310 THE VITAL FUNCTIONS. 



§ 3. Atmospheric Respiration. 



The next series of structures which are to come 

 under our review, comprehends all those adapted 

 to the respiration of atmospheric air in its 

 gaseous form ; and their physiology is no less 

 diversified than that of the organs by which 

 water is respired. 



Air may be respired by tracJiecBy or by pul- 

 monary cavities ; the first mode is exemplified in 

 insects ; the second is that adopted in the larger 

 terrestrial animals. 



The greater part of the blood of insects being 

 diffused by transudation through every internal 

 organ of their bodies, and a small portion only 

 being enclosed in vessels, and circulating in them, 

 the salutary influence of the air could not have 

 been generally extended to that fluid by any of 

 the ordinary modes of respiration, where the 

 function is carried on in an organ of limited extent. 

 As the blood could not be brought to the air, it 

 became necessary, therefore, that the air should 

 be brought to the blood. For this purpose there 

 has been provided, in all insects, a system of 

 continuous and ramified vessels, called trachece, 

 distributing air to every part of the body. The 

 external orifices, from which these air tubes 



