8 NERVE COMPONENTS OF BONY FISHES. 



manifestation, as one sense after another effects its 

 secondary connections with the cerebrum. Thus the fluc- 

 tuations of the relative importance of the organs of higher 

 sense, as the nose and the eye, from type to type are 

 clearly reflected in the size and organization of the cor- 

 responding primary and secondary brain centres. We 

 have, it is true, as yet only a few hints in these directions; 

 but yet enough has been gained to illustrate the exceed- 

 ing fruitfulness of this line of research. 



Now, in the domain of the peripheral nerves we have 

 as yet developed but few such illuminating generaliza- 

 tions, and our students still memorize the twelve pairs of 

 cranial nerves, their trunks, rami and ramuli, with the 

 distribution of each, much as one would learn a Greek 

 paradigm. If there is any morphological nexus between 

 the various nerves or any basis for a rational classification, 

 the average text-book gives no hint of it. 



In view of the present inchoate condition of the mor- 

 phology of the cranial nerves and of the fundamental 

 relation of this problem to the proper understanding of 

 the great afferent and efferent systems of the neuraxis 

 itself, it is most fitting that within very recent years there 

 has been a notable increase in the number of researches 

 centering about these questions. 



The literature of the cranial nerves is remarkably volu- 

 minous, but by far the larger part is either purely descrip- 

 tive or dominated by crude and false morphological 

 theories. It is only within the present decade that a 

 really practical standpoint has been introduced for the 

 proper morphological treatment of the cranial nerves — at 

 least their sensory portions. This is the doctrine of nerve 

 components, which had been earlier applied to the spinal 

 roots in the very suggestive "four- root theory," and which 



