X.] SENSATION AND THE SENSIFEROUS ORGANS. 269 



most convenient, sensation is a product of the sensifer- 

 ous 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 charac- 

 teristic 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. 1 



But, if this be true, if the recipient part of the 

 sensiferous apparatus is, in all cases, merely a mechan- 

 ism 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 modifica- 

 tion 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, 



1 " Charlie fibre est une espece de touche ou de marteau destine a 

 rendre un certain ton." Bonnet, " Essai de Psychologic," chap. iv. 



