178 Journal of Comparative Neurology and Psychology. 



arrangement of its cells and fibers serves as a guide to these 

 relationships. Further, by a study of its residual or vestigeal 

 structures and its ontogeny in various classes of animals we 

 gam hints of many of the past relationships between the other or- 

 gans. On account of its function as a go-between, the nervous 

 system becomes for us an interpreter. The truth of this has 

 long been realized but it has become of practical value only 

 since the work of Osborn, Allis and Ewart led the way to 

 the development of the theory of nerve components by Strong, 

 Herrick and others. According to this theory, those fibers 

 in the cranial nerves which supply the same kind of end organs 

 enter the same or comparable regions in the brain. For exam- 

 ple, all general cutaneous fibers (free nerve endings in the skin) 

 enter the spinal V tract in the brain and the dorsal tracts in the 

 cord. It happens that a nerve trunk is commonly composed of 

 two or more sets or kinds of fibers. All those fibers in the 

 several nerves which have the same central and peripheral con- 

 nections are said to belong to one and the same system of nerve 

 components. 



Although this theory concerns itself directly with the 

 analysis of peripheral nerve trunks, its value rests upon the ex- 

 istence of functionally distinct types of end organs on the one 

 hand and distinct nerve centers in the brain on the other hand. 

 The analysis of nerve trunks into nerve components neces- 

 sarily implies the analysis of the nervous system as a whole 

 from the same point of view, that is, on the basis of function. 

 In this new way of looking at the nervous system the brain, 

 which from the standpoint of structure has always been re- 

 garded as the most complex and obscure portion, becomes the 

 most illuminating — and this just because of its complex rela- 

 tions. Three considerations are important in this connection. 

 First, the reasoning on which the theory of nerve components 

 rested could not be made good unless it was shown that the fibers 

 of two components were independent in their central relations. 

 For example, unless it were clearly shown that the centers in 

 which the general cutaneous fibers end are structurally and 

 hence functionally distinct from those into which sensory fibers 



