680 



PHYSIOLOGY 



received in its food, which would correspond to a diminution of the protein 

 store of its body, reckoned as muscular substance, by 434 grm. Many such 

 experiments have been performed, and they all agree in showing that in 

 carnivora a very appreciable storage of nitrogen can take place in the body. 

 In cats it is sometimes possible to double the body weight by administration 

 of a large protein diet. Since no fat is laid on at the same time and the 

 animals are in a fine healthy condition, one must conclude that- the greater 

 portion of the storage takes place by a growth of muscle substance. The 

 degree to which the storage can take place is however variable and is 

 generally smaller in dogs than in cats. However much protein is given, the 

 limit is finally arrived at where no further laying on of protein tissues of the 

 body is possible, and the animal then enters into a state of nitrogenous 

 equilibrium, when he excretes a quantity of nitrogen exactly equal to that 

 taken in. This equivalence of income and output signifies that the extent 

 of the total metabolism of the body is affected by the amount of protein 

 supplied in the food, and, as a matter of fact, the total energy output of the 

 body rises and falls with the quantity of protein in the food. This is shown 

 in the following Table by Pettenkofer and Voit, in which the figures have 

 been recalculated by Pfliiger. 



We see therefore that carnivorous animals can satisfy their total energy 

 requirements at the expense of protein. When the protein income is in 

 excess of their requirements a small amount is laid on, probably. as increased 

 muscular tissue. The most marked effect is however an increased meta- 

 bolism, which rises in proportion to the nitrogenous income. The limit to 

 this increase is set by the powers of the alimentary canal to digest the 

 protein. The rise-in metabolism consequent on protein food is very rapid 

 and affects the gaseous exchanges as well as the output of nitrogen. Magnus 

 Levy and Falk found that a large protein meal might increase the respiratory 

 exchanges 40 per cent., an increase which lasted seven hours. The nitro- 

 genous output also rises immediately after a protein meal, so that 50 per 

 cent, of the nitrogen of the ingesta may appear in the urine within seven 

 hours after the meal. 



The whole of these results cannot [be strictly applied to omnivorous 

 animals, such as man. In these it is impossible to supply all the energy 

 requirements of the body on a pure protein diet . Even if a man eats as much 

 meat as he can, he will be unable to obtain sufficient CMHM^V for his daily 



