86 LAND MAMMALS IN THE WESTERN HEMISPHERE 



on the posterior side two large, rounded prominences, the 

 condyles, which articulate with the shin-bone to form the 

 knee-joint. On the anterior side is a broad, shal- 

 low groove, the rotular groove, in which glides 

 the patella, or knee-cap. The patella is a large 

 ossification, of varying shape, in the tendon com- 

 mon to the four great extensor muscles of the 

 thigh, the action of which is to straighten the leg. 

 The lower leg, like the fore-arm, has two 

 bones, which, however, are always parallel, never 

 i"'^"" crossed, and have no power of rotation. Of 



'TTur'if ■^' these, the inner one is the shin-bone, or tibia, 



mur of Wolf, in- ' ' ' 



side of lower end. which is always the larger and alone enters into 



ex^ con.., external ., ■, • • ± mi j_ i i • j_i /•? ? 



condyle, int. ^"^^ knec-joint. i he external bone is the jioula, 

 con., internal which is almost entirely suppressed in certain 

 uiar groove, highly Specialized forms, such as the horses and 

 Above, are two pumiuants, the tibia carrying the whole weight. 



views of the left i c i -i • • 



patella, anterior The uppcr end of the tibia IS enlarged and ex- 

 and internal ^gj^^jg Qygj. ^j^^^ ^f ^j^g fibula ; it has two slightly 



concave surfaces for articulation with the con- 

 dyles 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 rough- 

 ened and thickened to form the cnemial crest, to which is at- 

 tached 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 in- 

 ner 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 mam- 

 mals 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 coossified with the tibia at both ends, and in the 

 most highly specialized hoofed animals, such as the horses, 



