676 ANNUAL REPORT OF THE Off. Doc. 



enters into the structure of the body and serves as a source of energy. 

 But this process of digestion and assimilation by which the available 

 material of the food is separated from the unavailable and converted 

 into tissue constitutes work and consumes a portion of the energy 

 of the food. The case is not like that of a boiler fired by a mechani- 

 cal stoker, the power to drive which is derived ultimately from the 

 combustion of the very coal which it feeds into the boiler, or to still 

 continue the illustration of the reservoir, it is as if the water instead 

 of flowing into the reservoir w^as pumped up to it by a hydraulic 

 ram actuated by apart of the same water. This matter of the 

 amount of work expended in digesting and assimilating the food is 

 one which recent investigations have shown to be of great im- 

 portance and to have a very material bearing upon the relative values 

 of foods. Obviously a food like corn, for example, which requires 

 comparatively little digestive work will, other things being equal, 

 be of more value to the body than a food like straw whose digestion 

 involves a large amount of work. 



A portion of the food energy, then, is expended in the work of di- 

 gestion and assimilation and is, therefore, to be deducted from the 

 total energy of the food before we can find how much is available 

 for other purposes. The remainder we imxv call available energy. 

 It is used in part for the internal work of the body, such as the beat- 

 ing of the heart, the work of respiration and a great variety of other 

 processes going on in the body, and in part for the visible external 

 work; that is the useful work which the animal performs. Finally, 

 if the energy of the food is in excess of all these requirement — if 

 more water is running into the reservoir than is required to keep 

 the mill running — the excess is stored up in the body as growth of 

 tissue, chiefly of fat, to be used later when the supply of food may 

 be insufficient; — the h^vel (if the water in the reservoir rises. 



In order the<n to know how different feeding stuffs and rations 

 compare with each other as to their value as sources of energy to the 

 animal, we need to know, first, the total amount of energy which they 

 contain; second, the amount which needs to be deducted from this foi' 

 materials of the food which are unavailable to the body; and third, 

 the amount of work required for the digestion and assimilation of 

 the food. Fiirthermore, we need to know how the amount of enery 

 which is actually avail;)ble from a feeding stuff for ordinary pur- 

 poses is affected by the conditions under which it is fed. For ex- 

 ample, whether large and small amounts of tlie same feed have the 

 same degree of availahlity; whether the availability is affected by the 

 temperature of the stable, by the amount of water consumed, by the 

 age and state of fatness of the animal and other conditions which 

 might be named. It is this sort of information which it is expected 

 to obtain by the experiments with the respiration-calorimeter. The 



