406 



THE NBEVOTJS SYSTEM 



Nerve process 



Axis cylinder 



Neurilemma 



Nerve cells, wherever found, agree in this general struc- 

 ture. In some the production of dendrites is much less 

 than in others. Some also have one axon only, while 

 others may have from two to several. Those cells with one 



axon are called unipolar, with 

 two, bipolar, and with more 

 than two, multipolar. If we 

 trace the course of one of 

 these axones we find that it 

 may extend a long distance 

 from the jDarent cell with or 

 without branching, and, what 

 is most important, such ax- 

 ones are the axis cylinders 

 of the nerve fibers. 



We see then that a nerve 

 fiber is to be considered as an 

 elongated, axis cylinder, or 

 axon, which has become cov- 

 ered with one or more pro- 

 tective sheaths. When the 

 axis cylinder branches, these 

 branches are called collaterals, 

 while th^ terminal branches 

 in which they all end are 

 called the terminal brushes. 

 A nerve cell with its dendrites, 

 axones, sheaths, collaterals, 

 and terminal brushes, is 

 spoken of collectively as a 

 neuron. Such a unipolar neuron is shown in Figure. 193. 

 Theory of neuron action. — The action of an amceba 



Fig. 193 — Scheme of a. neurou; a, 

 free axis cylinder; 6, axis cylinder 

 surrounded by neurilemma alone; 

 c, axis cylinder surrounded by 

 medullary sheath alone; d, axis 

 cylinder surrounded by the sheath 

 and neurilemma and divided into 

 segments (by constrictions called 

 the nodes of Hanvier). 



