226 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



i. Spinal Column. — Observe (i) the double or S curva- 

 ture. This is peculiar to man, and the result of the 

 erect attitude. In this position it acts as a spring to 

 prevent shocks to the brain in falling, leaping, etc. Ob- 

 serve (2) the intervertebral substance. This is an elas- 

 tic cushion, of half an inch thickness, between all the 

 vertebras, and also acts to prevent concussion of the 

 brain. It is, however, not peculiar to man. 



Comparative Morphology of the Column. — The manner 

 of articulation of the vertebras is very characteristic of 



the different classes of ver- 



In mammals the 

 are short seg- 

 a cylinder with 



tebrates. 

 vertebras 

 ments of 



flat faces and intervertebral 

 substance between (Fig. 

 145, a). In reptiles the ar- 

 ticulation is by ball-and- 

 socket joint. The faces of 

 each vertebra are one of 

 them concave and the other 

 convex, and these fit over 

 one another, b. In some 

 reptiles the hollow face 

 looks forward (proccelian) ( 

 in others backward (opisthocoslian). In fishes the seg- 

 ments are double concave (amphiccelian), with double 

 convex intervertebral substance between, c. Some early 

 reptiles were like fishes in this regard. In the lowest 

 fishes there is a continuous unsegmented cord (noto- 

 chord), d. This is also the early condition of all ver- 

 tebrates, whether in the embryonic development or in 

 the evolution of vertebrates. This early notochord is 

 afterward changed into a vertebral column by ossifica- 

 tion in segments. These characteristics of the verte- 



FlG. 145. — Diagram showing the 

 characteristics of the vertebral 

 column in mammals (a), reptiles 

 (0), fishes (c), and lowest verte- 

 brates (d). 



