University of Kansas Publications
Museum of Natural History

Vol. 12, No. 9, pp. 363-474, 20 figs.

October 25, 1963

Variation in the Muscles and Nerves
of the Leg in Two Genera of Grouse
(Tympanuchus and Pedioecetes)

BY
E. BRUCE HOLMES

University of Kansas
Lawrence
1963

UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY

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University of Kansas Publications
Museum of Natural History

Vol. 12, No. 9, pp. 363-474, 20 figs.

October 25, 1963

Variation in the Muscles and Nerves
of the Leg in Two Genera of Grouse
(Tympanuchus and Pedioecetes)

BY
E. BRUCE HOLMES

University of Kansas
Lawrence
1963

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Theodore H. Eaton, Jr.

Volume 12, No. 9, pp. 363-474, 20 figs.
Published October 25, 1963

University of Kansas
Lawrence, Kansas

PRINTED BY
JEAN M. NEIBARGER, STATE PRINTER
TOPEKA, KANSAS
1963

29-5835

Variation in the Muscles and Nerves
of the Leg in Two Genera of Grouse
(Tympanuchus and Pedioecetes)

BY
E. BRUCE HOLMES

[CONTENTS]

[LIST OF ILLUSTRATIONS]

[INTRODUCTION]

The purposes of this study were: (1) to obtain information on individual variation in the anatomy of the muscles and nerves of the leg of Tympanuchus cupido pinnatus (Greater Prairie Chicken), T. c. attwateri (Attwater's Prairie Chicken), T. pallidicinctus (Lesser Prairie Chicken), and Pedioecetes phasianellus jamesi (Sharp-tailed Grouse); (2) to determine whether or not the two species of the genus Tympanuchus differ constantly in the myology of the leg; and (3) to determine what constant differences in the myology of the leg exist between the two closely related genera Tympanuchus and Pedioecetes.

These particular birds were chosen because they are closely related, and closely resemble one another in habitats occupied and in patterns of behavior. It was desired to study examples that showed as few adaptive differences as possible among the grouse. Series of each of the three species of grouse were readily obtainable, making it possible to draw comparisons at the level of individuals, subspecies, species, and genera.

The study here reported on was begun in the spring of 1957 and was completed in the autumn of 1961.

Prior work on the muscles of the leg of birds has been reviewed by Hudson (1937) and Hudson, et al. (1959). Only papers dealing with the innervation of the leg in birds are reviewed below.

DeMan (1873) treated the nerves of Paradisea papuana, Corvus monedula, and the chicken; he also commented briefly on a few other species. Jhering (Ihering, 1873) briefly described the lumbosacral plexus in approximately a dozen birds, but illustrated only two. Gadow (1880) described the nerves in Struthio, Rhea, and Casuarius; his paper contains some excellent illustrations of nerves. Unfortunately, the text is marred by numerous confusing typographical errors. Carlsson (1884) described the nerves of Eudyptes chrysolopha, Alca torda, Mergulus alle, and Mormon arcticus. Gadow (1891) described the nerves in a study that included a large variety of birds, but published few illustrations. DuToit (1913) described the lumbosacral plexus of the chicken. Romer (1927) gave the innervation of the hip and thigh muscles in the chicken, but did not cover the lumbosacral plexus. Appleton (1928) gave the innervation, in various birds, only of those muscles of the hip and thigh that are supplied by the tibial and peroneal nerves; he did not include the lumbosacral plexus. Sudilovskaya (1931) described the nerves of Struthio, Rhea, and Dromaeus (Dromiceius). Unfortunately, his illustrations are almost useless as far as the nerves are concerned. Boas (1933) described the lumbosacral plexus in a large number of birds. His extensive account includes numerous good illustrations. Howell (1938) listed the innervation of the hip and thigh muscles in the chicken; he did not include the lumbosacral plexus. Fisher (1946) listed the innervation of the muscles of vultures, but did not include the lumbosacral plexus. Wilcox (1948) gave the innervation of the muscles of Gavia immer, but did not include the lumbosacral plexus. Fisher and Goodman (1955) described the nerves in the Whooping Crane. Papers by Chomiak (1950) and Yasuda, et al. (1959), both dealing with the chicken, were not examined.

[MATERIALS AND METHODS]

Complete dissections of the muscles and nerves were made in eight legs (of five specimens) of the Lesser Prairie Chicken (Tympanuchus pallidicinctus), six legs (of four specimens) of the Greater Prairie Chicken (T. cupido pinnatus), three legs (of two specimens) of Attwater's Prairie Chicken (T. cupido attwateri), and six legs (of four specimens) of the Sharp-tailed Grouse (Pedioecetes phasianellus jamesi).

For convenience and simplicity of reference, each specimen has been designated by a symbol consisting of the first letter of the genus and of the species (and also of the subspecies in T. cupido) plus a number. The letter "L" or "R" is added to indicate the left or right leg. Thus the symbol T.p. 1L refers to the left leg of specimen number one of T. pallidicinctus.

All specimens are in the University of Kansas Museum of Natural History. The catalogue number of each specimen, and the legs of it that were dissected, are listed below.

The specimens were injected in the field either with formalin (10%) or embalming fluid, except for those of T. c. attwateri, which were frozen; the latter were later injected with embalming fluid. Injection in all the birds was by hypodermic syringe into all major muscle masses, into the body cavities, and subcutaneously in the neck, wings, and feet. In those specimens injected with embalming fluid, the body cavities were injected with formalin. The embalming fluid consisted of 70 per cent alcohol, glycerin (or propylene glycol), and formalin (full strength) in the approximate ratio of 78:20:2, respectively. This fluid gave good preservation; these specimens had the advantages of lacking almost entirely the irritating odor of formalin and of having pliable tissues. The skin of those specimens originally injected with formalin was slit in several places and they were transferred to crocks containing embalming fluid (without the formalin). After a period of many weeks, with two changes of fluid, most of the formalin odor was eliminated and the muscles were sufficiently pliable to be easily dissected. All specimens were kept in containers filled with embalming fluid. No mold ever appeared, even though no phenol or other chemical was added.

To facilitate comparison, two or three specimens were frequently dissected simultaneously. The nerves and smaller muscles were dissected with the aid of a stereoscopic microscope mounted on a long movable arm. In order satisfactorily to expose the lumbosacral plexus the posterior half of the sternum and pectoral muscles, as well as the abdominal viscera, were removed.

To insure more nearly accurate proportions, drawings of the pelvis and of some of the muscles were made with the aid of photographs of the several specimens listed above.

[TERMINOLOGY]

Skeleton

The majority of the osteological terms used in the present paper are those used by Howard (1929); however, many skeletal features are not named by Howard. Since names for most of these parts were not found in the other literature examined, it was necessary for me to propose terms for them. Most of this new terminology pertains to the pelvis. All of the osteological terms used in the present paper, whether used by Howard or not, are briefly defined below. Those of the pelvis are illustrated in [Fig. 1]. Most of the remaining terms are illustrated by Howard (1929).

Pelvis

The median dorsal ridge is the blunt ridge in the midline of the anterior part of the synsacrum formed by the neural spines of the vertebrae. The antitrochanter, on the posterodorsal rim of the acetabulum, is a pyramid-shaped projection that articulates with the proximal end of the femur. The anterior iliac crest is a ridge along the dorsomedial border of the ilium, beginning almost at the anterior end of that bone; the crest curves laterally as it extends posteriorly and (for purposes of the present definition) ends at the level of the posterior edge of the antitrochanter, where the crest is continuous with the lateral iliac process. The lateral iliac process is a pronounced, laterally or ventrolaterally, projecting ridge on the ventrolateral surface of the ilium posterior to the level of the antitrochanter; the process does not extend as far as the posterior end of the ilium. The lateral ischiatic ridge is a relatively slight ridge continuous with the posterior end of the lateral iliac process and curves posteroventrally across the lateral surface of the posterior part of the ischium; the ridge extends to the ventral edge of the ischium in some individuals and not in others. The dorsolateral iliac ridge begins at the lateral edge of the ilium near the posterior end of the lateral iliac process and curves posteromedially and somewhat dorsally, extending to the posterior edge of the ilium. The lateral iliac fossa is the concavity below the overhanging lateral iliac process. The ilio-ischiatic fenestra is a large oblong opening behind the acetabulum between the ilium and the ischium. The obturator foramen is a small oval opening posteroventral to the acetabulum between the ischium and the pubis. The ventral ischiatic tubercle is the angle formed by the ventrally projecting ischium at the point (near its midlength) where the ischium overlaps and lies lateral to (and fused to) the pubis. The pectineal process is an anterolaterally directed projection of the ventrolateral edge of the ilium anteroventral to the acetabulum. The femoral notch of the ilium is a shallow notch in the ventrolateral edge of the ilium approximately halfway between the last rib and the pectineal process. The oblique iliac crest is a pronounced blunt ridge on the ventral surface of the ilium and extends from the posterolateral corner of the last synsacro-thoraco-lumbar vertebra to near the anteroventral border of the ilio-ischiatic fenestra. The internal ilio-ischiatic crest is more or less continuous with the oblique iliac crest and extends posteriorly along the dorsal border of the ischium (forming the ventral border of the ilio-ischiatic fenestra), and then curves sharply dorsomedially onto the ventral surface of the ilium. The iliac recess is a concavity dorsolateral to the sharply curving posterior end of the internal ilio-ischiatic crest.

Fig. 1. Pelvis of Tympanuchus pallidicinctus. A. Lateral view. × 1. B. Ventral view. × 1⅛.

The terminology applied to the synsacral vertebrae by different authors varies. The terminology proposed by DuToit (1913) is employed in the present account. See my [Fig. 1]B. This terminology differs considerably from that used by Howard (1929). DuToit divides the fused synsacral vertebrae into the following five groups, listed in anteroposterior sequence: (1) synsacro-thoracic, which bear movable ribs; (2) synsacro-thoraco-lumbar, which lack movable ribs but possess well developed laterally directed parapophyses, in addition to the more dorsally directed diapophyses; (3) synsacro-lumbar, which lack parapophyses, although possessing inconspicuous diapophyses; these vertebrae are shortened anteroposteriorly and are so firmly fused together that often the number present can be determined only by counting the intervertebral foramina; (4) synsacro-sacral, which have much more pronounced transverse processes than do the synsacro-lumbar vertebrae; these transverse processes are expanded distally where they fuse with the ilium and represent both parapophyses and diapophyses partly or completely fused together plus sacral ribs (detectable only in the embryo); there are considered to be two of these vertebrae; they are situated at approximately the level of the acetabulum; (5) synsacro-caudal, which include the remainder of the fused vertebrae; no marked gross morphological features differentiate the synsacro-sacral and the synsacro-caudal groups of vertebrae. The boundaries between all but the last two groups of vertebrae are usually, but not always, easily determined. It may be difficult to determine whether a vertebra with rudimentary parapophyses belongs to the synsacro-thoraco-lumbar or the synsacro-lumbar group. Sometimes a parapophysis will be better developed on one side of a vertebra than on the other.

Femur

The trochanter is a large squarish tuberosity on the lateral surface of the proximal end of the femur. The trochanteric ridge is a sharp, longitudinal (relative to the femur) ridge forming the anterior edge of the trochanter. The obturator ridge is a short, blunt, longitudinal ridge forming the posterior edge of the trochanter. The anterior intermuscular line is a slight ridge extending distally from the trochanteric ridge. The posterolateral intermuscular line is a slight ridge extending distally from the obturator ridge. The posterior intermuscular line is a slight, longitudinal ridge on the mid-posterior surface of the femur. The internal condyle is a large rounded articular prominence on the medial side of the distal end of the femur. On the lateral side of the distal end of the femur are two articular prominences—the lateralmost, smaller one is the fibular condyle, separated by the fibular groove (visible from posterior aspect only) from the larger and more medial external condyle. The popliteal area is a depression on the posterior surface of the distal part of the femur immediately proximal to the condyles.

Tibiotarsus and Fibula

The inner cnemial crest is pronounced and directed anteriorly on the anterior surface of the proximal end of the tibiotarsus. The outer cnemial crest is pronounced and directed anterolaterally on the anterolateral surface of the proximal end of the tibiotarsus. The rotular crest is transverse and forms the anterior border of the proximal end of the tibiotarsus; the crest extends between the dorsal ends of the two cnemial crests and also extends medial to the inner cnemial crest. The fibular crest is longitudinal on the lateral surface of the tibiotarsus and fuses with the middle part of the fibula. The fibular tubercle is small and on the lateral surface of the fibula near the level of the middle of the fibular crest. The anteromedial intermuscular line is a slight ridge extending from the inner cnemial crest down the anteromedial surface of the tibiotarsus. The anterolateral intermuscular line is a slight ridge extending from the fibular crest down the anterolateral surface of the tibiotarsus. The supratendinal bridge is a transverse bony arch over a longitudinal groove near the distal end of the anterior surface of the tibiotarsus.

Tarsometatarsus

The hypotarsus is a large, pronounced, squarish protuberance on the posterior surface of the proximal end of the tarsometatarsus and contains grooves and canals for the passage of the flexor tendons. The longitudinal ridges forming the lateral and medial edges of the posterior surface of the hypotarsus are termed calcaneal ridges. The posterior metatarsal crest is long and sharp; it is continuous with the medial calcaneal ridge that extends most of the way down the posterior surface of the tarsometatarsus medial to the midline; there is an opening between this crest and the tarsometatarsus immediately distal to the hypotarsus. The medial metatarsal depression is large; it is on the medial surface of the proximal end of the tarsometatarsus. The anterior metatarsal groove is a longitudinal groove in the midline of the proximal part of the anterior surface of the tarsometatarsus. The three trochleae are large rounded articular prominences at the distal end of the tarsometatarsus; there is one at the base of each of the digits II, III, and IV. The term distal foramen (as used by Howard) refers to a short, anteroposteriorly directed canal that perforates the tarsometatarsus a short distance proximal to the intertrochlear notch between the trochleae for digits III and IV. Beginning at the middle of this canal and extending distally at a right angle to it is the intertrochlear canal, which opens via the terminal foramen into the intertrochlear notch between the trochleae for digits III and IV.

Nerves

For ease of description I have coined terms for the major divisions of the femoral and sciatic nerves.

Muscles

My terminology follows that of Fisher (1946) and Fisher and Goodman (1955) except for Mm. femoritibialis externus, flexor cruris lateralis (accessory head), and obturator internus et externus. Fisher (1946:547) states that most of his names for the hip and thigh muscles are those of Howell (1938) and the names for the shank and foot muscles are those of Hudson (1937). Fisher deviates, without explanation, from Howell's terminology in respect to Mm. vastus medialis and femoritibialis internus, M. caudofemoralis, M. flexor cruris lateralis, and Mm. obturator internus and obturator externus. Fisher's synonymy of these muscles (1946: table 42) is in error. Fisher understandably deviates from Hudson in respect to Mm. extensor brevis digiti III and extensor proprius digiti III (see Holmes, 1962), although Fisher's synonymy is in error here. See my table 1.

I am not using Fisher and Goodman's term femoritibialis externus; this muscle is here considered as a part of M. vastus lateralis. A great deal of confusion surrounds the terminology of the muscle complex here termed Mm. vastus lateralis and vastus medialis. Hudson (1937), Hudson, et al. (1959), Fisher (1946), and Fisher and Goodman (1955) have used different terminology for this complex. Most of the confusion stems from Gadow's (1891) unclear description of this complex, which he subdivided into two units termed Mm. femori-tibialis externus and femori-tibialis medius. Many birds have three parts to this complex. It is difficult to determine how to apply Gadow's two terms to these three parts. As nearly as I can determine, the correct method is that of Hudson, et al. (1959); but because Gadow's terms have been used in different ways (even by the same worker), it seems best to abandon these terms. Berger (1956:272) believes that the muscle unit that Fisher and Goodman term M. femoritibialis externus represents a head of M. vastus lateralis; I am accepting his opinion. For the three parts of the complex under discussion, I am using the terms M. vastus medialis and M. vastus lateralis pars lateralis and pars postica.

Fisher (Fisher, 1946; Fisher and Goodman, 1955) considers the muscle here termed M. femorocruralis as an accessory head of M. flexor cruris lateralis. The two muscle units in question are closely associated; they insert broadly on opposite sides of a common tendinous raphe. Howell (1938:73) considers this to be a secondary fusion of unrelated muscles. Romer (1927:366) states that in the chick embryo M. femorocruralis is in reality a shank muscle that migrates into the thigh during development. Therefore, Fisher's usage of a single name for these two unrelated muscles is unsatisfactory. I am using Howell's terminology in which the name flexor cruris lateralis represents the main head only of Fisher's M. flexor cruris lateralis and the name femorocruralis represents Fisher's accessory head.

Gadow (1891) divides the obturator complex into two muscles (or muscle groups), which he terms M. obturator and Mm. accessorii M. obturatoris. He states that the former is homologous with the mammalian obturator internus and the latter with the obturator externus. Hudson (1937), accepting Gadow's homologies, renamed these muscles M. obturator internus and M. obturator externus. Nearly all subsequent workers have followed Hudson's terminology, with its implication that these muscles are homologous with the mammalian muscles of the same name. Howell (1938) is an exception. He points out (pp. 78, 79) that the obturator internus of Hudson is homologous with the obturator externus of mammals. His evidence is convincing: "In origin the obturator is somewhat suggestive of the mammalian obturator internus, for which it has uniformly been mistaken. That the latter interpretation is incorrect, however, is attested by the facts that it receives twigs of n. obturatorius within the pelvis, passes through the obturator foramen rather than dorsal to the border of the ischium, and it is segregated from any muscle with tibial innervation. Insertion has shifted only to a slight and unimportant degree as compared with that of the mammalian obturator externus, and beyond question it is the equivalent of that muscle. The stimulus for a longer muscle, has been the same, resulting in the extension of origin to within the pelvis of the externus in birds and the internus in mammals, but the obturator internus is an extension of a part of the gemellus mass and this does not occur in any vertebrate class but Mammalia." Howell applies the term M. obturator to the entire obturator complex.

Romer (1927), studying the development of the thigh musculature in chick embryos, concluded that the entire obturator complex is homologous with the mammalian obturator externus plus quadratus femoris. He considered the avian M. flexor ischiofemoralis to be the homologue of the mammalian obturator internus.

Gadow, in his work on the ratites (1880:34), states that M. obturator (obturator internus of Hudson) cannot be homologous to the mammalian obturator internus, but must represent the obturator externus. His reasoning is as follows: "Als M. pectineus kann man diesen Muskel nicht auffassen, da er auf der Aussenfläche des Trochanter major inserirt, ferner auch nicht als M. obturator internus der menschlichen Anatomie, da er nicht vom Plexus ischiadicus, sondern vom Plexus cruralis aus innervirt wird. Seiner Innervation und Insertion nach wäre er nur mit dem M. obturator externus zu vergleichen, wobei er seinen Ursprung im Verhältniss zum Menschen nur bedeutend weiter auf das Os ischii und Os pubis distalwärts ausgedehnt hätte und so allerdings der Lage nach mit Ausnahme seines Insertionsdrittels ein 'internus' geworden wäre."

Since Gadow gives different homologues for M. obturator in two of his works (1880 and 1891), one would suspect that he had changed his opinion in the interim; however, there is no evidence that he did so. In 1880 he gives supporting evidence (quoted above) for his view; in 1891 he does not. After describing (1891:173) how the origin of M. obturator in bird ancestors presumably migrated from a location outside the pelvis to a position inside the pelvis prior to the meeting of the pubis and ischium external to the muscle, he states: "Eine ähnliche Entwicklung ist für den Obturator internus der Säugethiere anzunehmen, welchem der M. obturator der Vögel entspricht." A similar development in mammals is impossible, owing to the different relationship of the muscle to the pelvic bones in this class. Gadow says nothing more about the mammalian homologue of M. obturator. In view of this discrepancy, Gadow can hardly be considered as a supporter of the idea that the avian M. obturator is homologous with the mammalian obturator internus.

The evidence is conclusive, it seems to me, that the obturator internus of Hudson is not homologous with the mammalian obturator internus. Therefore, the term obturator internus is inappropriate for the avian muscle and must be abandoned. I shall follow Howell (1938) in naming the entire obturator complex M. obturator. This term, of course, is not used in the sense in which it is used by Gadow. The use of the term obturator externus for the entire complex is avoided because it may not correspond exactly to the mammalian obturator externus. As mentioned previously, Romer considers the avian muscle to be homologous not only with the mammalian obturator externus but also with the quadratus femoris.

I am following the policy of Wilcox (1948) and Berger (1952) in latinizing the term anterior, changing it to anticus. When preceded by the feminine word pars, the feminine ending is used (antica).

In table 1 my terminology is compared with that of Fisher and Goodman (1955), Howell (1938), Hudson (1937), and Gadow (1891). The terminology of Fisher (1946) is identical with that of Fisher and Goodman (1955) except that in his earlier work Fisher did not describe or name M. femoritibialis externus, and M. lumbricales of his earlier work is not mentioned in his later work. The terminology of Hudson, et al. (1959) is identical with that of Hudson (1937) except that the manner in which the femoritibialis complex is subdivided is identical with that of Gadow (1891) and different from that in Hudson's earlier work; also the abbreviations p. ext. and p. int. are substituted in his later paper for pars anterior and pars posterior, respectively, of M. adductor longus et brevis.

[ACKNOWLEDGMENTS]

I gratefully acknowledge the generous help of Professor A. Byron Leonard, under whose guidance this study was conducted and thank Professor E. Raymond Hall, Professor Howard A. Matzke, and Dr. Irwin Baird for numerous helpful suggestions and criticisms.

For help in collecting specimens I thank J. R. Alcorn, W. C. Glazener (through the courtesy of the Texas Game and Fish Commission), Dr. Harrison B. Tordoff, Jerry Tash, William Brecheisen, and Louis Brecheisen. I thank also Edwin Gebhard of the Kansas Forestry, Fish and Game Commission for help in locating the Lesser Prairie Chickens.

I am grateful for the assistance of Mrs. Chester Alexander and Dr. L. C. Dahl in translating a Russian and a Dutch reference, and thank George Young and James Bee for making equipment used in my study.

All of the original drawings except [Fig. 1] were made by me, although the final inking of [Figs. 12] through [19] was done by Bret Waller. [Fig. 1] was drawn by Kay Swearingen.

I was aided in this study during the summer of 1960 by a research grant from the University of Kansas.

[SKELETON]

Although no special study was made of the skeleton, certain conspicuous variations are discussed here.

There are a few pronounced differences between the pelvis of Tympanuchus and that of Pedioecetes. Whereas in the former the thick lateral iliac process has a pronounced overhang with the ventral edge lateral to the ischium ([Fig. 1]), in Pedioecetes there is no overhang at all and the edge of this process is much thinner. The ischium in Pedioecetes is wider (in dorsoventral extent), especially posteriorly, than in Tympanuchus. In Tympanuchus the posteroventral margin of the ischium is rounded and is free from the pubis, whereas in Pedioecetes it is pointed and fused with the pubis.

In Tympanuchus cupido (both subspecies) the lateral iliac process extends farther ventrally than in T. pallidicinctus, approaching or extending ventral to the level of the pubis in the former species; also the edge of this process is thicker in T. cupido.

All specimens studied have a single synsacro-thoracic vertebra. The number of combined synsacro-thoraco-lumbar and synsacro-lumbar vertebrae is eight in each specimen of Tympanuchus and in one specimen of Pedioecetes phasianellus jamesi and is seven in three specimens of the latter. In most specimens of Tympanuchus there are three synsacro-thoraco-lumbar and five synsacro-lumbar vertebrae, although in two specimens (T. pallidicinctus) there are four of each group; in one of these latter two specimens the parapophysis on one side of the fourth synsacro-thoraco-lumbar vertebra is small. The first (of five) synsacro-lumbar vertebra has a rudimentary parapophysis on one side in one specimen of Tympanuchus and on both sides in another specimen. One specimen of Pedioecetes phasianellus jamesi has five synsacro-lumbar vertebrae and the others have four; all have three synsacro-thoraco-lumbar vertebrae.

[NERVES]

For each nerve (or plexus) the condition found in most specimens of the Lesser Prairie Chicken (T. pallidicinctus) is described first. Following this, variations from the typical T. pallidicinctus condition are given for T. pallidicinctus, then for T. cupido (both subspecies considered together), and finally for P. p. jamesi.

Lumbosacral Plexus, [Figs. 2], [3]

T. pallidicinctus

Description.—Eight spinal nerves contribute to the lumbosacral plexus. These are the second through the ninth synsacral spinal nerves (S2 to S9). The entire ventral ramus of each of these nerves, excepting S2 and S9, contributes to this plexus. The ventral ramus of S2 divides into two branches, only the posterior of which contributes to the plexus; the anterior branch directly innervates muscles of the abdominal wall (as does the entire ventral ramus of S1). The ventral ramus of S9 divides into two branches, only the anterior of which contributes to this plexus; the posterior branch contributes to the more posteriorly situated pudendal plexus.

Each root of the plexus corresponds to a single spinal nerve except one spinal nerve (S5—the furcal) that contributes a root to both the femoral nerve and the sciatic nerve; thus typically the plexus has nine roots (but see below). The four anteriormost roots (S2 to S5) contribute to the femoral nerve, although the contribution from S2 is small. S3 and S4 contribute to the obturator nerve. The five posteriormost roots (S5 to S9) contribute to the sciatic nerve, although the contribution from S9 is relatively small.

Individual Variation.—In all specimens (of all species) examined, the right and left sides of the plexus in any one individual were practically identical. In T.p. 2 ([Fig. 2]B), there appear to be two furcal nerves; S5 is typical, but a small branch of S4 apparently also contributes to the sciatic nerve. In T.p. 5, S9 is unique in dividing into three branches; the anterior two join the sciatic nerve separately; the posterior one joins the pudendal plexus as usual.

T. cupido

Individual Variation.—S2 or S5, or both, may contribute to a limited extent to the obturator nerve. In T.c.p. 3 ([Fig. 3]A) and T.c.a. 1 and 2, much of the plexus has shifted one segment anteriorly, relative to the synsacral vertebrae (the so-called prefixed condition); the roots of the femoral nerve are S2, S3, and S4 (all large); the furcal nerve is S4 (in T.c.a. 1, S5 gives an extremely small root to the femoral nerve, thus making two furcal nerves); six roots (S4 to S9) contribute to the sciatic nerve; S3 and S4 remain as the main contributors to the obturator nerve except in T.c.a. 2 in which only S2 and S3 contribute to it.

P. p. jamesi

Individual Variation.—In P.p. 1, the plexus resembles the typical condition in T. pallidicinctus. In P.p. 2, 3, and 4, the plexus is prefixed. P.p. 2 resembles T.c.p. 3. In P.p. 3 and 4 ([Fig. 3]B) there are two furcal nerves (S4 and S5); S2 to S4 are the main contributors to the femoral nerve; only S2 and S3 contribute to the obturator nerve; S4 to S9 contribute to the sciatic nerve (the anteriormost and posteriormost roots are small).

Femoral Nerve, [Figs. 4], [5]

T. pallidicinctus

Description.—The femoral nerve is short, dividing inside the pelvis into six major divisions—anterior, middle, posterior, anterodorsal, dorsal, and posterodorsal. The anterodorsal and posterodorsal divisions are short, failing to extend so far laterally as the inguinal ligament; the posterodorsal division is also small and is usually covered by other divisions and is not visible when viewed from the ventral side.

The anterior division passes ventral to Mm. iliotrochantericus medius and iliacus and dorsal to the anterior end of the inguinal ligament. The division branches into two parts. The anterior part extends around the posterior border of M. extensor iliotibialis anticus and sends several twigs to the lateral surface of this muscle. The posterior part passes between the proximal parts of Mm. extensor iliotibialis anticus and extensor iliotibialis lateralis and supplies the skin.

The middle division passes ventral to Mm. iliotrochantericus medius and iliacus and dorsal to the inguinal ligament. The division branches into a large but variable number of parts. A variable number of branches (usually two) pass posterior to M. extensor iliotibialis anticus and penetrate the medial surface of M. extensor iliotibialis lateralis. Several branches supply the fused Mm. vastus lateralis and vastus medialis. The posteriormost branch of this division passes between Mm. ambiens and vastus medialis, giving twigs to the lateral surface of M. ambiens, and sometimes also to the medial surface of M. vastus medialis, and terminates in M. femoritibialis internus.

The posterior division, which does not subdivide, spirals completely around M. psoas (passing in turn anterior, dorsal, posterior, and ventral to it) and gives twigs into this muscle. This nerve then extends distally into the proximal part of the shank and there has a nonmuscular termination.

The short, thick anterodorsal division, partly covered by the anterior division, turns dorsally and passes through the femoral notch of the ilium and penetrates the deep surface of M. gluteus profundus.

The slender dorsal division passes ventral to M. iliotrochantericus medius and dorsal to the inguinal ligament and penetrates the ventral surface of M. iliacus.

The small, short posterodorsal division penetrates the ventral surface of M. iliotrochantericus medius.

Individual Variation.—In two legs the anterior division gives a twig or two twigs to M. extensor iliotibialis lateralis. The dorsal division may fuse proximally with either the anterior or middle division, thus appearing to be a branch of one of these divisions. In one leg ([Fig. 5]A), there are two separate branches (both fused with the middle division) to M. iliacus. On both sides of one specimen ([Fig. 5]A), the anteriormost branch of the middle division, which supplies M. extensor iliotibialis lateralis, gives off a twig that anastomoses with the branch of the anterior division that supplies M. extensor iliotibialis anticus. On both sides of another specimen, the anterodorsal division passes lateral to the anterior end of M. iliotrochantericus medius instead of through the femoral notch, which is lacking.

T. cupido

Individual Variation.—In three legs, the anterior division gives twigs into M. extensor iliotibialis lateralis. The dorsal division is fused proximally with the middle division in one instance. In three cases, a twig from the middle division anastomoses with the branch of the anterior division supplying M. extensor iliotibialis anticus. In the example shown in [Fig. 5]B, a twig comes off the cutaneous branch of the anterior division, perforates the ventral part of M. iliacus, and rejoins the cutaneous branch. In both legs of one specimen, the cutaneous branch of the anterior division perforates the anterior edge of M. extensor iliotibialis lateralis instead of passing between the latter and M. extensor iliotibialis anticus. The posteriormost branch of the middle division, which terminates in M. femoritibialis internus, perforates the medial part of M. vastus medialis in one leg. In another leg, one of the branches to the fused Mm. vastus lateralis and vastus medialis sends a twig into M. extensor iliotibialis lateralis.

P. p. jamesi

Individual Variation.—In three legs, the anterior branch of the anterior division is cutaneous and the posterior branch supplies M. extensor iliotibialis anticus. The dorsal division may fuse proximally with either the anterior or middle division. In one leg ([Fig. 4]B), there are two branches to M. iliacus, one associated with the anterior division and one with the middle division.

Obturator Nerve

T. pallidicinctus

Description.—The long slender obturator nerve passes along the oblique iliac crest and divides into several branches immediately before reaching the obturator foramen. One or two branches, which do not pass through the foramen, penetrate the superficial surface of M. obturator pars postica. Several small branches (variable in number and arrangement) pass through the obturator foramen and supply pars ventralis, pars dorsalis, and pars antica of M. obturator. When pars ventralis and pars dorsalis are fused, one branch perforates the proximal end of this mass and reaches pars antica. One large branch passes through the obturator foramen dorsal to the tendon of M. obturator pars postica, then turns ventrally, passing lateral to the latter; the branch passes between Mm. adductor superficialis and adductor profundus and gives twigs to each of these two muscles.

Individual Variation.—None of significance in any of the three species.

Sciatic Nerve, [Figs. 6], [7], [8], [9]

T. pallidicinctus

Description.—The sciatic nerve passes through the anterior part of the ilio-ischiatic fenestra. Several branches diverge from the nerve immediately after it emerges from the fenestra. The main trunk of the nerve then extends distally through the thigh deep to M. extensor iliofibularis and superficial (lateral) to Mm. flexor ischiofemoralis, caudofemoralis, adductor superficialis, and femorocruralis. The main trunk subdivides into two large nerves—peroneal and tibial—that are adjacent and bound to each other throughout the thigh; the peroneal nerve lies anterior to the tibial. At the distal end of the thigh the main trunk splits grossly into two large branches that diverge and enter the shank. This division does not represent the separation between peroneal and tibial nerves, as is sometimes assumed; the anterior branch includes a part of the tibial nerve as well as the entire peroneal nerve.

A longitudinal groove is visible grossly on the lateral surface of the main trunk, except at the proximal end; distally a second groove is visible posterior to the first one ([Fig. 6]). The long anterior groove indicates the boundary between the peroneal and tibial nerves; this groove may disappear distally, although the posterior groove is always visible distally. The posterior groove, which is continuous with the division of the sciatic nerve into anterior and posterior branches, represents the boundary between two divisions of the tibial nerve. (This is discussed in detail below.) In the middle of the thigh the peroneal and tibial nerves are enclosed in separate connective tissue sheaths, although the two sheaths are fused together; the point of fusion is marked by the anterior groove. If the two sheaths are slit open, the two nerves can be removed and can be seen to be entirely separate. In the proximal part of the main trunk the peroneal and tibial components are enclosed in a single sheath and appear as an undivided trunk; but if the sheath is removed, the two components can be pulled apart rather easily, although there may be some intermingling of a few fibers. This separation can be extended to a point proximal to the origin of all the branches of the sciatic nerve; thus it can be determined which branches arise from the peroneal component and which from the tibial. (These branches arise from the sciatic nerve as, or immediately before, the nerve passes through the ilio-ischiatic fenestra; since this level of the intact nerve could not be adequately observed, it was necessary to cut the nerve inside the pelvis and pull the intrapelvic part of the nerve out through the ilio-ischiatic fenestra. In doing this, care had to be taken to avoid damaging the most proximal branches.)

Three main branches arise from the peroneal component (apart from the main trunk) and two from the tibial. Including the peroneal and tibial components of the main trunk, the sciatic nerve can be divided into seven major divisions—anterior peroneal, middle peroneal, dorsal peroneal, posterior or main peroneal (contributes to main trunk), anterior or main tibial (contributes to main trunk), middle tibial, and posterior tibial. Farther distally, the posterior peroneal division becomes the peroneal nerve and the anterior tibial division becomes the tibial nerve. For descriptive purposes, the term peroneal (or tibial) nerve will be applied only where the nerve is enclosed in its own sheath, but regardless of whether or not the sheath is fused with another; proximal to this, where the separation may not be precise, the terms peroneal (or tibial) division or component will be used.

The small anterior peroneal division arises from the anterior edge of the sciatic nerve. Immediately after emerging from the ilio-ischiatic fenestra, the division turns anteriorly and passes deep to M. piriformis, to which the division gives a twig (in some cases more than one twig), then continues forward to supply the posterior part of M. gluteus profundus.

The middle peroneal division branches into two parts. One part penetrates the deep surface of the anteroproximal part of M. extensor iliofibularis. The other part emerges between the proximal ends of Mm. extensor iliofibularis and vastus lateralis and penetrates the deep surface of M. extensor iliotibialis lateralis.

The dorsal peroneal division arises from the posterodorsal part of the peroneal component, then angles posteriorly, crossing the dorsal surface of the anterior tibial division and superficially appears to arise from the tibial component. The dorsal peroneal division usually subdivides into two unequal branches, both of which penetrate the deep surface of the proximal end of M. extensor iliofibularis.

The large middle tibial division soon subdivides into two branches that pass posterodistally lateral to M. flexor ischiofemoralis. One branch (usually the anterior one) passes lateral to M. caudofemoralis (both heads) and emerges between Mm. extensor iliofibularis and flexor cruris lateralis and enters the skin. The other branch passes deep to M. caudofemoralis pars iliofemoralis, and divides into several branches. Several tiny branches penetrate the deep surface of M. caudofemoralis pars iliofemoralis. Another branch also enters the substance of the latter and emerges from the ventral edge of it, giving a twig to pars caudifemoralis, then passes lateral to pars caudifemoralis and enters M. flexor cruris lateralis. Still another branch passes deep to both heads of M. caudofemoralis and enters the anterior part of M. flexor cruris medialis.

The small posterior tibial division arises from the posterior edge of the sciatic nerve. The division diverges from the remainder of the nerve, as the latter passes through the ilio-ischiatic fenestra, and penetrates the dorsal surface of M. flexor ischiofemoralis.

Below the middle of the main trunk a bundle of fibers of moderate size separates from the anterior edge of the tibial nerve, leaves the tibial sheath, and enters its own sheath, lying superficially between the tibial and peroneal sheaths ([Fig. 6]). At the distal end of the thigh the sheath enclosing this bundle of fibers remains fused with the posterior edge of the peroneal nerve and passes with the latter (diverging from the remainder of the tibial nerve) through the tendinous guide loop for M. extensor iliofibularis, and then diverges from the peroneal nerve. Since this bundle of fibers is distributed with the peroneal nerve, and since the origin of the bundle may be easily overlooked, it has sometimes been misinterpreted as a branch of the peroneal nerve, whereas it almost certainly is a branch of the tibial nerve; this bundle will here be termed the paraperoneal branch of the tibial nerve.

A small but long branch separates from the posterior edge of the proximal end of the tibial nerve or from the tibial component proximal to this and extends distally for some distance adjacent to the tibial nerve, then passes posterodistally between Mm. extensor iliofibularis and flexor cruris lateralis and supplies the skin.

A small branch separates from the anterior edge of the peroneal nerve a short distance above the distal end of the main trunk and passes distolaterally between Mm. extensor iliotibialis lateralis and extensor iliofibularis and supplies the skin.

A twig comes off the medial surface of the tibial nerve near the distal end of the main trunk, passes anteriorly deep to the peroneal nerve, and penetrates the lateral surface of M. femorocruralis; in some cases two twigs enter this muscle.

Individual Variation.—In one leg ([Fig. 7]), the twig to M. caudofemoralis pars caudifemoralis arises more proximally than usual and perforates pars iliofemoralis independently of the branch to M. flexor cruris lateralis. The nerve supplying M. flexor cruris lateralis does not perforate M. caudofemoralis pars iliofemoralis, but passes deep to it in three legs. In half the legs, the paraperoneal branch of the tibial nerve, after extending a short distance in its own sheath, enters the sheath of the peroneal nerve and appears grossly to unite with it; if, however, the sheath is slit open, the paraperoneal branch can be easily pulled apart from the posterior edge of the peroneal nerve; the paraperoneal branch is again enclosed in its own sheath at the distal end of the thigh. In one leg, the cutaneous branch of the peroneal nerve perforates the posteroproximal part of M. gastrocnemius pars externa; in three others, this branch is absent. In one of these last three legs ([Fig. 7]), the distal cutaneous branch of the tibial nerve is also absent. In three legs (of different specimens), a minute twig from the middle tibial division passes posteriorly deep to M. caudofemoralis pars caudifemoralis toward the tail ([Fig. 7]); this twig joins the pudendal plexus in one leg; in the other two the twig could not be traced to its termination. Minute twigs come off the peroneal nerve near the middle of the thigh and enter M. extensor iliofibularis in some legs. In a few cases, a minute nonmuscular twig arises from the peroneal nerve near the distal end of the main trunk and passes anteriorly deep to M. vastus lateralis pars postica ([Fig. 7]).

T. cupido

Individual Variation.—In several legs, the nerve supplying M. flexor cruris lateralis does not perforate M. caudofemoralis pars iliofemoralis, but passes deep to it. The branch to M. flexor cruris medialis arises from the posterior (rather than the middle) tibial division in one instance ([Fig. 8]). In one leg, a minute twig from the middle tibial division passes posteriorly and joins the pudendal plexus; in another, a similar twig is present but could not be traced to its termination. In some specimens, minute twigs come off the peroneal nerve near the middle of the thigh and enter M. extensor iliofibularis. In one leg, a nonmuscular twig arises from the base of the cutaneous branch of the peroneal nerve and passes anteriorly deep to M. vastus lateralis pars postica. In another leg ([Fig. 8]), a tiny additional twig arises from the posterior edge of the tibial nerve and subdivides, one branch joining the cutaneous branch of the middle tibial division and the other joining the distal cutaneous branch of the tibial nerve.

P. p. jamesi

Individual Variation.—In both legs of one specimen, the branch to M. flexor cruris medialis arises from the posterior (rather than the middle) tibial division; in three legs, this branch arises as an independent division of the tibial nerve ([Fig. 6]). (Only in one leg does this branch arise as in T. pallidicinctus.) The branch to M. flexor cruris medialis perforates the lateral part of M. flexor ischiofemoralis in one instance. In all legs except one (nerve possibly destroyed), a second twig to M. flexor ischiofemoralis arises from the branch to M. flexor cruris medialis ([Fig. 6]). In one leg ([Fig. 9]), an additional branch, arising as an independent division of the sciatic nerve, enters M. extensor iliofibularis distal to the point of entrance of the dorsal peroneal division; this extra branch arises posterior (adjacent) to the dorsal peroneal division, but it could not be determined with certainty whether it arises from the peroneal or tibial component. A minute twig from the branch to M. flexor cruris medialis passes posteriorly and joins the pudendal plexus in one leg ([Fig. 6]); in another, a similar twig is present but could not be traced to its termination. In nearly all the legs, minute twigs come off the peroneal nerve near the middle of the thigh and enter M. extensor iliofibularis ([Fig. 6]). In both legs of one specimen, the paraperoneal branch enters the peroneal sheath (although separable from the peroneal nerve). The distal branch to M. femorocruralis gives off a long twig to M. gastrocnemius pars media in one instance ([Fig. 6]).

Peroneal Nerve, [Fig. 10]

T. pallidicinctus

Description.—The branch that is given off in the thigh has been discussed above. The peroneal nerve passes, with the paraperoneal branch of the tibial nerve, through the guide loop for M. extensor iliofibularis. The peroneal nerve diverges from the paraperoneal branch and passes along the anterior (proximal) edge of the tendon of M. extensor iliofibularis medial to the common tendon of the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II and lateral to the common tendon of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III.

The peroneal nerve soon gives off a spray of branches that supplies the following: femoral head of M. tibialis anticus, tibial head of M. tibialis anticus (branch passes deep to femoral head), M. extensor digitorum longus (branch passes deep to tibial head of M. tibialis anticus), and M. peroneus longus. A part of the nerve may or may not pass through a notch in the proximal end of the lateral head of M. flexor digitorum longus. The nerve then extends distally along the anterolateral edge of the latter muscle and subdivides into two long branches. Gadow (1891) termed these branches the superficial peroneal and the deep peroneal; his terminology will be used here.

The superficial peroneal branch, after giving off, near its proximal end, one or two twigs into M. peroneus brevis, passes lateral to the retinaculum for the tendon of M. tibialis anticus, then across the intratarsal joint lateral to the latter, then lateral to the insertion of M. tibialis anticus, where the branch subdivides. One of the two resulting branches gives one or two twigs into M. extensor brevis digiti IV, then terminates nonmuscularly in the digits. The other branch passes between the main and accessory insertions of M. tibialis anticus and joins the branch of the deep peroneal which supplies M. abductor digiti II. (See next paragraph.)

The deep peroneal branch passes through the retinaculum for the tendon of M. tibialis anticus, lying lateral, then deep, then medial to the latter; it crosses the intratarsal joint medial to the latter. Immediately above the insertion of M. tibialis anticus, the deep peroneal branch divides, one branch passing on each side of the main insertion. The branch passing lateral to the main insertion passes between the latter and the accessory insertion (medial to the latter) and is joined by a branch of the superficial peroneal nerve. This fused branch extends distally between Mm. extensor hallucis longus and extensor brevis digiti IV and medial to M. extensor brevis digiti III, giving twigs into the latter and into M. abductor digiti II before terminating nonmuscularly in the digits. The branch of the deep peroneal nerve that passes medial to the main insertion of M. tibialis anticus gives one or two twigs into the proximal head of M. extensor hallucis longus, then terminates nonmuscularly in the digits.

Individual Variation.—In four legs, the branch of the superficial peroneal nerve that usually joins the lateral branch of the deep peroneal nerve is lacking ([Fig. 10]B). In these legs it can be seen that Mm. extensor brevis digiti III and abductor digiti II are supplied by the deep peroneal nerve.

T. cupido

Individual Variation.—In two legs, the same branch that gives twigs into the proximal head of M. extensor hallucis longus also sends a twig into the distal head of this muscle ([Fig. 10]C).

P. p. jamesi

Individual Variation.—None of significance.

Tibial Nerve, [Fig. 11]

T. pallidicinctus

Description.—The branches given off in the thigh have been discussed in the account of the sciatic nerve. At the distal end of the thigh the peroneal nerve and the paraperoneal branch of the tibial nerve diverge from the remainder of the tibial nerve and pass through the tendinous guide loop for M. extensor iliofibularis whereas the remainder of the tibial nerve does not. This main part of the tibial nerve immediately divides into three main divisions—lateral, posterior, and medial.

The lateral division passes between Mm. flexor perforatus digiti IV and gastrocnemius pars externa and subdivides into two branches, one of which penetrates the medial surface of M. gastrocnemius pars externa. The other branch passes deep to the latter and sends twigs into the posterior head of M. flexor perforans et perforatus digiti II, then passes deep to the latter and enters M. flexor perforans et perforatus digiti III.

The posterior division sends a branch into the medial head of M. flexor perforatus digiti IV, then passes between the latter and the medial head of M. flexor perforatus digiti III, and extends distally giving off twigs to each of the three heads of M. flexor perforatus digiti IV, to each of the two heads of M. flexor perforatus digiti III, and to each of the three heads of M. flexor perforatus digiti II. The number and arrangement of these twigs is variable.

The medial division passes medial to the medial head of M. flexor perforatus digiti III, sends a twig to the lateral surface of M. gastrocnemius pars media, then passes into the shank musculature between Mm. plantaris and flexor hallucis longus, and sends a branch along the medial edge of M. flexor hallucis longus that gives several twigs into this muscle before terminating nonmuscularly. A small branch extends to M. popliteus, another to M. plantaris, and another to the posterior head of M. flexor digitorum longus. A nonmuscular branch passes between the medial and posterior heads of M. flexor digitorum longus and extends distally deep to this muscle. A long branch gives off near its proximal end a variable number of twigs that pass deep to M. plantaris and enter M. gastrocnemius pars interna; the branch then extends distally along the lateral edge of M. plantaris and terminates nonmuscularly.

The paraperoneal branch diverges from the peroneal nerve, passing medial and then distal to the insertion of M. extensor iliofibularis, whereas the peroneal nerve passes proximal and then lateral to this insertion. The paraperoneal branch passes deep to the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II and superficial to the tendon of the anterolateral head of M. flexor perforatus digiti IV and then passes distally along the anterolateral borders of the latter and the lateral head of M. flexor perforatus digiti III and the posterolateral border of M. flexor digitorum longus. This branch is thus separated from the peroneal nerve by M. flexor digitorum longus and by the fibula; the branch passes along the lateral surface of the tibial cartilage, continues lateral to the hypotarsus, then turns medially before extending distally between Mm. abductor digiti IV and flexor hallucis brevis, sending twigs into each of these muscles and a long twig into M. lumbricalis before terminating nonmuscularly.

Individual Variation.—In T.p. 3L,R ([Fig. 11]B), an extra branch arises from the tibial nerve as a separate (fourth) division; it enters the medial head of M. flexor perforatus digiti IV and also gives off a twig that anastomoses with the posterior division (left leg) or with the first branch of the posterior division (right leg). In T.p. 3R ([Fig. 11]B), a large extra branch arises from the proximal part of the medial division and passes medial and then deep to the medial head of M. flexor perforatus digiti III, perforates the tendinous part of the medial head of M. flexor perforatus digiti II, and joins the posterior division (lateral to the medial head of M. flexor perforatus digiti III). A similar branch is found in T.p. 3L except that it arises from the proximal part of the posterior (rather than the medial) division. In T.p. 3R ([Fig. 11]B), the branch to M. gastrocnemius pars externa arises so far proximally that it appears as a separate (fifth) division of the tibial nerve. In two legs, the branch of the medial division that supplies M. gastrocnemius pars media sends a twig into the distal end of M. femorocruralis ([Fig. 11]A).

T. cupido

Individual Variation.—In one leg, an extra branch of the medial division arises immediately distal to the branch to M. gastrocnemius pars media and enters the proximal end of the medial head of M. flexor perforatus digiti III. In one instance, the branch to M. gastrocnemius pars interna passes through a gap in the origin of M. plantaris rather than distal to the origin of the latter.

P. p. jamesi

Individual Variation.—The branch to M. gastrocnemius pars interna gives a minute twig to the deep surface of the free belly of M. plantaris in one leg.

Fig. 2. Ventral views of the lumbosacral plexus of Tympanuchus pallidicinctus. Sympathetic ganglionated chain removed. Numbers indicate synsacral spinal nerves. × 2. A. T.p. 1L. B. T.p. 2L.

Fig. 3. Ventral views of the lumbosacral plexus. Sympathetic ganglionated chain removed. Numbers indicate synsacral spinal nerves. × 2. A. Tympanuchus cupido pinnatus 3L. B. Pedioecetes phasianellus jamesi 4L.

Fig. 4. Semidiagrammatic ventral views of the femoral nerve, showing the distribution of the branches. × 3. 1,2, M. extensor iliotibialis anticus; 3, cutaneous; 4-6, M. extensor iliotibialis lateralis; 7,8, M. iliacus; 9, M. gluteus profundus; 10-12, fused Mm. vastus lateralis and vastus medialis; 13,14, M. vastus medialis; 15, M. ambiens; 16, M. femoritibialis internus; 17, nonmuscular; 18, M. psoas; 19, M. iliotrochantericus medius. A. Tympanuchus cupido pinnatus 3L. B. Pedioecetes phasianellus jamesi 3L.

Fig. 5. Semidiagrammatic ventral views of the femoral nerve, showing the distribution of the branches. × 3. 1,2, M. extensor iliotibialis anticus; 3, cutaneous; 5,6, M. extensor iliotibialis lateralis; 7,8, M. iliacus; 9, M. gluteus profundus; 10,11, fused Mm. vastus lateralis and vastus medialis; 13, M. vastus medialis; 15, M. ambiens; 16, M. femoritibialis internus; 17, nonmuscular; 18, M. psoas; 19, M. iliotrochantericus medius. A. Tympanuchus pallidicinctus 2L. B. Tympanuchus cupido attwateri 1R.

Fig. 6. Semidiagrammatic dorsolateral view of the sciatic nerve of Pedioecetes phasianellus jamesi 3R, showing the distribution of the branches. × 2½. 1, M. gluteus profundus; 2, M. piriformis; 3, M. extensor iliotibialis lateralis; 4-7, M. extensor iliofibularis; 8, M. flexor cruris medialis; 9, cutaneous; 10, to pudendal plexus; 11, M. flexor cruris lateralis; 12, M. caudofemoralis pars caudifemoralis; 13-15, M. caudofemoralis pars iliofemoralis; 16,17, M. flexor ischiofemoralis; 18,19, M. femorocruralis (branch of tibial nerve); 20, cutaneous; 21, M. gastrocnemius pars media (branch of tibial nerve); 22, cutaneous.

Fig. 7. Semidiagrammatic dorsolateral view of the sciatic nerve of Tympanuchus pallidicinctus 2L, showing the distribution of the branches. × 2½. 1, M. gluteus profundus; 2, M. piriformis; 3, M. extensor iliotibialis lateralis; 4, 7, M. extensor iliofibularis; 8, M. flexor cruris medialis; 9, cutaneous; 10, to pudendal plexus; 11, M. flexor cruris lateralis; 12, M. caudofemoralis pars caudifemoralis; 13-15, M. caudofemoralis pars iliofemoralis; 17, M. flexor ischiofemoralis; 18, M. femorocruralis (branch of tibial nerve); 22, cutaneous; 23, nonmuscular (branch of peroneal nerve).

Fig. 8. Semidiagrammatic dorsolateral view of the sciatic nerve of Tympanuchus cupido pinnatus 3L, showing the distribution of the branches. × 2½. 1, M. gluteus profundus; 2, M. piriformis; 3, M. extensor iliotibialis lateralis; 4,7, M. extensor iliofibularis; 8, M. flexor cruris medialis; 9, cutaneous; 11, M. flexor cruris lateralis; 12, M. caudofemoralis pars caudifemoralis; 13, M. caudofemoralis pars iliofemoralis; 17, M. flexor ischiofemoralis; 18, M. femorocruralis (branch of tibial nerve); 20, cutaneous; 22, cutaneous.

Fig. 9. Semidiagrammatic dorsolateral view of the sciatic nerve of Pedioecetes phasianellus jamesi 3L, showing the distribution of the branches. × 2½. 1, M. gluteus profundus; 2, M. piriformis; 3, M. extensor iliotibialis lateralis; 4,5,7, M. extensor iliofibularis; 8, M. flexor cruris medialis; 9, cutaneous; 11, M. flexor cruris lateralis; 13,14, M. caudofemoralis pars iliofemoralis; 16,17, M. flexor ischiofemoralis; 18,19, M. femorocruralis (branch of tibial nerve); 20, cutaneous; 22, cutaneous.

Fig. 10. A,B. Semidiagrammatic drawings of the peroneal nerve of Tympanuchus pallidicinctus 1L, showing the distribution of the branches. × 2. C. Semidiagrammatic drawing of the distal part of the peroneal nerve of Tympanuchus cupido attwateri 1R, showing the distribution of the branches. × 2. 1,2, M. tibialis anticus (tibial head); 3,4, M. tibialis anticus (femoral head); 5, M. extensor digitorum longus; 6, nonmuscular; 7,8, M. peroneus longus; 9, M. peroneus brevis; 10,11, M. extensor hallucis longus (proximal head); 12, M. extensor hallucis longus (distal head); 13-15, nonmuscular (to toes); 16, M. abductor digiti II; 17, M. extensor brevis digiti III; 18, M. extensor brevis digiti IV.

Fig. 11. A,B. Semidiagrammatic drawings of the tibial nerve (excluding the paraperoneal branch) of Tympanuchus pallidicinctus, showing the distribution of the branches. × 2. A. T.p. 1L. B. T.p. 3R. C. Semidiagrammatic drawing of the distal part of the paraperoneal branch of the tibial nerve of Pedioecetes phasianellus jamesi 2L, showing the distribution of the branches. × 2. 1, M. femorocruralis; 2, M. gastrocnemius pars media; 3, M. popliteus; 4, M. plantaris; 5, M. flexor digitorum longus; 6-8, nonmuscular; 9-11, M. gastrocnemius pars interna; 12,13, M. flexor hallucis longus; 14-16, M. flexor perforatus digiti IV (medial head); 17, M. flexor perforatus digiti III (medial head); 18-20, M. flexor perforatus digiti II; 21, M. flexor perforatus digiti IV (lateral head); 22-24, M. flexor perforatus digiti IV (anterolateral head); 25, M. flexor perforatus digiti III (anterolateral head); 26, M. flexor perforans et perforatus digiti III; 27,28, M. flexor perforans et perforatus digiti II; 29, M. gastrocnemius pars externa; 30,31, M. abductor digiti IV; 32,33, M. flexor hallucis brevis; 34,35, nonmuscular (to toes).

[MUSCLES]

In the accounts of the muscles the name used by Hudson, et al. (1959) for each muscle is given in parentheses after the name used by me if the two differ.

In the account of each muscle, the description of the condition found in most specimens of the Lesser Prairie Chicken (T. pallidicinctus) is given first. This is hereafter referred to as the typical condition for T. pallidicinctus. Then any individual variations found within this species are given. Under the heading T. cupido any constant differences between this species and typical T. pallidicinctus are given first, and any individual variations found within the species T. cupido (both subspecies considered together) are given second. Under the heading P. p. jamesi any constant differences between this subspecies and the typical condition for T. pallidicinctus (thus these differences are not necessarily constant between the two genera) are given first, and any individual variations found within the subspecies P. p. jamesi are given second.

In the bird embryo, according to the studies of Romer (1927) and Wortham (1948), the muscles within each segment of the leg differentiate from distinct dorsal or ventral mesenchymal masses. Presumably these represent the primitive dorsal extensor and ventral flexor muscle masses. The list below indicates the ontogenetic origin of the avian leg muscles, according to the studies of Romer and Wortham. The individual muscles are discussed in the order in which they are listed below.

Dorsal muscles of thigh

Ventral muscles of thigh

Ventral muscles of shank

Dorsal muscles of shank

Dorsal muscles of foot

Ventral muscles of foot

Fig. 12. Tympanuchus pallidicinctus 2L. Lateral view of the superficial muscles of the left leg. × 1.

M. Extensor Iliotibialis Lateralis (M. iliotibialis), [Figs. 12], [13], [20]F, G

T. pallidicinctus

Fig. 13. Tympanuchus pallidicinctus 2L. Medial view of the superficial muscles of the left leg. × 1. Articular capsule shown by concentrically arranged dashes.

General Description and Relations.—Most superficial muscle on lateral surface of thigh; broad, flat, and triangular; bounded anteriorly by M. extensor iliotibialis anticus and posteriorly by M. flexor cruris lateralis; posterior part considerably thicker than anterior part; anteroproximal and centrodistal parts aponeurotic; extreme posteroproximal corner also aponeurotic (could be considered tough sheet of connective tissue intimately fused with M. extensor iliotibialis lateralis, rather than part of muscle itself; see [Fig. 20]F); latter aponeurosis, as well as adjacent fleshy fibers, overlapped by M. flexor cruris lateralis; this aponeurosis fused with posterior end of underlying M. caudofemoralis pars iliofemoralis; centrodistal aponeurosis tightly fused to underlying Mm. vastus lateralis and vastus medialis; fleshy fibers posterior to this aponeurosis also fused with M. vastus lateralis, although posterior third of muscle free; fleshy part anterior to this aponeurosis bound by tough connective tissue to underlying M. vastus medialis, although no fusion of fibers; anterior edge tightly bound by strong connective tissue to M. extensor iliotibialis anticus, with some fusion of fibers (proximally); posteroproximal corner bound by tough connective tissue to adjacent muscles; anteroproximal aponeurosis fused with aponeurotic anteroproximal part of underlying M. extensor iliofibularis. Continuous proximal aponeurosis of M. extensor iliotibialis anticus and of M. extensor iliotibialis lateralis underlain by tough fascial sheet overlying M. gluteus profundus; anterior part of this fascia tightly fused to latter muscle but free from overlying aponeurosis; posterior part of this fascia tightly fused to overlying aponeurosis but free from M. gluteus profundus; middle part of fascia fused to both aponeurosis and M. gluteus profundus.

Fig. 14. Tympanuchus pallidicinctus 2L. Lateral view of the muscles of the left leg. The following muscles have been removed: extensor iliotibialis lateralis, extensor iliotibialis anticus, gastrocnemius pars externa and pars interna, and peroneus longus. × 1.

Fig. 15. Tympanuchus pallidicinctus 2L. Medial view of the muscles of the left leg. The following muscles have been removed: extensor iliotibialis lateralis, extensor iliotibialis anticus, ambiens, flexor cruris lateralis (in part), flexor cruris medialis (in part), gastrocnemius pars externa and pars interna, and peroneus longus. × 1.

Origin.—Approximately the anterior half attaches by an extensive aponeurosis, which is continuous anteriorly with that of M. extensor iliotibialis anticus, to the anterior iliac crest, ending posteriorly at the anterior end of the lateral iliac process; the posterior part attaches fleshily to the edge of the entire lateral iliac process and (posterior few mm.) aponeurotically to the entire lateral ischiatic ridge. The proximal part of the belly is much thicker than the fleshy origin. Two accessory aponeuroses associate with the anterior part of the muscle; the proximal one of these comes off the deep surface several mm. distal to the proximal end of the fleshy belly and passes medially between Mm. gluteus profundus and iliacus, fusing to both these muscles, and attaches to the lateral edge of M. iliotrochantericus medius and to the lateral edge of the ilium anterior to the latter; the aponeurosis actually splits into two sheets at the edge of M. iliotrochantericus medius; these sheets fuse to the dorsal and ventral surfaces of the latter muscle, enclosing it; the part of this aponeurosis between Mm. iliacus and iliotrochantericus medius is strongly fused with the underlying body wall. The distal accessory aponeurosis (sometimes weak) comes off the deep surface several mm. distal to the proximal one and passes medially along the ventral surface of M. iliacus, fusing with the latter, then joining the proximal accessory aponeurosis medial to M. iliacus.

Insertion.—The muscle inserts by a broad aponeurosis strongly fused to the underlying Mm. vastus lateralis and vastus medialis; the aponeurosis contributes superficially to the patellar tendon, attaching to the lateral half of the rotular crest.

Fig. 16. Tympanuchus pallidicinctus 2L. Lateral view of the muscles of the left leg. The following muscles, in addition to those listed for [Fig. 14], have been removed: ambiens, vastus lateralis pars lateralis, vastus medialis (except for part of patellar tendon), extensor iliofibularis, flexor cruris lateralis (in part), flexor perforans et perforatus digiti II, and flexor perforans et perforatus digiti III. × 1.

Innervation.—A variable number of branches (usually two) of the middle division of the femoral nerve pass ventral to M. iliacus and between Mm. extensor iliotibialis anticus and vastus medialis and enter the deep surface of the anteroproximal part of the muscle. The branch of the middle peroneal division of the sciatic nerve emerges between the proximal ends of Mm. extensor iliofibularis and vastus lateralis and sends twigs into the deep surface of M. extensor iliotibialis lateralis.

Individual Variation.—In two legs, the nerve supplying M. extensor iliotibialis anticus gives twigs into M. extensor iliotibialis lateralis.

T. cupido

Differences From T. pallidicinctus.—The fleshy origin from the lateral iliac process is considerably thicker (reflected in a thicker lateral iliac process).

Individual Variation.—In three legs the nerve supplying M. extensor iliotibialis anticus gives twigs into M. extensor iliotibialis lateralis. In another leg one of the branches to the fused Mm. vastus lateralis and vastus medialis sends a twig into M. extensor iliotibialis lateralis.

Fig. 17. Tympanuchus pallidicinctus 2L. Lateral view of the muscles of the left leg. The following muscles, in addition to those listed for [Fig. 16], have been removed: vastus lateralis pars postica, gluteus profundus, flexor cruris medialis (in part), caudofemoralis, flexor perforatus digiti IV, and tibialis anticus. × 1.

P. p. jamesi

Fig. 18. Tympanuchus pallidicinctus 2L. Lateral view of the muscles of the left leg. The following muscles, in addition to those listed for [Fig. 17], have been removed: patellar tendon, iliacus, iliotrochantericus medius, flexor cruris lateralis, flexor cruris medialis, flexor ischiofemoralis, adductor superficialis, femorocruralis, gastrocnemius pars media, flexor perforatus digiti III, flexor perforatus digiti II, flexor hallucis longus, plantaris, flexor digitorum longus, popliteus, and extensor digitorum longus. × 1.

Differences From Typical T. pallidicinctus.—The posteroproximal aponeurosis is more extensive, resulting in a narrower proximal fleshy end ([Fig. 20]G); the fleshy fibers adjacent to this aponeurosis are not overlapped by M. flexor cruris lateralis. There is a fusion of fibers between the anterodistal fleshy part of M. extensor iliotibialis lateralis and the underlying M. vastus medialis, but there is no fusion of fibers between the anterior edge of M. extensor iliotibialis lateralis and M. extensor iliotibialis anticus. The connective tissue binding the posteroproximal corner to adjacent muscles is stronger. The fleshy part of the origin is narrower, partly tendinous, and much thinner (reflected in a thin lateral iliac process). The proximal border is much more nearly straight, owing to a less pronounced lateral iliac process. The distal accessory aponeurosis is absent.

Fig. 19. Tympanuchus pallidicinctus 2L. A. Posterior view of the muscles of the left shank. The following shank muscles, in addition to those listed for [Fig. 17], have been removed: gastrocnemius pars media, flexor perforatus digiti III, and flexor perforatus digiti II. × 1. B. Posterior view of the proximal end of the shank, showing the most deeply situated muscle. × 1. C. Lateral view of the head of the left femur and the middle part of the pelvis, showing the deepest part of M. obturator. × 1. D. Medial view of the posteroventral part of the left side of the pelvis, showing the intrapelvic part of M. obturator. × 1. E. Anterior view of the left tarsometatarsus, showing the dorsal intrinsic muscles of the foot. × 1½. F. Posterior view of the left tarsometatarsus, showing the ventral intrinsic muscles of the foot. × 1½.