_+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.