OF RESPIRATION. 97 



maintain their elevated temperature during the whole year ; 

 but pass the winter in a sort of lethargy called HIBERNATION, 

 or the hibernating sleep. The marmot, the bear, the bat, 

 the crocodile and most reptiles, furnish examples. During 

 this state the animal takes no food ; and as it respires only 

 after very prolonged intervals, its heat is diminished, and its 

 vital functions generally are much reduced. The structural 

 cause of hibernation is not ascertained ; but the phenomena 

 attending it fully illustrate the laws already stated (254-8). 

 260. There is another point of view in which respiration 

 should be considered, namely, with reference to the specific 

 gravity of animals, or their power of rising in the atmo- 

 sphere, and of living at different depths in the water, under 

 a diminished or increased pressure. The organs of respira- 

 tion of birds and insects are remarkably adapted for the pur- 

 pose of admitting at will a greater quantity of air into their 

 body, the birds being provided with large pouches extending 

 into the abdominal cavity and into the bones of the wing ; 

 whilst in insects the whole body is penetrated by air tubes 

 enlarged at intervals into wider cells. Aquatic animals are 

 all provided with minute, almost microscopic water-tubes, 

 penetrating from the surface into their substance, or 

 the cavity, by which the body is adapted to pressures 

 which otherwise would crush the animal. In fishes, these 

 water-tubes penetrate through the bones of the skull, and 

 through skin and scales; in mollusks they are more nu- 

 merous in the fleshy parts, as for example, in the foot ; 

 in echinoderms they pass through the skin, and even 

 through the hard shell, whilst in polyps they perforate the 

 walls of the general cavity of the body. 



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