THE SOURCES OF ENERGY 57 



taining much free chemical energy, and leaves it in the form of 

 the excretions while containing almost no free chemical energy. 

 In the steam engine the difference between the quantities of 

 energy possessed by the working substance when entering and 

 leaving the mechanism is represented by the mechanical work 

 done and the heat lost by radiation, etc. Similarly, the difference 

 between the energy of the working substance (as the food) 

 when it enters the animal body and when it leaves it (as the 

 excretions) is represented by the mechanical work done by the 

 animal, and by the heat radiated away from its body and lost 

 in the excretions. 



The working substance, now degraded in respect of the energy 

 it contained, is taken up again by the green plants, reconverted 

 into carbohydrate, fat, and proteid, and the cycle of operations 

 recommences. 



The Digestive Process in Animals. — In the meantime, we 

 consider only the animal part of the life energy cycle. The 

 food matters exist, and the animal obtains these by the exercise 

 of its sensori- motor organs. But these food matters must be 

 digested, distributed to the tissues of the body, assimilated, 

 oxidised, and then excreted. It is this train of events that we 

 have now to study. 



Why must they be digested ? The foodstuffs must enter 

 into the blood-stream of the animal that eats them, and in order 

 that this may happen they must be dissolved. Now, as a rule, 

 the food of an animal is a complex and heterogeneous collection 

 of substances, how complex every healthy man and woman knows. 

 In the animal living in the wild it is even more complex, since 

 such creatures usually hunt down, kill, and devour other animals, 

 and often ingest flesh and bone, fur, feathers, scales, hair— the 

 edible as well as the inedible parts, which latter civilised man 

 tries to reject in the processes of the preparation of his food. 

 But even when the latter is carefully prepared and cooked it 

 still contains inedible parts — the cellulose or woody fibre of 

 vegetables, fruits, cereals, peas, and other plants, and the fibrous 

 substance of meat and certain fats which are not easily dissolved. 

 All these inedible constituents of the food are excreted in the 

 faeces, and, of course, they vary in different animals: thus her- 

 bivores can digest cellulose, while man cannot as a rule. 



Trituration, or mechanical disintegration of the substance of 

 the food, occurs mainly in the mouth. Finely divided particles 



