472 THE HUMAN BODY. 



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, might 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 temperature sensations were but extremes of one gene- 

 ral kind of feeling; that they were of the same "modal- 

 ity;" and comparable, 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 (p. 563). 

 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 if all clashed together may cause pain, 

 although the same brain-centres are stimulated in the two 

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

 merely due to the fact that it excites the nerves very 

 powerfully and gives rise to impulses which radiate farther 

 in the brain than those causing touch sensations, and so 

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

 tion. 



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- 



