Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 12

INDIVIDUAL VARIATION.--The nerve branch supplying M. gastrocnemius pars interna gives a minute twig to the deep surface of the free belly of M.

plantaris in one instance.

=_M. Flexor Digitorum Longus_=, Figs. 14, 16, 17, 19A

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Relatively broad; bipinnate; on posterolateral surface of tibiotarsus; bounded posteromedially by M.

flexor hallucis longus, posteriorly by M. flexor perforatus digiti II and anterolateral head of M. flexor perforatus digiti III, laterally by Mm. flexor perforans et perforatus digiti III and flexor perforans et perforatus digiti II, and anterolaterally by Mm. peroneus brevis and tibialis anticus; anterior surface of lateral part of distal half of common belly fused to M. peroneus brevis; divided into three heads--posterior (largest), lateral, and medial; _posterior head_ on posterior surface of head of fibula; overlapping and fused to lateral end of M. popliteus; proximomedial corner deep to latter; _lateral head_ on lateral surface of fibula; lateral and posterior heads separated by insertion of M. extensor iliofibularis; these two heads joined immediately distal to insertion of latter; _medial head_ on posterior surface of tibiotarsus; group of blood vessels and nerves pa.s.sing between medial and posterior heads; these two heads joined several mm.

distal to junction of lateral and posterior heads; deep surface of insertional tendon near distal end of tarsometatarsus serving as origin for M. lumbricalis.

ORIGIN.--_Posterior head_: This arises fleshily from the posterior surface of the fibula beginning almost at the proximal end and from the medial surface of the fibula beginning deep to the distal part of M.

popliteus. _Lateral head_: This arises fleshily (sometimes partly tendinously) from the lateral surface of the fibula proximal to the fibular tubercle. Some fibers arise from the distal edge of the tendon of insertion of M. extensor iliofibularis. _Medial head_: This arises fleshily from the posterior surface of the tibiotarsus just medial to the distal part of the posterior head, distal to M. popliteus, and either lateral or distolateral to the origin of M. plantaris. Distal to the junction of the three heads, the muscle arises fleshily from the posterior surface of the tibiotarsus (except the distal part) and from the medial and posterior surfaces of the fibula.

INSERTION.--The slender ossified tendon becomes flexible and pa.s.ses through the ca.n.a.l in the tibial cartilage that lies anterolateral to the ca.n.a.l for M. flexor perforans et perforatus digiti II and anteromedial to the ca.n.a.l for M. flexor perforatus digiti II, then pa.s.ses through the bony ca.n.a.l of the hypotarsus that is deep to all the other flexor tendons; the tendon ossifies again and lies adjacent (lateral) to the posterior metatarsal crest; the vinculum from the tendon of M. flexor hallucis longus fuses extensively to the superficial surface of the present tendon a short distance below the midpoint of the tarsometatarsus; the tendon is considerably broader below this point than above it. At the level of the first metatarsal, the tendon divides into three branches (unossified) that diverge, each pa.s.sing through a groove on the ventral surface of the subarticular cartilages ventral to the trochleae, then pa.s.s onto the ventral surfaces of digits II, III, and IV. On _digit IV_ the tendon gives off two dorsal fibro-elastic slips before attaching to the ventral surface of the ungual phalanx; one slip attaches to the subarticular cartilage ventral to the third interphalangeal joint, the other to the subarticular cartilage of the fourth joint and may also attach in part to the distal end of the fourth phalanx. On _digit III_ the tendon gives off two dorsal slips before attaching to the ventral surface of the ungual phalanx; one slip attaches to the subarticular cartilage of the second interphalangeal joint, the other to the subarticular cartilage of the third joint and may also attach in part to the distal end of the third phalanx. On _digit II_ the tendon gives off one dorsal slip before attaching to the ventral surface of the ungual phalanx; the slip attaches to the subarticular cartilage of the second interphalangeal joint and may also attach in part to the distal end of the second phalanx.

INNERVATION.--A branch of the medial division of the tibial nerve penetrates the medial surface of the posterior head.

INDIVIDUAL VARIATION.--In half the legs, the proximal end of the lateral head is notched for the pa.s.sage of the peroneal nerve; the main part of the head lies medial to this nerve; the short fleshy slip lateral to this nerve arises by a long, slender, and extremely weak tendon from connective tissue surrounding the femorotibiotarsal joint. In one leg, a bundle of fibers separates from the lateral head and attaches to the terminal four mm. of the anterior (proximal) edge of the tendon of M.

extensor iliofibularis. Each of the following variations occurs in several legs: a third dorsal slip on digit IV attaches to the distal end of the fourth phalanx in some legs and to the subarticular cartilage of the fourth joint in other legs; a third dorsal slip on digit III attaches to the distal end of the third phalanx in some legs and to the subarticular cartilage of the third joint in other legs; a second dorsal slip on digit II attaches to the distal end of the second phalanx in some legs and to the subarticular cartilage of the second joint in other legs.

_T. cupido_

INDIVIDUAL VARIATION.--The dorsal slips of insertion show variations similar to those noted above for _T. pallidicinctus_.

_P. p. jamesi_

INDIVIDUAL VARIATION.--In one leg, the proximal end of the lateral head is notched for the pa.s.sage of the peroneal nerve. The dorsal slips of insertion show variations similiar to those given above for _T.

pallidicinctus_. In four legs, a tiny vinculum extends from the lateral edge of the branch of the tendon on digit IV to the lateral edge of the underlying medial branch of the tendon of M. flexor perforatus digiti IV at the level of the second phalanx.

=_M. Popliteus_=, Fig. 19B

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Extremely short but relatively broad and thick; on posterior surface of proximal end of tibiotarsus; extending distomedially from proximal part of fibula; deep to M. flexor hallucis longus; lateral end overlapped by, and fused to, posterior head of M. flexor digitorum longus; medial end often slightly overlapped by, and fused to, M. plantaris; medial end (insertion) much wider than lateral end (origin).

ORIGIN.--The origin is fleshy and tendinous (superficial surface) from the medial surface of the fibula near the proximal end.

INSERTION.--The attachment is fleshy to the posterior surface of the proximal end of the tibiotarsus adjacent (lateral) to the origin of M.

plantaris.

INNERVATION.--A branch of the medial division of the tibial nerve penetrates the posterior surface.

INDIVIDUAL VARIATION.--None of significance in any of the three species studied.

=_M. Peroneus Longus_=, Figs. 12, 13

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Large; on anterolateral surface of shank; bounded medially by M. gastrocnemius pars interna and posterolaterally by Mm. flexor perforans et perforatus digiti III and flexor perforans et perforatus digiti II; proximal three fourths of posteromedial part (covered by M. gastrocnemius pars interna) aponeurotic and tightly fused to medial surfaces of underlying Mm.

tibialis anticus and extensor digitorum longus; proximal part of fleshy belly somewhat fused to anterior surface of underlying M. tibialis anticus; posterolateral surface strongly fused to aponeurotic medial head of M. flexor perforans et perforatus digiti II and slightly fused to anterolateral edge of M. flexor perforans et perforatus digiti III.

ORIGIN.--The muscle arises by fleshy and tendinous fibers from the edges of the inner and outer cnemial crests; the extreme proximal end arises either fleshily or aponeurotically from the rotular crest between the cnemial crests; the posteromedial edge (aponeurotic except distal one fourth fleshy) arises from the anteromedial intermuscular line.

INSERTION.--The narrow ossified tendon on the superficial surface of the distal part of the fleshy belly extends several mm. beyond the belly where it becomes flexible and divides into two branches. The short, broad posterior branch attaches broadly to the proximolateral corner of the tibial cartilage. The narrow anterior branch pa.s.ses along the lateral surface of the tibiotarsus, through a strong retinaculum immediately proximal to the external condyle, and crosses the lateral surface of the joint, where it is covered by connective tissue nearly as tough as, and continuous with, the retinaculum; the tendon attaches broadly to the lateral edge of the ossified tendon of M. flexor perforatus digiti III a short distance below the hypotarsus.

INNERVATION.--The peroneal nerve sends twigs to the deep surface.

INDIVIDUAL VARIATION.--In both legs of two specimens, the extreme proximal end extends proximal to the rotular crest and attaches fleshily to the superficial surface of the distal end of the patellar tendon.

_T. cupido_

INDIVIDUAL VARIATION.--None of significance.

_P. p. jamesi_

INDIVIDUAL VARIATION.--One leg shows the variation described above for _T. pallidicinctus_.

=_M. Tibialis Anticus_=, Figs. 14, 15, 16, 19E, 20N

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Thick; on anterior aspect of thigh deep to M. peroneus longus; bounded posteriorly by M. extensor digitorum longus and posterolaterally by Mm. flexor digitorum longus and peroneus brevis; divided into two heads--tibial and femoral; small femoral head adjacent to posterolateral surface of much larger tibial head; two heads joined near midpoint of fleshy part of muscle, forming bipinnate belly (pinnate structure most evident on deep surface); proximal part of femoral head situated between outer cnemial crest and head of fibula; proximal part of anterior surface of tibial head somewhat fused to overlying M. peroneus longus; medial surface fused to aponeurosis of latter.

ORIGIN.--_Tibial head_: This arises by fleshy and tendinous fibers from the edge of the inner cnemial crest, from the rotular crest between the inner and outer cnemial crests, and from the anterior surface, distal edge, and posterior surface of the outer cnemial crest; the attachment may or may not extend onto the superficial surface of the distal part of the patellar tendon; the attachment is adjacent to the origin of the underlying M. extensor digitorum longus. _Femoral head_: This arises by a slender tendon from the notch in the distal end of the external condyle of the femur.

INSERTION.--The slender ossified tendon extends along the anterior surface of the distal end of the tibiotarsus and pa.s.ses through a large, strong, oblique retinaculum (superficial to the supratendinal bridge); the lateral end of the retinaculum attaches to the lateral end of the supratendinal bridge; the medial end attaches immediately proximal to the medial end of the bridge. The tendon widens and becomes flexible as it pa.s.ses across the anterior surface of the intratarsal joint, then narrows and attaches to the tubercle on the anterior surface of the proximal part of the tarsometatarsus between Mm. extensor hallucis longus and extensor brevis digiti IV. The distalmost bundle of tendinous fibers does not attach to the tubercle, but extends distally along the anterior surface of the tarsometatarsus and attaches to the latter a few mm. distal to the tubercle, forming an accessory insertion.

A part of the peroneal nerve pa.s.ses between the main and accessory insertions.

INNERVATION.--A variable number of branches of the peroneal nerve penetrate the lateral surface of the femoral head; a variable number of branches of the same division pa.s.s deep to the femoral head and enter the posterior edge of the tibial head.

INDIVIDUAL VARIATION.--- In one leg, the accessory insertion is absent.

_T. cupido_

INDIVIDUAL VARIATION.--None of significance.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The origin of the tibial head does not extend onto the patellar tendon.

INDIVIDUAL VARIATION.--The accessory insertion is absent in one leg.

=_M. Extensor Digitorum Longus_=, Figs. 15, 17