142 



PRINCIPLES OF ANIMAL BIOLOGY 



relative size of the brain increases up through the vertebrate group, 

 reaching its maximum in man, whose brain includes more nerve tissue 

 than all the rest of his nervous system together. 



There is thus a tendency, in the animal scale, for complexity in 

 general to be accompanied by a massing or centralization of the nerve 

 tissue, and to emphasize this massing in the head region. The sug- 

 gestion is near that somehow a concentrated system is better fitted to 

 serve as a mechanism of control of a complex body than is a diffuse 

 system. Additional reasons for reaching this conclusion will appear as 

 the arrangement of cells in the larger masses of the system are examined. 





HYDRA 



V 



V 



FLATWORM EARTHWOI?M CRAYFISH FROG MAN 



Fig. 116. — Diagrams of nervous systems illustrating centralization and massing in the 



head region. 



The large masses of the nervous system, particularly the brain and 

 spinal cord, constitute the central nervous system. The position of the 

 central system in the body, and its structure, constitute fundamental 

 differences between vertebrate and invertebrate animals. In the inverte- 

 brates the nerve cord is below the digestive tract, in the vertebrates 

 above it. The cord is a double one (or there are two separate cords) in 

 the invertebrates, single in the vertebrates. Finally, the cords are 

 solid in invertebrates, hollow in vertebrates (resulting fi-om the system's 

 embryonic origin as a groove in the ectoderm which is pinched off below 

 as a tul)e). 



Peripheral Nervous System. — The nerves which pass out fiom the 

 central system and branch to all parts of the organism are collectively 

 called the peripheral nervous system. Of the principal nerves, a number 

 (10 in amphibia, 12 in the higher animals) arise from the brain within 

 the cranium; these are called cranial nerves. From the spinal cord there 



