The =ischium= (_II_) has the form of a triangular prism contracted at the middle. Its cranial end forms nearly two-thirds of the acetabulum. Its caudal end bears dorsad a rough thickening, the =tuberosity= of the ischium (_g_). From the caudal half of the ventral border of the bone a sickle-shaped process, the =ramus= (_h_) of the ischium, curves medioventrad and then craniad and joins the ramus of the pubis. Its medial border is rough and enters into the symphysis, forming the caudal one-third. The lateral angle of the bone is rounded. Its dorsal angle is marked near the cranial end by the =spine= (_e_) of the ischium. The concavity between this spine and the tuberosity corresponds to the =lesser sciatic notch= (_f_) of human anatomy. The acetabulum (_d_) is cup-shaped. The ventral one-sixth of its border is deficient and a broad groove extends from the deficiency to the bottom of the cup. The deficiency, =incisura acetabuli=, or =acetabular notch= (_d′_), is closed naturally by a ligament, and the groove gives origin to the ligament (ligamentum teres) which attaches the head of the femur. [Illustration: FIG. 56.--LEFT FEMUR, VENTRAL SIDE. _a_, head; _b_, neck; _c_, depression for round ligament; _d_, great trochanter; _e_, trochanteric fossa; _f_, intertrochanteric line; _g_, lesser trochanter; _h_, linea aspera; _i_, medial condyle; _j_, lateral condyle; _k_, intercondyloid fossa; _l_, lateral epicondyle.] =Femur= (Fig. 56).--The femur is the proximal bone of the posterior extremity. It consists of enlarged proximal and distal ends connected by a nearly cylindrical shaft. The proximal end presents on its medial side a hemispherical =head= (_a_) which fits into the acetabulum. It is supported by a =neck= (_b_) which is contracted near the head and expanded dorsoventrally where it joins the remainder of the bone. The medial surface of the head presents near its ventral border a depression (_c_) for the insertion of the round ligament of the femur. Ventrad the articular surface of the head extends as an acute projection onto the shaft, so that the whole articular surface appears somewhat pear-shaped. On the lateral side of the proximal end opposite the head is a projecting mass, the =great trochanter= (_d_), forming the end of the shaft. On the medial side of the great trochanter at its junction with the neck is a deep fossa, the =trochanteric= fossa or =digital= fossa (_e_) for the insertion of muscles. From the ventral surface of the great trochanter a ridge, the =intertrochanteric= line (_f_), is continued distad, ending in a pyramidal projection, the =lesser trochanter= (_g_), which serves for the insertion of muscles. A second ridge is continued to the lesser trochanter from the neck. A slight but well-marked ridge, the =spiral= ridge or line, runs round two sides of the neck parallel to the second ridge.