314 THE VITAL FUNCTIONS. 



air exhausted ; but they are generally more te- 

 nacious of life under these circumstances than 

 the larger animals, and often, after being appa- 

 rently dead, revive on the readmission of air. 



Aquatic insects have trachese, 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, 

 when 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 trachese, 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 

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

 cavity of the bladder. 



