The bony pelvis or hip girdle consists of two portions, termed coxae or ossa innominata, which, together with the sacrum and the front segments of the tail-bones, form the cavity of the pelvis.

The ossa innominata are flat, irregular bones, which, after forming the sides of the pelvic cavity, join together below to form its floor. In the fetus each innominate bone consists of three pieces, termed the ilium, the ischium, and the pubis. The ilium, the largest of the three, is that portion which unites with the sacrum above, with the acetabulum or femoral joint below, and outwardly forms the "point of the hip". The ischium is that portion which, on leaving the hip-joint, which it assists in forming, passes backward to the point of the buttock, and inward to join its fellow on the opposite side, thus contributing to the floor of the pelvis, and to the formation of a large opening there - the obturator foramen.

The pubis is the smallest of the three bones entering into the formation of the coxa. It is situated in front of the floor of the pelvis, which, together with the foramen just referred to, it concurs in forming. With the ilium and ischium it also joins in making up the acetabulum or cup-like cavity into which the head of the femur fits to form the nip-joint.

The two pubic bones unite on the floor of the pelvis to form a seam, or, as it is termed, the symphysis pubis.

The cavity of the pelvis is much larger at the front or inlet than at the outlet behind. The pelvic cavity of the mare is larger than that of the horse in every direction, but especially from side to side. The floor of the female pelvis is distinctly wider than that of the male animal.

Fig. 295. - Pelvis and Sacrum (Inferior Aspect).

1 Anterior Articular Body of Sacrum. 2 Posterior Spinous Process of Ilium. 3 Crista of Ilium. 4 Anterior Spinous Process of Ilium. 5 Neck of Ilium. 6 Acetabulum. 7 Obturator Foramen. 8 Lateral Ischiatic Notch. 9 Ischiatic Arch. 10 Tuberosity of Ischium. 11 Coccygeal Extremity of Sacrum. 12 Sub-sacral Foramina. 13 Anterior Articular Process of Sacrum.