THEORY OF NERVE COMPONENTS. I 7 



system may be defined as the sum of all fibres in tbe body 

 which possess certain physiological and morphological 

 characters in common, so that they may react in a com- 

 mon mode. Morphologically, each system is defined by 

 the terminal relations of its fibres — by the organs to which 

 they are related peripherally and by the centres in which 

 the fibres arise or terminate. The fibres of a single sys- 

 tem may appear in a large number of nerves, repeated 

 more or less uniformly in a metameric way (as in the 

 general cutaneous system of the spinal nerves), or they 

 may all be concentrated into a single nerve (as in the 

 optic nerve) . The post-optic systems are as follows : 



(i). The general cutaneous system of the head is 

 clearly the representative of the somatic afferent of the 

 trunk. Its fibres innervate the skin in general, without 

 specialized sense organs and all terminate in the spinal V 

 tract or cells associated with it, these being the continua- 

 tions into the head of the dorsal horns of the spinal 

 cord. 



The term somatic setisory system has been used for these compo- 

 nents. This usage, however, is ambiguous. Gaskell used this term 

 for all nerves to the outer skin, as distinguished from visceral nerves. 

 Cole would exclude the lateralis nerves, but apparently would in- 

 clude nerves for terminal buds of the skin, which we would exclude as 

 belonging to the communis system . It is better, therefore, to avoid 

 the use of this name altogether. 



(2). The communis system innervates visceral and 

 mucous surfaces, taste buds and other similar specialized 

 end organs (terminal buds) on the outer skin which do 

 not belong to the lateral line system. These fibres are 

 distinguished by their fine calibre and by the fact that 

 they all end in a single centre, the lobus vagi (which has, 

 however, in some teleosts a specialized pre-auditory por- 

 tion, the so-called lobus trigemini). They may enter this 



