284 THE EVOLUTION OF MAN. 



other. On the other hand, the above comparison of the 

 number of vertebrae in different tail-less and tailed Catarhines 

 shows considerable fluctuations in these numbers even : n 

 this one family. 178 



To understand the history of the development of the 

 human vertebral column, we must now study the form and 

 combination of the vertebrae in somewhat greater detail. 

 The main outline of each vertebra is that of a signet ring 

 (Figs. 254-256). The thicker part, which faces the ventral 

 side, is called the body of the vertebra, and it forms a short 

 disc of bone ; the thinner forms a semi-circular arch — the 

 vertebral arch, which is turned toward the dorsal side of the 

 body. The arches of all the consecutive vertebrae are so con- 

 nected by thin ligaments (ligamenta intercruralia) that the 

 space enclosed by them all in common forms a long canal. 

 In this spinal, vertebral canal lies, as we have seen, the hind 

 portion of the central nervous system, the spinal marrow. 

 The front part of this, the brain, is enclosed in the skull- 

 cavity, and hence the skull itself is merely the anterior 

 section of the vertebral column, modified in a peculiar way. 

 The base or ventral side of the bladder-shaped brain-capsule 

 was originally formed by a number of coalescent vertebral 

 bodies, the amalgamated upper vertebral arches of which 

 formed the arched or ventral side of the skull. 



While the firm, massive vertebral bodies constitute the 

 true central axis of the skeleton, the dorsal arches serve to 

 enclose and protect the central marrow. Analogous arches 

 also develop on the ventral side as a protection for the 

 thoracic and abdominal viscera. These inferior or ventral 

 vertebral arches, proceeding from the ventral side of the 

 vertebral bodies, form a canal in many low Vertebrates in ; 



