PHYSICAL BASIS OF SENSATION 645 



Referring to the excitatory changes on which sensation 

 depends, there has been much discussion as to whether the 

 effects of stimulus in efferent and afferent nerves are of the 

 same or of different natures. The difficulty in deciding this 

 point lay in the fact that the indications of the state of 

 excitation are different in the two cases, one exhibiting it 

 objectively by the motile effect, and the other subjectively 

 by sensation. It has been supposed, as we have seen, that 

 the excitatory changes transmitted by the nerves were un- 

 accompanied during their progress by any direct visible 

 indications. It has been shown, however, in the course of 

 previous chapters, that a change of form does in fact ac- 

 company the transmission of the wave of excitation along 

 the nerve. It was also shown that this mechanical indica- 

 tion could be rendered extremely delicate, ranking, in 

 degree of sensitiveness, between the galvanometer and the 

 brain. Employing this mode of investigation, then, we 

 found not only that the wave of excitation might be trans- 

 mitted in either direction in any given nerve, but also that 

 the changes induced by stimulus were similar in afferent and 

 efferent nerves (p. 529). 



Regarding the nature of this molecular change, again, 

 it has been supposed that the nerve under excitation exhi- 

 bited a specific variation, known as the neural, totally unlike 

 those changes which take place, for instance, in muscle. We 

 have seen, however, that this is not the case, the mechanical 

 and electrical expressions of the molecular changes in excited 

 nerve being of a nature essentially similar to those observed 

 in muscle also. Even in the matter of conduction, we have 

 seen that non-neural tissues transmit the state of excita- 

 tion to a certain distance beyond the point of stimulation. 

 The difference in this respect is one of degree, and not of 

 kind. 



We have next to deal with the question of sensation as 

 induced by molecular changes in the nerve. It is widely 

 admitted that the changes induced in the nerve by stimulus 

 will cause responsive sensations. But the relation between 



