314: ON SENSATION AND THE UNITY OF 



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 trans- 

 missive nerve fibres. The light, impinging 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 sen- 

 sation if indeed all animals which possess eyes 

 are endowed with what we understand as sensa- 

 tion. 



In the higher animals, a complicated apparatus 

 of lenses, arranged on the principle of a camera 

 obscura, serves at once to concentrate and to in- 

 dividualise the pencils of light proceeding from, 

 external bodies. But the essential part of the 

 organ of vision is still a layer of cells, which have 

 the form of rods with truncated or conical ends. 

 By what seems a strange anomaly, however, the 

 glassy ends of these are turned not towards, but 

 away from, the light: and the latter has to 

 traverse the layer of nervous tissues with which 

 their outer ends are connected, before it can affect 

 them. Moreover, the rods and cones of the 

 vertebrate retina are so deeply seated, and in 

 many respects so peculiar in character, that it 

 appears impossible, at first sight, that they can 



