394 'Journal of Comparative Neurology and Psychology. 



Accordingly, comparative anatomy, comparative embryology, 

 and comparative physiology must be appealed to before we can 

 feel that we are on safe ground in making a morphological sub- 

 division of the nervous system. The correlation of these subjects 

 is attempted in the modern functional analysis of the nervous 

 system as effected by recent students of comparative neurology. 

 All of these subjects are now sufficiently far advanced to justify 

 a reexamination of the question of the subdivision of the brain 

 from the phyletic standpoint and with especial reference to the 

 application of the BNA terms in comparative studies. 



The earliest form of regional differentiation of the nervous 

 system in the phylogeny w^as probably the concentration of a cen- 

 tralized or integrated system from the primitive diffuse type. 

 This differentiation appears in some form in all but the simplest 

 metazoa. In vertebrates the diffuse system also exhibits a certain 

 degree of integration on its own account and appears as the sym- 

 pathetic system. 



It is not intended to imply here the specific homology of the ganglia of the sympathetic chain or any other 

 part of the vertebrate sympathetic system with the diffuse ner\'aus system of any particular invertebrate 

 type — and much less with any invertebrate central ner\-ous system. It is very probable that the highly 

 complex sympathetic nervous system of vertebrates has been elaborated subsequently to the differentia- 

 tion of the central nervous system. It is maintained, however, that the diffuse vertebrate nervou s 

 system, especially the peripheral sporadic ganglia (and possibly the sympathetic as a whole), is in a 

 general way comparable with the diffuse ner\ous system of such animals as the Coelenterata. Even if 

 it should prove, as now seems probable, that the entire sympathetic system is in the ontogeny differen- 

 tiated from the same embryonic tissue (medullary plate) as the cerebro-spinal system, I think the com- 

 parison will still hold, though we must recognize that the ontogenetic development deviates far from 

 a recapitulation of the phylogeny of the sympathetic system. This deviation is in the direction of a 

 concentration of all of the embrj'onic ner\'ous rudiments, looking forward to the functional intimacy 

 between the cerebro-spinal and the sympathetic systems of the adult, and thus is parallel with the general 

 trend of the evolution of the whole nervous system. 



Our primary subdivisions, then, are (i) the sympathetic (auto- 

 nomic) and (2) the cerebro-spinal nervous systems. 



The cerebro-spinal or integrated nervous system in early phy- 

 logenesis separated from the diffuse nervous system to serve three 

 great types of bodily reactions, and gave rise to three systems of 

 sense organs (receptors) and their associated centers and return 

 motor paths. These, adopting Sherrington's nomenclature, we 

 may designate as follows: 



I. Exteroceptorsy systems for reaction to stimuli impinging 

 upon the outer surface of the body. The source of the stimulus 

 may not be in contact with the body, in which case the sense organs 

 in question are termed by Sherrington distance receptors. The 



