278 



STRUCTURE OF THE VERTEBRATES 



The cord tends to shorten in the reptiles, and in the most 

 primitive mammals barely reaches the sacrum. The tendency 

 of the nerve tissues to become concentrated in the anterior re- 

 gion is described as cephalization. This centralization of func- 

 tion is most prominent in the higher mammals. The fish, amphib- 

 ia and reptiles show a progressive development away from the 



Sacrum 



B. Human 

 Embryo 



C. Adult 

 Human 



A Turtle 



Fig. 157. Spinal Cords. Illustrating the shortening of the cord during evolu- 

 tion and during individual development. 



spinal type of animal. The extinct dinosaurs, however, had a 

 highly decentralized nervous system. The spinal centers were 

 more important than the brain, and the selective value of this 

 is apparent when one considers the great size of the animals. It 

 has been estimated that in the largest dinosaurs it would re- 

 quire approximately five seconds for an impulse to travel from 

 the tip of the tail to the brain, and back to the tail. 



