210 



THE GROWTH OF THE BRAIN. 



cells are interpolated in the pathway of the incoming 

 fibres, and serve to distribute the impulses which they 

 receive to other localities within the cord or brain, thus 

 increasing the possible number of pathways by which an 

 incoming impulse may find passage. Since these cells 

 are arranged in series, the impulses which take a long 

 course must often pass through several groups of them. 



FlG. 40. Cross-section of the spinal cord of the chick, X 100 

 diameters. (Van Gehuchten.) D., dorsal surface ; K, 

 ventral surface ; d.r., dorsal root ; ?/.r., ventral root ; g., 

 spinal ganglion. On the left the arrows indicate the 

 direction of the larger number of impulses. The small 

 arrow on the right dorsal root calls attention to the fact 

 that some neurons arising in the ventral plate emerge 

 through the dorsal root and convey impulses in the 

 directions indicated. 



But concerning the maximum number of times that the 

 impulse may thus be interrupted, no definite statement 

 can be made. Physiologically, then, the entire central 

 system may be regarded as made up of these afferent 

 cells, over whose neuron the incoming impulses arrive ; 

 of central cells which distribute them, and finally of a 

 group of efferent cells, over the neurons of which they 

 pass out. It is plain that, according to this scheme, the 

 complexity of any given nervous system would depend 



