WHY OUR SENSATIONS DIFFER. 471 



then, all that the optic nerve does is to send nervous impulses 

 to the brain, and all that the auditory and gustatory and 

 touch and olfactory nerve-fibres do is the same, and these 

 impulses are all alike in kind, we cannot explain the differ- 

 ence in quality of visual and other sensations by any dif- 

 ferences 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 decom- 

 poses 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 

 primarily differ because different central nerve-organs in 

 the brain aro concerned in their production. That just as 

 an efferent nerve-fibre will, when stimulated, cause a secre- 

 tion 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 proper- 

 ties of our own cerebral nervous system. For each special 

 sense we have a nervous apparatus with its peripheral 

 terminal organs, nerve-fibres, and brain-centres; and the 

 excitement of this apparatus, no matter in what way, causes 

 a sensation of a given modality, determined by the proper- 

 ties 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 stimulating 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, frequently so vivid, and halluci- 

 nations, are also probably in many cases due to direct 

 excitation of the central organs of sensory apparatuses, 

 though no doubt also often due to peripheral stimulation. 

 But no matter how or where the apparatus is excited, pro- 

 vided a sensation is produced it is always of the modality 

 of that sense apparatus. 



