so that, when we take a moderate draught, the whole of it is imbibed from the stomach in a very few minutes. Keeping in view this fact, and the above striking illustration of the influence of the condition of the blood upon the body at large, it becomes easy to conceive why, in a state of exhaustion from abstinence, drink should be more speedily restorative and refreshing than food.

Thirst, like appetite for food, is intended to direct us when and in what quantity we ought to drink; and so long as we lead a life of ordinary health and activity, and confine ourselves to the fluids with which nature provides us, there is little chance of our going far wrong by listening to its calls. But when we become indolent and dyspeptic, or resort to the use of fermented and stimulating liquors, which excite a thirst not recognised by Nature, the principle ceases to apply. At present, however, my observations refer entirely to such simple drinks as water, and to the state of health; and I shall touch upon other liquids when treating of diet in a subsequent part of the volume. Many persons without experiencing any real thirst, habitually indulge in potations of water or beer at all hours of the day, and to an extraordinary extent, and feel unhappy when suddenly restricted in the indulgence. But this temporary discomfort ought not to be considered as indicating that these potations are really necessary, because the same result happens in the analogous instances of smoking or snuffing. All three are abuses and perversions of Nature, and the uneasiness attending the sudden cessation of the beer or water drinking is no more a proof of either fluid being required, than that consequent on giving up cigar smoking is an indication that Nature designed the lungs for the reception of the impure effluvia of the tobacco leaf instead of the fresh breezes of heaven.

CHAPTER III.
MASTICATION, INSALIVATION, AND DEGLUTITION.

Mastication—The teeth—Teeth, being adapted to the kind of food, vary at different ages and in different animals—Teeth classed and described—Vitality of teeth and its advantages—Causes of disease in teeth—Means of protection—Insalivation and its uses—Gratification of taste in mastication—Deglutition.

Having seen that a regular supply of nourishment is carefully insured by the constantly returning impulses of Appetite, we come next to examine the mode in which the food is prepared for becoming a constituent part of the animal machine, and endowed with the properties of life.

The first important step in the complicated process of digestion, is that by which the food, after being received into the mouth, is mixed with the saliva and broken down till it becomes of a uniform pulpy consistence, fit for being easily swallowed and acted upon by the gastric juice on its arrival in the stomach. The term mastication or chewing is used to denote this operation; and the chief instruments by which it is performed are the teeth, the jaws, the muscles which move the jaws, the tongue, and the salivary glands. On each of these we shall offer a few observations.

The TEETH vary a good deal according to the kind of food on which the animal is destined to live; but in man and the higher orders of animals they may be divided into three distinct groups:—1st, The incisor or cutting teeth, being the eight broad and flat teeth with a sharp cutting edge seen in front of the upper and lower jaws, and marked I in the subjoined wood-cut, which represents one-half of the lower jaw, and consequently only one-fourth of the whole number of teeth. Thus we find only two incisors marked in the wood-cut, although there are eight of them in all, viz. two more are on the other side of the lower jaw, and four corresponding ones in the upper. 2d, The cuspidati, canine, or dog teeth, being the sharp-pointed roundish-bodied teeth, four in number, one, C, in contact with each of the outer incisor teeth, and called canine from being large in the dog and carnivorous animals, and used by them for the purpose of seizing and tearing their food; and, 3d, The molares or grinders, B G, twenty in number situated at the back part of the jaw, and so called from their office being to grind or bruise the food subjected to their action.[13] The term grinders, however, is sometimes restricted to the three back teeth on each side, marked G, and seen to have double roots and a broad grinding surface; and the two, B, intervening between them and the cuspidati are styled bicuspidati or double-speared, from bearing a greater resemblance to a double-headed canine tooth than to the other grinders.

The teeth are modified in different animals to suit their habits of life. In herbivorous animals, the canine teeth, for which they have no use, are comparatively undeveloped; whereas in carnivorous animals, which tear their prey in pieces, the canine teeth are large, powerful, and pointed, and the incisors comparatively small. In these animals they constitute what are properly called the tusks, and in some species they are of a truly formidable character. The molar or grinding teeth differ in like manner according to the nature of the food. In herbivorous and granivorous animals they are large and powerful, and to increase their efficiency the lower jaw admits of considerable lateral motion in a horizontal direction; whereas, in carnivorous animals, it admits of motion only upwards and downwards, as in opening and shutting the mouth. The lateral grinding motion is very evident in ruminating animals, such as the cow, which, after having filled its stomach with provender, is generally seen to lie down and ruminate, or chew the cud as it is called—the rumination consisting in bringing up small masses of herbage from the stomach, and submitting them to a thorough mastication or grinding between its molar teeth before being again swallowed and digested.

From this relation between the food and the organs of mastication, naturalists can tell with certainty, by simply inspecting the teeth, on what kind of food the animal to which they belong is intended to live; and as the teeth of man partake of the characters of those of both herbivorous and carnivorous animals, there cannot be a doubt that his diet was intended to be of a mixed kind, not confined exclusively to either the vegetable or the animal kingdom.

Hard and resisting as the teeth appear, they must still be regarded as living structures. Anatomically speaking, each tooth is divided into three parts: the fang or root, implanted in the socket of the jaw-bone; the neck, or portion encircled by the gum; and the white crown, appearing above the gum, and covered with enamel.

The root of each tooth is perforated longitudinally by a small canal, through which the bloodvessels and nerve are admitted to its central parts. From these bloodvessels the tooth derives its nourishment when growing; but they afterwards almost entirely disappear. From the nerve it derives that sensibility which makes us instantly aware of the contact of bodies either too hot or too cold with the teeth; and which, when the nerve is diseased, gives rise to the racking pain of toothach.

So effectually is life maintained in the teeth by this provision of vessels and nerves, that a tooth newly extracted from the socket of a young animal, and implanted in the fleshy comb of a cock, has been found to adhere and retain its vitality; and, in like manner, if, in early life, a tooth extracted by accident be immediately replaced in its socket, it will generally adhere and live.

The visible part or crown of the tooth is covered with a very hard white ivory-looking substance called enamel, which serves to prevent it from being worn down by friction, and into which neither bloodvessels nor nerves have been observed to penetrate. Owing to this structure, the tooth can be safely exposed without sustaining damage—a privilege on which most persons will be disposed to place a higher value after having experienced the pains consequent upon injury of the nerve from a portion of the enamel being broken off.

An obvious advantage attending the vitality of the teeth is, that it enables them to accommodate themselves to the growth of the jaw and the rest of the system at the different periods of life. In early infancy, when the human being is designed to live exclusively on his mother’s milk, which of course requires no mastication and consequently no teeth, the latter are still imperfectly formed and entirely hidden in the jaw: it is only at the end of some months that the front or cutting teeth begin to appear; and the whole set of milk, deciduous, or falling-out teeth, twenty in number, is not completed till about or after the third year. In the course of three or four years more, however, growth has advanced so far that the first set of teeth no longer fill the jaw; and they soon begin to be displaced by the second or permanent set, the gradual development of which commences at that period of life, and is not finished till the appearance of the last four grinders or wisdom-teeth, about the age of maturity.

It is a curious fact that the infant is born with the rudiments of both sets of teeth in the jaw at the same time, although neither makes its appearance till long after birth. The permanent teeth lie in a line under the milk-teeth, and it is from their growth causing the gradual absorption of the roots of the first teeth that the latter no longer retain their hold of the jaw, but drop out as soon as the others are ready to protrude. In the preceding wood-cut, the situation of the permanent teeth before they emerge from the jaw is rudely represented at A, where the outer surface of the jaw-bone has been removed on purpose, to shew the appearance of the roots. But nothing of this kind is to be found in the adult jaw, the parts marked A being inserted in the plate merely to illustrate what was once the position which the permanent teeth occupied.

The changes in the condition of the teeth, it may be remarked in passing, indicate clearly what species of food nature has intended for us at different ages. In early infancy, when no teeth exist, the mother’s milk is the only nutriment required; and in proportion as the teeth begin to appear, a small addition of soft farinaceous food prepared with milk may be made with propriety and gradually increased. But it is impossible to look at the small jaw, moderate muscle, and imperfect teeth of early life, without perceiving that only the mildest kinds and forms of animal food are yet admissible, and that the diet ought to consist essentially of soft and unirritating materials. It is not till the permanent teeth have appeared, that a full proportion of the ordinary kinds of butcher-meat becomes either beneficial or safe.

The teeth, being living parts and at the same time endowed with a mechanical function, are liable to injury in both capacities. Being composed chiefly of earthy matter, such as phosphate and carbonate of lime, the contact of strong acids decomposes their substance, and leads to their rapid decay. Hence the whiteness produced by acid tooth-powders and washes is not less deceitful than ruinous in its consequences; and hence also great caution is necessary in swallowing the acid drops frequently prescribed by the physician, which ought never to be allowed to come into contact with the teeth.

The teeth being constantly moistened with saliva, have a tendency to become incrusted with the tartar or earthy matter which it contains in solution, and which is separated from it partly by the evaporation of the more fluid constituents in breathing, and partly by chemical decomposition. As this incrustation not only destroys the beauty of the teeth, but also promotes their decay, it becomes an object of care to remove it as soon as it is formed; and the most effectual mode of doing so is to brush the teeth regularly twice a day—especially in the morning, when the quantity is greatest—with a brush dipped in soft water, till every particle is removed. The addition of any soft impalpable powder will assist in the effect; but nothing capable of acting chemically on the teeth, or of injuring them by friction, ought ever to be resorted to. Washing the mouth after every meal is also a good preservative.[14]

When the teeth are not used for a time, and when digestion is impaired, the quantity of tartar which accumulates on them is very great. Hence they are always most incrusted in the morning, and in fevers and other affections where no food is taken, and the stomach is at the same time disordered. I have seen one instance in which a thick crust of tartar was removed by a dentist in the belief of its being a diseased tooth—the tooth itself on which it was formed being left in the jaw perfectly sound.

When the tartar is not duly removed, its presence injures the teeth, irritates the gums, and generally leads, sooner or later, to considerable suffering. The regular washing and brushing above mentioned ought, therefore, to be sedulously practised at every period of life, and taught as a duty to the young. When digestion is very vigorous, the health good, and the diet plain and containing a full proportion of vegetable matter, the deposition of tartar seems to be diminished, and the teeth to be naturally of a purer white. Many rustics and savages thus possess teeth which would be envied in a town.

When digestion is impaired, and acidity prevails in the stomach, the mucous secretions in the mouth also become altered in character, and by their incessant contact injure and even destroy the teeth. From this cause we often see the teeth in young people in a state of complete decay. They are in reality the subjects of chemical decomposition, and eaten away by the morbid secretions of the mouth; and hence, in such cases, we generally find the individual complaining of heat and soreness of the tongue, gums, and mouth, and occasionally of the teeth being “set on edge.”

Considered as living parts, the teeth require some additional care. In that capacity they are exceedingly apt to suffer from sudden changes of temperature. Being from their solidity rapid conductors of heat, their internal nerve speedily becomes affected by the alternations of temperature to which they are daily exposed, both in taking food and in the change from a warm to a cold atmosphere. It is a not uncommon practice, for example, to take a glass of cold wine or water immediately after finishing a plateful of very hot soup; and it is quite usual to take tea and coffee and every kind of meat as hot as they can possibly be swallowed—than which practices it would be difficult to imagine any thing more hurtful to the teeth.

For the same reason, in going out at night from a warm room to the coil air, it is desirable to protect the teeth from the influence of the sudden change, by breathing through two or three folds of a silk handkerchief, or through a woollen comforter. When the teeth and lower part of the face are left exposed in such circumstances, rheumatism and toothach not unfrequently ensue from the direct impression of the cold air upon parts rendered more susceptible by the preceding heat.

The great source of injury to the teeth, however, both in childhood and in mature age, is disordered digestion. If the health be good, and the stomach perform its functions with vigour, the teeth will resist much exposure without sustaining injury. But if these conditions fail, they will rarely continue long unscathed.

It is almost always from the latter cause that, in infancy, teething so often gives rise to serious constitutional disorder.

Something more, however, than the mere action of the teeth and jaws is required to prepare the morsel for being swallowed. If we take a bit of dry biscuit or mealy potato into the mouth, and attempt to masticate it, we encounter at first no small difficulty from the stiffness and resistance of the dry mass, and feel instinctively that it would be in vain to attempt to swallow it, until moistened either by continued mastication or by the admixture of fluid from without. In ordinary states of the system, accordingly, a fluid called saliva or spittle is copiously secreted and poured into the mouth for this very purpose; and the process by which its due admixture with the contents of the mouth is accomplished is called the insalivation of the food.

To provide this necessary fluid, and to connect its supply directly with the process of mastication to which it is subservient, several glands for its secretion have been placed in the immediate neighbourhood of the mouth and jaws, in such a way that the latter cannot be opened and shut without affording them a stimulus, and still farther increasing the secretion which the presence of the morsel is itself sufficient to begin. From this arrangement it follows, that the more perfectly mastication is performed, the more thoroughly does the morsel become impregnated with the salivary fluid, and the better fitted is it rendered for subsequent deglutition and digestion.

The apparatus of mastication varies according to the kind of food on which the animal is destined to live; but in the higher orders of animals, it consists essentially of the parts already mentioned. In some animals, however, which live on soft gelatinous food—as the whale—no teeth are to be found, because their peculiar power is not required. In others—as the granivorous or grain-eating birds—the grinding or triturating process is effected not in the mouth but in the gizzard, where the food (mixed with gravel, which the animal is instinctively impelled to swallow for the purpose) is effectually bruised and softened down by the strong muscles which constitute the greater part of its substance. In these instances, the gravel is the grinding instrument, and without its presence digestion cannot be carried on any more than it could in man without the agency of teeth.

The degree of mastication required varies also according to the mode of life of the animal, and the digestibility of its food. Animal food, for example, being easy of digestion, requires less mastication than vegetable food, which is more difficult. This is so much the case, that most animals which live on fresh vegetable matter spend half their waking hours in ruminating or re-masticating the food, which they have already cropped and stored up for the purpose in one of their four stomachs. To this necessary act in them, Providence seems to have attached a high degree of gratification, for the very purpose of insuring its regular performance.

Man, being naturally omnivorous, or adapted for the digestion of both animal and vegetable substances, holds, as it were, an intermediate place in regard to the rapidity of mastication. He is neither obliged to ruminate like the cow, nor can he beneficially bolt his food with the rapidity displayed by birds of prey. His object is merely to reduce the alimentary mass to a soft and pulpy consistence, and digestion is promoted or retarded in exact proportion as he approaches or falls short of this point. Hasty mastication is consequently injurious, because it prevents the food from being sufficiently broken down and impregnated with saliva; and the more uncommon error of protracted mastication is also injurious, owing to the undue dilution which the mass sustains from the overflow of the salivary secretion.

Due mastication being thus essential to healthy digestion, the Creator, as if to insure its being adequately performed, has kindly so arranged, that the very act of mastication should lead to the gratification of taste—the mouth being the seat of that sensation. That this gratification of taste was intended, becomes obvious when we reflect that, even in eating, Nature makes it our interest to give attention to the process in which we are for the time engaged. It is well known, for example, that when food is presented to a hungry man, whose mind is concentrated on the indulgence of his appetite, the saliva begins to flow unbidden, and what he eats is consumed with a peculiar relish and is easily digested. Whereas, if food be presented to an individual who has fasted equally long, but whose soul is absorbed in some great undertaking or deep emotion, and who is consequently insensible to the gratification of taste, it will be swallowed almost without mastication, and without sufficient admixture with the saliva—now deficient in quantity—and therefore lie on the stomach for hours unchanged. In this point of view the peculiarly English custom of reading the newspapers or magazines during breakfast is more hurtful than one would suppose; and many dyspeptics have been surprised at the benefit resulting from its discontinuance. However, therefore, philosophy and morality condemn the undue cultivation of our bodily appetites, it cannot be denied that a certain degree of attention to taste, and to the pleasures of appetite, is both reasonable and beneficial; and it is only when these are abused that we oppose the intentions of Nature.

From the existence of this intentional relation between mastication and the salivary secretion, the latter is always most copious in those creatures whose food requires continued mastication. In ruminating animals, accordingly, the salivary glands are numerous and of great size, while they are at the same time so situated that the play of the muscles in the act of rumination communicates to them a proportionate stimulus. In those, again, which do not masticate at all, but swallow their food entire, there is scarcely any salivary secretion, and the glands appropriated to it are very small. Birds, and many fishes and reptiles, belong to the latter class.

From the foregoing explanation of the object and conditions of mastication, the reason will be apparent why fluids do not require to undergo that process, and also why dry mealy substances stand in need of protracted chewing before they can be easily swallowed. When hot spicy food is taken into the mouth, the secretion of saliva is immensely increased, obviously for the purpose of diluting the excess of stimulant before it shall be allowed to reach the stomach. But when the food is of a mild and unirritating quality, much dilution is unnecessary, and the secretion is accordingly moderate.

The chief purpose of mastication, then, is evidently the minute division of the aliment, so as to admit of its being easily acted upon by the gastric juice when received into the stomach. Dr Beaumont, however, seems to me to go too far in inferring, that “if the materia alimentaria could be introduced into the stomach in a finely divided state, the operations of mastication, insalivation, and deglutition, would not be necessary.” It would require a more extensive range of experiments than that which he has made, to prove that “aliment is as well digested and assimilated, and allays the sensation of hunger as perfectly, when introduced directly into the stomach (through an opening in the side) in a proper state of division, as when the usual previous steps have been taken.”[15] It is quite true that mastication and deglutition are chiefly mechanical processes; but it is difficult to believe that so much care would have been taken to provide a proper supply of fluid of a constant and peculiar character like saliva, if water were capable of answering the purpose as well, and if saliva were useful only in lubricating the food. There subsists, moreover, between the sense of taste and the power of digestion a certain relation, which renders it more than probable that the active gratification of the former during mastication, is favourable to the production and flow of nervous energy towards the stomach, and consequently in so far conducive to the healthy performance of digestion that even in that point of view insalivation could not easily be dispensed with. Dr Beaumont’s experiments, however, abundantly demonstrate that Montègre, and those who, along with him, consider the saliva as the principal agent in digestion, have not a shadow of foundation for their opinion.

When unmasticated food is introduced into the stomach, the gastric juice acts only upon its surface, and changes of a purely chemical nature sometimes commence in its substance before its digestion can be effected. Hence often arise, especially in children, those pains and troubles, that nausea and acidity, consequent on the continued presence of undigested aliment in the stomach. By a peculiarity of constitution, however, the stomach will not long retain food which it cannot dissolve. After a number of hours,—varying, according to the state of health, from one or two to ten, or even twenty,—it is either rejected by vomiting, or transmitted unchanged to the intestine, where its presence irritates and gives rise to colic, flatulence, bowel-complaints, and, in delicate children, not unfrequently to convulsions. Hence another proof of the importance of slow and deliberate mastication.

As soon as the morsel has been thoroughly masticated and impregnated with saliva, it is ready for transmission to the stomach. To this part of the process the term deglutition or swallowing is applied.

Immediately at the back part of the mouth several passages present themselves, leading in different directions—one upwards and forwards into the nose, another downwards and in front into the windpipe, and a third downwards and behind into the œsophagus, gullet or meat-pipe and stomach. The last is the passage taken by the food, and the violent coughing and occasional suffocation induced when it accidentally passes into the windpipe, are but a specimen of the serious evils which would be continually occurring if some provision were not made to obviate the danger, while the rarity with which such accidents actually happen, proves the almost unfailing efficacy of that which has been devised.

The passage of the food into the nostrils is prevented by the interposition of a moveable fleshy curtain or valve hanging down from the palate, and visible at the back part of the mouth; this, in the act of swallowing, is stretched backwards so as to extend to the back part of the throat, and thus entirely shut up the opening into the nostrils. The passage into the windpipe, again, is protected by a cartilaginous lid or covering called epiglottis (from επι, epi, upon, and γλωττις, glottis, the tongue), which projects backwards from the root of the tongue, and conducts the morsel over the glottis or opening of the windpipe. The epiglottis, however, is greatly assisted in this operation by that rising upwards and forwards of the gullet and windpipe to meet the morsel, of which we are conscious, and which can be felt by the hand in the act of swallowing, and the effect of which is in some degree to hide the glottis under the backward projection of the root of the tongue, and allow the morsel to drop past it into the gullet.

Once fairly in the gullet, the course of the food into the stomach is easy enough. The gullet is simply a round tube, made up of two rows of muscular or fleshy fibres, the one longitudinal and the other transverse and circular, with a soft moist lining membrane to facilitate the transmission of its contents. When the morsel is introduced, its upper part contracts involuntarily, and pushes the mass downwards; the portion now reached contracts in its turn, and propels it farther; and so on in succession till it arrives at the stomach.

Deglutition or swallowing is thus a more complicated operation than at first sight it appears to be. On looking at any person eating, one is apt to think that the morsel passes along the gullet into the stomach by its own weight; but we speedily perceive the error, when we recollect that, in the horse and the cow for example, the mouth is on a level with the ground when feeding, or drinking, and that the morsel or water is consequently propelled upwards into the stomach against its own gravity. It is well known also, and often made a matter of public exhibition, that a man can swallow even liquids when standing on the crown of his head, with the natural position of the stomach reversed.

Deglutition is easier and quicker when the appetite is keen, and the alimentary bolus or morsel is moist and properly softened. It is slow and difficult when the morsel is dry and mealy, and the appetite nauseated. In vomiting, the action of the muscular fibres is inverted, or proceeds from the lower end of the gullet towards the mouth; and hence the object is carried upwards instead of downwards, as in the natural order.

CHAPTER IV.
ORGANS OF DIGESTION—THE STOMACH—THE GASTRIC JUICE.

Surprising power of digestion—Variety of sources of food—All structures, however different, formed from the same blood—General view of digestion, chymification, chylification, sanguification, nutrition—The stomach in polypes, in quadrupeds, and in man—Its position, size, and complexity, in different animals—Its structure; its peritoneal, muscular, and villous coats; and uses of each—Its nerves and bloodvessels, their nature, origins, and uses—The former the medium of communication between the brain and stomach—Their relation to undigested food—Animals not conscious of what goes on in the stomach—Advantages of this arrangement—The gastric juice the grand agent in digestion—Its origin and nature—Singular case of gunshot wound making a permanent opening into the stomach—Instructive experiments made by Dr Beaumont—Important results.

If, in the whole animal economy, where all is admirable, there be one operation which on reflection appears more wonderful than another, and which evinces in a higher degree the prodigious resources and power of the Creator in fashioning every thing to His own will, it is perhaps that by which the same kind of nutriment is extracted from the most opposite varieties of food consumed by living beings. For, singular as it may appear, recent researches tend to establish the fact, that, even in animals differing so widely in their aliment as the herbivorous and carnivorous quadrupeds, the ultimate products of digestion in both—the chyle and the blood—are identical in composition, in so far at least as can be determined by their chemical analysis.[16]

Remarkable, however, as this uniformity of result undoubtedly is, it becomes still more striking when we contemplate the variety of sources from which food is derived for the support of animal life. To use the words of an able writer already quoted, “There is no part of the organized structure of an animal or vegetable, however dense its texture or acrid its qualities, that may not, under certain circumstances, become the food of some species of insect, or contribute in some mode to the support of animal life. The more succulent parts of plants, such as the leaves or softer stems, are the principal sources of nourishment to the greater number of larger quadrupeds, to multitudes of insects, as well as to numerous tribes of other animals. Some plants are more particularly assigned as the appropriate nutriment of particular species, which would perish if these ceased to grow: thus the silk-worm subsists almost exclusively upon the leaves of the mulberry-tree; and many species of caterpillars are attached each to a particular plant, which they prefer to all others. There are at least fifty different species of insects that feed upon the common nettle; and plants of which the juices are most acrid and poisonous to the generality of animals, such as euphorbium, henbane, and nightshade, afford a wholesome and delicious food to others.”[17] Nor are the precision and accuracy with which the same fluid—the blood—affords to every structure of the body the precise species of nourishment or secretion which its elementary composition requires, however different each may be from the rest in chemical qualities, less admirable and extraordinary than its own original formation from such a variety of materials. To bone, the blood furnishes the elements of bone with unerring accuracy; to muscle the same blood furnishes the elements of muscle,—to nerves the elements of nerve,—to skin the elements of skin,—and to vessels the elements of vessels;—and yet, while each of these differs somewhat in composition from the others, the constituent elements of the blood by which they are furnished are everywhere the same.

Similar phenomena, indeed, occur in the vegetable world; but this, instead of diminishing our wonder, tends rather to augment it. The same elements, extracted from the same soil, are converted into every variety of vegetable product—into leaves of every shade of green, flowers of every form and tint, and juices of every quality, from the deadly poison up to bland and life-supporting milk. Nay, even in the same plant—as in the poppy—we sometimes find the seeds and the capsule which covers them endowed with the most opposite properties.

It would be very interesting to discover by what resources Nature thus effects the production of the same kind of nutritive fluid or chyle, from so great a variety of substances, and apportions to every part the precise elements of which it stands in need; but it is doubtful whether the human faculties were ever designed to penetrate so far into the modes of vital action, and, in the mean time, it will be better for us to confine our attention to that branch of the inquiry which we can easily comprehend, and which bears a direct reference to our own welfare. We know already that certain organs are concerned in the processes above mentioned, and that these organs act under the regulation of certain general laws. If we make ourselves acquainted with, and carefully fulfil, these laws, we reap a rich reward in the enjoyment of sound and vigorous digestion. Whereas, if, either from ignorance or from carelessness, we neglect their fulfilment, we bring upon ourselves a severe punishment in the form of dyspeptic or nervous disease. Assuredly, then, alternatives like these ought to excite some desire for information in the minds even of the most indifferent.

Before, however, commencing a description of the organs concerned in digestion, it will be useful to take a general view of the different stages of preparation through which the food passes, between its reception into the stomach and its assimilation, or ultimate conversion into a constituent part of the animal body, and becoming endowed with the properties of life. The reader will thus be better enabled to understand the meaning of various terms and expressions, the frequent use of which it is almost impossible to avoid, even in the beginning of our exposition.

When the food is received into the stomach, it is there subjected to the action of a solvent fluid, called the gastric or stomach juice (from γαστηξ, gaster, stomach), by which it is gradually converted into a soft greyish and pultaceous mass, called chyme, (from χυμος, chymos, humour or juice); whence the process is called chymification, or chyme-making. The chyme, as fast as it is formed, is expelled by the contractile power of the stomach into the duodenum (from duodenus, consisting of twelve, because it is supposed to be about twelve inches long), or first portion of the intestines. It there meets with the bile from the liver, and with the pancreatic juice, which very much resembles the saliva, from the pancreas or sweet bread (πας, pas, all, and κρεας, kreas, flesh, it being of a fleshy consistence), a large gland which lies across the spine a little below the stomach, and is marked P in the wood-cut given in the chapter on Chylification. By the action of these two fluids, the chyme is converted into two distinct portions,—a milky white fluid named chyle (from χυλος, chylos, chyle), and a thick yellow residue. This process is called chylification or chyle-making. The chyle is then sucked in by absorbent vessels, extensively ramified on the inner membrane or lining of the bowels, and sometimes named, from the white colour of their contents, lacteals or milk-bearers (from lac, milk). These lacteals ultimately converge into one trunk, named the thoracic duct or chest-pipe (from its course lying through the thorax or chest), and which terminates, as will be seen in a cut in Chapter VI., in the great vein under the clavicle or collar-bone, hence called subclavian vein, just before the latter reaches the right side of the heart; and there the chyle is poured into the general current of venous blood.

But although thus mingled with the blood, the chyle is not yet sufficiently capacitated for its duties in the system. To complete its preparation, it still requires to be exposed to the action of the air during respiration. This is accordingly done by its passing through the lungs along with the dark and venous blood, which stands in need of the same change. In the course of this process, both the chyle and the venous blood are converted into red, arterial, or nutritive blood, which is afterwards distributed by the heart through the arteries, to supply nourishment and support to every part of the body. Hence the change which takes place in the lungs is properly enough named sanguification or blood-making.[18]

The thickish yellow residue left in the duodenum after the separation of the chyle from the chyme, is that portion of the food which affords no nourishment, and which, after traversing the whole length of the intestinal canal, and undergoing still farther change, is thrown out of the body in the shape of fæces or excrement. But in this course its bulk is increased, and its appearance changed, by the addition of much waste matter, which, having already served its purposes in the system, is at last, as will be afterwards shewn, thrown out by the same channel.

With this general view of the nature of Digestion before us, we can now examine more satisfactorily the structure and mode of action of each of the organs concerned in effecting it. Chymification being the first step in the complicated process, we shall begin with the organ by which it is performed, namely, the Stomach.

In the lowest class of animals,—the Hydra, for example, which belongs to the order of gelatinous polypi, and abounds in stagnant pools,—the stomach is like a simple bag, devoid of any peculiar organization; or, more properly speaking, the animal itself is nothing more than a living stomach; for the minutest inspection can discover in it no trace of any thing like vessels, nerves, brain, lungs, heart, or other known organ. Even the experienced eye of Cuvier, aided by a powerful microscope, could detect in their structure nothing more than a transparent parenchyma, full of darkish grains or points, and offering no trace of any distinguishable organs.[19] In form the animal somewhat resembles the finger of a glove, the hollow in the centre being appropriated for the reception of its food; and yet with all this simplicity of structure, it not only moves and swims, but seizes its prey by means of its tentacula, thrusts it into its cavity, and digests it visibly—“à vue d’œil.” And what is still more strange, when it is turned inside out, the surface which was formerly the exterior of the body, now digests as actively and efficiently as if it had never served any other purpose.

Bloodvessels being merely pipes serving for the conveyance of nourishment from the place where it is stored up and prepared, to those more distant parts which stand in need of it, it is obvious that in animals of this very simple description, where every part of the internal surface of the body is already in immediate contact with its food, and itself imbibes or absorbs directly all the nourishment which it requires, they would be entirely superfluous, and accordingly none are to be found.

Owing to this extreme simplicity of organization, a hydra may be cut into pieces, each of which will become a perfect animal. But in proportion as we ascend in the scale of creation, the organization becomes more complicated, and the functions more numerous and also more dependent on each other; so that, when we arrive at man—the highest of all—we find that the loss or injury of any vital organ puts a stop to, or impairs, the action of all the rest. It is impossible, however, in an elementary work like this, to trace the gradation through the different series of animals. We must confine our examination to man, and only borrow from other creatures such illustrations as may be necessary for throwing light upon the human structure.

In man, then, the stomach is a large membranous and muscular bag, lying under the convexity of the lower ribs of the left side, and stretching towards the right a little beyond the hollow commonly called the pit of the stomach. In shape it somewhat resembles the bag of a bagpipe, as will be seen from the figure on page [67]; its left or larger extremity being in contact with the ribs, and its right or narrow extremity situated under the pit of the stomach.

The position of the stomach (St^m.) relatively to the chest, bowels, and liver, will be understood by inspecting the figure on the next page. It is separated from the cavity of the chest by the diaphragm or midriff DD, with which its upper surface is in immediate contact, and through which the gullet passes to enter its left extremity. Its right or pyloric extremity, marked P in the cut on the opposite page, lies close to the lower surface of the liver (Liv^r.), the latter being a little displaced to shew its situation. On the lower surface it has the appearance of resting on the intestines, as if imbedded among their folds.

The parts of the stomach which have received names and require to be noticed, are,—the cardiac orifice (marked C in the next figure, and so named from being near the κεαρ, kear, or heart), in which the gullet terminates, and through which food and drink are introduced; P, the pylorus, or pyloric orifice (from πυλωρος, pyloros, a gate-keeper, because it allows none but digested food to pass out), where the intestine called the duodenum begins, and through which the chyme passes after digestion is completed, and which, when the stomach is full, is nearly on a level with the cardia, although when empty it is lower; SS, the smaller arch or curvature; and GGG, the greater arch or curvature. The spleen is attached to that part of the larger arch marked with dotted points. From the situation of the cardia C, and its connection with the gullet, it will be at once perceived that this forms one of the points of attachment by which the stomach is retained in its place.

In size the stomach varies much in different individuals, as well as in different animals, according to the bulk and quality of their food. As a general rule, it is larger among the labouring poor than among the rich, as the former require a larger quantity of their inferior food to obtain from it an equal amount of nourishment. For the same reason, animals which subsist on vegetable substances have a very capacious stomach, while those subsisting on animal or concentrated food have it simple and small. In man its capacity may be diminished or augmented within certain limits, by corresponding modifications of diet. In some gluttons, and in cases of diseased appetite, it has been found of enormous dimensions; but this rule does not always hold, for the stomach is sometimes smaller than usual in immoderate eaters, and then its contents pass through only partially digested.

In accordance with this relation between the capacity and structure of the organs of digestion and the quality of the food, the stomach and intestines are found to be very small and short in carnivorous quadrupeds and birds of prey, which are intended to live on concentrated aliment. The same is the case with the granivorous or grain-eating birds, as their food also is contained in a small bulk. But in herbivorous animals—in the food of which the nutritive principle forms a very small proportion of the whole, perhaps not one-twentieth, and which consequently require a large bulk of it for their sustenance—the digestive apparatus is on a large scale, as any one may conceive in a moment, by comparing the portly protuberance of the cow with the lank paunch of the greyhound. The cow, in fact, is little else but a living laboratory for the conversion of vegetable into animal matter; and accordingly, not only is its stomach large and complex, but its intestinal canal is nearly twenty-four times the length of its body; whereas, in some carnivorous animals, the whole intestine does not exceed once their own length.

In ruminating animals, such as the sheep and ox, the stomach, as will be seen from the annexed figure, not only is large, to adapt it to the bulky nature of their food, but is complicated in its structure, to fit it for effecting the great changes which vegetable aliment requires to undergo before it can be converted into blood. It may indeed be said to consist of four distinct stomachs conjoined. In the first of these, AA, termed the paunch, the herbage is deposited when first swallowed after hasty and ineffectual mastication. It there undergoes a kind of maceration or steeping in a fluid provided for the purpose; after which it passes from the paunch into a smaller bag, called the reticulum, or bonnet, B, which, in some animals, such as the camel and dromedary, is designed exclusively as a reservoir for water, which being there stored up in large quantities, ready for use when wanted, fits them in a wonderful manner for travelling through the arid deserts where no water is to be obtained, and where, without some such provision, they would of course soon perish. So admirably is the reticulum adapted for this special purpose, that the water contained in it undergoes little or no change either in quality or quantity, although if it were collected in the ordinary digesting stomach, it would be entirely absorbed in the course of a few minutes. It is not even mixed with the food which is swallowed after it, as the animal has the power of directing solids at once into the other cavities. From the reticulum the alimentary mass is again returned to the mouth, there to be thoroughly masticated and mixed with the saliva; after which it descends a second time through the gullet: but instead of passing, as before, into the paunch, it enters the third bag, omasum, or many-plies, C, where it undergoes farther changes, and is then transmitted to the fourth portion D, adjoining the pylorus, and named ab-omasum, or red-bag. The last portion is exactly similar in structure and in function to the simple stomach of man and the other mammalia, and is in fact the true stomach, the other three being merely preparatory organs.

The first part of the process, by which the food is taken hastily into the paunch and afterwards sent back to the mouth in detached portions for farther mastication, is called rumination or chewing the cud, and those species which perform it are thence called ruminating animals. Sheep and cows may be seen lying ruminating in pasture-fields after having cropped as much herbage as fills the paunch; and feeding is thus rendered to them a source of prolonged enjoyment.

In those birds, again, which live on hard grain and seeds, and possess no organs of mastication wherewith to bruise or grind them down, another modification of the digestive apparatus is found. Nature has furnished them with a membranous bag, called a crop or craw, into which the food is received, and where it is slightly softened by a mucous fluid secreted from the surface of the bag. Thus prepared, it is transmitted into an organ analogous to the stomach of other animals, and called the gizzard, which has a very singular structure. Its walls are composed of four distinct portions of thick tough muscular substance, a large one at each side of the cavity, and a small one at each end. The inner surface of the muscle is lined with an extremely callous cuticle, approaching in hardness to cartilage or horn. When the moistened grain is introduced into the gizzard from the crop, the muscular walls of the gizzard enter into powerful action, and, by their alternate contraction and relaxation, bruise the grains as between two grindstones. In some birds their action is assisted by a quantity of small gravel, purposely swallowed along with the food; and it is well known to seamen that poultry never thrive on a voyage, however well they may be fed, if gravel or coarse sand, as well as food, be not placed within their reach. Mr Hunter has counted as many as a thousand small stones in the gizzard of a common goose.[20]

The astonishing force with which the muscles of the gizzard act, and the resistance of its lining membrane, may be conceived from the experiments of Spallanzani and Reaumur, who compelled geese and other birds to swallow needles, lancets, and other sharp metallic bodies, and, on afterwards killing them, regularly found the points broken off and the edges blunted, without any injury having been sustained by the gizzard itself.

In STRUCTURE, the stomach of both man and animals consists of three membranous layers or coats, of follicles or glands, and of numerous bloodvessels and nerves.

The first or external layer is the smooth glistening whitish membrane, which is familiar to all who have ever seen an animal opened, or a fowl drawn for cooking. It is a fold of the tough shining membrane called peritoneum (from περιτεινω, periteino, I extend round), which lines the abdomen, and constitutes the outer covering of all the abdominal organs. Its use is obviously to strengthen the substance of the stomach, to assist in binding down this and the other organs in their respective situations, and, by the smoothness and constant moisture of their surfaces, to enable them to move upon each other, and adapt themselves freely to their different states of emptiness and distention.

The second, middle, or muscular coat consists of fleshy fibres, one layer of which, running longitudinally from the cardia to the pylorus, seems to be a continuation of the longitudinal muscular fibres of the gullet: another runs in a circular direction, embracing, as it were, the stomach from one curvature to the other, and constituting what are called the transverse fibres. A third and more internal layer of this coat, is spoken of by Sir Charles Bell as a continuation of the circular fibres of the gullet, which divide into two parcels, the one distributed over the left or larger end, and the other over the pyloric or narrower end.

The uses of the muscular coat have, as we shall afterwards see, a direct reference to the special function of digestion. By the joint action of its longitudinal and circular fibres, the stomach is enabled to contract, and shorten its diameter in every direction, so as to adapt its capacity to the volume of its contents; while, by their successive action, or alternate contraction and relaxation, a kind of churning motion is produced, which contributes greatly to digestion by the motion which it imparts to the food, and the consequent exposure which it effects of every portion of it in its turn to the contact of the gastric juice.

The force and rapidity of these muscular contractions are modified by the more or less stimulant nature of the food, the state of health, exercise, and other circumstances; but, according to Dr Beaumont, the ordinary direction in which they take place, and the course which they impart to the food, are as follows.

The alimentary bolus or morsel, on entering the cardiac orifice, turns to the left, follows the line of the great curvature of the stomach towards the pylorus, returns in the line of the smaller curvature, makes its appearance again at the cardia, and then descends as before to the great curvature, to undergo similar revolutions till digestion be completed. Each revolution occupies about from one to three minutes, and its rapidity increases as chymification advances.

In treating of muscular action in the former volume, I pointed out (p. [122]) the necessity of the co-operation of a nervous stimulus to produce the result; and remarked that there are two kinds of muscles, one called the voluntary, which contract at the command of the will, and the other the involuntary, over which the will has no control, and which act only in obedience to their own peculiar stimuli. Of the latter description are the muscular fibres of the stomach. They contract when the stimulus of food is applied to them, but we can neither contract nor relax them by an effort of the will, nor are we even conscious of their existence.

It is, indeed, fortunate for us that the necessary motions of the stomach are not entrusted to our guidance, like those of the hand or foot. Supposing that we were to eat three meals a-day, the digestion of each requiring three or four hours,—and that its management depended entirely upon our superintendence,—our whole attention would be required to the process, to the exclusion of every other duty, for ten or twelve hours a-day; and every time that our thoughts wandered for a few minutes, digestion would stand still, and the stomach be disordered by the chemical decomposition of the food which would ensue, so that it would be impossible for us to dedicate any time either to business or to social enjoyment. But from all these inconveniences we are entirely freed by the stomach being placed under the dominion of the involuntary nerves, and so constituted as to perform its functions without any aid from our will.

The third and innermost coat, called the mucous or villous, is that smooth, unequal, velvety membrane, of a reddish-white or pale pink colour, which lines the internal surface of the stomach. From being of much greater extent than the other two coats, its surface is thrown into rugæ, plicæ, folds, or wrinkles, which are simple in man, but very marked in some animals, as seen familiarly in tripe. The subjoined wood-cut, from the Library of Useful Knowledge, will give some notion of their appearance. Near the pyloric orifice the villous coat is doubled on itself, so as to form a ring, called the valve of the pylorus, the object of which is to prevent the too early exit of the food; this object, however, it accomplishes, not by any contractile power of its own, but by the aid of a layer of muscular fibres lying behind it. The villous coat is constantly covered with a very thin transparent viscid mucus, and its folds are always best seen in those who die suddenly. After disease, when the stomach is relaxed, they frequently disappear.

In addition to the folds just described, the mucous coat contains a great number of spheroidal glandular bodies or follicles, some of them scarcely larger than pin-heads, which lie immediately beneath and almost incorporated with it, and which are most numerous near the pylorus. Physiologists are not entirely agreed whether the fluid secreted by these follicles be the gastric juice or merely the mucus already referred to as lubricating the internal surface of the stomach. The latter, however, is the opinion generally entertained, and the one which is supported, as we shall afterwards see, by the strongest evidence; the gastric juice being, in fact, secreted directly from the capillary or hair-sized vessels in which the minute branches of the arteries terminate.

Of the nerves and bloodvessels supplying the stomach it is unnecessary to say much. We shall afterwards have occasion to notice the former at some length, and to the general reader the origin and distribution of the bloodvessels are as unimportant as they would be difficult of comprehension; for the nature of the red blood is the same by whatever artery it is supplied, and that of the dark blood the same by whatever vein it is returned to the heart. All that it is important to know is, that the stomach receives a large supply of blood by means of numerous bloodvessels, the principal of which, as represented on the wood-cut at page [67], follow the course of GG the greater and SS the smaller curvatures, and send off innumerable small branches as they proceed to every part of the stomach. The coronary artery and the pyloric branch of the hepatic or liver artery go to the smaller curvature, while another branch of the hepatic, and one from the splenic or spleen artery, are ramified on the larger curvature.

In determining the uses of the internal or villous coat of the stomach, we must begin by considering separately that of each of the elementary structures of which it is composed—its follicles, bloodvessels, and nerves—and the nature of the peculiar secretion, the gastric juice, to which it gives rise.

The FOLLICLES pour out the bland viscid mucus which lubricates the internal coat, and protects it in some degree from sustaining injury by the immediate contact of irritating bodies. When the follicles are diseased, as in what is called water-brash, they sometimes throw out a large quantity of a ropy transparent fluid, which oppresses the stomach and impairs digestion.

The BLOODVESSELS of the stomach, like those of every other part, are more or less active according to the energy of its functions at the time. In treating of the laws of exercise as applicable to all living parts,[21] I took considerable pains to point out the relation which the Creator has established between the activity of every organ and the energy of its vital functions. When the brain is exercised and the mind active, an augmented flow of blood takes place towards it to support its increased action, of which the throbbing temples and fiery complexion of a man in a paroxysm of rage are familiar examples. When it is inactive, and the mind indolent, a diminished flow of blood occurs. In like manner, when the muscles are called into vigorous action, the circulation of the blood through them is quickened, and their nerves are more than usually excited: greater waste of material is caused by the increase of activity, and more blood, consequently, is required to repair the waste and sustain their tone. This law was so well known to the older writers, that it was announced by them as an axiom in the very comprehensive phrase, Ubi stimulus, ibi affluxus—“Wherever a stimulus is, there is also an afflux.”

The stomach forms no exception to this general law of the animal economy. When it is empty and idle, it is contracted upon itself into comparatively small bulk; and its bloodvessels become shortened and tortuous in a corresponding degree. The result is both a diminution of their calibre and a slower circulation through their branches. But when the stomach is full and active, the bloodvessels have free scope, their tortuosity disappears, their diameter enlarges, and the circulation through them becomes quicker, and fit for the rapid secretion of the mucous and gastric fluids in the quantities which we have seen to be required for the fulfilment of digestion. Accordingly, when the latter process is going on, the small arterial branches ramified on the mucous coat of the stomach become so multiplied and distended, as to impart to it a deeper red colour than it has when the stomach is empty. The increased afflux of red or arterial blood to the stomach during digestion, is not merely inferred from the analogy of other organs. Many opportunities have occurred of ascertaining the fact; and, as I shall have occasion to mention, Dr Beaumont very often SAW it take place.

A corresponding change occurs in the veins of the stomach during digestion. Their diameter becomes enlarged, their course more straight, and the current of blood through them more rapid. As the minute or capillary extremities of the arteries open upon the inner surface of the stomach, and there exhale a fluid secretion, so the corresponding venous capillaries likewise open upon the same surface, and inhale or absorb fluid, which they carry into the general circulation. The rapidity with which this absorption sometimes takes place is almost incredible; for a large draught of water may be thus taken up in a few minutes. Fluids mixed with camphor or other strong-scented substance have been given to animals as an experiment, and, on killing them shortly afterwards, the peculiar smell has been detected in the blood. Most liquids are thus not digested, but simply absorbed.

Rapid, however, as the process is, poisons which enter the system by absorption do not act by any means so instantaneously as those which directly affect the nervous system.

In regard to the peculiar influence which each of the NERVES ramified on the stomach exercises on its functions, much difference of opinion still prevails. We may, however, gather some useful notions by adverting to the different sources whence they are derived, and comparing these with the purposes for which we know from analogy that different kinds of nerves are required.

Strictly speaking, the nervous filaments supplied to the stomach proceed from three distinct sources, and may be held to fulfil as many distinct uses. In apparent accordance with this, we observe three, if not four, distinct classes of operations going on in that organ, each of which may, from analogy, be presumed to require a distinct nerve for its performance. These are, first, the pleasurable consciousness attendant on the presence of wholesome food in a healthy stomach, and which becomes painful and disagreeable when the stomach is diseased or the food of improper quality; secondly, the peristaltic or muscular motion which commences the moment food is swallowed, and continues till digestion is completed; and, lastly, the different processes of circulation, nutrition, secretion, and absorption, which go on in the component tissues of the stomach and support its life. To these ought perhaps to be added the sensation in which the feeling of appetite originates, and which we have already seen to be connected with the pneumogastric nerve. But as it is still uncertain whether it and the first of the three now named may not be modifications of the same thing, I shall not insist on considering them as different.

Although we cannot state positively what particular nerve presides over each of these functions, it may be mentioned that strong presumptive evidence has been adduced, particularly by Brachet, to shew that the pneumogastric[22] nerve is charged with the involuntary motions of the stomach, as well as with the sense of its condition. Food being the natural stimulus of that organ, as light is of the eye, its presence alone, without and even against the will, suffices to produce the contraction of its muscular coat; and accordingly, the more stimulating the food, the more rapid and vigorous is the muscular contraction which it excites. So far indeed do the stomachic nerves respond to their own stimuli, that, if nauseous or other irritant and indigestible substances be swallowed, the action of the muscular coat becomes so violent as to excite sympathetically the simultaneous contraction of the diaphragm and abdominal muscles, to aid in their immediate expulsion by vomiting; and this is the reason why such substances are in common use as emetics.

Magendie doubts whether these movements are in any measure dependent upon nervous influence; but the fact of their being so seems to be proved by the experiments of Gmelin and Tiedemann, who found them constantly produced when the pneumogastric nerve was irritated either by the scalpel or by the contact of alcohol. Brachet also, who examined the subject with great care, obtained similar results; and the only plausible argument against their conclusiveness consists in the double function which seems thus to be assigned to a single nerve—that of conveying to the brain a sense of the state of the stomach, and that of imparting motion to its muscular fibres. Bracket, however, turns this charge into an additional proof; for, on careful dissection, it appears that the pneumogastric nerve is really a compound of two distinct sets of fibres, intimately connected no doubt in structure and in function, but each essentially distinct in its origin, and so far fitted for a peculiar office.

When the pneumogastric or chief nerve of the stomach is tied or cut through, and its ends separated so as to interrupt the flow of nervous energy towards that organ, digestion is either entirely arrested or greatly impaired. By the greater number of physiologists this result is considered to arise from the consequent stoppage of that gentle and continued agitation of the alimentary mass in the stomach which is necessary for its thorough impregnation with the gastric juice, and which we have seen to depend on a stimulus communicated to its muscular coat by that nerve. By some, however, this explanation is regarded as incorrect. Magendie and Dr Holland, for example, say that they have sometimes observed digestion continue even after the division of the nerve; and that, when it is retarded or impaired, the result arises only from the troubled state of respiration which the cutting of the nerve induces at the same time. To this, again, it is answered, that Dupuytren has divided the nerve below the part where the pulmonary branches are given off, and consequently left respiration unimpaired; but that still digestion was arrested, provided a portion of it was cut out so as not to allow the current of nervous influence to continue: for if the two ends of the nerve be left nearly in contact, it appears that little interruption of its action takes place.

Here, however, I cannot help observing, that, in drawing conclusions from experiments of this nature, the constitutional disturbance inseparable from the infliction of extensive wounds on living animals is seldom taken sufficiently into account. As regards digestion, for example, it is not to be expected that that function can be carried on with all the regularity of health when the animal is suffering severe pain, even although the stomach be left untouched. Brachet, indeed, has shewn, by direct experiment, that digestion may be interrupted almost as effectually by making an incision on the side or thigh (provided it be sufficiently deep and painful to excite constitutional disturbance), as by cutting the pneumogastric nerve itself. This being the case, we must not be too hasty in considering every apparent result as inseparably and exclusively connected with the individual injury under our notice at the time; we must repeat our observations under every variety of circumstances, and be careful to separate the accidental from the essential, before admitting the inference to be correct. It is, in fact, this unavoidable source of vagueness which so often renders experiments on living animals as inconclusive as they are inherently cruel.

But after making every allowance on this account, the experiments on digestion have been so frequently repeated, and so extensively varied, that the general results already noticed may safely be regarded as demonstrated. On all hands, accordingly, the necessity of the co-operation of the nervous energy in effecting it is admitted; for no one seriously denies the fact, that retardation or total cessation of digestion ensues, when the flow of the nervous fluid towards the stomach is prevented by the division and separation of the cut ends of the pneumogastric nerve, or by the operation of narcotics and the other disturbing causes already alluded to. It is true that the mode in which the nerve acts is not yet ascertained, although the fact of its necessary co-operation is rarely disputed. As, however, the direction of a current of galvanism to the cut end of the nerve next the stomach suffices to re-establish digestion after that process has been suspended by the interruption of the nervous influence consequent on its division, we may reasonably infer that, in the healthy state, the nerve merely transmits to the stomach a stimulus or energy generated for the purpose either in the brain or in the spinal marrow and ganglia—that the nerve, in short, acts only as a conductor, and does not originate the influence which it evidently imparts. In several of Brachet’s cases, indeed, as well as in those of Tiedemann, the continued irritation of the cut end of the nerve proved sufficient to carry on digestion to a certain point, by affording, in another way, the necessary stimulus to the muscular contractions of the stomach: for in all these experiments, digestion was found to have advanced almost in exact proportion to the degree of admixture which had been effected of the food with the gastric juice,—an admixture now ascertained to be produced chiefly by the contractile power of the stomach itself.

The muscular contractions of the stomach being thus under the guidance of the pneumogastric nerve, what are called its vital functions—those by which its life is sustained—viz. circulation, nutrition, secretion, and absorption, are generally considered to be carried on under the influence of the great sympathetic or ganglionic nerve, so called from its very extensive ramifications being supposed to bring the different parts of the system into relation with each other, and which, accordingly, is found to exist in animals that have neither brain nor spinal marrow, nor nerves of voluntary motion. In man, however, the sympathetic nerve receives filaments from the sentient or feeling part of the spinal marrow, probably for the purpose of connecting more intimately the organic with the higher functions of animal life. But as much obscurity still prevails on this subject, and moreover we have no direct control over the action of the ganglionic nerves, I shall not detain the reader with any conjectural discussion, but rather request his attention for a moment to the circumstance that it is through the medium of the nervous communications above hinted at, that the very remarkable influence which all must have experienced and observed as constantly exerted by the mind and stomach on each other, is kept up—an influence so powerful in its effects on both bodily and mental health, as to require special notice when we shall treat of the practical application of the present exposition.

The importance of the nervous agency in effecting digestion has been denied, because we are not conscious of the presence of food in the stomach. But in health the want of such consciousness is a privilege and not a defect; and it has been admirably pointed out by Dr Southwood Smith,[23] that in possessing, as we do, the distinct consciousness of a pleasurable feeling in the stomach after indulging in a suitable meal, we have all that is desirable for either utility or enjoyment. If we were aware of the presence of every portion of food which the stomach contains, and of the changes occurring in each, our attention would be so disagreeably and unprofitably taken up that we would pray to be delivered from the annoyance. Where, however, from disease or the food being inappropriate, the stomach is injured by what is eaten, consciousness then becomes painful for the express purpose of warning us that mischief has been done, and that we must take means for its removal. In some kinds of dyspepsia indeed, the sensibility becomes exalted to an extraordinary degree. Barras who suffered intensely from this cause, says of himself “the sensibility of the stomach increased to a surprising extent; instead of organic it became animal, to use the expression of Bichat. Every thing which took place in the principal organ of digestion became as palpable to sense as if it had taken place on the organ of touch, and the presence of aliment was perceived as clearly as if it been under my hand.”[24]

The nerves of the stomach, it ought to be remarked, have a direct relation to undigested but digestible substances; or in other words, undigested food forms their natural and appropriate stimulus. In consequence of this arrangement, when any body incapable of digestion is introduced into its cavity, distinct uneasiness is speedily excited, and an effort is soon made to expel it either upwards by the mouth or downwards by the bowels. It is in this way that bile in the stomach excites nausea, and that tartar emetic produces vomiting. The nerves of the bowels, on the other hand, are constituted with relation to the presence of digested food, and consequently, when any thing escapes into them from the stomach in an undigested state, it becomes to them a source of irritative excitement; and hence the colicky pains and bowel-complaints which so commonly attend the passage through the intestinal canal of such undigestible substances as fat, husks of fruits, berries, and cherry-stones.

Such, then, are the component parts of the stomach, and such the uses which they individually fulfil; but before we can consider them in their combined form, there is still another agent, and an important one in digestion, which has already been repeatedly named, and which, though not a portion of the stomach, yet plays too conspicuous a part in its operations not to require some separate notice—the GASTRIC OR STOMACH JUICE.

The existence of a solvent fluid in the stomach has long been known, and its uses suspected; but for our first accurate acquaintance with its properties and mode of action, we are indebted chiefly to the sagacity and persevering zeal of Spallanzani, who investigated the subject with great care and success about the middle of last century. Considering the peculiar difficulties by which the inquiry is surrounded, it is offering no trifling homage to that distinguished observer to say, that by means of numerous, varied, and well-devised experiments on man and animals, he succeeded in overcoming most of the obstacles which had baffled the ingenuity of his predecessors, and in obtaining results, the general accuracy and importance of which are now appreciated more and more highly, in proportion as our knowledge advances and opportunities present themselves of bringing them to the test of experience.

It is rarely, indeed, that we can actually see what is going on in a healthy stomach; but in a few instances this advantage has been enjoyed, and turned to account in investigating the phenomena of digestion. By far the most instructive example of this kind which has ever occurred, came under the observation of Dr Beaumont of the American army; and, as that gentleman eagerly embraced the opportunity so unexpectedly afforded him, of testing the prevailing doctrines by a series of experiments, continued during a period of several years, and under various conditions of health and external circumstances, I shall so frequently have occasion to refer to his observations, that it will be useful to give a brief outline of the case before entering farther upon the subject, in order that the reader may be enabled to judge for himself what weight is due to Dr Beaumont’s evidence on any disputed point.

Dr Beaumont, while stationed at Michillimackinac in the Michigan territory in 1822, in the military service of the United States, was called upon to take charge of Alexis St Martin, a young Canadian of eighteen years of age, good constitution, and robust health, who was accidentally wounded by the discharge of a musket on 6th June 1822.

“The charge,” says Dr Beaumont, “consisting of powder and duck-shot, was received in the left side, at the distance of one yard from the muzzel of the gun. The contents entered posteriorly, and in an oblique direction, forward and inward; literally blowing off integuments and muscles to the size of a man’s hand, fracturing and carrying away the anterior half of the sixth rib, fracturing the fifth, lacerating the lower portion of the left lobe of the lungs, the diaphragm, and PERFORATING THE STOMACH.”

On the fifth day, sloughing took place; lacerated portions of the lung and stomach separated, and left a perforation into the latter, “large enough to admit the whole length of the middle finger into its cavity; and also a passage into the chest half as large as his fist.” Violent fever and farther sloughing ensued; and for seventeen days every thing swallowed passed out through the wound, and the patient was kept alive chiefly by nourishing injections. By-and-by the fever subsided, the wound improved in appearance, and after the fourth week the appetite became good, digestion regular, the evacuations natural, and the health of the system complete. The orifice, however, never closed; and at every dressing the contents of the stomach flowed out, and its coats frequently became everted or protruded so far as to equal in size a hen’s egg, but they were always easily returned. The prefixed figure exhibits the appearance of the wound after it was healed. The circumference of the wound EEEL, extended to about twelve inches; and the opening into the stomach AAA, nearly in its centre, was about two inches below the left nipple F. The folds of the villous coat are visible at BC.

Some months after, St Martin suffered extremely from the death and exfoliation of portions of the injured ribs and their cartilages, and his life was often in jeopardy; but through the skill and unremitting care with which he was treated by Dr Beaumont, he ultimately recovered, and, in April 1823, was going about, doing light work and rapidly regaining strength.

On 6th June 1823, a year from the date of the accident, the injured parts were all sound, except the perforation into the stomach, which was now two and a-half inches in circumference. For some months thereafter the food could be retained only by constantly wearing a compress and bandage; but early in winter, a small fold or doubling of the villous coat began to appear, which gradually increased till it filled the aperture and acted as a valve, so as completely to prevent any efflux from within, but to admit of being easily pushed back by the finger from without.

Here, then, was an admirable opportunity for experimenting on the subject of digestion, and for observing the healthy and undisturbed operations of nature free from the agony of vivisections, and from the sources of fallacy inseparable from operating on animals. Dr Beaumont was sensible of its value, and accordingly pursued his inquiries with the most praiseworthy perseverance, and disinterestedness. Having been fortunate enough to obtain a copy of his work, I shall not hesitate to make free use of its contents.

Dr Beaumont began his experiments in May 1825, and continued them for four or five months, St Martin being then in high health. In the autumn, St Martin returned to Canada, married, had a family, worked hard, engaged as a voyageur with the Hudson’s Bay Fur Company, remained there four years, and was then engaged at a great expense by Dr Beaumont to come and reside near him on the Mississippi, for the purpose of enabling him to complete his investigations. He came accordingly in August 1829, and remained till March 1831. He then went a second time to Canada, but returned to Dr Beaumont in November 1832, when the experiments were once more resumed, and continued till March 1833, at which time he finally left Dr Beaumont. He now enjoys perfect health, but the orifice made by the wound remains in the same state as in 1824.

Dr Beaumont describes the aperture in St Martin’s stomach as being situated about three inches to the left of the cardia, near the left or superior termination of the great curvature. When the stomach was nearly empty, he was able to examine its cavity to the depth of five or six inches by artificial distention. When it was entirely empty, the stomach was always contracted on itself, and the valve generally forced through the orifice, together with a portion of the mucous membrane equal in bulk to a hen’s egg. After sleeping for a few hours on the left side, the protruded portion became so much larger, as to spread “over the neighbouring integuments five or six inches in circumference, fairly exhibiting the natural rugæ, villous membrane, and mucous coat, lining the gastric cavity. This appearance is almost invariably exhibited in the morning before rising from bed.” Such was the very favourable subject on whom Dr Beaumont’s observations and experiments were made, and such were the numerous opportunities which he enjoyed for repeating them, and verifying their accuracy. Having given this outline, we now return to the consideration of the gastric juice, on the origin and qualities of which it removes any uncertainty which previously existed.

In treating of the properties of the gastric juice, I shall on all disputed points give a decided preference to the observations of Dr Beaumont over those of any other physiologist; because, although a few cases have occurred, in which, from external wounds, direct access has been given to the interior of the stomach, and Richerand and others have availed themselves of the opportunities thus afforded of adding to our knowledge of the digestive process, still, in most of them which have been recorded, the patients were a comparatively short time under observation, and were not sufficiently re-established in health to admit of either extensive or conclusive experiments being made. Whereas in the case which Dr Beaumont had the good fortune to meet with, the patient remained under his eye for several years, and in the enjoyment of the most robust health; so that ample time and opportunity were afforded for every variety of experiments which reflection could suggest, and for their subsequent repetition under such modifications as seemed to be requisite for clearly distinguishing the accidental from the constant and essential result. In addition to these reasons, it ought to be added, in justice to the American physiologist, that, from the excellent judgment with which he carried on his investigations, and the scrupulous care with which he announces his results and separates facts from theory, it is impossible not to place great confidence both in his personal qualifications as an observer, and in the general accuracy of his statements. Moreover, as he enjoyed the rare advantage of seeing what he describes to have taken place in the stomach during healthy digestion, his evidence comes before us with the strongest possible claims on our attention.[25]

The first disputed point which is conclusively settled by Dr Beaumont is, that the gastric juice does not continue to be secreted between the intervals of digestion, and does not accumulate to be ready for acting upon the next meal. By inducing St Martin to fast for some hours, and then placing him with the opening in the left side exposed to a strong light, so as to give a distinct view of the cavity of the stomach, Dr Beaumont found its only contents to consist of a little viscid and occasionally slightly acidulated mucus mixed with saliva, and in no instance did he perceive any accumulation of the proper gastric juice. The same results had indeed been obtained by Tiedemann and other physiologists before the publication of Dr Beaumont’s memoir; but the evidence of the latter is so much more direct and incontrovertible, that it may justly be regarded as setting the question for ever at rest.

Having proceeded so far, Dr Beaumont next endeavoured to discover at what time the gastric juice begins to be poured out, and under what conditions its secretion is carried on; and here again ocular inspection afforded him satisfactory results.

It has already been remarked, that, on pushing back the valve which filled up the opening into the stomach, the cavity within became visible to a considerable extent; and that when St Martin lay over for a time on the left side, a portion of the villous coat, large enough to exhibit several inches of its surface, generally protruded. Owing to these circumstances, Dr Beaumont could easily observe what changes occurred, both when food was swallowed in the usual way, and when it was introduced at the opening left by the wound. Accordingly, on examining the surface of the villous coat with a magnifying glass, he perceived an immediate change of appearance ensue whenever any aliment was brought into contact with it. The action of the neighbouring bloodvessels was instantly increased, and their branches dilated so as to admit the red blood much more freely than before. The colour of the membrane consequently changed from a pale pink to a deeper red, the vermicular or worm-like motions of the stomach became excited, and innumerable minute lucid points and very fine nervous and vascular papillæ could be seen arising from the villous coat, from which distilled a pure, colourless, and slightly viscid fluid, which collected in drops on the very point of the papillæ and trickled down the sides of the stomach till it mingled with the food. This afterwards proved to be the secretion peculiar to that organ, or, in other words, the true gastric juice; the mucous fluid secreted by the follicles, which some have mistaken for it, is not only more viscid, but wants altogether the acid character by which it is generally distinguished.

Pursuing his experiments, Dr Beaumont then found that the contact not only of food but of any mechanical irritant, such as the bulb of a thermometer, or other indigestible body, invariably gave rise to the exudation of the gastric fluid from these vascular papillæ; but that, in the latter cases, the secretion always ceased in a short time, as soon apparently as the organ could ascertain that the foreign body was one over which the gastric juice had no power. But the small quantity obtainable in this way is perhaps more pure and free from admixture, and therefore better adapted for examination, than any which can be procured under any other circumstances.

Various methods have been employed for procuring the gastric fluid in a state of purity. Pieces of dry sponge, inclosed in a small hollow perforated ball with a string attached to it, have been swallowed both by man and by inferior animals, and afterwards withdrawn to have the juice expressed from them. In some instances the stomachs of criminals and animals killed after fasting have been opened, and the secretion collected. At other times the juice has been procured by voluntary or artificial vomiting. None of these methods is equal to that employed by Dr Beaumont; but of the three the first is unquestionably the best, because, although no gastric juice previously exists, the very contact of the ball excites the secretion of a quantity sufficient to moisten the sponge. In the second mode of proceeding, any portion of juice secreted in consequence of a stimulus applied after the stomach is opened, must necessarily be very small and rendered impure by the large admixture of mucus which it will contain; while, by the third method, as the gastric juice does not exist ready made in the stomach, either none but merely mucus will be procured, or it will be expelled mixed with the food or substance which had previously elicited its secretion.

Gastric juice, in its purest form, and unmixed with any thing except the small portion of mucus from which it can never be obtained entirely free, is described by Dr Beaumont to be a clear transparent fluid, without smell, slightly saltish (probably from the admixture of mucus), and very perceptibly acid. Its taste, he says, resembles that of thin mucilaginous water, slightly acidulated with muriatic acid. It is readily diffusible in water, wine, or spirits, and effervesces slightly with alkalis—a direct proof of its acid nature. It coagulates albumen, and is powerfully antiseptic, checking the progress of putrefaction in meat. When pure it will keep for many months, but when diluted with saliva it becomes fetid in a few days. According to Professor Dunglison, to whom some was submitted by Dr Beaumont for analysis, it contains free muriatic and acetic acids,—phosphates and muriates with bases of potassa, soda, magnesia, and lime,—together with an animal matter soluble in cold but insoluble in hot water. Tiedemann and Gmelin, again, describe it as composed principally of muriatic and acetic acids, mucus, saliva, osmazome, muriate, and sulphate of soda, with little or no albumen; and, according to the same physiologists, the proportion of acid is always greatest when vegetables or other substances of difficult digestion constitute the chief part of the diet. Other chemists give an analysis somewhat different from either of these; a circumstance which was, indeed, to be expected, considering not only the differences caused by variations of diet and of health, but also the necessarily different degrees of purity of the fluid submitted to examination.

The most remarkable property of the gastric juice is unquestionably the power which it possesses of dissolving and reducing to the appearance of a soft thickish fluid mass every thing in the shape of food which is submitted to its action,—while it exerts no perceptible influence on living or inorganic matter; for, so far as is yet known, nothing which is not organized, or which is still alive, can serve as nutriment for the animal frame. Water is the only inorganic body which is taken into the system for its own sake, and all mineral and other inorganic productions enter it as component parts of previously organized substances of either an animal or a vegetable nature. To a great extent, indeed, vegetation seems to be merely a process for the conversion of inorganic matter into a proper nutriment for the support of animal life; and many species of animals seem in their turn to be little else than living machines for the conversion of vegetable substances into a nutriment fit for other species by which they are intended to be devoured. It is true that, in some parts of South America, the natives, pressed by want, consume quantities of a soft unctuous clay, which is of course destitute of organization; but as there is every reason to believe that no nourishment is derived from it, and that it merely serves to allay the pangs of hunger, such instances form no exception to the general rule.

It would have been easy for the Creator to bestow such a structure on all animals, as to make them subsist entirely on vegetable aliment. But the arrangement which He has seen fit to adopt, is the source of an infinitely greater amount of active enjoyment than what could otherwise have existed. Had there been no beasts of prey, the world would soon have been overrun with herbivorous creatures to such an extent, that their numbers would speedily have become excessive in reference to the possible supply of food, and there would have been infinitely more suffering from starvation and disease, than what actually arises out of their existing relation to each other. On the present plan, there is ample food and enjoyment for all; and when the time does arrive when one animal must become the prey of another, the deprivation of life is in most cases unforeseen, and the suffering which attends it is in general only momentary in duration. There is thus both complete enjoyment of life while it lasts, and a great additional field opened for the support of an immense class of animals, which, with their present constitution, could not otherwise have existed at all.[26]

The gastric juice, as already remarked, has no power over living animal matter,—a most wise and admirable provision, since otherwise it would at once have attacked and destroyed the very organ which produces it. This is the reason why certain worms are able to exist in the stomach of man and other animals; and if it were possible for an oyster swallowed directly from the shell to continue to live, it would also effectually resist every attempt at digestion. But it, in common with most other beings, soon perishes in circumstances so foreign to its habits; and when once dead, the gastric juice assumes the mastery, and speedily converts it into chyme.

If any thing could have opened Montègre’s eyes to the fallacy under which he laboured in considering the gastric juice as almost identical with saliva, the circumstance we are now to mention would have sufficed. When a person, previously in good health, dies by a violent death, or when an animal is killed soon after a meal, it very often happens that, on opening the body after an interval of some hours, the stomach is found to be eroded, and its contents poured into the cavity of the abdomen, precisely as if a hole had been formed in it by ulceration. It was long before the reason of this was discovered; but at length it was ascertained to arise from the action of gastric juice (the abundant secretion of which was provoked by the immediately preceding meal) upon the substance of the stomach, now subjected to its power from being deprived of life. This fact has been so often verified, that it is by all admitted as incontrovertibly true.[27] If, therefore, the gastric juice be merely saliva and mucus, we might expect to find after death distinct traces of similar results from the contact of saliva within the mouth or gullet; but there no such erosion is ever witnessed, nor, as Montègre himself admits, does saliva exert any solvent power whatever over dead animal matter out of the body. These facts appear quite sufficient to convince any unprejudiced mind.

The power of coagulating milk, and albumen or the white of eggs, is another remarkable property of the gastric juice,—and one so familiarly known, that in dairies an infusion of the stomach of the calf is in common use, under the name of runnet or rennet, for curdling milk. In infants, also, we know that the nurse’s milk has scarcely reached the stomach before coagulation takes place; a fact which leads many experienced mothers to infer that the infant is already suffering from acidity, and to counteract the supposed evils by repeated doses of magnesia—which, of course, do more harm than good. The coagulation of milk in the stomach, is so far from being a morbid process, that milk cannot be properly digested without it. By the separation and absorption of the fluid whey, the curd is reduced to a proper consistence for being acted upon, both by the gastric juice itself and by the contractions of the muscular coat.

The gastric juice is also powerfully antiseptic; that is to say, it prevents animal substances from becoming putrid, and even renders sweet such as have advanced a considerable way towards putrefaction. Dr Beaumont mentions that the pure juice will keep unchanged for almost any length of time, and according to Spallanzani meat may be preserved in it without taint for five or six weeks, or even longer. This antiseptic tendency of the gastric fluid, accounts for the circumstance that little or no mischief results from the common practice among epicures, of not making use of game till the putrefactive process is advanced farther than is agreeable to the palates of the uninitiated.

The qualities of the gastric juice are so directly adapted to the natural food of the animal, that flesh introduced into the stomach of an ox or a sheep, for example, undergoes scarcely any change; while vegetable food, on the other hand, remains equally undigested in that of a beast of prey. Thus, “when a hawk or an owl has swallowed a small bird, in the stomach of which have been seeds, these bodies are not dissolved by the gastric fluid,”[28] but pass through the intestines unaltered. Man, the dog, and some other creatures, possess the power of digesting all sorts of aliment, whether vegetable or animal, and are hence called omnivorous or all-eating; but even in them, the relation which the properties of the gastric juice bear to the qualities of the food chiefly or exclusively used is so close, that, when a widely different kind is suddenly resorted to, indigestion is the almost inevitable consequence,—because then the gastric juice has not had time to acquire its requisite adaptation to the new materials on which it has to act. Hence also the danger arising from suddenly eating a full meal after having been long famished. The stomach accustomed for a time to the smallest quantity, is no longer able to provide gastric juice sufficient for the purposes of digestion; and being irritated by continued craving, its secretions are vitiated and its sensibility exalted to such a degree as to require the wisest management to enable it to regain its healthy tone.

The gastric juice of carnivorous animals differs even in its chemical constitution from that of the herbivorous; a circumstance which accounts for the difference observed in their action. In the former, such as that of birds of prey, serpents, and fishes, no free or uncombined acid can be detected, although it is invariably found in the gastric fluid of vegetable-eaters. In crows and dogs, on the other hand, and such animals as can live on either kind of aliment, it is never acid except when they have been fed chiefly on grain or plants. In man, the same relation has, by numerous experiments, been ascertained to exist.

But although, in every class of living beings, the gastric juice is constituted with a direct relation to its natural food, still its qualities may be so much modified by a very gradual change of diet, as to fit it for digesting aliment of a very dissimilar or opposite kind. Thus, in the natural state, the stomach of a sheep exerts scarcely any action on beef or mutton; but if the change from the one kind of food to the other be made by slow degrees, the gastric juice will in the end become so essentially altered, as to enable it to digest both. In this way, as is mentioned by Delabere Blaine,[29] a horse at the Veterinary College was supported for some time by animal matter alone; while others have subsisted on dried fish, or on milk. It has been shewn, also, by John Hunter, Spallanzani, and others, that eagles, falcons, owls, pigeons, and domestic fowls, may for a time be fed on aliments altogether foreign to their natural habits. But these facts only chew the extent to which Nature will go, on an emergency, for the preservation of life; and no more indicate the equal fitness of both kinds of aliment, than the fact of some men being able to stand for a few minutes on their heads proves an inverted position to be the natural attitude of the human race.

In consequence of this adaptation of the gastric juice to the nature of the food, it is obvious that sudden and extreme changes from one kind of diet to another must be injurious, because the stomach has not time to modify its secretions sufficiently to meet the altered demand made upon its powers. This, accordingly, is one of the reasons why so much caution is used in bringing horses into condition after having been for some time in the pasture-field. When they have previously been on dry food in the straw-yard, corn may be given with greater safety: so that it is the change, not so much in quantity as in kind of aliment, which causes the risk. And, on this account, when a horse is to be put upon hard food, after having been fed on grass or other succulent vegetables, Blaine recommends, not only that hay and corn should be given in very small quantity at first, but that the hay should be moistened, and the corn mixed with bran and mashed; by which means having acquired a greater analogy to grass, it will be more easily acted upon by the gastric juice, which has been previously adapted for green food.

Even in man, the gastric juice undergoes considerable modifications, not merely according to the kind of aliment habitually used, but also according to the time of life, the wants of the system, the season of the year, and the state of the health; so that while sudden and great changes from one kind of diet to another are positively hurtful on the one hand, absolute uniformity is not less objectionable on the other, because it leads to so great a uniformity in the quality of the gastric juice habitually secreted as to render it incapable of acting with due effect on any accidental variety of food, to which a change of circumstances may compel us.

Many attempts have been made to ascertain to which of the elements of the gastric juice its power is chiefly to be ascribed, and experiments have been instituted on them individually to discover which of them is most nearly analogous to it in effect. From the general results, it appears that acetic acid (vinegar) and muriatic acid have a wider range of influence, and produce solvent effects more closely resembling those of gastric juice, than any other known substances. Both of these acids, it will be recollected, are constituent elements of the gastric fluid; and it has, in consequence, been argued, that to them it is indebted for all its energy. And, indeed, without laying too much stress on this real or supposed analogy, it is impossible to overlook the well-known fact, that scurvy, and a highly alkaline state of the system, are generally induced by a diet restricted for a long time to animal food alone, and are prevented or cured most easily by a free use of lemon juice or of vegetable matter, either fresh or fermented. In these circumstances, the vegetable acid is probably efficacious both by directly improving digestion, and by combining with the excess of the alkaline salts already existing in the system. It is worthy of remark, too, that in weak stomachs acidity is almost invariably induced by the use of vegetable food, possibly to some extent for the very purpose of effecting its digestion; for it has been ascertained beyond a doubt, that in herbivorous animals the gastric juice always contains some free or uncombined acid—and in man also, after living much on vegetables for some time.

The necessity of acid for the chymification of vegetable food, affords an explanation of the fondness which the Germans and Dutch display for saur-kraut—or cabbage in a state of acetous fermentation—and of its alleged easy digestibility. It explains, also, the general use of vinegar along with salads, cucumbers, oysters, salmon, and other substances of difficult digestion, and shews that its utility is not imaginary, but loudly proclaimed by Nature’s own acts.

Another important principle, which Dr Beaumont conceives to be established by his numerous experiments, and which forced itself upon him by degrees, is, that in health THE GASTRIC SECRETION ALWAYS BEARS A DIRECT RELATION TO THE QUANTITY OF ALIMENT NATURALLY REQUIRED BY THE SYSTEM; so that, if more than this be taken, there will necessarily be too small a supply of the juice for the digestion of the whole. The principle here laid down is in perfect harmony with the sympathy which we have seen to exist between the stomach and the rest of the body, and therefore not only is highly probable in itself, but, if sound, will prove a most valuable guide in the practical regulation of diet. The number of phenomena which it explains, and its general applicability to daily use, afford no small presumption of its truth. When, for example, we eat more than the wants of the system require, indigestion will follow, because there will be more food in the stomach than what the quantity of gastric juice provided is able to dissolve; the proportion of the juice secreted being in relation, not to what we eat, but to the actual wants of the system, which, in the case supposed we have greatly exceeded. Here a remarkable harmony will be perceived between the quantity of the secretion and the true indications and uses of appetite as a guide to diet, explained in a preceding chapter (p. [21]).

The gastric secretion, and the appearance of the villous coat, undergo great modifications during disease, and on this subject also Dr Beaumont’s observations are highly valuable; because, instead of merely inferring, as others are obliged to do, he enjoyed the privilege of SEEING with his eyes what was actually going on. In the course of his attendance on St Martin, he found that, whenever a feverish state was induced, whether from obstructed perspiration, from undue excitement by stimulating liquors, from overloading the stomach, or from fear, anger, or other mental emotion depressing or disturbing the nervous system, the villous coat became sometimes red and dry, and at other times pale and moist, and lost altogether its smooth and healthy appearance. As a necessary consequence, the usual secretions became vitiated, impaired, or entirely suppressed; and the follicles from which, in health, the mucus which protects the tender surface of the villous coat is poured out, became flat and flaccid, and no longer yielded their usual bland secretion. The nervous and vascular papillæ, thus deprived of their defensive shield, were then subjected to undue irritation. When these diseased appearances were considerable, the system sympathized, and dryness of the mouth, thirst, quickened pulse, and other symptoms, shewed themselves; and NO GASTRIC JUICE COULD BE PROCURED OR EXTRACTED EVEN ON THE APPLICATION OF THE USUAL STIMULUS OF FOOD.

These facts, if correctly observed, are of extreme importance; and from the care with which Dr Beaumont pursued his investigations, and their accordance with the facts recorded by preceding physiologists, I do not think their general accuracy can be called in question. The dry irritated appearance of the villous coat, and the absence of the healthy gastric secretion in the febrile state, not only explain at once the want of appetite, nausea, and uneasiness generally felt in the region of the stomach, but shew the folly of attempting to sustain strength, by forcing the patient to eat when food cannot be digested, and when Nature instinctively refuses to receive it.

Before dismissing this part of the subject, it may be remarked, that the alleged sympathy of the stomach with the wants of the body has been denied, because the sense of hunger disappears the moment food is swallowed, or the stomach is distended even with clay or saw-dust, although the actual wants of the system cannot by possibility have been supplied in either case. But these facts seem to me rather to justify the inference that a sympathy does exist. Hunger ceases when food is taken, simply because now the condition of the stomach is in the desired relation to the state of the body, and the nerves consequently feel and transmit this impression to the more distant parts. In the other case, again, it ceases because the stomach cannot at first distinguish what is food from what is not; and, therefore, when distended, expresses content, because it feels satisfied that it has been honestly dealt with, and got what it wanted. But whenever it discovers the cheat, which it does in no long time, hunger returns, and can be properly appeased only by digestible substances. Dr Beaumont indeed expressly mentions, that, although the gastric secretion commences the moment any indigestible body touches the mucous surface of the stomach, it invariably ceases soon after discovering that the substance is one over which it has no power,—thus strongly confirming the existence of the sympathy. Here it may be also proper to observe, that from the frequent and pointed references which I have made to the results obtained by Dr Beaumont, some of my readers have imagined that I claimed for him the merit of the original discovery of all the truths which his experiments tend to establish. Nothing, however, was farther from my intention, and accordingly on page [87] I speak distinctly of his having eagerly embraced the opportunity afforded him of “testing the prevailing doctrines” on digestion; and on page [93] and other places I state that the same results had previously been arrived at by Tiedemann and other observers, but that the evidence in their favour adduced by Dr Beaumont was “more direct and incontrovertible,” and therefore more conclusive than their’s,—a position wholly at variance with any claim of discovery on his part. Indeed, the utter absence of pretension in Dr Beaumont’s work is one of its most pleasing characteristics.

CHAPTER V.
THEORY AND LAWS OF DIGESTION.

Different theories of Digestion—Concoction—Fermentation—Putrefaction—Trituration—Chemical solution—Conditions or laws of digestion—Influence of gastric juice—Experiments illustrative of its solvent power—Its mode of action on different kinds of aliment—beef, milk, eggs, soups, &c.—Influence of temperature—Heat of about 100° essential to digestion—Gentle and continued agitation necessary—Action of stomach in admitting food—Uses of its muscular motion—Gastric juice acts not only on the surface of the mass, but on every particle which it touches—Digestibility of different kinds of food—Table of results—Animal food most digestible—Farinaceous next—Vegetables and soups least digestible—Organs of digestion simple in proportion to concentration of nutriment—Digestibility depends on adaptation of food to gastric juice more than an analogy of composition—Illustrations—No increase of temperature during digestion—Dr Beaumont’s summary of inferences.

Before entering upon the consideration of the theory of digestion which naturally evolves itself from the facts expounded in the preceding chapter, it may be of advantage to turn for a moment to the various theories which have prevailed since the subject first attracted the attention of the learned.

Hippocrates regarded digestion as a kind of concoction or stewing; and many of his followers believed that it is effected in the stomach by the agency of heat alone, much in the same way as food is cooked over a fire. It is quite ascertained that heat favours the process, but it is pure absurdity to maintain that that agent alone will accomplish digestion.

Others of the older physiologists contended, that chymification results from simple fermentation of the alimentary mass, and referred to the gas disengaged during difficult digestion, as a proof that the process of fermentation goes on. But it is now demonstrated, that the tendency of healthy digestion is rather to arrest than to induce fermentation, and that the latter takes place only when disease exists, or when more food has been swallowed than the quantity of gastric juice secreted by the stomach is able to dissolve. Moreover, the products of digestion and of fermentation are so extremely different, that it is impossible to believe them to originate from the same chemical action.

The next theory which prevailed considered digestion to be the result of the putrefactive process. The single fact that the gastric juice not only arrests putrefaction, but even restores to sweetness meat in which that process is begun, is sufficient to demonstrate the wildness of such a supposition.

Another set of physiologists imagined that trituration would account best for all the changes occurring in the food during digestion; and consequently regarded the chyme as a sort of emulsion formed by the intimate mixture of the aliment with the juices of the stomach, just as an emulsion is formed by rubbing down almonds in a mortar. The advocates of this theory referred for proofs not only to the contractile motions of the stomach, already noticed, but to the muscular apparatus for trituration which forms so remarkable a feature in the gizzard of granivorous birds. But, in adopting this conclusion, they forgot that in birds the triturating apparatus does not digest, but serves, like the organs of mastication in man and quadrupeds, merely to bruise the grain on which the animal lives. In birds, in fact, digestion begins only after the trituration is finished.

A more recent and much more accurate view of digestion, is that which considers it as neither more nor less than a chemical solution of the food in the gastric juice. This theory is supported by a greater number of facts and experiments than any other; but although substantially correct, it is perhaps too exclusive and limited in its principles. It is true that, by the agency of gastric juice on food out of the body, a change very similar to chymification can be effected on it; but when we remember that chyme or the result of real digestion, is essentially the same in its elementary or component principles, whatever be the kind of food from which it is formed,—and that, as yet, we are acquainted with no purely chemical agent, which, applied to different substances, gives rise to the same uniform product,—we shall be more willing to believe that chymification is neither a purely mechanical nor a purely chemical operation; but the result of a vital process, to which both mechanical and chemical forces contribute, and which no action or combination of inanimate matter can either exactly imitate or supersede.

To enable ourselves to appreciate correctly the nature of digestion, we must begin by considering the conditions essential for its performance, or without which it cannot be carried on.

The FIRST indispensable requisite is an adequate supply of gastric juice, and its thorough admixture with every particle of the food on which it is to operate. The SECOND is a steady temperature of about 98° or 100° Fahr.; and the THIRD is the gentle and continued agitation of the alimentary mass in the stomach while digestion is going on.

In illustration of the influence of the FIRST condition. I may refer to the experiments already mentioned as having been made by Spallanzani, Stevens, and others, to shew the solvent power of the gastric juice on food even out of the body. Spallanzani states, that when small portions of well-masticated beef or mutton are placed in a vial with a due proportion of gastric juice, and the requisite temperature and gentle agitation are secured by placing the vial in the arm-pit, the appearances presented at the end of a few hours are extremely analogous to those observed in the natural process of chymification; the meat being in both cases converted into the soft greyish mass of a pultaceous consistence called chyme.

Dr Beaumont, who was well aware of the importance of Spallanzani’s researches, and of the almost universal adoption of his views by succeeding physiologists till confidence in their accuracy was for a time shaken by the bold and fallacious assertions of Montégre, felt that the opportunity afforded him by St Martin’s wound for verifying or disproving the experiments on which these views were founded, was much too valuable to be lost. He therefore entered upon a long series of investigations, of which the following is an imperfect, though I hope instructive, abstract.

To test the reality of the solvent powers ascribed to the gastric juice, Dr Beaumont withdrew from St Martin’s stomach about one ounce of it, obtained after a seventeen hours’ fast, by introducing first a thermometer to induce the secretion, and then a gum-elastic tube to carry it off. Into this quantity, placed in a vial, he introduced a piece of boiled recently-salted beef, weighing three drachms. He then corked the vial tightly, and immersed it in water raised to the temperature of 100°, which he had previously ascertained to be the heat of the stomach when the secretion was going on. In forty minutes, digestion had distinctly commenced on the surface of the beef. In fifty minutes, the fluid became quite opaque and cloudy, and the texture of the beef began to loosen and separate. In sixty minutes, chyme began to be formed. In one hour and a half, the muscular fibres hung loose and unconnected, and floated about in shreds. In three hours, they had diminished about one-half. In five hours, only a few remained undissolved. In seven hours, the muscular texture was no longer apparent; and in nine hours the solution was completed.

To compare the progress of digestion in the natural way with these results, Dr Beaumont, at the time of commencing the experiment just described, suspended a piece of the same beef, of equal weight and size, within the stomach by means of a string. At the end of the first half hour it presented the same appearances as the piece in the vial; but when Dr Beaumont drew out the string at the end of an hour and a half, the beef had been completely digested and disappeared, making a difference of result in point of time of nearly seven hours. In both, the solution began on the surface, and agitation accelerated its progress by removing the external coating of chyme as fast as it was formed. When the experiment was repeated with chicken instead of beef, the solution was slower, from the greater compactness of the chicken not allowing the gastric fluid to penetrate its substance so readily. Had the beef and chicken been masticated before being subjected to experiment, the difference between them in the rapidity of digestion would probably have been less.

To ascertain still more accurately the difference between natural and artificial digestion (the one in and the other out of the stomach), Dr Beaumont put twelve drachms of recently-salted boiled beef into a vial, with the same number of drachms of fresh gastric juice obtained through the opening of the stomach after a fast of eighteen hours; and then placed the vial in a basin of water on a sand-bath, where he kept it at the heat of 100° Fahr., and continued to agitate it gently. Digestion soon commenced and went on uniformly for about six hours, when it ceased. One-half of the meat was then dissolved, and the texture of the remainder loosened and tender,—resembling the same kind of aliment when ejected from the stomach partly digested some hours after a meal, as frequently seen in cases of indigestion. On weighing the undissolved portion which remained after all action had ceased, six drachms and twelve grains of the beef were found to have been digested by twelve drachms, or nearly double its weight, of gastric juice. It thus appears that a given quantity of gastric fluid can digest only a relative proportion of meat; so that, when more is eaten than what there is juice sufficient to dissolve, stomachic disorder must necessarily follow. In this latter case, Dr Beaumont found that the addition of fresh juice causes digestion to be resumed.

To discover what influence would be exerted on food masticated, swallowed, and mixed with the gastric juice in the usual way, and then withdrawn from the stomach, Dr Beaumont gave St Martin an ordinary dinner of boiled salted beef, bread, potatoes, and turnips, with a gill of pure water for drink; and twenty minutes afterwards drew off through the opening about a gill of the contents of the stomach into an open mouthed vial. In this short space of time digestion had already commenced, thus negativing the common notion that an hour elapses before it begins. The vial was now placed in a water-bath, at a temperature of 100°, and continued there for five hours. Examined at the end of that time, the whole contents were found to be dissolved. On then extracting an equal quantity of chyme from the stomach, and comparing it with the solution in the vial, little difference was observable between them, except that the process had been somewhat more rapid in than out of the stomach. But this experiment is remarkable in another point of view, as shewing that in the short space of twenty minutes enough of gastric juice had been secreted for the entire completion of digestion.

With a view to verify these results, and also to discover the comparative digestibility of different kinds of aliment, Dr Beaumont gave St Martin for dinner eight ounces of recently-salted lean beef, four ounces of potatoes, some bread, and four ounces of boiled turnips. After fifteen minutes he withdrew a portion of the contents of the stomach, and found that some of the meat had already been slightly digested. In a second portion, withdrawn at the end of forty-five minutes, fragments of the beef and bread were perceptible, and in a still more advanced state of digestion; the meat was in small shreds, soft and pulpy, and the fluid containing it had become more opaque and gruel-like in appearance. When two hours had elapsed, a third quantity was taken out, at which time nearly all the meat had become chymified and changed into a reddish-brown fluid; but small pieces of vegetable matter now presented themselves for the first time, but in a state of digestion so much less advanced than the meat, that their peculiar structure was still distinctly visible. Some of the second and third portions, put into a vial and treated in the usual way, advanced to complete digestion, as in the other experiment, except that the process was slower, and that a few vegetable fibres remained to the last undissolved; thus confirming the general opinion that vegetables are more difficult of digestion than animal substances.

The mode of solution by the gastric juice varies according to the nature of the food on which it acts. We have seen that it gradually reduces solids to a soft and fluid state; but its effect on milk and albumen is different. It begins by coagulating them so as to give them the requisite consistence for being affected by the muscular contractions of the stomach, and impregnated with the juice. Fifteen minutes after St Martin had drunk half a pint of milk, a portion taken out of the stomach by Dr Beaumont presented the appearance of a fine loosely-coagulated substance, mixed with a semi-transparent whey-coloured fluid. A drachm of warm gastric juice poured into two drachms of milk, at a temperature of 100°, produced a precisely similar appearance in twenty minutes. In another experiment, when four ounces of bread were given along with a pint of milk, and the contents were examined at the end of thirty minutes, the milk was coagulated, and the bread reduced to a soft pulp floating in a large proportion of fluid. In two hours the whole was digested.

When the white or albumen of two eggs was swallowed on an empty stomach, small white flakes began to be seen in about ten or fifteen minutes, and the mixture soon assumed an opaque whitish appearance. In an hour and a half the whole had disappeared. Two drachms of albumen, mixed with two of gastric juice out of the stomach, underwent the same changes, but in a rather longer time.

When the food is chiefly liquid, as when soup is taken either alone or in large proportion, the more fluid part is speedily absorbed, to fit the remaining nutritious portion for being better acted on by the gastric juice and muscular power of the stomach; but in impaired digestion, the requisite absorption of the fluid part does not go on so rapidly. Fifty minutes after St Martin had dined on vegetable soup, beef, and bread, Dr Beaumont found the stomach to contain a pulpous mass, like thick gruel in consistence, and of a semi-gelatinous aspect. The fluid portion had been absorbed to such an extent, that the remainder was even thicker than is usual after eating more solid food. From many similar observations, Dr Beaumont infers it to be a general law, that soups and liquids cannot be digested till they are formed into a thicker mass by the absorption of their watery part—as till then they are too liquid to be easily acted on by the gastric juice. Hence their unfitness for weak stomachs, and the impropriety of large libations of tea or coffee at breakfast by persons whose digestion is bad. During recovery from illness, chicken-tea, beef-tea, and soups, are often useful, simply because the system then requires the liquid to make up its lost blood.

Unfortunately Dr Beaumont made few experiments on the action of gastric juice upon vegetables; and, in the few recorded, he generally contents himself with noting the length of time required for their solution, which generally proved considerably longer than for animal substances. In one experiment, however, he states, that an hour after giving St Martin nine ounces of raw, ripe, sour, apples, the stomach was full of fluid and pulp, “quite acrid, and irritating the edges of the aperture, as is always the case when he eats acescent fruits or vegetables.” In an hour and a half the contents were still more sharp and acrid, and the pulp of the apple visible. At the end of two hours the stomach was empty, but the mucous membrane exhibited an irritated appearance. With farinaceous vegetables, however, the results were different. Thus, when a pint of thick, rich, boiled sago, sweetened with sugar, was given, the whole was digested in less than two hours, and there was neither acrimony of the gastric contents nor smarting of the edges of the wound; on the contrary, it seemed peculiarly grateful to the stomach, and rendered the mucous membrane soft, uniform, and healthy. The same results followed a repetition of the experiment, and also when a pint of soft custard was taken. In some states of the stomach, it is true, even farinaceous food excites acrimony and irritation, but rarely in the same degree as the other forms of vegetable aliment.

Such being the influence of gastric juice on different aliments at the natural heat of the body, we have now to ascertain, in the SECOND place, what share the high temperature has in the result.

To determine this point, Dr Beaumont took out two ounces of gastric juice, and divided it into two equal portions, in separate vials. He added to each an equal weight of masticated fresh beef; and placed the one in a bath at the temperature of 99°, and the other in the open air at 34°. As a contrast to these, he placed beside the latter a third vial, containing the same weight of masticated meat in an ounce of clear water.

In two hours the meat in the warm vial was partially digested; that in the cold gastric juice was scarcely changed; and the third portion, in the cold water, seemed only a little macerated. In six hours the meat in the warm vial was half digested, while that in the two others had undergone no farther alteration. The gastric juice in the first vial having by this time dissolved as much as it could of the beef, four drachms more were added from the stomach, and the vial was replaced in the bath. Digestion, which had previously ceased, was now resumed, and went on as steadily as if it had not been interrupted; thus shewing, in a striking manner, the impropriety of exceeding in our meals the quantity for which alone a sufficiency of gastric juice can be provided.

At the end of twenty-four hours, the three portions were examined. That contained in the warm juice was completely dissolved, and presented the usual appearances. The portions contained in the cold juice and in the cold water very much resembled each other, and exhibited no appearance whatever of chyme. They were macerated or softened, but not digested. These experiments, and others of a similar nature, shew clearly that a temperature equal to ordinary blood-heat is requisite for chymification.

To make sure that it was the low temperature alone which prevented the occurrence of digestion in the experiment detailed, Dr Beaumont now placed the vial containing the meat which had been exposed without effect for twenty hours to the action of the cold gastric juice on a water-bath at the ordinary blood-heat. In a very short time “digestion commenced, and advanced regularly as in the other parcels.” The same results were always obtained from a repetition of these experiments, so that they may be held as perfectly conclusive in establishing the essentiality of heat to the digestive process. Common observation, indeed, establishes this truth. Dr Kitchener, for example, after stating that “a certain degree of heat is absolutely necessary to excite and support a regular process of digestion,” remarks, that, “when the circulation is languid and the food difficult of solution in aged persons and invalids, even external heat will considerably assist concoction, and the application of the calefacient concave (stomach warmer) will enable the digestive organs to overcome refractory materials, and convert them into laudable chyle.”[30]

Thirdly.—The necessity of gentle and continued agitation for the accomplishment of digestion, is so obvious from the preceding exposition, that it requires no direct experiments to establish it. When portions of meat were suspended in the stomach, by a string so short as to prevent them from being fully subjected to the motion already described as always going on during digestion, the action of the gastric juice was confined almost entirely to their surface, and a longer time was consequently required for their solution than when they were left at liberty. In like manner, when meat out of the stomach was placed in a vial containing gastric juice, its solution was uniformly accelerated by gentle agitation, which acted simply by removing the coating of chyme as it formed on the surface, and thus affording to the gastric fluid an easier access to the undigested portions below. Accordingly, when in one of Dr Beaumont’s experiments two ounces of unmasticated roasted beef were introduced through the external aperture into the stomach, and held by a string, only one-half of it was digested in four hours, evidently from the want of mastication confining the action of the gastric juice to the surface of the mass, and because the string prevented it from following the regular motions of the stomach.

Having now made the reader sufficiently acquainted with the agents concerned in, and the conditions essential to the performance of digestion out of the body, we have next to exhibit the same agents and the same conditions in their ordinary operation in the living being, and to describe the beautiful arrangements by which they are respectively and unerringly regulated.

It has been already shewn, that, in endowing us with appetite, Nature has intended both to insure by its means a timely provision for the wants of the system, and to guard against our eating more than enough to supply them. We have also seen that, within certain limits, the quantity of gastric juice secreted bears a direct relation to the quantity of food consumed; that when the food exceeds considerably the real necessities of the system, a part of it remains undigested, because the stomach is unable to secrete a sufficiency of fluid for the solution of the whole; and that, as a necessary consequence, indigestion follows. This being the case, we may expect to find all the arrangements of Nature made with a view to prevent us from hastily filling the stomach to repletion, without being fully warned beforehand of the error we are committing. And such accordingly is the fact.

Considered in this light, the processes of mastication insalivation, and deglutition, are not only useful in preparing the morsel for the future action of the gastric juice, but, by transmitting the food to the stomach in small portions at a time, likewise serve the important secondary purpose of preventing its too rapid or excessive distention. To this good end, indeed, the stomach itself contributes, as has been distinctly shewn by Dr Beaumont. In the natural state of that organ, a regular and gentle contraction of its whole fibres and cavity follows the introduction of each individual morsel, and it is not till the relaxation consequent on that contraction takes place, that another is willingly admitted. This arrangement was more than suspected by other physiologists, but it remained for the American experimenter to demonstrate its existence and purposes. It is true that, during a hurried repast for example, food may be rapidly introduced into the stomach by an active effort of the will, but it is precisely in such circumstances that we are apt to eat too much, and that indigestion follows; because, from no time being allowed for the secretion of the requisite quantity of gastric juice, and its proper mixture with each portion of the aliment, the stomach is placed in an unnatural situation, and its nerves cannot receive the same impression of “enough eaten,” which is designed by Nature to arise only from the one being duly proportioned to and mixed with the other. The advantage of the natural arrangement is therefore confirmed rather than refuted by what may at first appear an exception.

When Dr Beaumont depressed the valve in St Martin’s stomach, and introduced a few spoonfuls of soup at the orifice, he observed the rugæ or folds of the mucous membrane to close gently upon it, gradually diffusing it through the gastric cavity, and completely preventing the entrance of a second quantity till this diffusion was effected—when relaxation again took place, and admitted of a farther supply. When solid food was introduced in the same way, either in larger pieces or finely divided, the same gentle contraction and grasping motion were excited, and continued from fifty to eighty seconds, so as to prevent more from being introduced without considerable force till the contraction was at an end. When St Martin was so placed as to admit of the cardia or upper orifice of the stomach being brought into view, and was then made to swallow a morsel of food in the natural way, a similar contraction of the stomach, and closing of its fibres upon the bolus, was invariably observed to take place; and till this was over, a second morsel could not be received without a considerable effort. And accordingly, when, either from haste or hunger, we disregard the order of Nature, and hurriedly gulp down food without due mastication, and without allowing time for the regular contraction of the stomach, we necessarily expose ourselves to the risk both of overloading it, and of ultimately impairing its digestive power.

Such being the provision made for insuring the gradual admission of food into the stomach, the next requisite is its proper admixture with the gastric juice.

Food being the appropriate stimulus of the secreting vessels of the stomach, the moment the alimentary morsel comes into contact with the mucous membrane, the action of the latter, as was formerly pointed out, becomes increased; its bloodvessels are distended, its colour deepens to a brighter red, and the gastric juice immediately begins to be poured out. The muscular fibres of the stomach, being acted upon by the same stimulus, next come into play, and execute their specific function of alternate contraction and relaxation. By these means the aliment speedily becomes impregnated with the gastric fluid and undergoes the influence of that continued gentle agitation already described as essential to digestion, and which seems to have for its chief object the careful admixture of every portion of the nutriment with the quantity of gastric juice necessary for its solution. The particles of food are thus continually changing place, and if the quantity taken be not too great for the power of the gastric juice which the stomach can supply, chymification goes on equally throughout; so that, if the contents of the stomach be withdrawn in from thirty minutes to an hour after a moderate meal, they will be found to consist of perfectly formed chyme and particles of food, intimately mixed and blended, in larger or smaller proportions, according to the vigorous or enfeebled state of the digestive organs, and the quality of the aliment itself. So effectually indeed has the admixture of food and solvent juice taken place in this short time, that, as already shewn, when a portion is removed from the stomach and placed in an appropriate vessel, digestion will commonly continue in it, provided it be placed in a proper temperature, and subjected to gentle and continued agitation.

It is the impossibility of its being adequately acted upon by the muscular contractions of the stomach, which renders fluid and highly concentrated nourishment, when exclusively used, so difficult of digestion; and hence the reason why a certain bulk and consistence given to whale oil, for example, by the admixture of such innutritious substances as vegetable fibre, bran, or even saw-dust, make it a more acceptable and digestible article of food to the inhabitants of the northern regions, than when it is consumed in its pure state. In like manner, in civilized society, bread, potatoes, and vegetables, are useful, not less by giving the requisite bulk and consistence to the rest of the food, than by the nutriment which they contain. Soups, jellies, arrow-root, and similar substances, are, for the same reason, more easily digested when eaten along with bread or some bulkier aliment, than when taken alone, especially if used for some time.

The motion which we have seen to be excited in the stomach by the entrance of aliment, is at first very gentle and slight; but in proportion as digestion proceeds, or the organ is distended, it becomes more rapid and energetic; and then it serves the additional purpose of gradually propelling the chyme through the pylorus into the intestine, there to be farther prepared and converted into chyle. The necessary churning or agitation of the food is, from the peculiar situation of the stomach, greatly assisted by the play of the diaphragm and abdominal muscles during inspiration and expiration; and the diminution of the vivacity and extent of the respiratory movement which always attends despondency and grief, is one source of the enfeebled digestion which notoriously accompanies or follows depression of mind. The same cause also leads necessarily to an unfavourable condition of the blood itself, which in its turn weakens digestion in common with every other function; but the muscular or mechanical influence is that which at present chiefly concerns us. On the other hand, the active and energetic respiration attendant on cheerfulness and buoyance of spirits, adds to the power of digestion, both by aiding the motions of the stomach, and by imparting to it a more richly constituted blood. If to these causes be added the increase of nervous stimulus which pleasing emotions occasion in the stomach (as in the muscles, and organs of secretion generally), we shall have no difficulty in perceiving why digestion goes on so well in parties where there is much jocularity and mirth. “Laughter,” says Professor Hufeland of Berlin, “is one of the greatest helps to digestion with which I am acquainted; and the custom prevalent among our forefathers, of exciting it at table by jesters and buffoons, was founded on true medical principles. In a word, endeavour to have cheerful and merry companions at your meals: what nourishment one receives amidst mirth and jollity will certainly produce good and light blood.”[31]

Exposed to the action of all the agents above enumerated, and to the increased nervous and vascular excitement which are necessary during digestion, a singular change soon commences in the food, and goes on till chymification is completed. After a little while the contents of the stomach, whatever their nature and variety, begin to be converted into a substance of a homogeneous, soft, greyish, and viscid appearance, of a sweetish, fade, and slightly acid taste, but which still preserves some of the qualities of the food, and is called chyme. The chyme always forms on that part of the food with which the gastric juice is in immediate contact; and, in proportion as it is produced, it is carried gradually onwards by the gentle motion of the stomach towards the pylorus, where consequently it always exists in the greatest quantity. At the cardiac or left end of the stomach it is most sparingly found, both because digestion is there only beginning, and because the chyme is speedily removed from it and carried away towards the pylorus.

The doctrine hitherto generally received, and held by Dr Wilson Philip and others as demonstrated, is, that “the layer of food lying next to the surface of the stomach is first digested, and in proportion as this undergoes the proper change, and is moved by the muscular action of the stomach, that next in turn succeeds, to undergo the same change.” Dr Beaumont, however, declares, that, whatever may be the case in rabbits and other animals on which Dr Philip’s experiments were made, such was not the order observed in the stomach of Alexis St Martin—and such, he naturally infers, is not the order in the human stomach in general. Nor is such, I may add, the order which either experience or a correct view of the antecedent circumstances and physiological conditions ought to lead us to anticipate. When vomiting, for instance, occurs an hour or two after a meal composed of different ingredients has been swallowed, no such separation into digested and undigested portions is to be found, but the whole are observed to have undergone changes proportioned to their degrees of digestibility, whether they were eaten first or last.

In conformity with this view, Dr Beaumont mentions, that, when St Martin swallowed a mouthful of any tenacious food after the digestion of the preceding meal was considerably advanced, he always saw it first pass towards the great curvature of the stomach, and then disappear. In a minute or two it re-appeared, more or less broken down, and mixed with the general alimentary mass; and in a short time longer it was so much changed as entirely to lose its identity. From these and numerous other facts, Dr Beaumont infers, that “there is a perfect admixture of the whole ingestæ during the period of alimentation and chymification;” and that “the whole contents of the stomach until chymification be nearly complete, exhibit a heterogeneous mass of solids and fluids—hard and soft, coarse and fine, crude and chymified—all intimately mixed, and circulating promiscuously through the gastric cavity, like the mixed contents of a close vessel, gently agitated or turned in the hand.” (P. 112.) As we proceed we shall meet with various indirect proofs of these statements being correct.

If Dr Beaumont’s observations on this subject are accurate, we may expect to find that chymification commences on the surface of each individual fragment of the food, and is not confined to the outer surface of the entire alimentary mass, as stated by Dr Philip. Such, accordingly, is the fact. When Dr Beaumont extracted a portion of the food through the opening half an hour or an hour after eating, he invariably found it composed of perfectly formed chyme and particles of food intimately mixed and blended; and in these experiments every portion of the aliment was already so completely supplied with gastric juice, that its chymification proceeded till the whole was digested with no other aid than that of the requisite heat and agitation.

When a meal is taken before the preceding one is out of the stomach, digestion is observed to be disturbed. Dr Wilson Philip explains this by stating, that the newly swallowed food becomes imbedded in, and occupies the centre of, the older and half-digested mass, where it remains distinct and untouched till the former meal is entirely disposed of. A more logical explanation, however, and one more in harmony with facts, is offered by Dr Beaumont, who ascribes the disturbed digestion to the supply of gastric juice having been entirely taken up by the first meal, and to the stomach, now comparatively exhausted, being unable to secrete a fresh supply for the second—seeing that, in ordinary circumstances, its vessels secrete only enough to meet the real wants of the system. Dr Beaumont adduces in confirmation of this view, the fact that many children, and most cooks, are in the habit of eating small quantities of food almost every hour or two without their digestion suffering materially, because then the amount of gastric juice secreted is quite equal to the chymification of the whole quantity taken.

Reasonable as this inference appears from the facts stated by Dr Beaumont, I cannot help thinking that there is something more in the constitution of the stomach than the mere deficiency of gastric juice, which renders the too early intrusion of new food hurtful. We know, for example, that, at the commencement of digestion, the muscular contractions of the stomach are comparatively slow and feeble, and that as chymification advances they become rapid and energetic, as if to remove the chyme as fast as it is formed. If then new food, for which the feebler movement is best adapted, be introduced when digestion is far advanced and the energetic motion going on, may not this disproportion be itself an impediment, and co-operate with the deficiency of the gastric juice in disordering digestion? Moreover, as the stomach, in common with every other organ intended for alternate activity and repose, is always more or less fatigued by the active fulfilment of its function, its premature excitement by new food must add to its exhaustion, and weaken its power (in the same way in which fresh muscular exertion adds to the exhaustion of muscles already sufficiently exercised), and consequently lead to imperfect digestion.

The same principle which explains the necessity of repose for repairing the vascular and nervous energy of the stomach, when these have been excited and exhausted by the labour of digestion, also affords a solution of the question why the appetite does not return as soon as the stomach is empty, but begins to be felt only after the latter has enjoyed a period of repose, varying in duration with the mode of life, the state of health, and the nature of the previous meal.

If we regard chymification as going on at the surface of every individual portion of the food, we at once see that the constant motion of the stomach serves, not only to affect the original mixture of the food with the gastric juice, but to remove the chyme from the surface of each little fragment of the alimentary mass in proportion as it is formed, and by this very removal to expose a fresh surface of the fragment to the contact of the mucous membrane, thereby enabling it to excite a farther secretion of the gastric juice, where, as sometimes happens, the stomach is unable to provide a sufficient quantity from the beginning. In this way the formation and removal of chyme go on from the very first, although, of course, more slowly than after the gastric solvent has had time to act. It is generally said that an hour elapses before any chyme is formed; but Dr Beaumont has detected its existence at a much earlier period, and is of opinion that, from the uniform and constant decrease in the contents of the stomach, which begins as soon as the food is swallowed, chymification commences almost immediately. This decrease, though slow at first, becomes gradually accelerated, till the whole mass is converted into chyme. Apparently in harmony with this more energetic action, the acidity of the gastric fluid also becomes greater, and affords a greater stimulus, in proportion as digestion advances.

As formerly explained, the thickish, semi-fluid, greyish, chyme, into which the aliment is converted, is gradually impelled towards the pyloric extremity of the stomach. On its arrival there, the pylorus, or valve between the stomach and the intestine called the duodenum, opens and allows the chyme to pass into the intestine. But, by a curious mode of sensibility, if any portion of undigested food be mixed with it, the pylorus contracts upon it, refuses it egress, and throws it back into the stomach for farther digestion.

If, however, any thing really indigestible finds its way into the stomach, and presents itself at the pylorus—or if the stomach has temporarily lost its digestive power, and the food remains in it for many hours unchanged—then the pylorus, after repeatedly refusing egress, at last opens and allows it to pass into the gut. So marked is the contractile impulse towards the pylorus when digestion is going on, that Dr Beaumont found even the bulb of his thermometer carried down with a steady and considerable force.

Such are the direct conditions requisite for the fulfilment of digestion: but there remain others, of an indirect kind, which also require to be noticed. Of these, a due supply of arterial blood and nervous energy is the most remarkable; but as both produce their effect by modifying the secretions and motions of the stomach, already described as the direct requisites of digestion, it will save a good deal of repetition if, for the present, we take their influence for granted, and reserve their further elucidation till we come to treat of the practical applications of the preceding exposition.

Having thus obtained a comprehensive view of the agents employed in effecting digestion, and of the changes produced by it on different kinds of food, we find another important subject of investigation, immediately connected with the process, presenting itself—the comparative digestibility of different kinds of food. Dr Beaumont did not neglect this branch of the inquiry; but the experiments which he performed for its elucidation are, like those of most of his predecessors, deprived of great part of their value by the vague way in which they seem to have been conducted, and the common omission of all particulars in regard to those conditions which are known to exert a powerful influence on the progress of digestion.

The following Table, which I have arranged in a more lucid order than Dr Beaumont has done, exhibits the general results of all the experiments made upon St Martin posterior to 1825; and the average is deduced from those which were performed when the stomach was considered by Dr Beaumont to be in its natural state, and St Martin himself subjected to ordinary exercise.

TABLE shewing the Mean Time of Digestion of the different Articles of Diet.

This table is very interesting, but the results must not be too much relied upon, or regarded as representing the uniform rate of digestibility. We have already seen that chymification is greatly influenced by the interval which has elapsed since the preceding meal, the amount of exercise taken, the keenness of the appetite, the state of the health and mind, the completeness of the mastication, the state of rest or exercise after eating, and various other circumstances; and, above all, the quantity swallowed in proportion to the gastric juice secreted. And consequently, if an experiment be made without regard to these conditions, and without any thing being recorded except the time occupied in digestion in the individual case, the conclusions deduced from it may be most fallacious. The very aliment which, taken in full quantity, remains on the stomach for hours, may, in a smaller quantity, be entirely digested in one-third of the time. Thus, in the foregoing table, two and a half hours are set down as the average period required for the chymification of jelly; and yet, in one of Dr Beaumont’s experiments, we find that eight ounces of that substance were entirely digested in ONE hour. So that, if all the other conditions are not carefully kept in view at each trial, the results cannot possibly be held as conclusive.

It may be said that, on the occasion just alluded to, St Martin’s digestion must have been particularly good—and, in truth, it seems to have been so; for at nine o’clock A. M. he breakfasted on soused tripe, pig’s feet, bread and coffee, and yet, only one hour later, no vestige of any of these savoury things remained in the stomach. What renders this result the more remarkable, is the fact, that, in another table, a simple breakfast of coffee and bread is set down as having required FOUR hours for its digestion. The rapid disposal of the same elements with the addition of soused tripe and pig’s feet, instead of disproving my position, evidently strengthens it, by shewing that, if from any cause the digesting power varies in intensity, the result obtained from the experiment on one kind of food, cannot, with any shew of reason, be considered as an accurate index of its rate of digestibility in comparison with that of other kinds.

This neglect of the other conditions is accordingly the circumstance which throws a doubt over the results not only of Dr Beaumont’s experiments, but of those of every other inquirer. Dr Beaumont indeed candidly admits, that his were performed for the purpose of demonstrating other important principles connected with digestion, and not at all with the view of determining the comparative rates of digestibility of different kinds of aliment; and in alluding to the various requisites for a satisfactory series of experiments, he himself justly states, that this would be a Herculean task which it would take years to accomplish. In considering the following general results, then, the reader ought to bear in mind that they are only probable and approximative, and not strictly demonstrated or certain.

As a general rule, animal food is more easily and speedily digested, contains a greater quantity of nutriment in a given bulk, and therefore satisfies hunger for a longer time, than either herbaceous or farinaceous food; but, apparently from the same cause, it is also more heating and stimulating. Minuteness of division, and tenderness of fibre, are shewn by Dr Beaumont’s experiments to be two grand essentials for the easy digestion of butcher-meat; and the different kinds of fish, flesh, fowl, and game, are found to vary in digestibility chiefly in proportion as they approach or depart from these two standard qualities.

Farinaceous food, such as rice, sago, arrow-root or gruel, is also rapidly assimilated, and proves less stimulating to the system than concentrated animal food; but as it affords no scope for the due action of the muscular coat of the stomach, its exclusive use for any length of time seldom fails to weaken that organ and impair digestion. For the same reason, however, it becomes a very appropriate aliment, where stomachic irritation already exists. When the stomach is in a healthy state, milk is digested almost as easily as farinaceous food and is equally unstimulating.

The other kinds of vegetable substance are the slowest of all in undergoing digestion, and very frequently pass out of the stomach and through the bowels comparatively little changed; and hence the uneasiness which their presence so often excites in the intestinal canal, especially in persons of weak digestion, owing to the nerves of the intestines having a relation to chyme or digested food, and not to substances which resist the action of the gastric juice. In a given bulk they contain less nutriment, and excite the system less, than any other kind of food; so that they are well adapted for the diet of those in whom it is necessary to avoid every kind of stimulus, and who are not subjected to great muscular exertion; but to a person undergoing hard labour, they afford inadequate support.

Liquids—soup, for example—do not call into play the muscular coat of the stomach, and are so slow of digestion where that organ is already weakened, that they often give rise to acidity; and hence they are unfit for most dyspeptic patients. Before the gastric juice can act upon them, the fluid part must be absorbed, and the mass thickened to a proper consistence for undergoing the usual churning motion. On examining the contents of the stomach an hour after St Martin had dined on beef-soup, Dr Beaumont found that on one occasion the absorption of the watery part had been carried so far as to leave the remainder of even a thicker consistence than after an ordinary solid meal, but a similar result follows only when the digestive powers are very vigorous. When drink is swallowed, it also is carried off by absorption, and is not digested or allowed to pass through the pylorus. One purpose of this provision seems to be to prevent the gastric juice from being rendered inefficient by too much dilution.

When the food on which an animal lives is of a highly concentrated kind, and contains much nourishment in a small bulk, the apparatus of organs provided for its digestion is on a correspondingly small scale in point of extent. Thus, in carnivorous animals, whose food is, bulk for bulk, the most nutritious of all, the stomach and intestines are simple and short, the latter not exceeding in length more than from one to four or five times that of the body. In herbivorous animals, on the other hand, whose food is sparingly nutritious, and therefore requires to have a large bulk or volume, the stomach, as we saw in a former chapter, is greatly more complicated, and the length of the intestines enormously increased. Man, being intended to feed on both animal and vegetable substances, possesses an organization which holds an intermediate place between the two extremes. In him neither are the intestines so short as in carnivorous animals, nor have they the complexity and length characteristic of the herbivorous—thus clearly shewing the intentions of Nature in regard to his food, and at the same time allowing him a considerable latitude of adaptation when the force of circumstances for a time denies him access to any variety.

Animal food being in general more quickly digested than vegetable, and a simpler organization being sufficient for its conversion into chyme, many physiologists have inferred that this is owing solely to its being already of an animal nature, and therefore requiring scarcely any change to fit it for becoming a constituent part of the living fibre. But I agree with Dr Beaumont in thinking that this explanation is more gratuitous than philosophical, and that the process of chymification implies almost as complete a change in the one instance as in the other. In both, the operation of the gastric juice seems to be entirely analogous. In both, a complete solution takes place, and the chyle into which animal food is ultimately converted bears no greater resemblance to the future animal fibre, than does that produced from vegetable aliment. Thus the chyle of a horse, which lives exclusively on vegetables, has quite as great a resemblance to its future muscle, as that of a tiger, a lion, or a fox has to its future produce. Besides, whether the food be animal or vegetable, the ultimate result of digestion is always the formation of new animal matter; but in the former case, the nutritive particles are mixed up with a smaller proportion of innutritious matter than in the latter, and consequently a larger quantity of them can be extracted from a given bulk in a shorter time, than in the case of vegetables. There are most probably also minute differences in the chemical composition of the chyle derived from different kinds of food; but its general nature—its fitness for forming new animal tissue—and that of the process by which it is produced, are always the same.

Animal food, it is true, affords a more stimulating nutriment than farinaceous and other kinds of vegetable aliment, and hence it is avoided in diseases of excitement. But it seems to me that this stimulus is owing not only to its own inherent properties, but also to its more highly concentrated state, and to the much greater quantity of chyle which is derived from it than from an equal bulk of vegetable aliment. From the numerous experiments of injecting water, poisons, and other substances into the veins, performed by Magendie and others, we have direct proofs that the same agent which, introduced rapidly into the system, will sometimes act so powerfully as to destroy life, will excite scarcely any perceptible disorder if introduced very slowly, Analogy, therefore, bears us out in believing that the rapid admixture of very nutritious chyle with the blood may over-stimulate the system, when its more gradual introduction would have produced no such effect. At the same time, there can be no doubt that there is also a greater inherent stimulus in animal than in vegetable aliment.