SECT. i. WASTE AND REPAIR. 3 



to all the tissues during its circulation ; for with every 

 breath, with every effort, muscular or mental, with 

 every motion, voluntary or involuntary, at every instant 

 of life, asleep or awake, part of the muscular and ner- 

 vous substances becomes dead, separates from the living 

 part, is returned to the circulation, combines with the 

 oxygen of the blood, and is removed from the system, 

 the waste being ordinarily in exact proportion to the 

 exertion, mental and physical. Hence food, assimilated 

 into blood, is necessary to supply nourishment to the 

 muscles, and to restore strength to the nervous system, 

 on which all our vital motions depend ; for, by the nerves, 

 volition acts upon living matter. Waste and repair is a 

 law of nature, but when nature begins to decay, the 

 waste exceeds the supply. 



However, something more than food is necessary, for 

 the oxygen in the blood would soon be exhausted were 

 it not constantly restored by inspiration of atmospheric 

 air. The perpetual combination of the oxygen of the 

 air with the carbon of the blood derived from the food 

 is a real combustion, and the cause of animal heat ; but 

 if the carbonic acid gas produced by that chemical union 

 were not continually given out by the respiratory organs, 

 it would become injurious to the animal system. Thus 

 respiration and the circulation of the blood are mutually 

 dependent; the activity of the one is exactly propor- 

 tional to that of the other : both are increased by exer- 

 cise and nervous excitement. 



External heat is no less essential to animals than to 

 vegetables ; the development of a germ or egg is as de- 

 pendent on heat as that of a seed. The amount of 

 heat generated by respiration and that carried off by 

 the air is a more or less constant quantity ; hence, in 

 hot countries, rice and other vegetable diet is sufficient, 

 but as the cold increases with the latitude, more and 



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