Part 12 (1/2)

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 fles.h.i.+ly 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 fles.h.i.+ly (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 fles.h.i.+ly 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 fles.h.i.+ly 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