268 SENSATION AND THE SENSIFEROUS ORGANS. [LECT. 



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, 

 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 have anything to do with 

 that epidermis of which gustatory and tactile, and at 

 any rate the lower forms of auditory and visual, 

 organs are obvious modifications. 



Whatever be the apparent diversities among the 

 sensiferous apparatuses, however, they share certain 

 common characters. Each consists of a receptive, a 

 transmissive, and a sensificatory portion. The essen- 

 tial part of the first is an epithelium, of the second, 

 nerve fibres, of the third, a part of the brain ; the sensa- 

 tion is always the consequence of the mode of motion 

 excited in the receptive, and sent along the trans- 

 missive, to the sensificatory part of the sensiferous 

 apparatus. And, in all the senses, there is no like- 

 ness whatever between the object of sense, which is 

 matter in motion, and the sensation, which is an 

 immaterial phenomenon. 



On the hypothesis which appears to me to be the 



