METABOLISM OF MATTER AND ENERGY. 1015 



This is often the case when there is no increased excretion of nitrogen 

 and when the nitrogen of the excretory product does not exceed that 

 of the food. In these cases carbon compounds (fat and carbohydrates) 

 must be concerned in the production of muscular energy. It does not 

 necessarily follow that this is true for all forms of energy or for the 

 production of the total muscular energy of the body. It has also been 

 claimed that when work is severe the amount of nitrogen excreted in 

 the urine is increased, or, in other words, protein is broken down to 

 furnish energy for muscular work. The different theories of the source 

 of energy were briefly mentioned above. More investigations are 

 needed before positive deductions can be drawn. 



Since the total income and outgo of matter are measured in respira- 

 tion experiments, the data are available for discussing the conservation 

 of matter in the animal body. Although this would hardly be ques- 

 tioned to-day, the experimental proof of the law as applied to the 

 animal body is not only a matter of abstract scientiiic interest, but of 

 fundamental biological importance. 



What was said of the importance of studying the balance of nitro- 

 gen under abnormal or unusual conditions applies with equal force to 

 the metabolism of carbon and of energy. 



METABOLISM OF ENERGY. 



The previous discussion has been chiefly concerned with only one of 

 the functions of food, namely, supplying the body with material for the 

 building and repair of tissue. Food is also a source of energy. It 

 supplies the body with the necessary heat and energy for internal and 

 external muscular work. The complex compounds consumed in the food 

 possess high potential energy. Before being excreted they are broken 

 down in the body into simpler compounds and part with this energy. 

 The elements unite with the oxygen of the air and undergo combustion 

 in the body. 



In ordinary combustion the final products are carbon dioxid, water, 

 and free nitrogen. In the body carbon and hydrogen are also burned 

 to carbon dioxid and water, although the process is not so simple as in 

 the direct oxidation of material in the air. The nitrogen combustion 

 is, however, not so complete, since nitrogen is excreted in the forms of 

 urea and other cleavage products. 



It is believed, in accordance with the principle of the conservation of 

 energy, that the amount of potential energy which is transformed into 

 kinetic energy when food is burned in the body is the same as that 

 which would be produced if an equal amount of the same material was 

 burned outside the body. Provided the final products are the same the 

 intermediate steps may be disregarded. Further, in accordance with 

 the principle of maximum work the tendency is toward those changes 

 which result in the greatest evolution of heat or other form of kinetic 

 energy. Accordingly the heats of combustion of the nutrients of food 



