The Sesamoid Bones

The two proximal or great sesamoids (Ossa sesamoidea phalangis primæ) are situated behind the distal end of the large metacarpal bone, and are closely attached to the first phalanx by strong ligaments. Each has the form of a three-sided pyramid. The anterior or articular surface conforms to the corresponding part of the distal end of the large metacarpal bone. The posterior or flexor surface is flattened and oblique; in the fresh state it is covered by a layer of cartilage which also fills the interval between the opposed borders of the two bones, and forms a smooth groove for the deep flexor tendon. The abaxial surface is concave, and gives attachment to part of the suspensory ligament; it is separated from the posterior surface by a rough everted border. The base faces downward, and furnishes attachment to the inferior sesamoidean ligaments. The apex is directed upward and is rounded.

The third sesamoid or navicular bone (Os sesamoideum phalangis tertiæ) is shuttle-shaped, and is situated behind the junction of the second and third phalanges. Its long axis is transverse, and it possesses two surfaces, two borders, and two extremities. The articular surface (Facies articularis) faces upward and forward; it consists of a central eminence, flanked by concave areas, and articulates with the distal end of the second phalanx. The flexor or tendon surface (Facies flexoria) is directed downward and backward. It resembles the articular surface in form, but is more extensive and not so smooth. In the fresh state it is coated with cartilage and the deep flexor tendon plays over it. The proximal border (Margo liber) is wide and grooved in its middle, narrower and rounded on either side. The distal border (Margo ligamenti) bears in front a narrow facet for articulation with the third phalanx. Behind this is a groove, which contains a number of relatively large foramina, and is bounded behind by a prominent edge. The extremities are blunt-pointed.

Development.—It ossifies from a single center.

The Bones of the Pelvic Limb

The pelvic girdle consists of the ossa coxæ, which unite ventrally at the symphysis pelvis, and articulate with the sacrum dorsally.