396 'Journal of Comparative Neurology and Psychology. 



the stimulus is in contact with the body (taste) or at a distance (smell) — a practically important distinc- 

 tion from the standpoint of the type of reaction which should follow, though one which may rest on a 

 relatively trivial physico-chemical difference in the stimuli themselves. Accordingly, the cerebral centers 

 for smell, which normally call forth locomotor responses, have differentiated far from those of taste, 

 where typically visceral responses alone are involved. (Cf. the article last cited, On the Phylogenetic 

 Differentiation of the Organs of Smell and Taste.) 



Attention should be called in this connection to the fact that in the hypothetical ancestral vertebrate 

 from which the olfactory and gustatory reflex systems were evolved both the effectors and receptors were in 

 an unspecialized condition and in later phylogeny their elaboration occurred simultaneously. Responses 

 to chemical stimuli of an unspecialized or "total" sort would follow in such an animal and the differ- 

 entiation of the responses into visceral and somatic types occurred pari passu with the functional differ- 

 entiation of the sense organs into interoceptors and distance receptors. And of course the cerebral con- 

 duction pathways for smell and taste became clearly defined and separate from the general unspecialized 

 central gray only gradually as the functional need arose. In a similar way all of the well defined reflex 

 paths within the higher brains are shown by comparative studies to have taken form from the more 

 diffuse type of central nervous structure found in the lowest vertebrates. The central gray and forma- 

 tio reticularis grisea et alba are survivals in higher brains of this primitive unspecialized nervous tissue. 



The first two of Sherrington's physiological groups, then, 

 compromise in general the systems of nerves and end organs 

 designated by recent morphologists the somatic systems; his third 

 group includes the visceral or splanchnic systems. 



From the preceding analysis it clearly appears that the basis 

 for the most fundamental divisions of the cerebro-spinal nervous 

 system is found in the contact of the organism with the environ- 

 ment. In other words, the organization of the cerebro-spinal 

 nervous system has been shaped by its peripheral end organs. 



Early in the process of the resultant differentiation the verte- 

 brate central nervous svstem responded to these peripheral influ- 

 ences by developing a series of structurally defined longitudinal 

 zones. In the first place, the dorsal part of the nerve tube and the 

 related nerves were devoted to the afferent or sensory limb of the 

 reflex arc and the ventral part to the efferent or motor limb, sep- 

 arated by the sulcus limitans (His). The early recognition of this 

 relation in the spinal cord by Sir Charles Bell led to the formu- 

 lation of Bell's law of the composition of the spinal nerve roots. 

 These have been further subdivided by Gaskell and his followers, 

 so that in passing from the dorsal to the ventral surface of the spinal 

 cord we now recognize successively, in both gray and white matter, 

 the following longitudinal zones: 



1. Somatic sensory, including exteroceptive and propriocep- 

 tive centers and conducting paths. 



2. Visceral sensory, including the interoceptive centers and 

 their pathways. These are feebly developed and still very imper- 

 fectly understood in the spinal cord. Afferent impulses enter by 



