356 A MANUAL OF VETERINARY PHYSIOLOGY 



Whatever this may be — and it appears in the adult, as we have 

 already seen, to be small — the embodiment of newly-arrived pro- 

 tein into the body tissues is associated with an equal output of 

 worn-out material containing the same proportion of nitrogen, 

 so that, though the nitrogen does not vary, the tissues do. 



All true proteins are capable of being used as food, but when 

 albuminoids, such as gelatin, are given they produce the same 

 amount of urea as an assimilable protein, but the animal loses 

 flesh. The gelatin story is interesting. Originally this substance 

 held a high nutritive position in a diet, mainly owing to its 

 chemical analysis and its close relationship to true protein. It 

 took years to find out that from a dietetic point of view it was 

 worthless as a nutrient. Something similar occurred in the 

 feeding of horses. Bran yields an excellent analysis, and, in 

 consequence, has frequently formed a prominent part of badly- 

 arranged horse dietaries. As a matter of fact, the nitrogen in 

 bran is worthless. 



What we have learnt regarding nitrogenous food may here be 

 summarised : 



1. The body requires nitrogen ; no diet is complete without 

 it, nor can life be permanently supported in its absence. 



2. The body having obtained its nitrogen, stores up the small 

 amount required to replace wear and tear, and excretes the whole 

 of the remainder mainly in the form of urea. 



3. The assumption that the proteins are the source of muscular 

 energy is incorrect ; this is the function of non-nitrogenous food, 

 yet increased muscular effort must be met by an increased 

 nitrogenous ration. In some unknown way nitrogen directs the 

 production of energy in the muscle machine. 



Carbon Equilibrium. — This condition implies that the total 

 carbon in the excreta is balanced by that received in the food. 

 We have seen that an animal in nitrogenous equilibrium may be 

 gaining or losing weight. This gain or loss must be fat. An 

 attempt to establish carbon equilibrium on a purely protein diet, 

 such as may be used for nitrogenous experiments, is difficult. 

 We have seen how the system struggles on protein to obtain the 

 necessary carbon, and that, in consequence, far more protein 

 has to be consumed than is actually necessary to establish nitro- 

 genous equilibrium. It has also been seen that much less 

 protein food is necessary to establish nitrogenous equilibrium 

 when a small quantity of fat or carbohydrate is added to the 

 diet. The importance of carbon equilibrium is nothing like so 

 great as that of nitrogenous, and this is explained by the fact 

 that the amount of fat in the body varies within wide limits 

 compatible with apparently perfect health. 



