376 THE SENSORIAL FUNCTIONS. 



which take place in the organs interposed be- 

 tween the external impressions and the nerves, 

 are still more remarkable and important, and 

 will be respectively the subjects of separate 

 inquiries. The objects of these senses, as well 

 as those of smell, being situated at a distance, 

 produce their first impressions by the aid of 

 some medium, 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 phe- 

 nomena belonging 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. 



