CHAPTER VII. 



RESPIRATION. 



THE study of digestion and circulation has taught the reader the 

 nature of the methods and the avenues along which ingested mate- 

 rials must pass in the processes of their elaboration in order to main- 

 tain the requirements of life. It has also made him acquainted with 

 the various forms under which those materials became absorbable and 

 miscible with the blood, and which must necessarily be renewed in 

 proportion as the latter is changed by the nutrient movement. It is 

 known, too, that the liquid and soluble products of digestion and the 

 lymph itself, when poured into the venous blood, do not have the qual- 

 ities of a directly nutrient fluid immediately after their mixture 

 with the blood. In order that these qualities should develop it is 

 necessary that there should occur the intervention of an essential ele- 

 ment, which animals find in, and incessantly draw from, the envelop- 

 ing atmosphere oxygen. The latter is the great agent in the final 

 transformations which the various organic matters must undergo. 

 The introduction of a certain proportion of oxygen into the economy 

 is, therefore, the first aim of the function of respiration. 



The general tendency of the various gases to mingle even when 

 wet membranes separate them has been pointed out. Looked at in 

 its essential character, the respiration of animals consists in a single 

 exchange of gases which takes place during the action exercised by 

 the air upon the blood. In fact, atmospheric oxygen, brought into 

 contact with a thin, membranous wall, passes through it and pene- 

 trates the blood, while the carbonic-acid gas contained in that liquid 

 is freed from it through the same membrane. Therefore, if respira- 

 tion, on the one hand, takes something away from the blood, on the 

 other, it communicates to it a principle which renders it suitable to 

 complete the organs, furnish material for their secretions, or to repair 

 their losses, while, at the same time, it gives rise to a disengage- 

 ment of heat indispensable to the free exercise of the functions. It 

 is this vivifying principle which combines with the organic matters 

 of the blood to form the water and carbonic acid that are unceas- 

 ingly eliminated by expiration and which are soon decomposed in 

 the atmosphere under the influence of solar radiation, to furnish car- 

 bon and hydrogen to vegetation. 

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