476 THE MECHANICAL FUNCTIONS. 



The articular surfaces of the knee joint are 

 broader, and admit of greater extent of motion in 

 man than in quadrupeds : hence the leg can be 

 brought into the same line with the thigh, and form 

 with it a straight and firm column of support to the 

 trunk ; and the long neck of the thigh bone allows 

 of more complete rotation. The widely spread basin 

 of the pelvis effectually sustains the weight of the 

 digestive organs, and they rest more particularly 

 on the broad expansion of the iliac bones ; in qua- 

 drupeds, these bones, having no such weight to 

 support, are much narrower. 



The base on which the whole body is supported 

 in the erect position is constituted by the toes and 

 by the heel, the bone of which projects backwards 

 at right angles to the leg. Between these points 

 the sole of the foot has a concavity in two direc- 

 tions, the one longitudinal, the other transverse, 

 constituting a double arch. This construction, 

 besides conferring strength and elasticity, provides 

 room for the convenient passage of the tendons of 

 the toes, which proceed downwards from the larger 

 muscles of the leg; and also for the lodgement of 

 smaller muscles affixed to each individual joint, and 

 for the protection of the various nerves and blood 

 vessels distributed to all these parts. The con- 

 cavity of the foot adapts it, also, to retain a firmer 

 hold of the inequalities of the ground on which we 

 tread. The muscles which raise the heel, and 

 compose the calf of the leg, are of great size and 

 strength, and derive a considerable increase of 

 power from the projection of the bone of the heel, 

 into which their united tendons are inserted. In 

 all these respects the human structure possesses 



