X.] SENSATION AND THE SENSIFEROUS ORGANS. 267 



vibrations originated by sonorous or luminous bodies. 

 Nevertheless, the receptive apparatus still consists of 

 nothing but specially modified epithelial cells. In the 

 labyrinth of the ear of the higher animals, the free 

 ends of these cells terminate in excessively delicate 

 hair -like filaments ; while, in the lower forms of 

 auditory organ, its free surface is beset with delicate 

 hairs like those of the surface of the body, and the 

 transmissive nerves are connected with the bases of 

 these hairs. Thus there is an insensible gradation in 

 the forms of the receptive apparatus, from the organ 

 of touch, on the one hand, to those of taste and smell ; 

 and, on the other hand, to that of hearing. Even in 

 the case of the most refined of all the sense organs, 

 that of vision, the receptive apparatus departs but little 

 from the general type. The only essential constituent 

 of the visual sense organ is the retina, which forms so 

 small a part of the eyes of the higher animals ; and 

 the simplest eyes are nothing but portions of the 

 integument, in which the cells of the epidermis have 

 become converted into glassy rod-like retinal corpuscles. 

 The outer ends of these are turned towards the light ; 

 their sides are more or less extensively coated with 

 a dark pigment, and their inner ends are connected 

 with the transmissive nerve fibres. The light, imping- 

 ing on these visual rods, produces a change in them 

 which is communicated to the nerve fibres, and, being 

 transmitted to the sensorium, gives rise to the sensa- 

 tion if indeed all animals which possess eyes are 

 endowed with what we understand as sensation. 



In the higher animals, a complicated apparatus of 



