The lower leg, like the fore-arm, has two bones, which, however, are always parallel, never crossed, and have no power of rotation. Of these, the inner one is the shin-bone, or _tibia_, which is always the larger and alone enters into the knee-joint. The external bone is the _fibula_, which is almost entirely suppressed in certain highly specialized forms, such as the horses and ruminants, the tibia carrying the whole weight. The upper end of the tibia is enlarged and extends over that of the fibula; it has two slightly concave surfaces for articulation with the condyles of the femur, the approximate edges of which are raised into a bifid _spine_. The upper part of the shaft is triangular, with one edge directed forward, and the superior end of this edge is roughened and thickened to form the _cnemial crest_, to which is attached the patellar ligament. The middle portion of the shaft is rounded and the lower end broad and usually divided by a ridge into two grooves or concavities for the ankle-bone; from the inner side of this end projects downward a tongue-like process, the _internal malleolus_, which prevents inward dislocation of the ankle. The fibula is relatively stoutest in the less advanced mammals and is usually straight and slender, with enlarged ends, the lower one forming the _external malleolus_, which serves to prevent outward dislocation of the ankle. In many forms the fibula is coössified with the tibia at both ends, and in the most highly specialized hoofed animals, such as the horses, camels and true ruminants, the bone has apparently disappeared. The young animal, however, shows that the ends of the fibula have been retained and the shaft completely lost; the upper end is in the adult firmly fused with the tibia and, in the horses, the lower end is also, but this remains separate in the ruminants and camels, forming the _malleolar bone_, which is wedged in between the tibia and the heel-bone. Because of its importance in holding the ankle-bone in place, this lower end of the fibula is never lost in any land mammal. [Illustration: FIG. 37.—Bones of left lower leg of Wolf, front side. _T._, tibia. _F._, fibula. _sp._ spine of tibia. _cn._ cnemial crest. _i.m._, internal malleolus. _e.m._, external malleolus.] [Illustration: FIG. 38.—Bones of left lower leg of Horse (much more reduced). _cn._ cnemial crest. _F._, lower end of fibula, coössified with tibia. Other letters as in Fig. 37.] [Illustration: FIG. 39.—Bones of lower leg, left side, of Tapir. _T._, tibia. _F._, fibula. _sp._, spine of tibia. _cn._, cnemial crest. _i.m._, internal malleolus. _e.m._, external malleolus. N.B. This figure is on a much larger scale than Fig. 38.]