542 



THE CHANGING WORLD 



After the upright posture was hit upon in the course of evolution, 

 however, this particular disadvantage largely disappeared, since the 

 increasing heaviness of the encased brain was amply provided for by 

 poising the whole head on the top of a supporting pillar-like vertebral 

 column. 



Experiments in assuming an upright posture and in going about 

 on the hind legs did not originate with the primates. There were 

 Mesozoic dinosaurs, for example, that habitually reared up their 

 gigantic bodies on end, also the whole class of birds, as well as vari- 

 ous kinds of jumping animals such as kangaroos. 



In these cases, however, while stand- 

 ing up, the legs are bent in a sitting 

 attitude, with the knees projecting for- 

 ward. It remained for man to become 

 the most straight-legged upright biped 

 of them all, and this fact has resulted 

 in the modification of practically every 

 part of his anatomy. 



The single archlike curving backbone 

 of a typical quadruped, from which the 

 weight of the body hangs suspended, 

 became in man a vertical supporting 

 column, partially straightened by a new 

 curve in the opposite direction, forming 

 the "small of the back." Young babies 

 are flat-backed at first, and only acquire 

 this compensating curvature later when 

 they develop into walking bipeds. 



The centra, or bodies of the vertebrae, 

 become flat-faced, thus stacking up into 

 a firmer column than would have been 

 possible with the original ball-and-socket centra, while the spinous 

 processes of the vertebrae all come to slope backward and downward, 

 instead of in the anticlinal fashion, as in quadrupeds generally. 



Furthermore, the axis and atlas of the cervical vertebrae become 

 modified to permit easy rotation of the head, allowing the eyes of 

 erect man to sweep the horizon, as no quadruped can easily do, 

 at the same time permitting the eyes themselves to gaze straight 

 out from the vertical face instead of looking downward along a 

 projecting snout. 



Adult 



Infant 



Diagrams showing the dif- 

 ference in the curvature of the 

 backbone between the infant 

 and the adult. (From Walter, 

 The Human Skeleton. By per- 

 mission of The Macmillan 

 Company, pubUshers.) 



