INTRODUCTION. 



man and other animals. If it were possible for 

 you to run as swiftly as the birds can fly, in a 

 few minutes you would become almost breathless, 

 and quite exhausted with the exertion. In order 

 that they may be able to sustain the great efforts 

 they make in flight, and the wonderful speed 

 with which they move, they are formed in such 

 a manner as to have a store of breath provided 

 for their great necessities. To secure this ad- 

 vantage, there are air-vessels, or little cavities 

 for the reception of air, almost all over their 

 bodies; even in their bones, where air often sup- 

 plies the place of marrow, as you may observe 

 in the bones of a chicken, which have a much 

 smaller proportion of marrow in them than the 

 bones of quadrupeds. This singular provision 

 of nature, besides enabling them to breathe more 

 freely, increases their bulk without adding to 

 their weight, and by that means gives a larger 

 space for the muscles to act on, and so pro- 

 motes the facility of their flight in another way 

 This extension of the air-vessels, also effects more 

 speedily the changes in the blood, and thus fits it 

 for a more rapid circulation ; by this means greatly 

 increasing the heat of their little bodies, and ena- 

 bling them to meet, without injury, all the changes 

 of temperature to which they are subject in their 

 passage through the air. The better to fit them 



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