122 Professor Huxley [March 7, 



In the higher animals, a complicated apparatus of lenses arranged 

 on the principle of a camera obscura, serves at once to concentrate 

 and to individualize 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 firsisight, 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 appara- 

 tuses, however, they share certain common characters. Each consists 

 of a receptive, a transmissive, 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 trans- 

 missive, to the sensorial 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 sensation, which is an 

 immaterial phenomenon. 



On the hypothesis which appears to me to be the most convenient, 

 sensation is a product of the sensiferous apparatus caused by certain 

 modes of motion which are set up in it by impulses from without. 

 The sensiferous apparatuses are, as it were, factories, all of which at 

 the one end receive raw materials of a similar kind — namely, modes 

 of motion — while, at the other, each turns out a special product, the 

 feeling which constitutes the kind of sensation characteristic of it. 



Or, to make use of a closer comparison, each sensiferous apparatus 

 is comparable to a musical-box wound up; with as many tunes as 

 there are separate sensations. The object of a simple sensation is the 

 agent which presses down the stop of one of these tunes, and the more 

 feeble the agent, the more delicate must be the mobility of the stop. 



But, if this be the case, if the recipient part of the sensiferous 

 apparatus is, in all cases, merely a mechanism affected by coarser or 

 finer kinds of material motion, we might expect to find that all sense 

 organs are fundamentally alike, and result from the modification of 

 the same morphological elements. And this is exactly what does 

 result from all recent histological and embryological investigations. 



It has been seen that the receptive part of the olfactory apparatus 

 is a slightly modified epithelium, which lines an olfactory chamber 

 deeply seated between the orbits in adult human beings. But, if we 

 trace back the nasal chambers to their origin in the embryo, we find 

 that, to begin with, they are mere depressions of the skin of the fore 



