SENSE-ORGANS 345 



We may distinguish in each sense-organ an essential 

 and an accessory part. 



The essential part of each sense-organ is composed 

 of minute organs, which upon examination appear to be 

 in reality modified epithelial cells ; and the delicate ter- 

 minations of the nerve filaments distributed to the sense- 

 or-an may, ^vith more or less distinctness, be traced to 

 these modified cells, in which indeed they seem to end. 

 These minute organs, these modified epithelial cells, may 

 be spoken of as sense-organules ; they serve as inter- 

 mediators in each case between the physical agent of the 

 sensation and the sensory nerve. The physical agent is 

 by itself unable to produce in the fibres of the sensory 

 nerve those changes which, reaching the brain as nervous 

 impulses, give rise to the special sensations. Thus, as we 

 shall presently see, rays of light falling upon the optic 

 nerve cannot give rise to a sensation of sight. The physical 

 a-ent must act first on the^ sense-organules, and these in 

 turn act upon the filaments of the nerve. Thus light 

 fallina upon the sense-organules, situated in that essential 

 part o'f the eye called the retina, sets up changes in them, 

 these changes set up corresponding changes m the delicate 

 nerve filaments which with the sense-organules go to 

 make up the retina, and the changes in the nerve filaments 

 propagated along the optic nerve to the brain give rise, 

 in the latter, to sensations of sight. 



Hence in the essential part in each sense-organ we have 

 to distinguish between the sense organules, i.e., the modi- 

 fied epithelium, and the terminal expansion of the sensory 

 nerve ■ and further, in each sense-organ, there is added to 

 this essential part a more or less complicated accessory 



part. . 



Lastly, in all these special sensations, there are certain 

 phenomena which arise out of the structure of the sense- 

 organ and others which result from the operation of the 

 central apparatus of the nervous system upon the materials 

 supplied to it by the sense-organ. 



