SENSATION AND THE SENSIFEROUS ORGANS. 2T5 



In tbe higher animals, a complicated apparatus of 

 lenses, arranged on the principle of a camera obscura, 

 serves at once to concentrate and to individualise 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, how- 

 ever, 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 

 have anything to do with that epidermis of which gus- 

 tatory and tactile, and at any rate the lower forms of 

 auditory and visual, organs are obvious modifications. 



"Whatever be the apparent diversities among the sen- 

 siferous apparatuses, however, they share certain com- 

 mon characters. Each consists of a receptive, a trans- 

 missive, and a sensificatory portion. The essential part 

 of the first is an epithelium, of the second, nerve fibres, 

 of the third, a part of the brain ; the sensation is always 

 the consequence of the mode of motion excited in the 

 receptive, and sent along the transmissive, to the sen- 

 sificatory part of the sensiferous apparatus. And, in all 

 the senses, there is no likeness whatever between the 

 object of sense, which is matter in motion, and the sen-- 

 sation, which is an immaterial phenomenon. 



