CHEMICAL EFFECTS OF RESPIRATION. 157 



description of the circulation. What a masterly ! what 

 a wonderful piece of mechanism this ! A surface of 

 air several hundred feet in extent, and a surface of 

 blood of similar dimensions, all within the bulk of eight 

 or ten inches long, and five or six in diameter. Where 

 shall we look for a parallel, even among the effects of 

 Infinite wisdom ? 



CHEMICAL EFFECTS OF RESPIRATION. 



^ atmosphere, as already stated, acts upon the 

 blood through the tissue of vessels in which both fluids 

 are confined. The most obvious effects resulting from 

 this action, is the change of color from the dark purple 

 hue which the venous blood has, when it comes to the 

 lungs, to the bright vermillion which it assumes when it 

 returns to the left ventricle of the heart. 



The atmospheric air which produces this change, is 

 composed of 20 parts of oxygen gas, and 80 parts of 

 nitrogen gas, with a variable portion of carbonic acid 

 gas. After the action of the air upon the blood, and 

 when it is again thrown out of the lungs, it is found that 

 a portion of the oxygen which it contained has disap- 

 peared, and that it is replaced, or nearly so by carbonic 

 acid gas. There is also a quantity of watery vapor, 

 always emitted from the lungs at every expiration. 



The quantity of oxygen consumed in respiration, not 

 only depends on the kind of animal, but also on the dif- 

 ferent conditions of the same animal. Thus animals of 

 the lower order, as the Mollusca, require very little ox- 

 ygen, and will live for a long time in an atmosphere in 

 which Birds, and Mammalia have perished for want of 

 it. The same animal also, when exercising vigorously, 

 consumes, and requires more oxygen than when at rest. 



Now carbonic acid is composed of carbon and oxy- 

 gen, and since the oxygen disappears in the act of respi- 

 ration, and there is a like quantity of the former gas 

 found in its place, it is inferred that the oxygen has com- 



What is the most obvious effect of the action of the air on the blood 

 of the lungs ? What is the composition of air ? What is the effect of 

 respiration of the composition of the air respired ? 

 14 



