ANIMAL RESPIRATION-THE BREATH 



OF LIFE 



CHAPTER XXXI 



ANIMAL RESPIRATION— GENERAL PRINCIPLES- 

 BREATHERS IN WATER AND BREATHERS IN AIR 



GENERAL PRINCIPLES 



The essential nature and purpose of breathing or respiration 

 have been explained elsewhere (vol. i, p. 45), but it may be de- 

 sirable in this section to call attention to a few points of general 

 interest. The living substance of which the bodies of animals are 

 more or less composed is of exceedingly unstable nature, and is 

 constantly breaking down into simpler substances (see p. 3), this 

 process of waste being continually counterbalanced by the taking 

 in of food, which is built up into fresh body-substance. An animal 

 may, in fact, be regarded as a self-repairing machine. In the last 

 section we have considered very fully the food and feeding of 

 animals, and have seen that the necessity for repair of waste, and 

 for growth to a certain size, exert a very far-reaching influence 

 upon bodily characteristics and habits. 



We are now more particularly concerned with the other part of 

 the cycle of chemical changes which incessantly goes on within 

 the living body, i.e. with the down-breaking processes or processes 

 of waste. It is these which yield the obvious or actual energy 

 necessary for the performance of all the vital actions, including 

 the heat which is so characteristic of warm-blooded active creatures 

 like mammals and birds. Such down-grade chemical processes, as 

 they may perhaps be called, are quite comparable to those which 

 go on in a burning candle or lamp, i.e. they are a kind of combus- 

 tion. And both in the case of the animal body, and in the case of 



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