316 ON SENSATION AND THE UNITY OP 



sensiferous apparatus is comparable to a musical- 

 box wound up; with as many tunes as there are 

 separate sensations. The objects of a simple sen- 

 sation 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 true, if the recipient part of the 

 sensiferous apparatus is in all cases, merely a me- 

 chanism 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 in- 

 vestigations. 



It has been seen that the receptive part of the 

 olfactory apparatus is a slightly modified epithe- 

 lium, 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 adjacent parts, 



* " Chaque fibre est une espece de touche ou de marteau 

 destine a rendre un certain ton." Bonnet, Esaai de 

 chologie, chap. iv. 



