Phalanges.—Between the phalanges the joints possess capsules, and each has a radial and an ulnar lateral ligament.

VI. BONES OF THE PELVIC EXTREMITIES.

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.

[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).