ON ZOOLOGY. 129 



or more properly its membranous expansion, that 

 of hearing, the sensation of sound is communi- 

 cated to the brain, and by a repetition of this sen- 

 sation, they learn to comprehend its varieties so as 

 to increase their means of discriminating objects 

 or events, by which they can still further recognize 

 each other, or avoid danger upon its approach. 

 By a third organ, that of smell, they possess a 

 nerve, which is excited by the gas, or effluvia, 

 of certain bodies produced by chemical agency 

 or by natural evaporation, through which they 

 can distinguish agreeable or disagreeable, inju- 

 rious or salutary substances, and trace their ene- 

 mies, or more readily pursue their prey. By the 

 excitement of a fourth nerve, they acquire the 

 attribute of taste, by which the selection of their 

 food is regulated, and (as already explained) one 

 of their principal enjoyments produced. And by 

 a general expansion of nervous membrane over 

 the whole of the extreme surface, namely the 

 skin, they acquire the sense of common feeling, 

 which conveys to them a consciousness of injury 

 when any external part is unduly excited, and 

 qualifies some of them to distinguish substances 

 by the touch. 



Independently of these five sources of sensa- 

 tion, most animals are made up of certain fleshy 

 portions, called muscles, to which nerves, com- 

 municating directly or indirectly with the brain, 

 have access. And these, by some unknown 



