314 THE VITAL FUNCTIONS. 



more tenacious of life under these circum- 

 stances than the larger animals, and often, after 

 being apparently dead, revive on the readmis- 

 sion of air. 



Aquatic insects have tracheae, like those living 

 in air, and are frequently provided with tubes, 

 which are of sufficient length to reach the sur- 

 face of the water, where they absorb air for res- 

 piration. In a few tribes a complicated mode 

 of respiration is practised ; aerated water is 

 taken into the body, and introduced into cavities, 

 where the air is extracted from it, and trans- 

 mitted by the ordinary tracheae to the different 

 parts of the system.* 



Such, then, is the extensive apparatus for 

 aeration in animals, which have either no circu- 

 lation of their nutritious juices, or a very im- 

 perfect one ; but no sooner do we arrive at the 

 examination of animals possessing an enlarged 

 system of blood vessels, than we find nature 

 abandoning the system of tracheae, and employ- 

 ing more simple means of effecting the aeration 



* Mr. Dutrochet conceives that the principle on which this 

 operation is conducted is the same with that by which gases are 

 reciprocally transmitted through moistened membranes ; as in 

 the experiments of Humboldt and Gay Lussac, who, on enclosing 

 mixtures of oxygen, nitrogen, and carbonic acid gases, in any 

 proportion, in a membranous bladder, which was then immersed 

 in aerated water, found that there is a reciprocal transit of 

 the gases ; until at length pure atmospheric air remains in the 

 cavity of the bladder. 



