378 ANNUAL REPORT OF THE Off. Doc. 



and intimately conuected with the formation of this heat, is the use 

 ot enough energy to keep the heart and lungw in action. Here is 

 shown one of the greatest economics of nature. The body demands 

 the use of a certain amount of energy to carry on the operations of 

 life, but all this energy in fulfilling its office in the body is at the 

 same time changed into heat and performs the double office of doing 

 work and producing heat. Thus when the heart beats it is using up 

 force, but at the same time all this force is changed into heat and 

 helps to warm the body. Just as when one pounds a nail on an 

 anvil the nail becomes hot from the force of the blow that is changed 

 iLto heat. 



Here also comes in one of the finest and most difficult of the prob- 

 lems yet to be solved in animal nutrition. If the body needs more 

 heat than force, then all of the force so far as it is expended inter- 

 nally will be no loss to the body because it will furnish just as much 

 heat after producing the force as it would if it had been burned at 

 once for fuel. If, however, more force is used in the body, than is 

 needed in its equivalent of heat, then all the extra heat is so much 

 sheer loss. We know that when the body is working hard there is 

 much more heat produced than the body needs. Then the body 

 breaks out in a profuse perspiration to try and get rid of some extra 

 heat. 



There is also another way in which heat is produced in the body. 

 Much of the digestion is in the nature of fermentation, and just 

 as the materials in the soil heat when they begin fermenting, so all 

 of these fermentations in the alimentary canal cause the giving off 

 of considerable heat. If the body needs this heat, then the fermenta- 

 tion causes no loss, but if as is usually the case, the actual muscular 

 energy used gives out all the heat necessary for the body, then all 

 the material that is used up in the fermentation is just so much clear 

 loss to the body. We do not know just how much of the heat of the 

 body is really utilized, but in the absence of definite knowledge, the 

 best we can do is to suppose that on a maintenance ration the food 

 is used in the most economical way and there is no waste heat pro- 

 duced. We know that this supposition is not exactly correct, but in 

 the present state of knowledge it is probably the best supposition 

 that can be made. 



On this basis it is quite easy to estimate the relative value of 

 foods, because all we have to know is their total digestibility, and 

 the kind of constituents that are digested. 



One correction has to be made. When food is digested by a rumi- 

 nant, part of the actual food value is actually destroyed and passed 

 off as a gas. In the horse the same fermentation takes place in 

 the large intestines. This much is all loss and must be subtracted 

 from the value of the digested portion. As it is not possible to de- 

 termine directly by actual measurement the amount of food value 



