_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.]
The =ilium= (_I_) is somewhat contracted at the middle and broader at its ends. One end enters into the acetabulum (Fig. 55, _d_) and forms about one-fifth the articular surface. This end is also the thickest part of the bone. The lateral surface of the ilium is concave for the attachment of muscles. The medial surface is smooth over its acetabular half and rough over its sacral half. The rough portion is marked at its junction with the smooth portion by the ear-shaped =auricular impression= by which the bone articulates directly with the sacrum. The caudal half of that part of the medial surface craniad of the auricular impression gives attachment to the ilio-sacral ligaments which bind the ilium to the sacrum. The dorsal border is straight at its cranial end and concave and rounded at its caudal end. Between the two portions and at the dorsal edge of the auricular surface is a protuberance corresponding to the =posterior inferior spine= (Fig. 55, _b_) of the human ilium. The concavity of the dorsal border (_c_) corresponds to the great sciatic notch of the human ilium. At its caudal end is the short =spine of the ischium= (_e_), which is not a part of the ilium. The ventral border of the ilium is broad caudad, becoming narrower craniad. The lateral margin of the ventral border is continued to a tuberosity at the edge of the acetabulum; its medial margin is called the =iliopectineal line= (_l_) and extends on the pubis to the symphysis. An eminence, the =iliopectineal eminence= (_l′_), on the iliopectineal line, lies opposite to the acetabulum at about the junction of the ilium and pubis. The cranial end of the bone is thickened, forming the =crest= (_a_) of the ilium. At the junction of the crest with the ventral border is a projection, the =anterior superior process= (_m_) of human anatomy.