Part 3

Action.--Moves femur forward and rotates it anteriorly.

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

+Musculus iliacus+ (Figs. 4, 5).--Arising from a fleshy origin on the ventral edge of the ilium just posterior to the origin of the m. iliotrochantericus medius, this small slender muscle passes posteroventrally to its fleshy insertion on the posteromedial surface of the femur just proximal to the origin of the m. femorotibialis internus.

Action.--Moves femur forward and rotates it posteriorly.

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

+Musculus sartorius+ (Figs. 1, 4).--A long, straplike muscle, the sartorius forms the anterior edge of the thigh. The origin is fleshy, half from the anterior edge of the ilium and from the median dorsal ridge of this bone and half from the posterior one or two free dorsal vertebrae. The insertion is fleshy along a narrow line on the anteromedial edge of the head of the tibia and on the medial region of the patellar tendon.

Action.--Moves thigh forward and upward and extends shank.

Comparison.--In Loxia and Spinus, only one-third of the origin is from the last free dorsal vertebra. In Hesperiphona, Carpodacus, Pinicola, and Leucosticte, only one-fifth of the origin is from this vertebra.

+Musculus iliotibialis+ (Fig. 1).--Broad and triangular, this muscle covers most of the deeper muscles of the lateral aspect of the thigh. The middle region is fused with the underlying femorotibialis muscles. In the distal half of this muscle there are three distinct parts; the anterior and posterior edges are fleshy and the central part is aponeurotic. The origin is from a narrow line along the iliac crests--from the origin of the m. sartorius, anteriorly, to the origin of the m. semitendinosus posteriorly. The origin is aponeurotic in the preacetabular region but fleshy in the postacetabular region. The distal part of the muscle is aponeurotic and joins with the femorotibialis muscles in the formation of the patellar tendon. This tendon incloses the patella and inserts on a line along the proximal edges of the cnemial crests of the tibiotarsus.

Action.--Extends crus.

Comparison.--In Vireo the central aponeurotic portion of this muscle is absent.

+Musculus femorotibialis externus+ (Fig. 2).--Covering the lateral and anterolateral surfaces of the femur, this large muscle has a fleshy origin from the lateral edge of the proximal three-fourths of the femur. The origin separates the insertion of the m. iliotrochantericus anticus from that of the m. ischiofemoralis and, in turn, is separated from the origin of the m. femorotibialis medius by the insertions of the m. iliotrochantericus anticus and m. iliotrochantericus medius. Approximately midway of the length of the femur this muscle fuses anteromesially with the m. femorotibialis medius. Distally, the m. femorotibialis externus contributes to the formation of the patellar tendon which inserts on a line along the proximal edges of the cnemial crests of the tibiotarsus.

Action.--Extends crus.

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

+Musculus femorotibialis medius+ (Figs. 2, 4).--The origin of this muscle, which lies along the anterior edge of the femur, is fleshy from the entire length of the femur proximal to the level of attachment of the proximal arm of the biceps loop. Laterally this muscle is completely fused for most of its length with the m. femorotibialis externus and contributes to the formation of the patellar tendon, which inserts on a line along the proximal edges of the cnemial crests of the tibiotarsus. Many of the fibers, nevertheless, insert on the proximal edge of the patella.

Action.--Extends crus.

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

+Musculus femorotibialis internus+ (Fig. 4).--One of the most superficial muscles lying on the medial surface of the thigh, this muscle is divided, especially near the distal end, into two parts, lateral and medial. The origin of the lateral part is fleshy from a line on the medial surface of the femur; the origin begins proximally at a point near the insertion of the m. iliacus. The medial, bulkier part of the muscle has a fleshy origin on the medial surface of the lower one-third of the femur. The two parts fuse to some extent above the points of insertion and insert on the medial edge of the head of the tibia.

Action.--Rotates tibia anteriorly.

Comparison.--Two parts of this muscle variously fused; otherwise, no significant differences in the species studied.

+Musculus piriformis+ (Fig. 3).--This muscle is represented by the pars caudifemoralis only, the pars iliofemoralis being absent in passerine birds as far as is known. The pars caudifemoralis is flat, somewhat spindle-shaped, and passes anteroventrally from the pygostyle to the femur. The origin is tendinous from the anteroventral edge of the pygostyle, and the insertion is semitendinous on the posterolateral surface of the shaft of the femur about one-fourth its length from the proximal end.

Action.--Moves femur posteriorly and rotates it in this direction; moves tail laterally and depresses it.

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

+Musculus semitendinosus+ (Figs. 2, 3, 5).--The origin from the extreme posterior edge of the posterior iliac crest of the ilium is fleshy and is aponeurotic from the last vertebra of the synsacrum and the transverse processes of several caudal vertebrae. The straplike belly passes along the posterolateral margin of the thigh. Immediately posterior to the knee, the muscle is divided transversely by a ligament. That portion passing anteriorly from the ligament is the m. accessorius semitendinosi (here considered a part of the m. semitendinosus) and is discussed below. The ligament continues distally in two parts; one part inserts on the medial surface of the pars media of the m. gastrocnemius and the other part fuses with the tendon of insertion of the m. semimembranosus.

The m. accessorius semitendinosi extends anteriorly from the above mentioned ligament to a fleshy insertion on the posterolateral surface of the femur immediately proximal to the condyles.

Action.--Moves femur posteriorly, flexes the crus and aids in extending the tarsometatarsus.

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

+Musculus semimembranosus+ (Figs. 3, 4, 5).--This straplike muscle passes along the posteromedial surface of the thigh. The origin is semitendinous along a line on the ischium, from a point dorsal to the middle of the ischiopubic fenestra to the posterior end of the ischium, and from a small area of the abdominal musculature posterior to the ischium. The insertion is by means of a broad, thin tendon on a ridge on the medial surface of the tibia immediately distal to the head of this bone. The tendon of insertion passes between the head of the pars media and pars interna of the m. gastrocnemius and is fused with the tendon of the m. semitendinosus.