=Innominate Bones. Ossa innominata.= (=Os coxæ BNA=) (Figs. 54 and 55).--The two innominate bones articulate with the sacrum and extend thence caudoventrad and finally turn mediad and unite in the middle line, forming the =symphysis pubis=. They thus form an arch, =the pelvic arch=, =pelvic girdle= or =pelvis=, which is closed dorsad by the sacrum. In the middle of the lateral surface of each bone is a hemispherical depression, the =acetabulum= (Fig. 55, _d_), which receives the head of the femur. [Illustration: FIG. 54.--INNOMINATE BONE OF KITTEN, VENTROLATERAL VIEW. _I_, ilium; _II_, ischium; _III_, pubis; _IV_, acetabular bone. _a_, acetabulum; _b_, obturator foramen.] In the kitten each innominate bone is composed of three principal parts united by sutures (Fig. 54). From the sacrum to the acetabulum is a single bar, the =ilium= (_I_). Caudad of the acetabulum are two bars. The dorsal one of these is the =ischium= (_II_), and the ventral one is the =pubis= (_III_). The ischium enters into the formation of the acetabulum (_a_), but the pubis does not. The two bones, however, are in contact at the ventral edge of the acetabulum. From this point they diverge, but unite with one another again near the middle line and thus enclose an oval foramen, the =obturator foramen= (_b_). Wedged between the ilium, ischium, and pubis at their point of junction and helping to form the acetabulum is a small irregular bone, the =acetabular bone= (_IV_). In the adult cat these four parts are united into a single bone which is nevertheless usually described, ignoring the acetabular piece, as made up of ilium (Fig. 55, _I_), ischium (_II_), and pubis (_III_). [Illustration: FIG. 55.--INNOMINATE BONE OF ADULT CAT, VENTROLATERAL VIEW. _I_, ilium; _II_, ischium; _III_, pubis. _a_, crest of the ilium; _b_, posterior inferior spine; _c_, great sciatic notch; _d_, acetabulum; _d′_, incisura acetabuli; _e_, spine of the ischium; _f_, lesser sciatic notch; _g_, tuberosity of the ischium; _h_, ramus of the ischium; _i_, ramus of the pubis; _j_, obturator foramen; _k_, pubic tubercle; _l_, iliopectineal line; _l′_, ilio-pectineal eminence; _m_, anterior superior process.]