98 THE POPULAR SCIENCE MONTHLY. 



are turned not toward, 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 appara- 

 tuses, 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-fibers, 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 feel- 

 ing 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- 

 part of the head, lined by a continuation of the general epidermis. 

 These depressions become pits, and the pits, by the growth of the ad- 

 jacent parts, gradually acquire the position which they finally occupy. 



* " Chaque fibre est une espece de touche ou de marteau destine a rendre un certain 

 ton." Bonnet, "Essai de Psychologie," chap. iv. 



