12 A COURSE ON ZOOLOGY. 



All living beings require oxygen for the accomplishment 

 of the various phenomena of which their organizations 

 form the theatre ; they derive this gas, which is absolutely 

 necessary for their existence, directly or indirectly from 

 the atmosphere. Animals and terrestrial vegetables 

 are in direct contact with the air; aquatic animals 

 and vegetables support themselves by the air held dis- 

 solved in water. This dissolved air represents ordina- 

 rily only one-eighth the volume of water, but it contains 

 a proportion of oxygen very much greater than that 

 found in the atmosphere ; hence there is a sort of com- 

 pensation. 



If for our study we select an animal breathing the 

 atmosphere by the aid of lungs, there is found a very 

 marked difference between the composition of the air 

 which enters the respiratory organs and that which is 

 expelled from the same organs. In the last case but 

 sixteen or sixteen and a half per cent, of oxygen are 

 found, instead of the twenty per cent, present in the 

 former ; on the other hand, carbonic acid gas, which is 

 found in an almost negligible proportion in the atmos- 

 phere, represents at least four per cent, of the air that 

 leaves the lungs ; in addition, vapor of water is present 

 in as large a proportion as the relatively high tempera- 

 ture of the exhaled air will permit, while the atmosphere 

 rarely contains a quantity bordering on saturation. Dis- 

 appearance of part of the oxygen, and notable increase 

 in the proportions of carbonic acid gas and water 

 vapor, are, then, the phenomena which seem to result 

 from the passage of air through the respiratory organs. 



These phenomena are exactly the same as those pro- 

 duced by all ordinary combustion, and it is not surprising 

 that at a very early date the two orders of phenomena 



