PHYSICAL BASIS OF SENSATION 651 



If the sensation be in fact due to definite and ascertainable 

 physico-physiological changes in the nerve, then the various 

 modifications of sensation must, in like manner, be traceable 

 to corresponding modifications in the physico-physiological 

 process. In that case, the particular relation which is known 

 to exist as between stimulus and sensation expressed as 

 Weber-Fechner's Law must be demonstrable as directly 

 dependent upon molecular changes induced, and not on the 

 existence of some assumed psychic factor. This molecular 

 theory, further, if it expresses a truth of universal applica- 

 tion, ought to be capable of explaining not only the 

 quantitative relation between stimulus and sensation, but 

 also that qualitative variation of which Weber-Fechner's 

 Law is unable to take account. Should the Molecular 

 Theory prove adequate to this, its truth may be regarded as 

 demonstrated. 



We shall, however, subject this theory to further and still 

 more crucial tests. If it be true that our sensations, 

 painful and pleasurable, are due to nervous impulses of 

 opposite character, then any modification of either of these 

 impulses by any given agent should appropriately modify 

 the resultant sensation. We have seen, for instance, that 

 the negative wave is complex, and contains within it the 

 masked positive. We have also seen that by appropriate 

 means these two waves may be made to exhibit themselves 

 separately ; or the positive, by the total suppression of the 

 negative, may be displayed alone. I shall therefore show 

 that, by the employment of the same means, the subjective 

 sensation of painful or negative tone may also be analysed 

 into its component parts, which may thus be made to exhibit 

 themselves in succession ; and, on the other hand, that by 



changes as they occur is the emotion. Certain experiments furnish evidence 

 not highly satisfactory that all pleasurable states of consciousness are accom- 

 panied by bodily movements of extension, and all painful by movements of 

 flexion. These movements may be very slight. Miinsterberg concludes that 

 the feeling of agreeableness is the mental accompaniment and outcome of 

 reflexly-produced movements of extension, and disagreeableness of the move- 

 ments of flexion.' Schafer, Text- Book of Physiology , vol. ii. (1900), p. 975. 



