=M. semitendinosus= (Fig. 90, _j_).--A long slender muscle on the ventral (caudal) border of the thigh, between the semimembranosus (_i_) and the biceps femoris (Fig. 68, _t_). _Origin_ from the apex of the tuberosity of the ischium beneath the origin of the biceps femoris. The muscle passes to the medial side of the shank and ends in a thin but strong tendon (Fig. 92, _i′_) about five millimeters broad. The tendon curves proximad and passes beneath the gracilis tendon (Fig. 91, _b_) to its _Insertion_ into the crest (dorsal border) of the tibia one or two centimeters from its proximal end. _Relations._--Lateral surface with the biceps femoris (Fig. 68, _t_), the integument, a mass of fat, and distad with the gastrocnemius (Fig. 92, _j_) and popliteus (Fig. 92, _k_). Caudal surface with the integument. Medial surface with the semimembranosus (Fig. 90, _i_) and distad with the integument of the lower leg. _Action._--Flexor of the shank. =M. semimembranosus= (Figs. 90 and 91, _i_; Fig. 92, _h_).--A thick prismatic muscle lying along the ventral (or caudal) side of the thigh between the semitendinosus (_j_) and the gracilis (Fig. 91, _b_). _Origin_ by short tendon-fibres from the caudal border of the tuberosity and the ramus of the ischium. The muscle mass is divided throughout most of its length into two portions (Fig. 92, _h_ and _h′_), one of which (_h′_) lies laterad and caudad of the other. The two portions pass to the medial side of the knee. The caudal portion (_h′_) ends in a strong flat tendon five millimeters broad, the _Insertion_ of which is into the medial surface of the femur on the medial epicondyle, at the middle of its distal border and into the adjacent medial surface of the tibia behind the lateral ligament. The dorsal part has its _insertion_ into the distal one and one-half to two centimeters of the ridge which is continued from the medial epicondyle of the femur onto the shaft; and into the sesamoid bone of the medial epicondyle. _Relations._--Lateral surface with the semitendinosus (Fig. 90, _j_), the biceps femoris (Fig. 68, _t_), and distad with a small part of the adductor femoris (Fig. 92, _g_), and with the gastrocnemius (Fig. 92, _j_). Medial surface with the gracilis (Fig. 91, _b_) and distad with the sartorius (Fig. 91, _a_). Cranial (or dorsal) border at the origin with the quadratus femoris (Fig. 90, _f_); for the remainder of its length with the adductor femoris (Fig. 90, _h_). Caudal border with the integument proximad. _Action._--Extensor of the thigh. [Illustration: FIG. 91.--MUSCLES ON THE MEDIAL SIDE OF THE LEG.