Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 10

=_M. Gastrocnemius_=, Figs. 12, 13, 15

_T. pallidicinctus_

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 fles.h.i.+ly 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 fles.h.i.+ly 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 pa.s.ses 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 pa.s.s 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.

_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 fles.h.i.+ly 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 pa.s.ses from the femorofibular fascia to the deep surface of the patellar tendon; the extreme proximal end usually attaches fles.h.i.+ly 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 pa.s.sing through the ca.n.a.l in the tibial cartilage that lies posteromedial to the ca.n.a.l for M. flexor digitorum longus. The tendon pa.s.ses with the tendon of M. flexor perforatus digiti II (medial to the latter) through a ca.n.a.l 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 pa.s.sing 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.

INDIVIDUAL VARIATION.--In one leg, the fleshy part of the anterior head is unusually long. In another leg, the anterior head is entirely tendinous. In one leg, a bundle of fibers of the posterior head attaches to the deep surface of the distal part of the patellar tendon. In one leg, near the middle of the tarsometatarsus a rather long and narrow but thick and strong vinculum arises from the tendon of M. flexor perforatus digiti II and, farther distally, joins the tendon of M. flexor perforans et perforatus digiti II.

_T. cupido_

INDIVIDUAL VARIATION.--In one leg, the posterior head arises in part from the distolateral edge of the patellar tendon and in another, in part from the superficial surface of the distolateral corner of the patellar tendon.

_P. p. jamesi_

INDIVIDUAL VARIATION.--None of significance.

=_M. Flexor Perforans et Perforatus Digiti III_=, Figs. 12, 14

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Thick, bipinnate; on lateral surface of proximal part of shank between two heads of M. flexor perforans et perforatus digiti II; bounded anteriorly by M. peroneus longus; anterior surface fused with tendinous anterior head of M. flexor perforans et perforatus digiti II; anterolateral edge somewhat fused to posterior edge of M. peroneus longus superficial to latter tendon; posterior surface fused to posterior head of M. flexor perforans et perforatus digiti II; distal part of belly covered by common belly of latter muscle; posteromedial edge fused to underlying lateral head of M. flexor perforatus digiti IV; anteromedial edge usually somewhat fused to underlying M. flexor digitorum longus.

ORIGIN.--The origin is fleshy and tendinous from the edge of the outer cnemial crest and fleshy from the superficial surface of the distolateral part of the patellar tendon; the posteroproximal corner arises tendinously from the femorofibular fascia.

INSERTION.--The belly narrows abruptly, terminating approximately at the middle of the shank; the slender ossified tendon extends posterodistally along the shank, becoming flexible before pa.s.sing posterior to the tibial cartilage deep to the tendon of M. gastrocnemius, medial to the tendon of M. flexor perforatus digiti IV, and superficial to the medial half of the tendon of M. flexor perforatus digiti III; a thin sheet of connective tissue covers the tendon and attaches by its edges to the underlying tendon of M. flexor perforatus digiti III (thus the latter tendon forms a sheath for the tendon of M. flexor perforans et perforatus digiti II); the tendon is ossified for most of the length of the tarsometatarsus; at midlength of the latter, the tendon lies between the tendons of Mm. flexor perforatus digiti IV and flexor perforatus digiti III; near the distal end of the tarsometatarsus, the tendon becomes lateral and then deep to the tendon of M. flexor perforatus digiti III and is connected by a vinculum to the latter (which see). The tendon enters the ventral surface of digiti III between the tendons of Mm. flexor perforatus digiti III and flexor digitorum longus; after sending a dorsal slip (lateral to the tendon of M. flexor digitorum longus) to the subarticular cartilage ventral to the first interphalangeal joint, the tendon divides into two branches, between which emerges the tendon of M. flexor digitorum longus; the lateral branch attaches to the subarticular cartilage of the second interphalangeal joint and to the lateral surface of the distal end of the second phalanx; the medial branch has similar attachments on the medial side of the digit.

INNERVATION.--A branch of the lateral division of the tibial nerve pa.s.ses deep to the posterior head of M. flexor perforans et perforatus digiti II and enters the posteromedial edge of M. flexor perforans et perforatus digiti III.

INDIVIDUAL VARIATION.--In both legs of one specimen, the part arising from the femorofibular fascia appears as a distinct but short accessory head. There is no significant individual variation in _T. cupido_ or _P.

p. jamesi_.

=_M. Flexor Perforatus Digiti IV_=, Figs. 14, 16

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--On posterolateral aspect of shank deep to M. gastrocnemius pars externa; bounded medially by medial head of M. flexor perforatus digiti III, anterolaterally by posterior head of M. flexor perforans et perforatus digiti II, and anteriorly by M. flexor digitorum longus; divided into three heads--medial (largest), lateral, and anterolateral (smallest); tendon of insertion of M. extensor iliofibularis pa.s.sing between medial and lateral heads; proximal and anteroproximal parts of _lateral head_ an extremely thin, flat tendon; anterodistal part of tendon fused to lateral surface of fleshy part of underlying lateral head of M. flexor perforatus digiti II; proximal part of tendon fused indistinguishably to tendinous part of underlying lateral head of M. flexor perforatus digiti II; fleshy part of _anterolateral head_ anterodistal to lateral head; proximal part of former a long slender tendon anterior to lateral head; anterior surface of anterolateral head (both fleshy and tendinous parts) fused to tendon of anterolateral head of M. flexor perforatus digiti III; deep surface fused to underlying anterolateral head (fleshy) of M. flexor perforatus digiti II; common tendon of anterolateral heads of M. flexor perforatus digiti IV and M. flexor perforatus digiti III pa.s.sing medial to tendon of insertion of M. extensor iliofibularis, to peroneal nerve, and to fibular arm of guide loop for M. extensor iliofibularis; tendon of M.

ambiens inserting on anterolateral surface of this common tendon; _medial head_ entirely fleshy; medial surface fused to medial head of M.

flexor perforatus digiti III; deep surface fused to medial edge of underlying medial head of M. flexor perforatus digiti II; medial and lateral heads joined, forming bipinnate belly (pinnate structure most evident on deep surface); anterolateral head joined to distolateral part of belly.

ORIGIN.--The _medial head_ attaches fles.h.i.+ly to the proximal part of the popliteal area proximal (adjacent) to the origin of M. flexor hallucis longus and distolateral to the distal end of the origin of M.

femorocruralis; the attachment extends laterally onto the posterolateral surface of the femur proximal (adjacent) to the common attachment of M.

gastrocnemius pars externa and the distal arm of the guide loop for M.

extensor iliofibularis; the medial edge of the origin is fused with part of the tendinous origin of the medial head of M. flexor perforatus digiti III.

The broad flat common tendon of the _lateral head_ and the lateral head of M. flexor perforatus digiti II fuses to the superficial surface of the fibular arm of the guide loop for M. extensor iliofibularis and contributes to the femorofibular fascia; consequently the ultimate origin would be the external femoral condyle and the head of the fibula.

The slender common tendon of the _anterolateral head_ and the anterolateral heads of Mm. flexor perforatus digiti II and flexor perforatus digiti III pa.s.ses deep to the insertional tendon of M.

extensor iliofibularis and to the fibular arm of the guide loop for the latter muscle (to which it partly fuses); the tendon attaches to a narrow line on the head of the fibula adjacent to the attachment of the fibular arm of the guide loop and to the deep part of the femorofibular fascia.

INSERTION.--The slender ossified tendon becomes flexible before it pa.s.ses posterior to the tibial cartilage deep to the tendon of M.

gastrocnemius, lateral to the tendon of M. flexor perforans et perforatus digiti III, and superficial to the lateral half of the tendon of M. flexor perforatus digiti III; a thin sheet of connective tissue covers the tendon and attaches by its edges to the underlying tendon of M. flexor perforatus digiti III (thus the latter tendon forms a sheath for the tendon of M. flexor perforatus digiti IV; this sheath is separate from a similar sheath surrounding the tendon of M. flexor perforans et perforatus digiti III); the tendon is again ossified where it pa.s.ses along the posterolateral surface of the tarsometatarsus posterolateral to the tendon of M. flexor perforans et perforatus digiti III; near the distal end of the tarsometatarsus the tendon becomes flexible and expands greatly in width and thickness, and sends a small slip dorsally, medial to the underlying tendons, that attaches to the subarticular cartilage ventral to the trochlea for digit IV; sometimes this slip is continuous with the retinaculum ventral to the tendon at the level of the proximal end of the digit. Several more or less distinct sheets of tough connective tissue lie ventral to all of the flexor tendons at the level of the trochleae and the proximal end of the digits, holding them in place. The tendon narrows as it pa.s.ses onto the ventral surface of digit IV and soon divides into three branches; the tendon of M. flexor digitorum longus emerges between the medial and middle branches. The lateral branch attaches to the subarticular cartilage ventral to the first interphalangeal joint and is also bound by connective tissue to the ventrolateral surface of the first phalanx.

A dorsal slip arises at the point of divergence of the lateral and middle branches and attaches to the subarticular cartilage of the first interphalangeal joint. The middle branch attaches to the subarticular cartilage of the second joint. The medial branch, after sending dorsal slips to each of the first two subarticular cartilages, attaches to the subarticular cartilage of the third interphalangeal joint.

INNERVATION.--The posterior division of the tibial nerve sends a branch into the posterior edge of the medial head, then pa.s.ses between the latter and the medial head of M. flexor perforatus digiti III; as it extends distally it gives off twigs to the medial surface of the medial head, to the deep surface of the lateral head, and to the deep surface of the anterolateral head.

INDIVIDUAL VARIATION.--In one leg, an additional branch arises from the tibial nerve at the level of origin of the posterior division and enters the posterior surface of the medial head; a twig from this branch anastomoses with the first twig of the posterior division to the same head; a branch of the medial division joins the posterior division distal to the origin of the twigs to the medial head but proximal to the origin of the twigs to the other heads.