274: SENSATION AND THE SENSIFEROUS ORGANS. 



ous or luminous bodies. Nevertheless, the receptive ap- 

 paratus 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 gen- 

 eral 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, impinging on these visual rods, pro- 

 duces a change in them which is communicated to the 

 nerve fibres, and, being transmitted to the sensorium, 

 gives rise to the sensation if indeed all animals which 

 possess eyes are endowed with what we understand as 

 sensation. 



