CHAPTER IX 



NUTRITION 



THE USE OF FOOD 



FOOD may be defined as any substance taken in by an organism and made use 

 of for any purpose. The uses of food may be said to be threefold. When an 

 organism is increasing in size, it is clear that the additional matter laid on 

 must be obtained from without. In the adult organism, the main part of the 

 food is used to afford energy for muscular movement, production of heat, etc. 

 But there is also a small but essential part required for the repair of wear and 

 tear on the part of the tissues themselves, even in the adult. This may be 

 thought to be identical with growth, but we shall see later that there is 

 evidence to show that certain things may be necessary for growth, although 

 not so for maintenance. It is as if, after a machine has been constructed, 

 certain working parts only require repair. 



Perhaps an illustration may help to make these differences clear. A petrol 

 motor in process of construction needs the supply of iron, steel, brass, copper, 

 porcelain, insulation material, asbestos, and so on. Some of these cannot be 

 replaced by any other ; insulating material, for example, cannot be replaced 

 by metal. We shall find analogous conditions in the growth of living things. 

 When the engine is completed, fuel must be given in order that work may 

 be done by it. This fuel does not enter as a constituent of the fabric, and 

 corresponds to that part of our food which is utilised for the giving of energy. If 

 the motor is kept at work, certain parts require replacement from time to time, 

 owing to their wearing out : such are piston rings, linings of bearings, etc. These 

 are the analogous parts to that fraction of our food which is needed for repair of 

 tissue waste, or maintenance. We may note that certain parts practically never 

 require renewal, such as the fly-wheel or the framework. The reader will probably 

 ask, what does the lubricating oil represent 1 ? We must not, of course, expect to 

 be able to push our simile to all details and it seems to fail here. The agents 

 known as enzymes in some ways correspond to the lubricating oil, but these are 

 formed by the organism itself. On the whole, water and salts are the nearest food 

 constituents representing the function of the lubricant ; they afford no energy, but 

 are indispensable to the working of the living machine. We may also compare the 

 waste products in the two cases. The products of combustion of petrol escape in 

 the exhaust gases as carbon dioxide and water vapour, just as the same substances are 

 given out in the gases expired from the lungs. The waste lubricating oil carries with 

 it fine particles of metal, worn from the cylinder and bearings ; in a similar way, 

 the water excreted by the kidneys removes the products formed in the wear 

 and tear of the tissues, in addition to other things, whose meaning will become 

 plainer presently. For the present, attention may be called to our nitrogen 

 food, the proteins, of which a part only is used for the giving of energy, the 

 nitrogenous part being excreted as a waste product, urea, by the kidneys. We 

 can imagine something of this kind in the case of the petrol motor; suppose 

 that there were an incombustible impurity in the fuel, and that it were con- 

 verted by the heat of the explosion into something very soluble in the 

 lubricating oil of the cylinder, it would then pass out with the waste oil, 

 which represents the urine. 



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