(58 ZOOL<<-\. 



nuitad with the atmospheric air, parts with its carbonic 

 acid, and becomes charged with oxygen.* 



As regards the source of the carbonic acid gas contained ill 

 the blood, and thus exhaled during the respiratory act, then- 

 is reason to believe that it originates in the union of the 

 oxygen absorbed with the carbon of the organic particles in 

 all parts of the body, whether contained in the blood or 

 removed from the living tissues. The essential act, then, of 

 respiration seems to be a combustion going on in the depth of 

 t he t issues, and the exchanges effected in the lungs are only the 

 preliminaries to this work. 



J127. Activity of Respiration. Respiration, essential 

 I life, varies in activity in different animals. 



In birds it is the most active; they consume more air in a 

 given time, proportionally, than any other class of animals, 

 and they soonest die asphyxiated when deprived of it. 



Mammals have also a very active respiration, and many 

 experiments have been made to determine the quantity of 

 oxygen required by man in a given time. Now this has 

 been found to vary with age and a variety of circumstances ; 

 about 500 quarts, or rather more per day, may be assumed to 

 be the average. Now, oxygen forms only 21 per cent, of the 

 atmospheric air : hence about 2750 quarts of air are required 

 per hour for the support of this respiration. 



Animals of the inferior classes generally, and especially 

 those living in water, have the respiration much less active. 



To meet this enormous consumption of oxygen, which 

 would surely end in the destruction of life on the globe if 

 not obviated, nature employs the respiration of plants. 



Vegetables absorb the carbonic acid gas spread through 

 the atmosphere, and under the influence of the sun's rays 

 they extract the carbon, and set free the oxygen. Thus 

 animals supply carbon to vegetables, while the latter return 

 oxygen to animals.f 



128. There exists a kind of relation between the viva- 



* It is right to observe, that the quantity of carbonic acid gas contained in 

 the blood, though small, iasuilicicnt to account for the volume of this gas set 

 free during MqpmtiOB. Thus, in man the blood contain- at lra-t Mh of its 

 volume of gas, and as the quantity of blood which traverses the lungs in a 

 ininulf may !> valuc<l at \iM cubic inches, at.out fifty cubic inches of this 

 gas must pass in the same time. Now tin- highest valuation of the _ 

 otl in rrspiratwn during a minute does not rvcrol t wrnt v ->r\ en cubic inches. 



t It might be supposed that the air of cities must be less pure in respect 

 ol' tin- amount of oxygen than that of the country; but experiment shows 

 that thin is not the case. 



