CHAPTER II 

 FOOD-DIGESTION AND RESPIRATION 



THE first question that occurs to us in this connection is Why 

 do living beings require to take food, that is, some material 

 from the outer world which supplies something that they are in 

 need of? 



When such a creature is actually engaged in making greater 

 the amount of substance in its body, is growing, as we say, 

 it is quite clear that this extra substance must be obtained from 

 outside, and what is taken in this way must contain the correct 

 chemical constituents that are wanted to make up the new body 

 tissues. 



Further, even in the adult, when growth has ceased, there is a 

 certain loss of material, due to wear and tear in the process of 

 activity, as well as the growth of some parts, such as hair, which 

 continues to take place. As a motor car uses up tyres, piston 

 rings, bearings, etc., so the cell machinery requires replacement 

 of parts worn out. This is sometimes called maintenance^ and 

 has practically the same requirements as growth. But not 

 altogether, since there is evidence that some parts, once con- 

 structed, never require replacement, somewhat like the fly-wheel 

 of a petrol motor, which lasts as long as the engine itself, apart 

 from accidents. 



The amount of food needed by the adult for the purpose of 

 replacing wear and tear is very small. It might be expected, for 

 example, that the structure of muscle would be worn away to some 

 perceptible degree by vigorous exercise. It is a rather remarkable 

 fact that it has been found impossible to obtain evidence of any 

 loss of the actual muscular structure itself, except after such severe 

 work as to be abnormal. There is more evidence of wear and tear 

 in some peculiar forms of muscular work, as we shall see later. It 

 might, perhaps, be said that in ordinary muscular work the products 

 of the wear and tear are used up again to repair the machine. 

 This may be so, but, as far as the necessity of supply from the 

 outside is concerned, the result is the same. 



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