Variation in the Muscles and Nerves of the Leg in Two Genera of Grouse Part 13

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Bipinnate; on anterior surface of tibiotarsus deep to M. tibialis anticus; bounded laterally by M.

peroneus brevis; lateral edge usually slightly fused to proximal half of latter; medial surface fused to aponeurosis of M. peroneus longus.

ORIGIN.--The muscle arises fleshily from the lateral surface of the inner cnemial crest, from the rotular crest between the cnemial crests (deep to the attachment of M. tibialis anticus), from the basal (medial) half of the anterior surface of the outer cnemial crest, and from the anterior surface of the tibiotarsus (except the distal part) between the anteromedial and anterolateral intermuscular lines; proximal to the anterolateral intermuscular line, the origin usually extends almost to the lateral edge of the tibiotarsus.

INSERTION.--The ossified tendon extends along the mid-anterior surface of the distal part of the tibiotarsus deep to the tendon of M. tibialis anticus and pa.s.ses under the supratendinal bridge, becoming flexible and widening slightly as it crosses the anterior surface of the intratarsal joint; the tendon narrows again and pa.s.ses through a small but strong retinaculum on the anterior surface (medial to midline) of the proximal part of the tarsometatarsus; the retinaculum is immediately proximal and medial to the insertion of M. tibialis anticus. The tendon ossifies again as it pa.s.ses down the anterior surface of the tarsometatarsus and bifurcates near the midpoint of the latter; the lateral branch soon bifurcates again; of these three branches, which are ossified for some distance, the lateral one pa.s.ses onto the dorsal surface of digit IV, the middle one pa.s.ses onto the dorsolateral surface of digit III, and the medial one subdivides (at the level of the trochleae) into three branches--one pa.s.sing onto the dorsal surface of digit III and two pa.s.sing onto the dorsal surface of digit II. At the level of the metatarsophalangeal joints, all of these tendons are interconnected by strong sheets of connective tissue and it is often difficult exactly to delimit the tendons at this level. On the digits, tough connective tissue binds the tendons to the phalanges; this is most p.r.o.nounced at the interphalangeal joints. The tendons are distinct on the first phalanx of each digit, but are often poorly defined farther distally. On _digit IV_ the tendon subdivides into branches that attach to the proximal ends of the ungual, fourth, third, and (usually) second phalanges. On _digit III_ the lateralmost tendon bifurcates, with one branch attaching to the ungual phalanx and the other to the proximal end of the third phalanx; the medial tendon attaches to the proximal end of the second phalanx. On _digit II_ the originally medial tendon pa.s.ses underneath and then lateral to the other tendon and attaches to the ungual phalanx; the other tendon attaches to the proximal end of the second phalanx.

INNERVATION.--One or more branches of the peroneal nerve enter the lateral edge.

INDIVIDUAL VARIATION.--In four legs, the lateral branch of the trifurcated tendon is not ossified at all.

_T. cupido_

INDIVIDUAL VARIATION.--In a few cases, the muscle does not come in contact with M. peroneus brevis.

_P. p. jamesi_

DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The belly is shorter. The lateral branch of the tendon on the tarsometatarsus is not ossified (true also of some legs of _Tympanuchus_).

INDIVIDUAL VARIATION.--In several legs, the muscle also arises from the distal part of the posterior surface of the outer cnemial crest.

=_M. Peroneus Brevis_=, Figs. 14, 16, 17, 18, 19A

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Small; on lateral surface of distal part of tibiotarsus; mainly anterior to fibula; bounded posteriorly and laterally by M. flexor digitorum longus (fused with latter), anteriorly by M. tibialis anticus, and anteromedially by M. extensor digitorum longus (usually slightly fused to latter).

ORIGIN.--The muscle arises by fleshy and tendinous fibers from the medial and anterior surfaces of the fibula beginning a short distance below the distal end of the fibular crest and from the anterolateral surface of the tibiotarsus anterior to the fibula; the anteromedial edge attaches to the anterolateral intermuscular line.

INSERTION.--The short, slender, ossified tendon pa.s.ses along the anterolateral surface of the tibiotarsus and through a retinaculum immediately proximal and anteromedial to the retinaculum for the anterior branch of the tendon of M. peroneus longus; the tendon becomes flexible and widens as it pa.s.ses across the lateral surface of the intratarsal joint deep to the tendon of M. peroneus longus, turning posteriorly and attaching to the proximolateral corner of the hypotarsus.

INNERVATION.--The superficial peroneal branch of the peroneal nerve gives one or two twigs to the anterior surface of the proximal part.

INDIVIDUAL VARIATION.--None of significance.

_T. cupido_

INDIVIDUAL VARIATION.--In a few legs, the muscle does not come in contact with M. extensor digitorum longus.

_P. p. jamesi_

INDIVIDUAL VARIATION.--None of significance.

=_M. Extensor Hallucis Longus_=, Figs. 19E, 20N

_T. pallidicinctus_

GENERAL DESCRIPTION AND RELATIONS.--Slender and elongate; proximal part on anterior surface of tarsometatarsus medial to anterior metatarsal groove; near midlength of tarsometatarsus, muscle twisted onto medial surface of latter; divisible into two heads--proximal and distal; belly of proximal head (largest) ending at level of twisting onto medial surface of bone; short distal head beginning at this point deep to tendon of proximal head and soon joining latter tendon.

ORIGIN.--_Proximal head_: This arises fleshily 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 fleshily 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 relationship 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 relationship 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