Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 16

In the Whooping Crane, Fisher and Goodman (1955) found a peroneal, rather than a femoral, nerve supply for pars postica of M. vastus lateralis. They also report a peroneal nerve supply for M. flexor ischiofemoralis (in contrast to the usual tibial nerve supply) and for M. adductor superficialis (in addition to the usual supply from the obturator nerve). The innervation was not given for the intrinsic foot musculature.

Fisher (1946), studying vultures, reports the following: tibial branches, in addition to the main sciatic branch, supplying M. extensor iliofibularis (typically supplied by the peroneal nerve); an obturator supply, in addition to the usual tibial supply, to M. flexor cruris medialis; a tibial supply, in addition to the typical obturator supply, to M. obturator pars postica; a possible obturator supply, in addition to the typical femoral supply, to M. ambiens; a possible peroneal supply, in addition to the typical tibial supply, to M. flexor digitorum longus; and a peroneal supply to Mm. abductor digiti IV, flexor hallucis brevis, and adductor digiti II (which are typically supplied by the paraperoneal branch of the tibial nerve). Fisher's postfibular branch of the peroneal nerve, which supplies the latter three muscles, apparently represents the paraperoneal branch of the tibial nerve.

Carlsson (1884) did not find a femoral nerve supply for M. gluteus profundus. He found an obturator supply, in addition to the usual sciatic supply, to M. flexor ischiofemoralis in _Eudyptes chrysolopha_ and _Mergulus alle_ but not in the other two forms studied. He reported a peroneal supply, rather than the expected tibial (paraperoneal) supply, to Mm. abductor digiti IV and adductor digiti IV.

DeMan (1873) found a twig of the obturator nerve supplying M. flexor ischiofemoralis, in addition to the typical innervation, in _Corvus monedula_, but not in the few other forms studied. He did not distinguish tibial and peroneal components in the thigh.

Wilc.o.x (1948), studying a loon, did not find any peroneal supply to M.

extensor iliotibialis lateralis or to M. gluteus profundus. He found a femoral, rather than a peroneal, supply to M. piriformis. He found an obturator, instead of a tibial, supply to M. flexor ischiofemoralis. (In some of my specimens I found a tiny blood vessel, appearing much like a nerve, emerging from the obturator foramen and entering M. flexor ischiofemoralis.) Wilc.o.x reports an innervation of M. caudofemoralis pars caudifemoralis from the pudendal plexus, in addition to the usual sciatic one. Wilc.o.x did not distinguish tibial and peroneal components in the thigh. In the shank and foot he misidentified the peroneal nerve as the tibial nerve and therefore gave erroneous innervations for all the muscles supplied by this nerve, except for M. adductor digiti IV, which actually should be supplied by the tibial nerve.

Howell (1938) studied only the hip and thigh musculature of the chicken.

He overlooked the femoral nerve supply for M. gluteus profundus.

Romer (1927) studied only the hip and thigh muscles of the chick. He did not distinguish tibial and peroneal components in the thigh. He did not mention any sciatic supply for M. gluteus profundus.

Appleton (1928), studied (in various birds) only those muscles of the hip and thigh that are innervated by the tibial and peroneal nerves. He terms the former "ischiadicus ventralis" and the latter "ischiadicus dorsalis." His findings did not differ from mine.

Many differences in the innervation of specific muscles are reported in the literature, even in the same species (by different workers). Some of these differences may be real; others are probably misinterpretations.

Consequently more work needs to be done before a complete understanding can be obtained of the innervation of the leg muscles of birds.

Especially needed are studies of the tibial-peroneal nerve relationship, perhaps approached by a method other than gross dissection.

SUMMARY

The muscles and nerves were dissected in eight legs of the Lesser Prairie Chicken (_Tympanuchus pallidicinctus_), six legs of the Greater Prairie Chicken (_T. cupido pinnatus_), three legs of Att.w.a.ter's Prairie Chicken (_T. c. att.w.a.teri_), and six legs of the Sharp-tailed Grouse (_Pedioecetes phasianellus jamesi_) for the purpose of obtaining information on individual variation as well as variation between these closely related species. Relatively little information is available regarding the nerves of the leg of birds and little is known about individual variation and variation between closely related forms in the muscles of the leg of birds.

All osteological terms used in the present paper are defined and those of the pelvis are ill.u.s.trated. New terms were coined for some structures for which no names could be found in the literature. Terms were also coined for the major divisions of the femoral and sciatic nerves. With three exceptions, my muscle terminology follows that of Fisher (1946) and Fisher and Goodman (1955). Their term femoritibialis externus is not used here; the muscle so named is considered to be a part of M. vastus lateralis. Fisher's accessory head of M. flexor cruris lateralis is considered to be a distinct muscle--M. femorocruralis. Usage of the term obturator internus is avoided because the muscle so named is considered not to be h.o.m.ologous with the mammalian muscle of the same name; the entire obturator complex is called M. obturator, and is subdivided into four parts.

The typical (most common) condition of the nerves and muscles in _Tympanuchus pallidicinctus_ is described in detail. Variations from this condition among the other birds studied are then described. All muscles of one leg of _T. pallidicinctus_ are ill.u.s.trated. Several variations in the muscles are also ill.u.s.trated. The lumbosacral plexus and nerves of the leg in several specimens that show variations are ill.u.s.trated.

Considerable individual variation was found in both the muscles and the nerves of the leg of the species studied. Certain muscles were more variable than others. Mm. flexor digitorum longus, obturator, caudofemoralis, and extensor hallucis longus showed the greatest number of variations. Mm. vastus medialis, femoritibialis internus, flexor perforatus digiti III, extensor brevis digiti III, and abductor digiti IV did not exhibit any variations considered significant. Certain legs showed a greater number of variations from the typical condition than did others.

Although most of the variations were minor, some were major. M. extensor proprius digiti III was present in two legs of _Pedioecetes_ but absent in the other legs studied. A fleshy muscle slip connected M.

caudofemoralis pars caudifemoralis with the tendinous raphe between Mm.

flexor cruris lateralis and femorocruralis in two legs, whereas in others this connection was tendinous or even absent altogether. M.

flexor cruris lateralis had an accessory slip arising from the caudal musculature in one leg. A vinculum connected the insertional tendons of Mm. flexor perforans et perforatus digiti II and flexor perforatus digiti II in one leg.

In most specimens there was as much variation between the muscles of the right and left legs of one individual as there was between individuals.

The same was true for the nerves, except for the lumbosacral plexus, in which there was no significant variation between the right and left sides of any individual. The peroneal and obturator nerves varied less than the other nerves.

No constant differences in the muscles or nerves was found between _T.

cupido pinnatus_ and _T. c. att.w.a.teri_. One constant difference was found between _T. cupido_ and _T. pallidicinctus_: the fleshy origin of M. extensor iliotibialis lateralis in _T. cupido_ was thicker (a.s.sociated with a thicker edge of the lateral iliac process).

Although no constant differences in the nerves were found between _Pedioecetes_ and _Tympanuchus_ (both species), 17 constant differences in the muscles were found between these two genera. Study of additional specimens possibly would show enough individual variation in some of these differences to reduce the number of constant differences to fewer than 17. Seven of these differences pertain to features of a single muscle--M. flexor cruris medialis. Some of the other differences are a.s.sociated with the thinner and much less p.r.o.nounced lateral iliac process in _Pedioecetes_. The picture of the differences between _Tympanuchus_ and _Pedioecetes_ that this study presents is radically different from that presented by the study of Hudson, _et al._ (1959).

The important differences in innervation between previous studies and the present one are discussed.

All of the muscles under consideration have been grouped as either dorsal or ventral muscles, according to their embryonic origin, as described by Romer (1927) and Wortham (1948). This grouping probably represents accurately the phylogenetic origin of these muscles. The dorsal muscles probably were originally supplied by dorsal nerves--the femoral and peroneal--and the ventral muscles probably were originally supplied by ventral nerves--the obturator and tibial. This primitive muscle-nerve relationship has been relatively constant.

Several previous workers have reported some dorsal muscles supplied by ventral nerves and _vice versa_. Those findings should be viewed with suspicion until verified, because the proximal parts of the tibial and peroneal nerves are intimately a.s.sociated and their relationship is easily misinterpreted. I found a branch of the tibial nerve that is closely a.s.sociated with, and distributed with, the peroneal nerve. That branch of the tibial nerve has been mistakenly considered a part of the peroneal nerve by some workers. My study revealed no definite exceptions to the expected innervation.