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

by E. Bruce Holmes

P. p. jamesi

Differences from Typical T. pallidicinctus.—The proximal end of pars interna does not reach the patella.

Individual Variation.—In one leg, an additional twig to pars media arises from the distal branch to M. femorocruralis.

M. Flexor Perforans et Perforatus Digiti II, [Figs. 12], [14]

T. pallidicinctus

General Description and Relations.—Long, slender, and Y-shaped; on lateral surface of shank; the two heads enclosing M. flexor perforans et perforatus digiti III; posterior head bounded posteriorly by M. gastrocnemius pars externa; extreme proximal end deep to M. vastus lateralis pars postica; anterior surface fused to posterior surface of M. flexor perforans et perforatus digiti III; deep surface fused to tendinous part of lateral head of M. flexor perforatus digiti IV; anterior head tendinous except for extreme distal end; covered by, and fused to, posterior edge of M. peroneus longus; fused to anterior surface of M. flexor perforans et perforatus digiti III; two heads join above middle of shank; anteroproximal and posterodistal parts of common belly usually tendinous.

Origin.—Anterior head: This arises by a narrow tendon (partly ossified) from the distal tip of the outer cnemial crest. The tendon is so intimately fused with a connective tissue sheet fused to the deep and posterior surfaces of M. peroneus longus and to the anterior surface of M. flexor perforans et perforatus digiti III that M. flexor perforans et perforatus digiti II could be considered to arise from these two muscles. Posterior head: This arises mostly fleshily from the lateral surface of a compound sheet of tough connective tissue formed by the fusion of the tendinous posteroproximal corner of M. flexor perforans et perforatus digiti III, the proximal parts of the tendons of origin of the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II, the fibular and distal arms of the guide loop for M. extensor iliofibularis, and the lateral part of the articular capsule; a part 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 also contributes to this sheet, which attaches to the lateral surface of the external condyle of the femur and to the anterolateral surface of the head of the fibula; for convenience in description, this complex connective tissue sheet will hereafter be termed the femorofibular fascia. The anteroproximal corner of the posterior head of M. flexor perforans et perforatus digiti II often attaches to the lateral surface of the vinculum that passes from the femorofibular fascia to the deep surface of the patellar tendon; the extreme proximal end usually attaches fleshily to a small area on the femur immediately proximal to the fibular condyle and adjacent (distal) to the attachment of the distal arm of the guide loop for M. extensor iliofibularis.

Insertion.—The common belly terminates approximately two thirds of the way down the shank; the slender ossified tendon begins along the posteromedial edge of the common belly, continues distally along the posterior surface of the shank, and becomes flexible before passing through the canal in the tibial cartilage that lies posteromedial to the canal for M. flexor digitorum longus. The tendon passes with the tendon of M. flexor perforatus digiti II (medial to the latter) through a canal in the hypotarsus (see M. flexor perforatus digiti II); just below the hypotarsus, the tendon becomes superficial to the tendon of M. flexor perforatus digiti II and farther distally becomes lateral and finally deep to the latter; the tendon is ossified for most of the length of the tarsometatarsus. At the distal end of this bone, the tendon expands before passing onto the ventral surface of digit II between the tendons of Mm. flexor perforatus digiti II and flexor digitorum longus; at the level of the first phalanx, the edges of the tendon extend dorsally around the tendon of M. flexor digitorum longus and fuse, forming a sheath around the latter; the latter emerges from the sheath near the distal end of the first phalanx; the tendon attaches to the proximal end of the subarticular cartilage ventral to the first interphalangeal joint (the strongest attachment is on the medial side).

Innervation.—The lateral division of the tibial nerve sends twigs into the posteromedial edge of the posterior head.