496 THE HUMAN BODY. 



here also c.oser examination shows that we must seeK farther. 

 Sensation is not produced in a sense-organ, but far away 

 from it in the brain; the organ is merely an apparatus for 

 generating nervous impulses. If the optic nerves be divided, 

 no matter how perfect the eyeballs, no amount of light will 

 arouse visual sensations; if the spinal cord be cut in the 

 middle of the back no pressure on the feet will cause a tactile 

 or other feeling; though the skin, and its nerves and the 

 lower half of the spinal cord be all intact. In all cases we 

 find that if the nerve-paths between a sense-organ 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 sensa- 

 tion 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 do. 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 

 excited. 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, already referred to, 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-feelings 

 following stimulation of those parts of the skin which are still 

 possessed. It is, then, clear that the modality of our sensa- 

 tions is to be sought deeper than in properties of the end- 

 organs of the nerves of each sense. 



Properties of external forces and properties of periph- 

 eral 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 dif- 

 ferent from sonorous ? This question must be answered in 



