470 THE HUMAN BODY. 



and the brain be severed no stimulation of the organ will 

 call forth a sensation. The final production of this clearly 

 depends, then, on something occurring in the brain, and so 

 the kind of a sensation is presumably dependent upon brain 

 events rather than on occurrences in sense-organs. Still it 

 might be that something in the sense-organ caused one sen- 

 sation to differ from another. Each organ might excite the 

 brain in a different way and cause a different sensation, and 

 so our sensations differ because our sense-organs did. Such a 

 view is, however, negatived by observations which show that 

 perfectly characteristic sensations can be felt in the absence 

 of the sense-organs through which they are normally ex- 

 cited. Persons whose eyeballs have been removed by the 

 surgeon, or completely destroyed by disease, have frequently 

 afterwards definite and unmistakable visual sensations, quite 

 as characteristic as those which they had while still possess- 

 ing the visual end organs. The tactile sensations felt in am- 

 putated limbs, referred to on p. 464, afford another example 

 of the same fact. The persons still feel things touching their 

 legs or lying between their long-lost toes; and the sen- 

 sations are distinctly tactile and not in any way less different 

 from visual or auditory sensations than are the touch-feel- 

 ings following stimulation of those parts of the skin which 

 are still possessed. It is, then, clear_thai.the modality of our 

 sensations is to be sought deeper than^ injjroperties of the 

 end oi^aiiS"rrHiienn^'v^s~oTeach~ sense. 



Properties ""61 external: -forces-a-nd properties of periphe- 

 ral nerve-organs being excluded as causes of differences in 

 kind of sensation, we come next to the sensory nerve-fibres 

 themselves. Is it because optic nerve-fibres are different 

 from auditory nerve-fibres that luminous sensations are 

 different from sonorous? This question must be answered 

 in the negative, for we have already (p. 193) seen reason to 

 believe that all nerve-fibres are alike in essential structure 

 and that their properties are everywhere the same; thatVaJL 

 they do is to transmit "nervous impulses" when j?xci ted, 

 andTHat, no matter" What the excitant, these impulses are 

 molecular movements, always alike in kind, though they 

 may differ in amount and in rate of succession. Since, 



