172 Journal of Comparative Neurology. 



dropped out, new ones have been added and primitive re- 

 lations have been distorted by the usurpation of vast areas 

 of the head by nerves of distant segments. It is to this 

 problem that the author has addressed himself primarily 

 in the following pages. 



The doctrine of nerve components dates properly from 

 the systematic separation of sensory and motor roots and 

 the formulation of Bell's law. Gaskell's suggestive 

 "four-root theory" has been a stimulus to further 

 advances and now it is customary to recognize in the 

 spinal nerves of the vertebrata four types of fibres: 

 (i) somatic efferent and (2) somatic afferent (general 

 cutaneous), making up the major part of the ventral and 

 dorsal roots respectively, and (3) visceral efferent and (4) 

 visceral afferent. It is probable that the visceral efferent 

 fibres go out with both roots and the visceral afferent 

 enter by the dorsal root. 



The proper analysis of the cranial nerves has been 

 retarded by various uncritical attempts to conform them 

 rigidly to Bell's law. These attempts resulted only in 

 confusion so long as qualitative differences other than 

 sensory and motor in the nerve fibres were not recognized 

 and all sensory cranial nerves were compared directly 

 with dorsal spinal roots (general cutaneous) , and all motor 

 cranial nerves were compared with the somatic motor 

 fibres of the trunk. Our precise knowledge of the sensory 

 components in the cranial nerves of the lower vertebrates 

 begins with Strong's paper on the Cranial Nerves of 

 Amphibia ('95), and the present research was earned out 

 upon the basis of that work. 



Throughout the Ichthyopsida we can at present distin- 

 guish in the cranial nerves three sensory systems of com- 

 ponents and two motor, aside from the sympathetic. Each 



