6 14 "Journal of Comparative Neurology and Psychology. 



If the above conclusions are accepted, how must the nature of 

 the nerve impulse be pictured ? As a physico-chemical change 

 in the neurone w^hich is propagated from part to part within the 

 neurone, not merely conducted. Such a change must be regarded 

 as an interaction among the substances of the neurone. In such 

 an interaction both the neurofibrillae and the neuroplasm doubt- 

 less play their parts. Under this conception of the nerve impulse, 

 it might travel through a fine fiber and still be delivered in effective 

 strength to a large number of tissue elements, provided only that 

 the caliber of the fiber or the combined caliber of its branches 

 increased sufficiently to multiply the impulse by the number of 

 elements to be supplied. This is exactly what appears to take 

 place in the motor axones in the lamprey. Each motor axone 

 appears to constitute a cone w^iose apex is at the cell of origin 

 in the spinal cord or brain and whose base is formed by the com- 

 bined end branches. 



The conclusions which this discussion renders probable are that 

 the nerve impulse is a physico-chemical process which consists 

 in an interaction between different substances in the neurone; 

 that it is propagated from part* to part of the neurone by the pro- 

 gressive interaction of these substances; that it is not merely con- 

 ducted by any specific substance in the neurone; that it increases 

 in strength with the increase in caliber of the axone carrying it; 

 and that this last is the chief function of motor end plates and 

 enlargements at the ends of axones wherever such enlargements 

 occur. It may be necessary to add that the practical freedom 

 of the nerve fiber from fatigue and the very slight amount of 

 metabolism connected with the transmission of an impulse must be 

 taken into account in a theory of the nature of the nerve impulse. 

 And it should be said that Bethe, one of the foremost exponents 

 of the important role played by neurofibrillae, recognizes in his 

 fibril acid hypothesis the interaction of fibrillae and neuroplasm 

 and that in so far this hypothesis conforms to the conclusions 

 reached above. 



Significance of the size of the cell-body and dendrites. 



If the caliber of the axones is important, may there not be some 

 special significance in the volume of the others parts of the neurone 

 at least in certain cases ^ Why do the mitral neurones of the 

 olfactory bulb have so large cell-bodies and dendrites ? The 



