Chapter II: Part 2

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. 10B). 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. 10C).

_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. 11B), 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. 11B), 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. 11B), 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. 11A).

_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.

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

M. extensor iliotibialis lateralis M. extensor iliofibularis
M. extensor iliotibialis anticus M. piriformis
M. ambiens M. gluteus profundus
M. vastus lateralis M. iliacus
M. vastus medialis M. iliotrochantericus medius
M. femoritibialis internus M. psoas

Ventral muscles of thigh

M. flexor cruris lateralis M. adductor superficialis
M. flexor cruris medialis M. adductor profundus
M. caudofemoralis M. obturator
M. flexor ischiofemoralis M. femorocruralis

Ventral muscles of shank

M. gastrocnemius M. flexor perforatus digiti III
M. flexor perforans et perforatus M. flexor perforatus digiti II
digiti II M. flexor hallucis longus
M. flexor perforans et perforatus M. plantaris
digiti III M. flexor digitorum longus
M. flexor perforatus digiti IV M. popliteus

Dorsal muscles of shank

M. peroneus longus M. extensor digitorum longus
M. tibialis anticus M. peroneus brevis

Dorsal muscles of foot

M. extensor hallucis longus M. extensor proprius digiti III
M. abductor digiti II M. extensor brevis digiti IV
M. extensor brevis digiti III

Ventral muscles of foot

M. lumbricalis (M. adductor digiti II--not
M. abductor digiti IV present)
M. flexor hallucis brevis (M. adductor digiti IV--not
present)

=_M. Extensor Iliotibialis Lateralis_= (M. iliotibialis), Figs. 12, 13, 20F, G

_T. pallidicinctus_

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. 20F); 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.

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.

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.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The posteroproximal aponeurosis is more extensive, resulting in a narrower proximal fleshy end (fig. 20G); 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.

INDIVIDUAL VARIATION.--The muscle is usually somewhat fused to the posteroproximal and anteroproximal fleshy corners of the underlying M. extensor iliofibularis.

=_M. Extensor Iliotibialis Anticus_= (M. sartorius), Figs. 12, 13

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Anteriormost muscle of thigh; long and strap-shaped; proximal part entirely anterior (adjacent) to M. extensor iliotibialis lateralis; posterior edge of middle part medial to latter muscle; distal part mostly medial to Mm. extensor iliotibialis lateralis and vastus medialis; proximal part aponeurotic, continuous posteriorly with anteroproximal aponeurosis of M. extensor iliotibialis lateralis; anterior edge of M. extensor iliotibialis lateralis bound by strong connective tissue to adjacent part of M. extensor iliotibialis anticus; some fusion of fibers (proximally) between these two muscles; anteroproximal corner of fleshy part of muscle sometimes fused to underlying anterior edge of ilium and fascia covering body wall musculature adjacent (anterior) to ilium.

ORIGIN.--The muscle arises aponeurotically from the anterior part of the anterior iliac crest and (anteroproximal corner) from the anterior end of the median dorsal ridge.

INSERTION.--The flat tendon, continuous posteriorly with the superficial tendon of M. femoritibialis internus, fuses to the tendon of M. vastus medialis, contributing superficially to the medial part of the patellar tendon, which attaches to the medial half of the rotular crest; most of the tendon is overlapped by the edge of M. gastrocnemius pars interna.

INNERVATION.--A branch of the anterior division of the femoral nerve gives twigs into the lateral surface of the posterior part.

INDIVIDUAL VARIATION.--In two legs, a twig from the anteriormost branch of the middle division of the femoral nerve anastomoses with the typical branch to M. extensor iliotibialis anticus.

_T. cupido_

INDIVIDUAL VARIATION.--In several legs, the anterior edge of origin extends forward onto the neural spine of the last free thoracic vertebra. A twig from the middle division of the femoral nerve anastomoses with the typical branch to M. extensor iliotibialis anticus in three legs.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--There is no fusion of fibers between M. extensor iliotibialis anticus and M. extensor iliotibialis lateralis.

INDIVIDUAL VARIATION.--The anterior edge of origin extends forward onto the neural spine of the last free thoracic vertebra in some legs.

EXPLANATION OF FIGURE 20

A-D. Dorsal views of M. iliotrochantericus medius, showing its relationship to femoral notch. × 1. In D, note absence of femoral notch and location of branch of femoral nerve. A. _Tympanuchus pallidicinctus_ 2L. B. _T. cupido pinnatus_ 4L. C. _Pedioecetes phasianellus jamesi_ 1L. D. _T. pallidicinctus_ 3L.

E. Medial view of distal end of M. flexor cruris medialis of _P. p. jamesi_ 4L. × 1. Part of insertion is covered by medial collateral ligament.

F,G. Lateral views of posteroproximal corner of M. extensor iliotibialis lateralis (removed from specimen). × 1. F. _T. pallidicinctus_ 2L. G. _P. p. jamesi_ 3L.

H,I. Dorsolateral views of M. piriformis. × 1. H. _P. p. jamesi_ 1L. I. _T. cupido attwateri_ 1L.

J. Lateral view of M. caudofemoralis pars caudifemoralis (removed from specimen) of _T. c. pinnatus_ 4L. × 1.

K. Lateral view of extrapelvic part of M. obturator of _T. pallidicinctus_ 3L (bones not shown). × 2.

L,M. Region surrounding obturator foramen of _T. pallidicinctus_ 3L, showing points of attachment of three parts of M. obturator (muscles removed). × 3. L. Lateral view. M. Medial view.

N. Anterior view of left tarsometatarsus of _P. p. jamesi_ 4L, showing dorsal intrinsic muscles of foot. × 1-1/2. Tendon of M. extensor digitorum longus has been removed.

=_M. Ambiens_=, Figs. 13, 16, 17

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Thin and elongate; on medial surface of thigh; broadest above middle of belly; belly narrowed distally, forming long slender tendon passing lateral to distal part of M. extensor iliotibialis anticus; bounded anterolaterally by M. vastus medialis and posterolaterally by Mm. femoritibialis internus and psoas (proximally).

ORIGIN.--The muscle arises by a short flat tendon from the pectineal process.

INSERTION.--The long slender tendon enters an elongate channel within the patellar tendon; the point of entrance is at the proximal end of the latter tendon just medial to the patella; the tendon passes distolaterally (within the channel) below the patella and emerges from the distolateral edge of the patellar tendon and then extends distally along the anterolateral surface of the head of the fibula, superficial to the fibular arm of the guide loop for M. extensor iliofibularis, and joins the anterolateral surface of the common tendon of origin of the anterolateral heads of Mm. flexor perforatus digiti III, flexor perforatus digiti IV, and flexor perforatus digiti II; the point of junction is usually immediately proximal to the proximal end of the lateral head of M. flexor digitorum longus.

INNERVATION.--The branch of the middle division of the femoral nerve that supplies M. femoritibialis internus gives off a tiny twig or twigs that penetrate the lateral surface of the proximal part of M. ambiens.

INDIVIDUAL VARIATION.--None of significance in _T. pallidicinctus_ or in _P. p. jamesi_; in _T. cupido_ the origin is partly fleshy in one leg.

=_M. Vastus Lateralis_= (M. femoritibialis externus + part of M. femoritibialis medius), Figs. 14, 16

Fisher and Goodman (1955) apply the name femoritibialis externus to the muscle unit that I here term the pars postica of M. vastus lateralis. The reasons for this change are discussed in the section on terminology.

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Thick; on lateral surface of femur deep to M. extensor iliotibialis lateralis; anterior to M. extensor iliofibularis and lateral to M. vastus medialis; much of lateral surface, except proximal part, fused with overlying M. extensor iliotibialis lateralis; deep surface of anterior half fused with M. vastus medialis; proximal part overlapping, but usually not fusing with, insertions of Mm. iliacus and caudofemoralis; partially separable into two parts--pars lateralis and pars postica, former constituting main part of muscle; latter considerably smaller and situated deep to posterodistal part of pars lateralis, except for posterodistal part extending posterior to edge of pars lateralis; proximal part of pars postica strongly fused with pars lateralis; posterodistal tendinous edge of pars lateralis fused or not fused with lateral surface of pars postica; proximal end (narrow) of pars postica tendinous and variable in length.

ORIGIN.--_Pars lateralis_: This arises fleshily from most of the lateral surface and (distally) from the anterior surface of the femur, extending anteriorly to the anterior intermuscular line, fusing with M. vastus medialis, and extending posteriorly to the posterolateral intermuscular line (proximally) and the origin of pars postica (distally); the proximal end begins at the level of the distal edge of the insertion of M. iliotrochantericus medius, contacting the insertions of Mm. iliotrochantericus medius, piriformis, and flexor ischiofemoralis, and terminates distally at the level of the proximal ends of the femoral condyles.

_Pars postica_: This arises fleshily and tendinously (proximal end and deep surface) from the posterolateral surface of approximately the distal half of the femur, extends posteromedially to the posterolateral intermuscular line where it contacts the origin of M. femorocruralis, and extends anteriorly to the level of a line drawn diagonally across the femur from the proximal end of the origin (at the posterolateral intermuscular line) to the proximal end of the external condyle; the distal end is anterior (adjacent) to the attachment of the proximal arm of the tendinous guide loop for M. extensor iliofibularis; the origin is adjacent to, but distinct from, the origin of pars lateralis.

INSERTION.--_Pars lateralis_ is fused indistinguishably with M. vastus medialis; these two muscles form the main (middle) part of the patellar tendon, which also receives contributions from pars postica and Mm. femoritibialis internus, extensor iliotibialis lateralis, and extensor iliotibialis anticus; the patellar tendon attaches to the entire rotular crest of the tibia; the patella is situated in the proximal part of this tendon; some deep fleshy fibers of M. vastus lateralis pars lateralis and M. vastus medialis attach to the proximal edge of the patella. _Pars postica_ forms a short narrow tendon that fuses to the lateral part of the tendon of pars lateralis, forming the lateralmost part of the patellar tendon. A broad flat vinculum extends from the lateral surface of the femorofibular fascia (defined under M. flexor perforans et perforatus digiti II) to the deep surface of the lateral part of the patellar tendon; a similar vinculum extends from the medial surface of the internal condyle to the deep surface of the medial part of the patellar tendon.

INNERVATION.--Two or more branches of the middle division of the femoral nerve penetrate the anterior surface of the fused Mm. vastus lateralis (pars lateralis) and vastus medialis; short twigs emerge from the deep surface of pars lateralis and penetrate the superficial surface of the anteroproximal part of pars postica.

INDIVIDUAL VARIATION.--The proximal ends of M. vastus medialis and M. vastus lateralis are usually separated by a deep notch. In some legs, a small bundle of fibers forming the anteroproximal part of M. vastus lateralis attaches to the lateral surface of M. vastus medialis anterior to this notch.

_T. cupido_

INDIVIDUAL VARIATION.--One leg shows the same variation found in _T. pallidicinctus_ (see above). In several legs, pars lateralis does not extend so far proximally as usual, but begins at the level of insertion of M. piriformis (does not contact the insertion of M. iliotrochantericus medius) and may not overlap M. iliacus. In a few legs, no vincula are associated with the patellar tendon.

_P. p. jamesi_

INDIVIDUAL VARIATION.--Pars lateralis often begins proximally at the level of the insertion of M. piriformis.

=_M. Vastus Medialis_= (Part of M. femoritibialis medius), Figs. 13, 14, 15

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Thick; on anteromedial surface of femur medial to anterior part of M. vastus lateralis pars lateralis; bounded medially by Mm. ambiens and extensor iliotibialis anticus (distally); bounded posteromedially by M. femoritibialis internus; proximal part medial to posterior ends of Mm. iliacus, iliotrochantericus medius, and gluteus profundus; lateral surface, except proximal part, fused with anterior part of M. vastus lateralis pars lateralis; part of lateral surface of M. vastus medialis covered by sheet of fascia attaching to anterior intermuscular line; M. vastus lateralis separable from this fascia, but fascia absent anteriorly and distally and these two muscles indistinguishably fused.

ORIGIN.--The proximal third is attached narrowly by its lateral edge; the distal two thirds is attached broadly by its entire deep surface. The proximal third arises tendinously from the trochanteric ridge and the proximal end of the anterior intermuscular line and fleshily from a narrow area of the femur adjacent (medial) to the latter; the distal part arises tendinously from the anterior intermuscular line and fleshily from a broad adjacent area on the anteromedial surface of the femur, terminating distally at the level of the proximal end of the internal condyle; the posterior edge contacts the origin of M. femoritibialis internus.

INSERTION.--Attachment is in common with M. vastus lateralis pars lateralis, which see.

INNERVATION.--Two or more branches of the middle division of the femoral nerve penetrate the anterior surface of the fused Mm. vastus medialis and vastus lateralis pars lateralis; a variable number of branches of the same division penetrate the medial surface of the proximal part of M. vastus medialis.

INDIVIDUAL VARIATION.--None of significance in any of the three species studied.

=_M. Femoritibialis Internus_=, Figs. 13, 15

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Elongate; on posteromedial surface of femur; bounded anteriorly by M. vastus medialis and posteriorly by M. adductor profundus (overlapping anterior edge of latter); anteroproximal part lateral to M. ambiens; anterodistal corner deep to distal end of M. extensor iliotibialis anticus; distal part of muscle split into superficial and deep layers; superficial layer thin, narrow, and tendinous except for proximal end; deep layer wider, much thicker, and fleshy except for distal end taking form of flat tendon; anterior edge of latter somewhat fused to medial edge of tendon of M. vastus medialis; deep layer widest near distal end of fleshy part; posterior edge of superficial layer fused to underlying deep layer, and anterior edge fused to (continuous with) posterior edge of tendon of M. extensor iliotibialis anticus.

ORIGIN.--The origin is mostly fleshy from the posteromedial surface of the femur between the origin of M. vastus medialis and the posterior intermuscular line, terminating immediately proximal to the internal condyle.

INSERTION.--The tendons of both superficial and deep layers attach to the medial part of the rotular crest, forming the medialmost part of the patellar tendon.

INNERVATION.--The posteriormost branch of the middle division of the femoral nerve penetrates the medial surface of the muscle near the proximal end.

INDIVIDUAL VARIATION.--None of significance in any of the three species studied.

=_M. Extensor Iliofibularis_= (M. biceps femoris), Figs. 12, 14, 16, 17

The term extensor in the name of this muscle does not refer to its function. Howell (1938) used the term extensor to indicate derivation of the muscle from the primitive dorsal extensor muscle mass. (Likewise he used the term flexor to indicate derivation from the primitive ventral flexor muscle mass.)

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Deep to M. extensor iliotibialis lateralis and posterior to femur; broad proximally and narrow distally; posterior to M. vastus lateralis and anterior to proximal part of M. flexor cruris lateralis (superficial to distal part of latter); anteroproximal part aponeurotic, fused to deep surface of aponeurosis of M. extensor iliotibialis lateralis; proximal part of aponeurosis of M. extensor iliofibularis also fused to dorsal edges of underlying Mm. gluteus profundus and piriformis.

ORIGIN.--The posterior part is fleshy from the ventromedial surface of the entire lateral iliac process; the anterior part is aponeurotic from the posterior part of the anterior iliac crest.

INSERTION.--The tendon forms along the posterodistal edge of the belly and continues beyond the end of the belly as a cylindrical tendon that passes through the tendinous guide loop (the belly terminates approximately at the level of the guide loop), then extends anterodistally into the shank musculature; the tendon passes between the medial and lateral heads of M. flexor perforatus digiti IV, between the medial and lateral heads of M. flexor perforatus digiti II, lateral to the common tendon of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III, and between the posterior and lateral heads of M. flexor digitorum longus, attaching to the fibular tubercle.

The tendinous guide loop has three arms--proximal femoral, distal femoral, and fibular; the proximal and distal femoral arms unite posterior to the tendon of M. extensor iliofibularis; the proximal arm is medial to, and the distal arm is lateral to, the latter; the fibular arm joins the distal edge of the distal arm lateral to the tendon of M. extensor iliofibularis. The proximal arm extends anteroproximally lateral to the medial head of M. flexor perforatus digiti IV and medial to M. vastus lateralis pars postica, attaching to a narrow line on the anterolateral surface of the femur a short distance proximal to the external condyle and adjacent (posterior) to the origin of M. vastus lateralis pars postica. The distal arm extends anteriorly medial to the posterior head of M. flexor perforans et perforatus digiti II and medial to M. vastus lateralis pars postica, attaching in common with the tendon of origin of M. gastrocnemius pars externa to a small oval area on the posterolateral surface of the femur a short distance proximal to the fibular groove; the arm is also fused to the underlying articular capsule. The fibular arm (broadest of the three) passes deep to, and fused with, the common tendon of origin of the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II, superficial to the common tendon of origin of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III, and deep to the tendon of M. ambiens, attaching broadly to a narrow line on the anterolateral surface of the proximal part of the fibula; the arm is also fused to the underlying articular capsule.

INNERVATION.--A branch of the middle peroneal division of the sciatic nerve sends twigs to the deep surface of the anteroproximal part; the dorsal peroneal division of the sciatic nerve penetrates the deep surface of the proximal end.

INDIVIDUAL VARIATION.--In some instances a variable number of twigs arises from the peroneal nerve near the middle of the thigh and enters the deep surface of the muscle. They are difficult to expose without breaking and may have been overlooked in some specimens.

_T. cupido_

INDIVIDUAL VARIATION.--The same variation is found as in _T. pallidicinctus_ (see above). In one leg, the tendon of insertion bifurcates into proximal and distal arms before attaching.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--It arises from the ventral rather than the ventromedial surface of the lateral iliac process (there is no ventromedial surface to this process).

INDIVIDUAL VARIATION.--In nearly all of the legs, minute twigs to M. extensor iliofibularis come off the peroneal nerve near the middle of the thigh. The insertional tendon tends toward doubleness in two legs.

=_M. Piriformis_= (M. gluteus medius et minimus), Figs. 16, 20H, I

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Small, thin, and triangular; lateral to antitrochanter and posterior part of trochanter; deep to M. extensor iliofibularis and posterior (adjacent) to M. gluteus profundus; distal half (or more) tendinous.

ORIGIN.--The muscle arises fleshily from the posterior end of the anterior iliac crest (ventral to the origins of Mm. extensor iliotibialis lateralis and extensor iliofibularis) beginning adjacent to the posterior end of M. gluteus profundus.

INSERTION.--The flat tendon narrows, overlaps the anteroproximal corner of insertion of M. flexor ischiofemoralis, and attaches to the lateral surface of the proximal part of the femur immediately anterior to the insertion of M. flexor ischiofemoralis and posterior to the proximal end of M. vastus lateralis; the attachment is posterodistal to the insertion of M. iliotrochantericus medius and posteroproximal to the insertion of M. iliacus.

INNERVATION.--The small anterior peroneal division of the sciatic nerve turns anteriorly immediately after emerging from the ilio-ischiatic fenestra and passes deep to M. piriformis, giving twigs to the deep surface.

INDIVIDUAL VARIATION.--In both legs of one specimen, the insertion does not overlap the insertion of M. flexor ischiofemoralis. The posteroproximal corner of the muscle is tendinous in one leg.

_T. cupido_

INDIVIDUAL VARIATION.--The anterior border is somewhat fused with the posterior edge of M. gluteus profundus in one leg, while in another there is a slight gap between the origins of M. gluteus profundus and M. piriformis. In one leg, the posterior edge of the origin is aponeurotic. On both sides of one specimen, an accessory tendinous band arises several mm. posterior to the main part of M. piriformis and joins the proximal part of the insertional tendon, thus forming a Y-shaped unit (fig. 20I); the accessory tendon arises from the anterior end of the lateral iliac process (left side) or from the anterior part of the lateral iliac fossa (right side). The insertion may be proximal (rather than posterior) to the proximal end of M. vastus lateralis. In one leg, the insertional tendon is partly fused to the insertional tendon of M. flexor ischiofemoralis.

_P. p. jamesi_

INDIVIDUAL VARIATION.--There is often a gap between the origins of M. gluteus profundus and M. piriformis. In one leg (fig. 20H), the posteroproximal corner of the muscle is aponeurotic. The insertion is often proximal (rather than posterior) to the proximal end of M. vastus lateralis. In one instance, the insertion does not overlap the insertion of M. flexor ischiofemoralis.

=_M. Gluteus Profundus_= (M. iliotrochantericus posterior), Figs. 14, 16

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Large and thick; covering dorsolateral surface of entire preacetabular part of ilium; deep to Mm. extensor iliotibialis lateralis and extensor iliotibialis anticus; bounded posteriorly by M. piriformis and ventrally by M. iliacus; ventral edge fused with anterior part of latter and with proximal accessory aponeurosis of M. extensor iliotibialis lateralis; tough sheet of fascia strongly fused to anterior two thirds of lateral surface; posterior to this, fascia overlying muscle but not attaching to it; posterior half of fascia fused to overlying aponeurosis of M. extensor iliotibialis lateralis; deep surface of muscle somewhat fused to proximal part of M. iliotrochantericus medius.

ORIGIN.--The superficial surface is tendinous from the entire anterior iliac crest except the posterior end and from the crest forming the anterior and anterolateral edges of the ilium; the muscle arises fleshily from the entire dorsolateral surface of the preacetabular ilium as far posteriorly as the level of the pectineal process; the dorsal edge is adjacent (anterior) to the origin of M. piriformis.

INSERTION.--The attachment is by a short, wide, thick tendon to a curved line (convex anteriorly) on the lateral surface of the femoral trochanter.

INNERVATION.--The anterodorsal division of the femoral nerve turns dorsally through the femoral notch of the ilium and penetrates the deep surface of the ventral part of the muscle midway of its length; the anterior peroneal division of the sciatic nerve passes deep to M. piriformis and terminates near the posterodorsal edge of M. gluteus profundus.

INDIVIDUAL VARIATION.--On both sides of one specimen, the branch from the femoral nerve passes lateral to the extreme anteroproximal corner of M. iliotrochantericus medius instead of through the femoral notch.

_T. cupido_

INDIVIDUAL VARIATION.--In one leg, the insertional tendon is strongly fused to the insertional tendon of M. iliotrochantericus medius.

_P. p. jamesi_

INDIVIDUAL VARIATION.--None of significance.

=_M. Iliacus_= (M. iliotrochantericus anterior), Figs. 13, 14, 15, 16, 17

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Adjacent ventrally to ventrolateral edge of M. gluteus profundus; lateral edge much thicker than medial edge; deep to M. extensor iliotibialis lateralis and anterolateral to M. iliotrochantericus medius; distal (posterior) end passing between proximal ends of Mm. vastus medialis and vastus lateralis pars lateralis; insertion overlapped by latter; dorsal surface of anterior part fused with ventrolateral edge of M. gluteus profundus and with ventral surface of proximal accessory aponeurosis of M. extensor iliotibialis lateralis; ventral surface partly fused with distal accessory aponeurosis of latter muscle.

ORIGIN.--The origin is fleshy and tendinous from the lateral edge of the anterior part of the ilium.

INSERTION.--The attachment is by a short flat tendon to the lateral surface of the femur distal to the trochanter and anterodistal to the insertion of M. piriformis and deep to the proximal part of M. vastus lateralis pars lateralis.

INNERVATION.--The dorsal division of the femoral nerve penetrates the ventral surface.

INDIVIDUAL VARIATION.--The dorsal division of the femoral nerve may fuse proximally with either the anterior or middle division. In one leg, there are two separate branches to the muscle.

_T. cupido_

INDIVIDUAL VARIATION.--The insertion may not be overlapped by M. vastus lateralis. The dorsal division of the femoral nerve is fused proximally with the middle division in one leg.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The fleshy origin is wider.

INDIVIDUAL VARIATION.--The dorsal division of the femoral nerve may fuse proximally with either the anterior or middle division. In one leg, there are two branches to M. iliacus, one fused with the anterior division and the other with the middle division.

=_M. Iliotrochantericus Medius_=, Figs. 17, 20A, B, C, D

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Small and triangular; ventral to posterior half of M. gluteus profundus; all but posteroventral corner deep to latter; posteromedial to M. iliacus, anterior to neck of femur, and dorsolateral (adjacent proximally) to M. psoas; proximal end notched at level of femoral notch for passage of anterodorsal division of femoral nerve; part anterior to femoral notch mainly tendinous; dorsal surface of proximal part somewhat fused to M. gluteus profundus, proximal accessory aponeurosis of M. extensor iliotibialis lateralis split into two sheets enclosing and fusing with M. iliotrochantericus medius, ultimately attaching to lateral edge of ilium in common with origin of latter muscle.

ORIGIN.--The muscle arises from the ventrolateral surface of the ilium anterior to the acetabulum and posterior to the origin of M. iliacus; the anterior part attaches to the ventrolateral edge of the ilium and the posterior part attaches just above the ventral edge. The muscle is not attached to the concavity of the femoral notch (the origin is notched here). The part attaching anterior to the femoral notch is narrow, tendinous, and continuous anteriorly with the accessory aponeurosis of M. extensor iliotibialis lateralis (thus the anterior border of the muscle cannot be exactly delimited). The part attaching posterior to the femoral notch is wider and fleshy (fig. 20A).

INSERTION.--The short flat tendon attaches to the lateral surface of the distal end of the trochanter slightly anterior and immediately distal to the insertion of M. gluteus profundus; the attachment is proximal to the origin of M. vastus lateralis, anteroproximal to the insertion of M. piriformis, and several mm. proximal to the insertion of M. iliacus.

INNERVATION.--The small posterodorsal division of the femoral nerve penetrates the ventral surface.

INDIVIDUAL VARIATION.--On both sides of one specimen, the femoral notch is absent and the proximal end of the muscle is not notched; the proximal part is entirely fleshy and the anterior border is well defined (fig. 20D).

_T. cupido_

INDIVIDUAL VARIATION.--The part attaching anterior to the femoral notch has a fleshy origin in one leg (fig. 20B), but in another, no part attaches anterior to the femoral notch (thus the muscle is not notched). In one leg, the insertional tendon is strongly fused to, and continuous with, the ventral edge of the insertional tendon of M. gluteus profundus.

_P. p. jamesi_

INDIVIDUAL VARIATION.--The part attaching anterior to the femoral notch may be mainly or entirely fleshy. In one leg, the part attaching anterior to the femoral notch is entirely separate from, although overlapped by, the main part of the muscle for the entire length of the fleshy belly (fig. 20C); both parts have a common insertional tendon.

=_M. Psoas_= (M. iliacus), Figs. 13, 15, 18

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Small and slender; on medial aspect of proximal end of thigh lateral to proximal end of M. ambiens; ventromedial to M. iliotrochantericus medius; proximal end visible from inside pelvis (medial to inguinal ligament); passes dorsolateral to inguinal ligament.

ORIGIN.--The muscle arises fleshily from the ventrolateral edge of the ilium posterior to the femoral notch and ventral (adjacent) to the origin of M. iliotrochantericus medius.

INSERTION.--The attachment is tendinous to the medial surface of the femur a short distance proximal to the origin of M. femoritibialis internus.

INNERVATION.--The posterior division of the femoral nerve, which spirals completely around M. psoas, gives several twigs into the proximal part.

INDIVIDUAL VARIATION.--None of significance.

_T. cupido_

INDIVIDUAL VARIATION.--In two legs the insertion is partly fleshy.

_P. p. jamesi_

INDIVIDUAL VARIATION.--In one leg the insertion is partly fleshy. The posterior division of the femoral nerve perforates the muscle in one instance.

=_M. Flexor Cruris Lateralis_= (M. semitendinosus), Figs. 12, 13, 14, 15, 16, 17

This muscle represents only the main head of the muscle for which Fisher and Goodman (1955) used the same name. Their accessory head of M. flexor cruris lateralis is here termed M. femorocruralis.

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Large, thick, and strap-shaped; on posterior surface of thigh; proximal part bounded anteriorly by Mm. extensor iliotibialis lateralis and extensor iliofibularis; anterodistal part deep to latter; bounded medially by Mm. caudofemoralis (proximally) and flexor cruris medialis (distally); proximal end much narrower than remainder and posterior to ilium; fused to underlying tough membrane, which forms body wall posterior to ilium; proximal half of narrow part aponeurotic; distal part of muscle posterior to M. femorocruralis; separated from latter by common raphe to which both attach; caudal muscle (M. transversoanalis) attached aponeurotically to superficial surface of posteroproximal fleshy part of M. flexor cruris lateralis.

ORIGIN.--The origin is tendinous (superficial surface) and fleshy from the entire dorsolateral iliac ridge and fleshy from an area of the ilium below this ridge, also tendinous from the posterior edge of the ilium medial to the dorsolateral iliac ridge, and also tendinous from the transverse processes of the first free caudal vertebra and the vertebra either anterior or posterior to the latter.

INSERTION.--M. flexor cruris lateralis and M. femorocruralis insert broadly on opposite sides of a long tendinous raphe that extends parallel to, but some distance posterior to, the distal half of the femur; the distal end of this tendon broadens somewhat and fuses to the medial surface of M. gastrocnemius pars media (continuous with the tendon of the latter); the superficial part of this tendon continues toward the tibiotarsus, soon fusing to the deep surface of the overlying tendon of M. flexor cruris medialis; thus the common tendon of M. flexor cruris lateralis and M. femorocruralis insert in common with both M. flexor cruris medialis and M. gastrocnemius pars media.

INNERVATION.--A branch of the middle tibial division of the sciatic nerve enters the substance of M. caudofemoralis pars iliofemoralis, and emerges near its ventral edge, then passes lateral to M. caudofemoralis pars caudifemoralis and enters the anterior part of M. flexor cruris lateralis.

INDIVIDUAL VARIATION.--In three legs, the nerve does not perforate M. caudofemoralis pars iliofemoralis, but passes deep to it.

_T. cupido_

INDIVIDUAL VARIATION.--In one leg, a small accessory slip arises from the ventrolateral surface of the caudal musculature and joins the posterior edge of the main part of M. flexor cruris lateralis a short distance dorsal to the pubis. In several legs, the nerve does not perforate M. caudofemoralis pars iliofemoralis, but passes deep to it.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The muscle is wider. The extreme proximal end is fleshy up to its origin, which is fleshy and tendinous from the vertebrae. The common insertional tendon of M. flexor cruris lateralis and M. femorocruralis fuses with the distal end of the fleshy part (instead of tendon) of M. flexor cruris medialis.

INDIVIDUAL VARIATION.--None of significance.

=_M. Flexor Cruris Medialis_= (M. semimembranosus), Figs. 12, 13, 14, 15, 16, 17, 20E

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Most posterior muscle on medial surface of thigh; long and strap-shaped; bounded anteriorly by M. adductor profundus; posteroproximal corner of latter medial to anteroproximal part of M. flexor cruris medialis; bounded laterally by Mm. caudofemoralis (proximally) and flexor cruris lateralis (distally); anteroproximal corner adjacent to posteroventral corner of M. flexor ischiofemoralis and lateral to extreme posteroproximal corner of M. adductor superficialis; distal end tendinous, extending into proximal part of shank; bounded medially by M. gastrocnemius pars interna and laterally by Mm. gastrocnemius pars media and plantaris.

ORIGIN.--The muscle arises by a wide flat tendon from a narrow line on the lateral surface of the ischium dorsal to the ventral ischiatic tubercle.