M(. 6. DEPARTMENT OF AGRICULTURE. 676 



ing animal is constantly burning up its own tissues and it is the 

 energy set free in this burning which enables it to do work, including 

 both the visible external work which we can see and also the internal 

 work required by the beating of the heart, the movements of respira- 

 tion and the various activities of the i-nternal organs. As we know, 

 all these external and internal activities may go on for a time in an 

 animal deprived of food and we, therefore, conclude that they are 

 at the expense of the burning of tissue. Indirectly, however, all 

 this energy is derived from the food of the animal, since it is the 

 food which supplies the material for building up the body tissue. 

 We may again recur to the illustration of the reservoir. Its poten- 

 tial energy, that is, its power of doing work, lies in the fact that it 

 contains a considerable amount of water at a certain level, just as 

 the body of the animal contains a considerable amount of potential 

 energy in the form of tissue which can be burned. If we are to make 

 continuous use of the reservoir, we must have a stream of water 

 running into it as well as a stream running out. It is this inflow- 

 ing water which corresponds to the food of the animal, and just as 

 the potential energy of the reservoir is supplied ultimately by the 

 water which runs in at the upper end so the energy which the ani- 

 mal is able to exert is derived ultimately from its food. This is 

 what is meant by stating that the food of the animal is a source of 

 energy. It is the inflowiug stream which keeps up a store of poten- 

 tial energy in the body, and just as the work that can be done by the 

 mill depends in the long run upon the rate at which water runs into 

 the reservoir, so in the body, the amount of w'ork that can be done 

 depends upon the supply of energy in the food. In other words, the 

 question is one of the balance between the income and expenditure 

 of the energy of the body. 



Looking at the matter in this light, let us take up a little more in 

 detail the items making up the income and the expenditure of energy 

 by the animal organism. 



As just stated, the food constitutes the income of energy. It is 

 a fact of common observation, however, as well as established on a 

 quantitative basis by large numbers of experiments, that only a por- 

 tion of the material actually eaten is really available for the pur- 

 poses of the body. In the case of our common domestic animals, in 

 particular, a relatively large proportion of the food is not food at all 

 in the true sense, but simply ballast which passes through the ani- 

 mal unacted upon. To carry out still further our illustration of the 

 water in the reservoir, the case is as if the inflowing stream of water 

 were muddy and had to be filtered or run through a settling basin be- 

 fore being admitted to the reservoir. The mud thus removed from 

 the water would correspond to the excreta of the animal, while the 

 pure water would represent that portion of the food which actually 

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