THEORY OF NERVE COMPONENTS. I 5 



US in the analysis of the sensory components of the 

 cranial nerves by the comparative study of the correlated 

 variation between the sense organ and the centre. It 

 should be remembered, too, that these variations in the 

 primary centres involve corresponding changes in the 

 secondary tracts and centres, and thus contribute to some 

 of the most difficult problems of morphology. Much re- 

 mains to be done in working out» these principles; 

 it is, moreover, the most sound morphology, and the 

 most economical as well, to continue this line of re- 

 search among the fishes until its most fruitful sug- 

 gestions have been gathered in before attempting the 

 application in detail to human anatomy, for we find no 

 other group of the vertebrates so diversely specialized in 

 these respects as the fishes, and hence presenting so 

 varied an assortment of stages in the development of the 

 several systems of components. 



The research which follows is an attempt to solve some 

 of these problems in the fishes, and it is believed that the 

 results will contribute something toward the development 

 of a true philosophical understanding of the human nerv- 

 ous system. The author feels that at the present time the 

 most important step in this direction is a clear insight into 

 the nature of the several components of the cranial nerves, 

 their relations to each other and to the spinal nerves, and 

 particularly their significance to metamerism and the 

 allied head problems. Until each component can be 

 isolated and treated as a morphological unit and then un- 

 raveled in its peripheral courses through the various nerve 

 roots and rami — until this is possible no further great 

 advances in cranial nerve morphology can be looked for 

 even among the lower vertebrates, still less in man. For 

 in the human subject whole systems of nerves have been 



