Each half is firmly united to its fellow in a ventral symphysis behind, and is in front expanded and united to the sacrum. Each half or innominate bone is seen in the young animal to consist of four distinct parts, the ilium or dorsal element, the pubis or anterior ventral element, the ischium or posterior ventral element, and a small fourth part, the acetabular or cotyloid bone, wedged in between the three others. These parts, though all distinct in the young animal, are in the adult so completely fused that their respective boundaries cannot be distinguished. At about the middle of the outer surface of the innominate bone is a very deep cavity, the acetabulum (fig. 78, A, 1) with which the head of the femur articulates; all the bones except the pubis take part in its formation.

The ilium is a rather long bone, expanded in front and contracted behind; it forms about half the acetabulum. On its inner or sacral surface (fig. 78, 4) is a large roughened patch for articulation with the sacrum; its outer or gluteal surface is concave. The posterior part of the bone is flattened below, forming the narrow iliac surface (fig. 78, A, 5).

Fig. 78. Right innominate bone, A, of a full-grown Terrier, B, of

a Collie Puppy. × 1.

A is seen from the ventral side, B from the inner or sacral side.

1. acetabulum.8. ischial border.
2. thyroid foramen.9. ischium.
3. supra-iliac border of ilium.10. tuberosity of ischium.
4. sacral surface.11. ischial symphysis.
5. iliac surface.12. pubis.
6. acetabular border.13. pubic symphysis.
7. pubic border.14. cotyloid or acetabular bone.

The ischium (fig. 78, 9) is a wide flattened bone forming the posterior part of the innominate bone. It meets the pubis ventrally, but is separated from it for the greater part of its length by the large obturator or thyroid foramen (fig. 78, 2). At its posterior end externally it bears a rather prominent roughened ischial tuberosity (fig. 78, A, 10). The ischium meets its fellow in a ventral symphysis, and forms about one-third of the acetabulum.

Fig. 79. Front view of the left leg bones of a Dog