Part 13 (2/2)

ORIGIN.--_Proximal head_: This arises fles.h.i.+ly from the anterior surface of approximately the proximal half of the tarsometatarsus medial to the anterior metatarsal groove; the proximal end is partly medial to and partly deep to the retinaculum for M. extensor digitorum longus; some fibers arise from the extreme distal edge of the main insertion of M.

tibialis anticus; the distal end of the belly is unattached. _Distal head_: This arises fles.h.i.+ly from the medial surface of the tarsometatarsus proximal to the first metatarsal and deep to the tendon of the proximal head.

INSERTION.--The slender tendon of the proximal head, which begins along the medial edge of the distal part of the belly, soon fuses with the superficial surface of the distal head (ossified here); the common tendon (unossified) pa.s.ses onto the dorsal (proximal) surface of the first metatarsal, where it pa.s.ses through a retinaculum, then pa.s.ses along the dorsal surface of the hallux (bound by strong connective tissue to the metatarsophalangeal joint), attaching to the dorsal surface of the ungual phalanx.

INNERVATION.--The branch of the deep peroneal nerve that pa.s.ses medial to the main insertion of M. tibialis anticus gives one or two twigs into the proximal part of the proximal head. No supply to the distal head was found, but see below.

INDIVIDUAL VARIATION.--In one leg, the proximal end of the distal head is fused to the distal end of the belly of the proximal head, whereas in three legs, a distinct gap separates the fleshy parts of the two heads.

The following variations, each found in one leg, pertain to the relations.h.i.+p of the origin of the proximal head to the retinaculum for M. extensor digitorum longus: the origin does not extend proximally medial to the retinaculum; the origin does not extend proximally deep to this retinaculum; a part of the proximal end extends proximally lateral to this retinaculum (in this instance there is an unusually wide gap between the retinaculum and the insertion of M. tibialis anticus). In one leg, the distalmost fibers of the distal head do not join the common tendon but insert independently on the articular capsule of the metatarsophalangeal joint (deep to the common tendon).

_T. cupido_

INDIVIDUAL VARIATION.--The relations.h.i.+p between the two heads varies as follows: the proximal end of the distal head may be fused to the distal end of the belly of the proximal head; the proximal end of the distal head may begin anterior (adjacent) to the distal end of the belly of the proximal head; there may be a distinct gap between the fleshy parts of the two heads. In two legs, there is no origin from the insertion of M.

tibialis anticus. In one leg, a small accessory bundle of fleshy fibers arises from the proximal end of the first metatarsal (widely separated from the origin of the distal head), pa.s.ses through the retinaculum deep to the common tendon and attaches to the dorsal surface of the articular capsule of the metatarsophalangeal joint; thus this bundle is completely separate from the remainder of the muscle. In two legs, the same nerve branch that gives twigs into the proximal head also gives off (much farther distally) a twig that enters the distal head.

_P. p. jamesi_

INDIVIDUAL VARIATION.--The proximal end of the distal head may begin anterior (adjacent) to the distal end of the belly of the proximal head.

In four legs, the origin of the proximal head does not extend proximally medial to the retinaculum for M. extensor digitorum longus; in one of these legs, a part of the proximal end extends proximally lateral to this retinaculum. The distalmost fibers of the distal head do not join the common tendon but insert independently on the dorsal surface of the articular capsule of the metatarsophalangeal joint in four legs; in another leg, the entire distal head has the latter insertion (consequently the two heads are completely separate).

=_M. Abductor Digiti II_= Figs. 19E, 20N

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Short; on medial surface of distal part of tarsometatarsus; proximal end adjacent (anterior) to distal head of M. extensor hallucis longus.

ORIGIN.--The origin is fleshy from the medial surface of the distal part of the tarsometatarsus anterior (adjacent) to the first metatarsal and from the anteromedial surface of the basal half of the first metatarsal.

INSERTION.--The flat tendon pa.s.ses over the medial surface of the trochlea for digit II and attaches to the medial surface of the proximal end of the first phalanx of digit II; the tendon is fused with the articular capsule.

INNERVATION.--The compound nerve formed by the fusion of a branch of the superficial peroneal nerve with a branch of the deep peroneal nerve gives a twig to the anterolateral edge of the muscle.

INDIVIDUAL VARIATION.--In some cases, the twig arises from the deep peroneal branch alone (which is not joined by the superficial peroneal nerve).

_T. cupido_

INDIVIDUAL VARIATION.--In one leg, some of the fleshy fibers arising from the first metatarsal insert independently on the medial surface of the trochlea for digit II (deep to the main part of the muscle).

_P. p. jamesi_

INDIVIDUAL VARIATION.--None of significance.

=_M. Extensor Brevis Digiti III_= (M. extensor proprius digiti III), Figs. 19E, 20N

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