2 THE FOOD OF ANIMALS 



dependent on this sort of waste. In other words, the chemical 

 changes embraced under the term " waste " furnish the power by 

 which the organism is worked, just as a locomotive is kept going 

 by consumption of fuel, or an electric bell made to sound by the 

 chemical action going on in a battery. 



Yet, despite this incessant chemical disintegration in nearly all 

 parts of its body an animal maintains a constant shape and ap- 

 pearance for long periods of time, or, if young, may exhibit those 

 increases in size and weight which constitute growth. 



It is therefore evident that waste must be counterbalanced by a 

 corresponding process of renewal, while in such cases as those of 

 a growing child or a fattening ox there is not merely bare renewal, 

 but a formation of new substance, often to a very considerable 

 extent. Increase in size does not, however, consist in the addition 

 of external layers, for the new material must be intimately em- 

 bodied with that already existing. The raw material from which 

 repair and increase 'are effected is food, which is worked up by 

 the digestive organs into a suitable condition, those parts of it 

 which are not, or cannot be, digested being cast out of the body 

 as useless. 



One of our most distinguished zoologists (the late Professor 

 Milnes Marshall) describes the chemical changes by which 

 the living substance of the body is constantly being built up 

 and as constantly broken down, in a very picturesque way, and 

 his words may well be quoted here: — "This conception of proto- 

 plasm, or the living matter of animals and plants, as undergoing 

 incessant change, or metabolism as it is called, is one of much 

 importance. Living protoplasm has been compared to a foun- 

 tain in which the form remains constant though each component 

 particle of water is in constant movement. In protoplasm, as 

 in the fountain, we distinguish two main processes, an uphill 

 or anabolic process, as the water rises to the crest of the wave, 

 or the food is being built up into the living tissues; and a 

 downhill or katabolic process, as the water falls from the crest 

 back into the basin, or as the living brain, muscle, &c., become 

 broken down into the various excretory products. The uphill or 

 anabolic processes are synthetic, and require or absorb energy. . . . 

 On the other hand, the downhill or katabolic processes are 

 analytic, and are sources of energy. Protoplasm may, to adopt 

 the simile given above, be regarded as the topmost point, the 



