=Development.=—The tibia has the usual three chief centers of ossification and supplementary ones for the tuberosity and the external malleolus. The latter is really the distal end of the fibula; it is a separate piece at birth, and the line of union is commonly quite evident in the adult in the external articular groove. The proximal end unites with the shaft at about three and a half years, and the distal end at about two years of age. THE FIBULA The =fibula= of the horse is a much reduced long bone, situated along the outer side of the tibia. The =shaft= or =body= (Corpus fibulæ) is a slender rod which forms the outer boundary of the interosseous space; it usually terminates below in a pointed end about one-half to two-thirds of the way down the external border of the tibia. The =proximal extremity= or =head= (Capitulum fibulæ) is relatively large, and is flattened transversely. Its internal surface presents a narrow area along the upper border for articulation with the tibia. The external surface is rough and gives attachment to the external lateral ligament of the stifle joint. It has rounded anterior and posterior borders. [Illustration: FIG. 71.—PROXIMAL EXTREMITY OF RIGHT TIBIA OF HORSE, END VIEW. _l.c.a._, _l.c.p._, Depressions for attachment of anterior and posterior crucial ligaments; _l_, _m_, _m_, depressions for attachment of semilunar cartilages. ] The =distal extremity= is fused with the tibia, constituting the external malleolus. =Development.=—This resembles that of the ulna. The embryonic cartilaginous fibula extends the entire length of the leg, but does not articulate with the femur. The lower part of the shaft is usually reduced to a fibrous band. Three centers of ossification appear, one each for the shaft and the extremities. The distal end unites early with the tibia, forming the external malleolus. It is interesting to note that in some cases the entire shaft of the fibula develops, a reversion to the condition in the Miocene ancestors of the present horse. THE PATELLA The =patella= is a large sesamoid bone which articulates with the trochlea of the femur. It presents for description two surfaces, two borders, a base, and an apex. The =anterior= or =free surface= (Facies libera) is irregularly quadrilateral, convex, and rough for muscular and ligamentous attachment. The =posterior= or =articular surface= is smaller and is triangular in outline. It presents a vertical rounded ridge, which corresponds to the groove on the trochlea of the femur, and separates two concave areas. Of the latter, the inner cavity is much the larger, and is not very well adapted to the inner lip of the trochlea; in the fresh state, however, it is completed and rendered more congruent by the curved =accessory fibro-cartilage=. [Illustration: FIG. 72.—FRONTAL SECTION OF RIGHT TIBIA OF HORSE, ANTERIOR VIEW. ]