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

by E. Bruce Holmes

T. pallidicinctus

General Description and Relations.—Deep to M. extensor iliotibialis lateralis and posterior to femur; broad proximally and narrow distally; posterior to M. vastus lateralis and anterior to proximal part of M. flexor cruris lateralis (superficial to distal part of latter); anteroproximal part aponeurotic, fused to deep surface of aponeurosis of M. extensor iliotibialis lateralis; proximal part of aponeurosis of M. extensor iliofibularis also fused to dorsal edges of underlying Mm. gluteus profundus and piriformis.

Origin.—The posterior part is fleshy from the ventromedial surface of the entire lateral iliac process; the anterior part is aponeurotic from the posterior part of the anterior iliac crest.

Insertion.—The tendon forms along the posterodistal edge of the belly and continues beyond the end of the belly as a cylindrical tendon that passes through the tendinous guide loop (the belly terminates approximately at the level of the guide loop), then extends anterodistally into the shank musculature; the tendon passes between the medial and lateral heads of M. flexor perforatus digiti IV, between the medial and lateral heads of M. flexor perforatus digiti II, lateral to the common tendon of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III, and between the posterior and lateral heads of M. flexor digitorum longus, attaching to the fibular tubercle.

The tendinous guide loop has three arms—proximal femoral, distal femoral, and fibular; the proximal and distal femoral arms unite posterior to the tendon of M. extensor iliofibularis; the proximal arm is medial to, and the distal arm is lateral to, the latter; the fibular arm joins the distal edge of the distal arm lateral to the tendon of M. extensor iliofibularis. The proximal arm extends anteroproximally lateral to the medial head of M. flexor perforatus digiti IV and medial to M. vastus lateralis pars postica, attaching to a narrow line on the anterolateral surface of the femur a short distance proximal to the external condyle and adjacent (posterior) to the origin of M. vastus lateralis pars postica. The distal arm extends anteriorly medial to the posterior head of M. flexor perforans et perforatus digiti II and medial to M. vastus lateralis pars postica, attaching in common with the tendon of origin of M. gastrocnemius pars externa to a small oval area on the posterolateral surface of the femur a short distance proximal to the fibular groove; the arm is also fused to the underlying articular capsule. The fibular arm (broadest of the three) passes deep to, and fused with, the common tendon of origin of the lateral heads of Mm. flexor perforatus digiti IV and flexor perforatus digiti II, superficial to the common tendon of origin of the anterolateral heads of Mm. flexor perforatus digiti IV, flexor perforatus digiti II, and flexor perforatus digiti III, and deep to the tendon of M. ambiens, attaching broadly to a narrow line on the anterolateral surface of the proximal part of the fibula; the arm is also fused to the underlying articular capsule.

Innervation.—A branch of the middle peroneal division of the sciatic nerve sends twigs to the deep surface of the anteroproximal part; the dorsal peroneal division of the sciatic nerve penetrates the deep surface of the proximal end.

Individual Variation.—In some instances a variable number of twigs arises from the peroneal nerve near the middle of the thigh and enters the deep surface of the muscle. They are difficult to expose without breaking and may have been overlooked in some specimens.

T. cupido

Individual Variation.—The same variation is found as in T. pallidicinctus (see above). In one leg, the tendon of insertion bifurcates into proximal and distal arms before attaching.

P. p. jamesi