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 Attwater's Prairie Chicken (T. c. attwateri), 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 illustrated. 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 homologous 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 illustrated. Several variations in the muscles are also illustrated. The lumbosacral plexus and nerves of the leg in several specimens that show variations are illustrated.

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. attwateri. One constant difference was found between T. cupido and T. pallidicinctus: the fleshy origin of M. extensor iliotibialis lateralis in T. cupido was thicker (associated 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 associated with the thinner and much less pronounced 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 associated and their relationship is easily misinterpreted. I found a branch of the tibial nerve that is closely associated 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.