1 66 THE ORIGIN OF THE NERVOUS SYSTEM 



physiological conditions and the persistence of some and 

 the resorption of others by certain other conditions. 



In many other neurons dendrites arise from all parts 

 of the cell surface and grow in all directions without 

 orientation at any stage (Fig. 45). In some cases in 

 which the axon of the mature neuron arises from a 

 dendritic outgrowth, this outgrowth is secondary and 

 the axon developed originally directly from the cell 

 body. The spinal ganglion cell in early stages shows 

 two outgrowths localized at opposite poles (see Fig. 60, 

 p. 1 86), but later the bases of the two outgrowths 

 approach each other and unite, and the cell gives rise to 

 a single secondary outgrowth from the end of which the 

 two primary outgrowths extend in opposite directions. 

 In some spinal ganglion cells in mature condition this 

 secondary outgrowth is not single in its origin, but the 

 cell gives rise to two or more outgrowths which branch 

 and anastomose inside the connective tissue capsule 

 which surrounds the cell, but finally unite to form the 

 single outgrowth and in other cells several regions of 

 branching and anastomosis appear in the course of this 

 outgrowth. 



Frequently axons or dendrites on reaching a certain 

 point in their course show a sudden and definite change 

 in direction as if they had come under the influence of 

 some environmental factor acting in a direction different 

 from that to which they had previously been reacting. 

 For example, the longer dendrites of the pyramidal 

 cells of the cerebral cortex grow vertically toward the 

 surface, but as they approach it they branch and the 

 branches become parallel to the surface (Fig. 46). In 

 the mature Purkinje cell of man the chief dendrites 



