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. [Illustration: FIG. 59.—THIRD SESAMOID OR NAVICULAR BONE OF HORSE. (After Schmaltz, Atlas d. Anat. d. Pferdes.) ] =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. OS COXÆ The =os coxæ= (or os innominatum) forms the skeleton of the hip or haunch, and is the largest of the flat bones. It consists primarily of three parts, the =ilium=, =ischium=, and =pubis=, which meet to form the =acetabulum=, a large cotyloid cavity for articulation with the head of the femur. These parts are fused at about one year of age, but it is convenient to describe them separately. THE ILIUM The =ilium= (Os ilium) is the largest of the three parts. It is irregularly triangular and presents two surfaces, three borders, and three angles. The =gluteal surface= (Facies glutæa) faces upward, backward, and outward. It is wide and concave in front, narrower and convex behind. The wide part is crossed by the curved =gluteal line= (Linea glutæa), which extends from the middle of the inner border toward the external angle. This surface gives attachment to the middle and deep gluteal muscles. [Illustration: FIG. 60.—OSSA COXARUM OF MARE, DORSAL VIEW.