THEORY OF NERVE COMPONENTS. 9 



now affirms that in a similar way the cranial nerve trunks 

 may contain several varieties of sensory fibres which have 

 different functional and morphological relations and 

 several of which may be present in a single segmental 

 nerve. Since these systems of components are defined by 

 both the peripheral and central relations of their fibres, it 

 is obvious that the ordinary methods of research are inade- 

 quate for their study, since these methods have usually 

 examined the proximal termini microscopically and the 

 peripheral courses macroscopically, with but slender basis 

 for an exact correlation of the two sets of findings. The 

 only case thus far published in which both central and 

 peripheral relations of the sensory components have been 

 fully worked out in the same type is Strong's research 

 ('95) upon the cranial nerves of the tadpole of the frog. 



Numerous other students, both in this country and in 

 Europe, are now at work upon different phases of the 

 problem, and this activity is expended mainly upon the 

 fishes. The reasons for this are evident, for not only do 

 these primitive types present the problem in its simplest 

 terms and in terms easily assimilable to the paradigm 

 given by Strong, but the extreme diversity among the 

 various groups of fishes in the relative development of the 

 several nerve systems gives us a remarkably beautiful 

 morphological series which sheds much light upon the 

 relationships of the components. 



Nature has, as it were, performed for us in the fishes a 

 series of experiments which reveal as clearly what are the 

 primary and secondary anatomical centres for the several 

 systems of sense organs as the experimental method of 

 V. Gudden or even this in combination with the Nissl stain. 

 The working out of the details of this scheme proves 

 more difficult than would at first sight be supposed 



