EXPERIMENTS ON VERTEBRATES 147 



causes a change in the conditions of the se^mental 



o -> 



ganglia behind the seat of the operation. This change 

 is the shock-effect. 



Finally, the most important difference between the 

 segmental conception of the central nervous system 

 and the centre-theory may be pointed out. Accord- 

 ing to the latter theory, the central nervous system 

 consists of a series of centres for as many different 

 "functions." Each "function" is determined by the 

 structure of its "centre." According to the seg- 

 mental theory, there are only indifferent segmental 

 ganglia in the central nervous system, and the dif- 

 ferent reactions or reflexes are due to the different 

 peripheral organs and the arrangement of muscles. 

 The centre-theory must remain satisfied with the 

 mere problem of localising the apparent " seat " of a 

 " function " without being able to give the dynamics 

 of the reactions of an animal, as the latter depend 

 in reality upon the peripheral structures, and not on 

 the structures of the ganglia. For this reason the 

 segmental theory alone will be able to lead to a 

 dynamical conception of the functions of the central 

 nervous system. 



This difference may be made more apparent by 

 comparison of these functions with those of the re- 

 tina. The optical perception of forms consists in 

 the power of single elements to determine, accord- 

 ing to their position on the retina, different space- 

 sensations. One retinal element may aid in bringing 

 about many different pictures. Viewed from the 



