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

by E. Bruce Holmes

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.