SEC. 3. ON THE MODE OE DEVELOPMENT OF 

 CUTANEOUS SENSATIONS. 



§ 654. Our studies so far point to the conclusion that sensa- 

 tions of touch and temperature stand on the same footing as 

 visual, auditory and other special sensations , and it will be profit- 

 able now to compare in some detail the former with the latter. 

 In doing so we may, in order to make the matter more simple, 

 confine ourselves in the first instance to sensations of touch 

 proper, that is to sensations of mere contact and pressure, dis- 

 cussing later on the relations of these to sensations of heat and 

 cold. 



In studying vision we came to the conclusion that the undula- 

 tions of the ether so affect the rods and cones and other retinal 

 structures as to give rise to visual impulses, and that these visual 

 impulses, travelling up the fibres of the optic nerve to the visual 

 centres, gave rise by means of those centres to the affections of 

 consciousness which we call visual sensations ; we may leave 

 aside in the present instance all reference to the complexity of 

 the visual centres. 



We obtained absolute proof that the only way in which light 

 can give rise to visual impulses in the optic fibres is by acting on 

 the retinal structures. Since the optic fibres are the only nerve 

 fibres in direct connection with the retinal structures visual 

 impulses can be carried by them alone. As we pointed out we 

 know absolutely nothing about the nature of visual impulses 

 themselves ; our conclusions concerning the various characters 

 and kinds of visual impulses are simply deductions from the psy- 

 chological examination of our sensations ; our objective know- 

 ledge of them is limited to the fact that when light falls on a 

 functionally active retina an electric change is developed in the 

 optic fibres. As we mentioned in § 553 the statement that stim- 

 ulation of the optic fibres themselves, as when the optic nerve is 

 cut with a knife, gives rise to visual sensations, has led to the 

 adoption of the view that any impulse passing along the optic 

 fibres, however started, whether by the action of light on the 

 retina, or by direct stimulation of the fibres themselves, gives rise 



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