Myology and Serology of the Avian Family Fringillidae: A Taxonomic Study

by William B. Stallcup

Action.—Flexes thigh; may flex crus also and may extend tarsometatarsus.

Comparison.—In Vireo olivaceous, the origin of this muscle does not extend the length of the ischiopubic fenestra. The origin, furthermore, is along the dorsal edge of the ischiopubic fenestra and not from the membrane covering the fenestra. Finally, in this species, the origin of the pars posticus is fleshy.

Musculus tibialis anticus (Figs.[ 2],[ 5]).—Lying along the anterior edge of the crus, a part of this muscle is covered by the m. peroneus longus. The origin is by two distinct heads, each of which is pinnate. The anterior head arises directly from the edges of the outer and inner cnemial crests. The posterior head arises by a short, strong tendon from a small pit on the anterodistal edge of the external condyle of the femur. This tendon and the proximal end of the muscle pass between the head of the fibula and the outer cnemial crest. The two heads of the muscle fuse at a place slightly more than one-half of the distance down the crus. At the distal end of the crus this muscle gives rise to a strong tendon which passes under a fibrous loop immediately proximal to the external condyle in company with the m. extensor digitorum longus and which passes between the condyles of the tibia and inserts on a tubercle on the anteromedial edge of the proximal end of the tarsometatarsus.

Action.—Flexes tarsometatarsus.

Comparison.—No significant differences noted among the species studied.

Musculus extensor digitorum longus (Figs.[ 3],[ 5], [8]).—Slender and pinnate, this muscle lies along the anteromedial surface of the tibia. The origin is fleshy from most of the region between the cnemial crests and from a line along the anterior surface of the proximal fourth of the tibia. Approximately two-thirds of the distance down the crus the muscle gives rise to the tendon of insertion which passes through the fibrous loop near the distal end of the tibia in company with the m. tibialis anticus. The tendon then passes along beneath the supratendinal bridge at the distal end of the tibia, traverses the anterior intercondylar fossa, and passes beneath a bony bridge on the anteromedial surface of the proximal end of the tarsometatarsus. The tendon continues along the anterior surface of the tarsometatarsus to a point immediately above the bases of the toes and there gives rise to three branches, one to the anterior surface of each foretoe. The insertions of each branch are on the anterior surfaces of the phalanges as shown in Fig.[ 8].

Action.—Extends foretoes.

Comparison.—This muscle is weakly developed in Leucosticte and Calvarius; the belly is slender and extends only half way down the crus before giving rise to the tendon of insertion. The functional significance of this variation is difficult to understand. The convergence in muscle pattern shown by these two genera, however, is in all probability the result of similarities in behavior patterns. These birds perch less frequently than do the other birds studied. Thus, the toes are neither flexed nor extended as often; the smaller size of the m. extensor digitorum longus may have resulted in part from this lessened activity. Except for the variations just noted, there are no significant differences among the species studied; even the rather complex patterns of insertion are identical.

Musculus peroneus longus (Fig.[ 1]).—Relatively thin and straplike, this muscle lies on the anterolateral surface of the crus and is intimately attached to the underlying muscles. The part of the origin from the proximal edges of the inner and outer cnemial crests is semitendinous but the part of the origin from the lateral edge of the shaft of the fibula is tendinous. Approximately two-thirds the distance down the crus the muscle gives rise to the tendon of insertion. Immediately above the external condyle of the tibiotarsus this tendon divides. The posterior branch inserts on the proximal end of the lateral edge of the tibial cartilage. The anterior branch passes over the lateral surface of the external condyle to the posterior surface of the tarsometatarsus and there unites with the tendon of the m. flexor perforatus digiti III.

Action.—Extends tarsometatarsus and flexes third digit.