CHEMICAL EFFECTS OF RESPIRATION. 153 



scription 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 1 



CHEMICAL EFFECTS OF RESPIRATION. 



458. The 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. 



459. The atmospheric air which produces this change, 

 is composed of twenty parts of oxygen gas, and eighty 

 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 disappear- 

 ed, 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. 



460. The quantity of oxygen consumed in respiration, 

 not only depends on the kind of animal, but also on the 

 different conditions of the same animal. Thus animals of 

 the lower order, as the mollusca, require very little oxygen, 

 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. 



461. Now carbonic acid is composed of carbon and 

 oxygen, 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 combined 



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 respi- 

 ration of the composition of the air respired ? 



