SENSATION AND SENSE-ORGANS. 497 



the negative, for we have already seen reason to believe 

 that all nerve-fibres are alike in essential structure and that 

 their properties are everywhere the same; that all they do is 

 to transmit "nervous impulses" when excited, and that, no 

 matter what the excitant, these impulses are molecular move- 

 ments, always alike in kind, though they may differ in 

 amount and in rate of succession. Since, then, all that the 

 optic nerve does is to send nervous impulses to the brain, 

 and all that the auditory and gustatory and tactile and olfac- 

 tory nerve-fibres do is the same, and these impulses are all 

 alike in kind, we cannot explain the difference in quality of 

 visual and other sensations by any differences in property of 

 the nerve-trunks concerned, any more than we could attempt 

 to explain the facts that, in one case, an electric current sent 

 through a thin platinum wire heats it, and, in another, sent 

 through a solution of a salt decomposes it, by assuming that 

 the different results depend on differences in the conducting 

 copper wires, which may be absolutely alike in the two cases. 

 We are thus driven to conclude that our sensations pri- 

 marily differ because different central nerve-organs in the 

 brain are concerned in their production. That just as an 

 efferent nerve-fibre will, when stimulated, cause a secretion if 

 it go to a gland-cell, and a contraction if it go to a muscle- 

 fibre, so an optic nerve-fibre, carrying impulses to one brain 

 apparatus and exciting it, will cause a visual sensation, and a 

 gustatory nerve-fibre, connected with another brain-centre, a 

 taste sensation. In other words, our kinds of sensation 

 depend fundamentally on the properties of our own cerebral 

 nervous system. For each special sense we have a nervous 

 apparatus with its peripheral terminal organs, its nerve-fibres, 

 and its brain-centres ; and the excitement of this apparatus, no 

 matter in what way, causes a sensation of a given modality, 

 determined by the properties of its central portion. Usually 

 the apparatus is excited by one particular force acting first 

 on its peripheral organs, but it may be aroused by stimulat- 

 ing its nerve-fibres directly or, as in certain diseased states 

 (delirium), or under the action of certain drugs, by direct 

 excitation of the centres. The sensations of dreams, fre- 

 quently so vivid, and hallucinations, are also probably in 

 many cases due to direct excitation of the central organs of 

 sensory apparatuses, though no doubt also often due to periph- 

 eral stimulation. But no matter how or where the appa 



