OF RESPIRATION. 123 



uniform distribution of heat through the body, has not yet 

 been satisfactorily ascertained. 



259. Some of the higher warm-blooded animals do not 

 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 buoy- 

 ancy of animals, or their power of rising in the atmosphere, 

 and their ability to live at different depths in the water, under 

 a diminished or increased pressure. The organs of res- 

 piration of birds and insects are remarkably adapted for the 

 purpose of admitting at will a greater quantity of air into 

 their body, the birds being provided with large pouches ex- 

 tending from the lungs into the abdominal cavity and into 

 the bones of the wing. In insects the whole be dy is pene- 

 trated by air tubes, the ramifications of their tracheae, which 

 are enlarged at intervals into wider cells ; whilst most of the 

 aquatic' animals are provided with minute, almost micro- 

 scopic tubes, penetrating from the surface into the substance, 

 or the cavities of the body, admitting water into the interior, 

 by which they thus adapt their whole system to pressures 

 which would otherwise crush them. These tubes may with 

 propriety be called water-tubes. In fishes, they penetrate 

 through the bones of the head and shoulder, through skin 

 and scales, and communicate with the blood vessels and 

 heart, into which they pour water ; in mollusks they are 

 more numerous in the fleshy parts, as, for example, in the 



