ON THE MAMMALIAN NERVOUS SYSTEM. 



495 



side of a centre, the amount of the flow and the intensity of the processes which 

 emerge from the efferent side is small. A comparatively intense excitation is necessary 

 for the etherised centre to discharge at all, and the emerging impulses are so altered 

 in their time relations and intensity, or limited to so few channels, that but little 

 evidence of their presence is indicated by the galvanometric method. 



To facilitate further explanations we would call attention to the following diagram, 

 fig. 22, in which are represented the hypothetical elements of a nerve centre. 



Fig. 22. 



Sectional Surface 

 of Cord A 



Efferent 

 Side. 



CO 



Dt 



Afferent 

 Side. 



Ex 



C s P-9 



Fietd 



.f 

 Corytinetzot? 



In fig. 22 are shown as simply as possible the three constituent parts of a simple 

 nerve centre, namely, the afferent side, field of conjunction, and efferent side. In 

 addition, the diagram represents the known part of the afferent channel which forms 

 the direct ascending fibres in the spinal cord (Di), the internuncial fibres (I), which 

 also ascend, but which are not yet absolutely known, and the fibres of the pyramidal 

 tract (P). 



If a posterior root be excited at the point marked Ex. in fig. 22, we can obtain a 

 record of the electrical change evoked by such excitation, in the following parts, 

 namely, the portion of the cord (A) above the excitation, in the outgoing channel (B) 

 (em.), and in the channel of excitation (C). On taking the average of all such records 

 we find that they are as follows : 



Average at A 198. 



B 26. 



C 250. 



