PHYSICAL BASIS OF SENSATION 647 



stimulus. The question, however, remains, whether or not 

 these opposite sensation-tones could be demonstrated to be 

 dependent upon characteristic nervous changes of opposed 

 characters. If we should succeed in making such a demon- 

 stration a physico-physiological basis of psychical effects 

 would have been established which would unquestionably 

 prove to be of great value. 



Now we have seen, referring to previous investigations 

 on nerves, (i) that a feeble stimulus applied to the nerve is 

 transmitted as a pulse of expansion. This we have 

 designated the positive wave. The propagation of this 

 wave being more or less of the nature of a hydrostatic 

 disturbance, we have seen that its transmission is not affected 

 to any great extent, even when the conductivity of the tissue 

 is diminished. (2) A more intense stimulus we have found 

 competent to give rise to a disturbance of opposite or negative 

 sign that is to say, to a pulse of contraction. The velocity with 

 which this second, or, as we have called it, the true excitatory 

 wave, was conducted, we found to increase with the intensity 

 of the stimulus. While with feeble stimulus the positive 

 wave alone was transmitted, with stronger, both negative and 

 positive were propagated, but the more intense negative was 

 liable to mask the feeble positive. As the negative wave 

 was dependent on the conductivity of the tissue for its 

 propagation, we have seen that it was possible to separate 

 the two by any means which would diminish the conductivity 

 of the tissue. By such means the negative could be made to 

 lag behind the positive ; or, by its complete suppression, it 

 was even possible to exhibit the positive alone. Thus a 

 tissue which normally gave only negative response, owing to 

 the masking of the positive, might, by the depression of its 

 conductivity, be made to give diphasic, or positive response 

 alone (p. 530). 



So far then, as regards the detection of two nervous im- 

 pulses of opposite sign by means of the delicate mechanical 

 method. The same facts may also be demonstrated by the 

 less sensitive method of electrical response, according to 



