THE SKELETON 65 



nance of the erect posture, which is more complete and habitual 

 in man than in the animals most closely allied to him anatomically. 

 These peculiarities, however, only appear fully in the adult. In 

 the infant the head is proportionately larger, which gives the 

 center of gravity of the Body a comparatively very high position 

 and renders the maintenance of the erect posture difficult and in- 

 secure. The curves of the vertebral column are nearly absent, 

 and the posterior limbs are relatively very short. In all these 

 points the infant approaches more closely than the adult to the 

 ape. The subsequent great relative length of the posterior limbs, 

 which grow disproportionately fast in childhood as compared with 

 the anterior, makes progression on them more rapid by giving a 

 longer stride and at the same time makes it almost impossible to 

 go on " all fours " except by crawling on the hands and knees. In 

 other Primates this disproportion between the anterior and pos- 

 terior limbs does not occur to nearly the same extent. 



In man the skull is nearly balanced on the top of the vertebral 

 column, the occipital condyles which articulate with the atlas 

 being about its middle (Fig. 28) , so that but little effort is needed 

 to keep the head erect. In four-footed beasts, on the contrary, the 

 skull is carried on the front end of the horizontal vertebral column 

 and needs special ligaments to sustain it. * For instance, in the ox 

 and sheep there is a great elastic cord running from the cervical 

 vertebrae to the back of the skull and helping to hold up the head. 

 Even in the highest apes the skull does not balance on the top of 

 the spinal column; the face part is much heavier than the back, 

 while in man the face parts are relatively smaller and the cranium 

 larger, so that the two nearly equipoise. To keep the head erect 

 and look things straight in the face, "like a man," is for the apes 

 far more fatiguing, and so they cannot long maintain that position. 



The human spinal column, gradually widening from the neck to 

 the sacrum, is well fitted to sustain the weight of the head, upper 

 limbs, etc., carried by it; and its curvatures, which are peculiarly 

 human, give it considerable elasticity combined with strength. 

 The pelvis, to the sides of which the lower limbs are attached, is 

 proportionately very broad in man, so that the balance can be 

 more readily maintained during lateral bending of the trunk. The 

 arched instep and broad sole of the human foot are also very 

 characteristic. The majority of four-footed beasts, as horses, 



