72 ZOOLOGY. 



equals pretty nearly the amount of the oxygen absorbed ; the 

 azote exhaled and absorbed are nearly equal, and in addition 

 there is the vapour or pulmonary transpiration. Thus the 

 blood loses carbonic acid gas, azote, and water, whilst it 

 becomes charged with oxygen and azote ; and thus it may be 

 proved that arterial blood holds dissolved much more oxygen 

 than venous blood, and that it is to this gaseous fluid that 

 arterial blood owes its colour and qualities. Respiration 

 consists, then, in the phenomena of absorption and exhalation, 

 by means of which the venous blood, coming in contact with 

 the atmospheric air, parts with its carbonic acid, and becomes 

 charged with oxygen.* 



As regards the source of the carbonic acid gas contained in 

 the blood, and thus exhaled during the respiratory act, there 

 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 

 the tissues, and the exchanges effected in the lungs are only 

 the preliminaries to this work. 



127. Activity of Respiration. Respiration, essential 

 to all 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 



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

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

 free during respiration. Thus, in man the blood contains at least Ath of its 

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

 minute may be valued at 250 cubic inches, about fifty cubic inches of this 

 gas must pass in the same time. Now the highest valuation of the gas given 

 off in respiration during a minute does not exceed twenty-seven cubic inches. 



