498 THE HUMAN BODY. 



ratus is excited, provided a sensation is produced it is always 

 of the modality of that sense apparatus. 



While in the more specialized senses the modality of the 

 sensation can be ascribed only to brain properties (so that 

 we may be pretty sure that a man, the inner end of whose 

 optic nerve was in physiological continuity with the outer 

 end of his auditory, and the inner end of his auditory with 

 the outer end of his optic, would hear a picture and see a 

 symphony), yet, conceivably, differences in the rhythm or 

 intensity of afferent nervous impulses might cause differ- 

 ences in modality in less differentiated senses. Until quite 

 recently it has been considered possible that tactile and tem- 

 perature sensations were but extremes of one general kind of 

 feeling; that they were of the same "modality;" and com- 

 parable, for example, to the sensations of yellow and blue in 

 the visual set of feelings. This view has now been definitely 

 proved to be inadmissible (Chap. XXXV). The points of the 

 skin which arouse in us the sensations of touch, heat, and cold 

 are all distinct; each one when stimulated gives rise to only 

 one kind of sensation, if any; and always the same kind. A 

 heavy pressure, gradually increased, arouses sensations which 

 pass imperceptibly from touch to pain, and this result may 

 be due to the fact that regular and orderly afferent impulses, 

 determined through tactile nerve-endings, excite the centre 

 in one way; while irregular, disorderly, and violent impulses, 

 originated when the pressure is great enough to directly 

 excite nerve-trunks beneath the skin, may cause a different 

 sensation; much as musical notes properly combined may 

 cause pleasure, but all clashed together may cause suffering, 

 although the same brain-centres are stimulated in the two 

 cases. The pain from a heavy weight may, however, be due 

 to the fact that it excites a different set of nerve-fibres than 

 those connected with tactile feeling, and gives rise to impulses 

 which excite new centres, the modality of which is a pain 

 sensation so great as to cloak concomitant touch sensations. 



However differences in nervous rhythm may account for 

 minor differences in sensation, it remains clear that the 

 characters of our sensations are creations of our own organ- 

 i^m; they depend on properties of our Bodies and not on 

 properties of external things, except in so far as these may 

 or may not be adapted to arouse our different sensory appa- 

 ratuses to activity. From the kind of the sensation we can- 



