I accept the following concept of muscle-nerve relationship. All muscles of the pelvic limb of birds have developed phylogenetically from either the dorsal extensor muscle mass or the ventral flexor muscle mass. The former was (at least originally) supplied by only the femoral and peroneal nerves ("dorsal" nerves), the latter by only the obturator and tibial nerves ("ventral" nerves). The best guide for determining which muscles are phylogenetically dorsal and which are ventral seems to be their embryogeny (as shown in the studies of Romer, 1927, and Wortham, 1948). In the phylogenetic changes undergone by the muscles under consideration, the innervation may have changed in some instances, although this is less likely to occur than changes in the attachment or function of the muscles. If a change in innervation has occurred, it would be more likely to be a change from one dorsal nerve to the other or from one ventral nerve to the other rather than from a dorsal nerve to a ventral one or vice versa.
Thus, in my opinion, a report of a dorsal muscle supplied by a ventral nerve, or vice versa, should be viewed with suspicion until it is verified. I suspect that many previous workers have ignored this concept of muscle-nerve relationship, or else do not accept it, since they report, without comment, dorsal muscles (as determined embryologically) innervated by ventral nerves, or vice versa. Owing to the intimate association between the proximal parts of the tibial and peroneal nerves, the true relationship may be difficult to determine. I suspect that this relationship has been misinterpreted by a number of workers. I found in Tympanuchus and Pedioecetes a branch of the tibial nerve that is closely associated with, and distributed with, the peroneal nerve and has been mistakenly considered a part of the peroneal nerve by some workers. In the study here reported on, I have found no definite exceptions to the expected innervation. The only possible exception is an extra branch, which could not be traced to its origin, supplying M. extensor iliofibularis in one leg. Thus my study of innervation agrees with the embryological determination of the (phylogenetic) dorsal and ventral muscles and lends strong support to the above-stated concept of muscle-nerve relationship.
I have compared my findings on the nerves with those of other workers, who have studied the nerves with a varying degree of thoroughness. The important differences in innervation between these studies and the present one are discussed below.
In neither of Gadow's works did he distinguish tibial and peroneal components in the thigh. In his later work (1891), covering a wide variety of birds, he found that M. piriformis sometimes has a femoral innervation in addition to the constant sciatic one and that M. gluteus profundus may or may not have a sciatic supply in addition to the femoral one. A comparison of Gadow's terminology of the sciatic nerve branches in the shank and foot (in both works) with mine shows that his branch I represents my peroneal nerve plus my paraperoneal branch of the tibial nerve (Ic); his branch II represents my medial division of the tibial nerve; and his branch III represents my posterior (IIIa) and lateral (IIIb) divisions of the tibial nerve.
Gadow's study (1880) on the ratites included Struthio, Rhea, and Casuarius. Only in Casuarius did Gadow find a branch (IIe) of the sciatic nerve supplying Mm. lumbricalis, adductor digiti II, and abductor digiti II. The two former muscles are typically supplied (as in Rhea) by the paraperoneal branch of the tibial nerve; Gadow's branch IIe presumably represents a segregated branch of this nerve. More surprising is his finding that M. abductor digiti II is innervated in Casuarius by both the deep peroneal nerve and branch IIe and in Rhea by branch Ic (paraperoneal branch of tibial nerve). The deep peroneal innervation is typical. Also unexpected is his finding that the posterior division of the femoral nerve gives minute twigs into M. gastrocnemius pars interna in Struthio and Casuarius. Since the other terminal branches of this nerve in these birds are nonmuscular, since this muscle is chiefly supplied by other nerves, and since the innervation from the femoral nerve is apparently atypical for most birds, the possibility should be considered that the femoral twigs are sensory rather than motor.
Sudilovskaya (1931), studying Struthio, Rhea, and Dromaeus (Dromiceius), used the same terminology as Gadow except that he designates as branch III Gadow's branch Ic. Sudilovskaya's discussion of the main branches of the sciatic nerve is confusing. He states that in Struthio, branches I, II, and III all pass through the tendinous guide loop for M. extensor iliofibularis; this is hard to believe. As near as I can determine, he has mistakenly given the same designation (branch III) to two separate branches (Gadow's Ic and III). There is no problem, however, in determining to which of these two branches he is referring when he is describing the innervation of a particular muscle, since one supplies only muscles of the shank and the other only intrinsic foot muscles. Sudilovskaya found M. abductor digiti II to be innervated by branch III (Ic of Gadow); thus the innervation of this muscle corresponds to that found in Rhea by Gadow. Although M. adductor digiti II had the expected innervation from branch III (paraperoneal branch of tibial nerve) in Dromaeus, that muscle was found to be supplied by branch II in Rhea. (Gadow, on the other hand, reports a typical innervation for this muscle in Rhea.) Sudilovskaya found M. peroneus brevis to be supplied by the deep peroneal branch (in contrast to the superficial peroneal supply that I found in Tympanuchus and Pedioecetes). He found M. gastrocnemius pars interna to be supplied in Struthio by twigs of the femoral nerve in addition to its typical innervation from branch II of the sciatic nerve; this agrees with Gadow's findings in the same genus. Sudilovskaya reports that M. gastrocnemius pars externa was innervated by branches II and III in Struthio and Rhea and by branches I and III in Dromaeus. (Gadow found only the typical innervation—branch III.)
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.