SENSATION. 267 



the object and the nerve on which the impression is to be 

 made. The object is never- allowed to come into direct con- 

 tact with the nerves; not even in the case of touch, where 

 the organ is defended by the cuticle, through which the im- 

 pression is made, and by which that impression is modified 

 so as to produce the proper effect on the subjacent nerves. 

 This observation applies with equal force to the organs of 

 taste and of smell, the nerves of which are not only sheathed 

 with cuticle, but defended from too violent an action by a 

 secretion expressly for that purpose. In the senses of hearing 

 and of vision, the changes which take place in the organs 

 interposed between the external impressions and the nerves, 

 are still more remarkable and important, and will be re- 

 spectively the subjects of separate inquiries. The objects of 

 these senses, as well as those of smell, being situated at a dis- 

 tance, produce their first impressions by the aid of some me- 

 dium exterior to our bodies, through which their influence 

 extends: thus, the air is the usual medium through which 

 both light and sound are conveyed to our organs. Hence, 

 in order to understand the whole series of phenemena be-' 

 longing to sensation, regard must be had to the physical 

 laws which regulate the transmission of these agents. We 

 are now to consider these intermediate processes in the case 

 of each of the senses. 



