THE NERVOUS SYSTEM 307 



cells are essentially alike, in that each consists of a bit of proto 

 plasm, containing a nucleus. Nerve cells differ from others, 

 of course, in their peculiar function, and it would be interest- 

 ing indeed to know how they do their work of thinking, 

 memorizing, inventing, etc., but this no one can tell. They 

 also frequently differ from others in their very irregular shape 

 (Fig. 159) and in the outgrowth from most of them of a long 

 process, the axon, or axis cylinder, the work of no other cells 

 requiring such connection with parts of the body at a dis- 

 tance from them. 



A portion of a nerve fibre is represented in Figure 160. It 

 consists of a very fine central thread, the above-mentioned 



^Primitive Sheath _^ tleduttan/fhteft 



Cylinder 



\nucieui 



FlG. 160. A SHORT PIECE OF A NERVE FIBR& 

 Very highly magnitieck 



axis cylinder, which is continuous from its point of exit from 

 a cell in the brain or spinal cord, to its end, e. g. in some 

 muscle, gland or skin cell. Covering this axis cylinder, which 

 is the real conducting fibre of the nerve, is the medullary 

 sheath, which is of material different from that of the fibre and 

 contains considerable fat. It seems to act as a covering to 

 prevent impulses which are passing along the fibre from 

 jumping across into other fibres lying close by, thus serving 

 something the same purpose as does the insulating covering 

 of gutta percha around an electric wire. This medullary 

 sheath is interrupted at short intervals called nodes (Fig. 

 160) and between every two nodes there occurs a nucleus 

 showing the medullary sheath to be made up of many cells. 

 Outside the whole is a thin covering, the primitive sheath. 



On the other hand, since this sheath contains fats, the 

 nutrition of the nerve fibre has been regarded as its main, 

 function. There is little to support this view, however. 



