SEC. 4. ON THE MODE OF DEVELOPMENT OF 

 CUTANEOUS SENSATIONS. 



886. 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 profitable 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 our- 

 selves in the first instance to sensations of touch proper, that is to 

 sensations of mere contact and pressure, discussing 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 im- 

 pulses 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 psychological 

 examination of our sensations ; our objective knowledge 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 750 the statement that stimulation 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 stimula- 

 tion of the fibres themselves, gives rise to a visual sensation and 

 must therefore be regarded as a visual impulse. 



This view, under the title of the doctrine of " the specific energy 

 of nerves," has been extended to the nerves of the other special 



