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

by E. Bruce Holmes

General Description and Relations.—Divided into three distinct, widely separated parts—pars externa, pars interna, and pars media; pars externa: large; on posterolateral surface of shank; narrow proximally and distally; bounded anterolaterally by M. flexor perforans et perforatus digiti II and anteromedially by medial head of M. flexor perforatus digiti III; completely separate from pars interna and media except for common tendon of insertion; pars interna: large; on anteromedial surface of shank; narrow distally; bounded anterolaterally by M. peroneus longus and posteromedially by pars media (proximally) and medial head of M. flexor perforatus digiti III; broad sheet of tough connective tissue extending between distal parts of pars externa and pars interna; covering underlying M. flexor perforatus digiti III (medial head), somewhat fused with anteroproximal edge of M. peroneus longus; pars media: small and short; on medial surface of proximal part of shank; deep to tendon of insertion of M. flexor cruris medialis; bounded anteromedially by pars interna, posterolaterally by medial head of M. flexor perforatus digiti III, and proximally by M. femorocruralis; fused to latter, and boundary between the two difficult to locate.

Origin.—Pars externa: The short cylindrical tendon fuses with the anterior half of the distal arm of the tendinous guide loop for M. extensor iliofibularis and attaches in common with the latter to the posterolateral surface of the femur immediately proximal to the fibular condyle; the attachment is proximal (adjacent) to the origin of M. flexor perforans et perforatus digiti II and distal (adjacent) to the origin of M. flexor perforatus digiti IV and is fused to the articular capsule.

Pars interna: The proximal end is partly separable into two layers—a superficial longer one and a deep shorter one. The superficial layer attaches fleshily to the ventral part of the anterior surface of the patella and to the medial half of the superficial surface of the patellar tendon; this layer slightly overlaps the distal fleshy end of M. extensor iliotibialis anticus. The deep layer (overlapped by the superficial layer) attaches to the medial surface of the inner cnemial crest, to the rotular crest medial to the latter, to the medial surface of the proximal part of the tibiotarsus, and (posteroproximal corner) to the distomedial edge of the patellar tendon and to the articular capsule posteromedial to the rotular crest; the entire ventral edge is tendinous, the remainder fleshy.

Pars media: This arises fleshily from an oblique line beginning at the distal end of the origin of M. femorocruralis (continuous with the latter) and extending distomedially across the proximal part of the popliteal area to the proximal edge of the internal condyle, then attaching to the adjacent part of the articular capsule; this part is adjacent (distal) to the insertion of M. adductor profundus and adjacent (proximomedial) to the medial head of M. flexor perforatus digiti IV.

Insertion.—Pars media narrows distally with a narrow tendon along the posterior edge of the fleshy belly; approximately one third of the way down the tibiotarsus the fleshy part terminates and the tendon joins the posterior edge of pars interna, continuing distally in this position. The ossified tendon on the superficial surface of the distal part of pars interna, continuous posteriorly with the tendon of pars media, is joined approximately two thirds of the way down the tibiotarsus by the tendon of pars externa; the fleshy belly of pars interna ends just below the junction. The ossified tendon on the superficial surface of the distal part of pars externa extends beyond the fleshy belly and becomes flexible before joining the tendon of pars interna and media. The common tendon (partly ossified) extends along the posterior surface of the tibiotarsus and widens as it passes posterior to the tibial cartilage, bound to the latter by a thin tough sheet of connective tissue which attaches to the edges of the tibial cartilage, thus forming a sheath for the tendon; the tendon attaches by its edges to the posterior edges of the calcaneal ridges of the hypotarsus, then continues distally (much reduced in thickness) along the posterior surface of the tarsometatarsus, enclosing the flexor tendons; the lateral edge of the tendon attaches to the posterolateral edge of the tarsometatarsus, terminating immediately above the level of the hallux; the medial edge attaches to the edge of the posterior metatarsal crest; the tendon terminates as a thin sheet that attaches to the fascia on the sole of the foot. (Hudson, et al., 1959 consider the posterior metatarsal crest to be an ossified part of the tendon of M. gastrocnemius.)

Innervation.—A branch of the lateral division of the tibial nerve penetrates the proximal part of the medial surface of pars externa. One or two branches of the medial division of the tibial nerve pass deep to M. plantaris and penetrate the deep surface of the posterior part of pars interna. The most proximal branch of the medial division of the tibial nerve penetrates the lateral surface of pars media.

Individual Variation.—None of significance.

T. cupido

Individual Variation.—In one leg, the lateral edge of pars interna overlaps the proximomedial edge of M. peroneus longus; some fibers attach to the lateral surface of the inner cnemial crest.