ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 439 



Such a regeneration of fibers does not differ in any way 

 from the manner in which nerve fibers arise in the embryo. 

 The spinal nerves grow out from the spinal cord and pene- 

 trate further and further until they finally reach their in- 

 tended terminations. The optic nerve (together with the 

 retina) is a direct outgrowth of the brain. The auditory 

 nerve is a growth from the brain, which finally reaches and 

 connects with the developing ear. 



Neuroglia. 



Ill the brain and spinal cord where the neurons are 

 closely packed they seem to be supported and held in place 

 by a peculiar kind of connective tissue differing entirely 

 from that which supports other tissues. The supporting 

 tissue here consists of large, many-branched cells, looking 

 very much indeed like nerve cells. The many branches of 

 these cells seem to connect and form a supporting mesh- 

 work for the more delicate nervous tissue. These support- 

 ing cells and the network which arises from them are spoken 

 of as the neuroglia. These cells are not connective tissue 

 cells, but are in origin related to the nervous cells, having 

 been derived from the same primitive source. In sections 

 of the spinal cord these cells may be easily mistaken for 

 nerve cells, differing, however, from them in the absence of 

 all nervous functions and so, of course, anatomically in the 

 absence of nerves. 



GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 



Before going into the physiology of special portions of 

 the system it is desirable to treat of those physiological 

 properties which all nervous tissues have in common. In 

 fact, the specific physiologies of the special portions are due 

 more to the origin and distribution of the nerves than to 

 any physiological difference in their activities. 



The general physiology of a nerve may be summed up in 

 the statement that it is to convey a nervous impulse. This 

 impulse may arise either in the cell of which the nerve is a 



