Part 4

Action.--Flexes crus.

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

+Musculus biceps femoris+ (Fig. 2).--Long, thin, and somewhat triangular, this muscle lies on the lateral side of the thigh just underneath the m. iliotibialis. Its origin is from a line along the anterior and posterior iliac crests underneath the origin of the m. iliotibialis. Anterior to the acetabulum the origin is aponeurotic, and the edge of this aponeurosis passes over the proximal end of the femur. The origin posterior to the acetabulum is fleshy. The most anterior point of origin is difficult to ascertain but it lies near the center of the anterior iliac crest. The most posterior point of origin is immediately dorsal to the posterior end of the ilioischiatic fenestra. Behind the knee the fibers of this muscle converge to form the strong tendon of insertion which passes through the biceps loop, under the tendon of origin of the m. flexor perforatus digiti II, and inserts on a small tubercle on the posterolateral edge of the fibula at the point of the tibia-fibula fusion.

The biceps loop is tendinous and the distal end attaches to a protuberance on the posterolateral edge of the femur at the proximal edge of the external condyle. The proximal end attaches to the anterolateral edge of the femur immediately proximal to the distal end of the loop, which extends posterior to the femur. The distal arm of this loop is connected with the tendon of origin of the m. flexor perforatus digiti II by a strong tendon.

Action.--Flexes crus.

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

+Musculus ischiofemoralis+ (Fig. 3).--Short and thick, this muscle arises directly from the lateral surface of the ischium between the posterior iliac crest and the ischiopubic fenestra. The area of origin extends to the posterior edge of the ischium. The insertion is tendinous on the lateral surface of the trochanter opposite the insertion of the m. iliotrochantericus medius.

Action.--Moves femur posteriorly and rotates it in this direction.

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

+Musculus obturator internus+ (Figs. 4, 7).--Lying on the inside of the pelvis and covering the medial surface of the ischiopubic fenestra, is this flat, pinnate, leaf-shaped muscle. The origin is fleshy and is from the ischium and pubis around the edges of this fenestra; none of the fibers arises from the membrane stretched across the fenestra. Anteriorly the fibers converge and form a strong tendon that passes through the obturator foramen and inserts on the posterolateral surface of the trochanter of the femur.

Action.--Rotates femur posteriorly.

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

+Musculus obturator externus+ (Fig. 7).--Short and fleshy, this muscle consists of two parts which are not easily separable but which may be traced throughout its length. The parts are more nearly distinct at the origin. The dorsal part arises directly from the ischium along the dorsal edge of the obturator foramen. The larger ventral part arises directly from the anterior and ventral edges of the obturator foramen. The fibers of the dorsal part pass anteriorly, cover the tendon of the m. obturator internus laterally, and insert on the trochanter around the point of insertion of the latter muscle. The fibers of the ventral part pass parallel with the tendon of the m. obturator internus and insert on the trochanter immediately distal and posterior to the tendon of the latter muscle.

Action.--Rotates femur posteriorly.

Comparison.--In Passer, Estrilda, Poephila, Hesperiphona, Carpodacus, Pinicola, Leucosticte, Spinus and Loxia, this muscle is undivided and, in its position, origin, and insertion, resembles the ventral part of the bipartite muscle described above. The origin is from the anterior and ventral edges of the obturator foramen and the insertion is on the trochanter of the femur immediately distal and posterior to the insertion of the m. obturator internus. In all other genera examined, the muscle is bipartite. In Chlorura the dorsal part is larger and better developed than it is in the other genera.

+Musculus adductor longus et brevis+ (Figs. 3, 4, 5).--Consisting of two distinct, straplike parts, this large muscle lies on the medial surface of the thigh, posterior to the femur.

The pars anticus has a semitendinous origin on a line that extends posteriorly from the posteroventral edge of the obturator foramen to a point half way across the membrane that covers the ischiopubic fenestra. The insertion is fleshy along the posterior surface of the femur from the level of the insertion of the m. piriformis distally to the medial surface of the internal condyle.

The pars posticus originates by a broad, flat tendon on a line across the posterior half of the membrane that covers the ischiopubic fenestra. The insertion is at the point of origin of the pars media of the m. gastrocnemius on the posteromedial surface of the proximal end of the internal condyle of the femur. There is a broad tendinous connection with the proximal end of the pars media of the m. gastrocnemius. The anterior edge of the pars posticus is overlapped medially by the posterior edge of the pars anticus.

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.