CHAPTER VII 



THE GENERAL ANATOMY AND SUBDIVISION OF THE 

 NERVOUS SYSTEM 



ON merely topographic grounds the nervous organs are divided 

 into the central nervous system, or axial nervous system, compris- 

 ing the brain and spinal cord, and the peripheral nervous system, 

 including the cranial and spinal nerves, their ganglia and periph- 

 eral end-organs, and the sympathetic nervous system. The 

 nerves are simply conductors, putting the end-organs into phys*- 

 iological connection with their respective centers. The general 

 form of the human central nervous system and its connections 

 with the peripheral nerves are seen in Fig. 41. The nerves 

 connected with the spinal cord are the spinal nerves, those con- 

 nected with the brain are the cranial or cerebral nerves, and 

 both of these systems of nerves together are called the cerebro- 

 spinal nerves, in contrast with the sympathetic nerves, which 

 latter may or may not be connected with the central nervous 

 system (see p. 225). 



The central nervous system is the great organ of correlation 

 and integration of bodily processes. Its primitive form in verte- 

 brates is a simple tube, and this is the form shown in an early 

 human embryo (see Fig. 46, p. 116). The original tubular form 

 is but little modified in the trunk region of all vertebrates, where 

 the spinal cord (medulla spinalis) is formed by a tolerably uni- 

 form thickening of the lateral walls of the tube (see Figs. 41, 58). 

 But in the head region the brain (enccphalon) is formed by the 

 very unequal thickening of different parts of the walls of the tube 

 and by various foldings brought about thereby. The general 

 arrangement of the human central nervous system at successive 

 stages of development is seen in Figs. 47-51. 



The external form of the brain has been shaped by the space 

 requirements of the nerve-cells and fibers which make up its 

 substance. A group of nerve-cells which performs a single 

 function is often spoken of as the "center" of that function; but 



106 



