The =pollex= (fig. 80, A, I ) is far shorter than the other digits, and normally does not touch the ground in walking. It has only two phalanges, while each of the other digits has three. A pair of small sesamoid bones are developed on the ventral or flexor side of the metacarpo-phalangeal articulations of all the digits except the pollex. Frequently similar sesamoid bones occur also on the dorsal side of the phalangeal articulations. THE PELVIC GIRDLE. The =pelvic girdle= consists of two halves, which lie nearly parallel to the vertebral column. 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). [Illustration 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. 2. thyroid foramen. 3. supra-iliac border of ilium. 4. sacral surface. 5. iliac surface. 6. acetabular border. 7. pubic border. 8. ischial border. 9. ischium. 10. tuberosity of ischium. 11. ischial symphysis. 12. pubis. 13. pubic symphysis. 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. [Illustration FIG. 79. FRONT VIEW OF THE LEFT LEG BONES OF A DOG (_Canis familiaris_) × 1/2. A femur, B tibia, C fibula, D patella.