CHAP, i.] SENSORY NERVES. 489 



sense-organ in the course of the changes through which the sensory 

 impulses give rise to a sensation. Thus a stimulus of any kind 

 applied to the optic nerve along any part of its course, if it is able 

 to start any impulses at all, gives rise to a sensation of light, and 

 precisely the same stimulus applied to the acoustic nerve along any 

 part of its course gives rise to a sensation of sound; and so on. All 

 the evidence we possess goes against the view that a piece of optic 

 nerve, deprived of both its peripheral and central endings, differs in 

 function from a similarly isolated piece of acoustic nerve; such facts 

 as are within our knowledge go to shew that the disturbances 

 generated in a piece of optic nerve by a galvanic current are the 

 same as those generated in a piece of acoustic nerve. We are 

 therefore driven to the conclusion that the difference which appears 

 when the central endings are intact arises in the central organs. 



In all these differentiated sensory mechanisms, or special senses 

 as they are called, we have then to deal with two elements : the 

 peripheral sense-organ, in which we have to study how the special 

 physical agent gives rise to special sensory impulses; and the 

 central sense-organs, in which our study is confined to the manner 

 in which these special impulses modify the operations of the 

 central nervous system. Inasmuch as in a normal body the 

 peripheral organ remains in connection with the central organ, and 

 our study of the special senses is carried on chiefly by subjective 

 observations in which we make use of our own consciousness, it 

 frequently becomes very difficult to distinguish in any given 

 sensation the peripheral from the central element. The two 

 become more distinct, the more complex the sense and the more 

 highly organised the sense-organs. For this reason it will be most 

 convenient to commence our study of the special senses with the 

 sense of vision. 



