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THE SENSORIAL FUNCTIONS. 



from too violent an action by a secretion ex- 

 pressly provided for that purpose. In the senses 

 of hearing and of vision, the changes which take 

 place in the organs interposed between the ex- 

 ternal impressions and the nerves, are still more 

 remarkable and important, and will be respec- 

 tively 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 con- 

 veyed to our organs. Hence, in order to under- 

 stand the whole series of phenomena belonging 

 to sensation, regard must be had to the physical 

 laws which regulate the transmission of these 

 agents. We are now to consider these inter- 

 mediate processes in the case of each of the 

 senses. 



