SENSATION AND THE SENSIFEROUS ORGANS 311 



sun which stamps the sense organ; and this, 

 translated into modern language, means nearly 

 the same thing as Hartley's vibrations. Thus we 

 are prepared for what happens in the case of the 

 auditory and the visual senses. For neither the 

 ear, nor the eye, receives anything but the impulses 

 or vibrations originated by sonorous or luminous 

 bodies. Nevertheless, the receptive apparatus 

 still consists of specially modified epithelial cells. 

 In the labyrinth, or essential part of the ear of 

 the higher animals, the free ends of these cells 

 terminate in excessively delicate hair-like fila- 

 ments; 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 in- 

 sensible 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 ap- 

 paratus 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 



