CoGHll.L, TJic Reaction to Tactile Stimuli. loi 



Emphasis, properly, lias boon placed, by authorities generally, 

 iqion the principle of cephalization as correlated with the organs of 

 speeial sense; but these early movements of the embryo show that, 

 so far as functional development is concerned, the most primitive 

 centralization of the nervous system, ontogenetically, is in direct 

 response to the demands of the motor sj^stem in its relation to loco- 

 motion, while the sensory system involved is not the special sensory 

 but the most primitive, diffuse, exteroceptive field. It remains to 

 locate exactly this primitive center of the cerebro-spinal system by 

 correlated anatomical and experimental studies ; but from the ex- 

 periments alone, this center would seem to be in close relation to the 

 cephalic musculature of the trunk. This is inferred particularly 

 from the fact that a flexure in response to a touch on the tail bud 

 begins in the head region and progresses caudad and is the same 

 in form (without reference to the initial direction of the movement) 

 as the flexure that follows stimulation of the head. All movements, 

 then, regardless of the point of stimulation, must emanate from the 

 same center.* Into this center all impulses W'Ould seem to flow in 

 order to be directed in such a way upon the musculature of the trunk 

 as to give rise to locomotion. Clearly the development of an eye or 

 ear as such in its earliest functional condition has no part in deter- 

 mining this region of centralization. The controlling factor in this 

 centralization is the motor system : a cephalization in response to 

 the prepotency of the requirements of effectors and not in response 

 to the demands of the cephalic receptive fields. 



Phylogenetically, then, the most primitive cephalization of the 

 nervous system may have occurred, also, in response to the demands 

 for locomotion and have given rise to a center of control in the region 

 corresponding to the lower portion of the myelencephalon or the 

 upper portion of the medulla spinalis. Quite in harmony with this 

 suggestion is the convincing evidence that Johnston^ presents for the 

 migration caudad of the afferent roots of the cranial nerves. Such 

 a change in their course would lead them more directly into this 

 primitive locomotor center. Upon this hypothesis, also, the economy 



'"The Nervous System of Vertebrates," Chapter III. 



