X.] PROTEIN REQUIRED B Y ANIMALS 203 



its equilibrium is to be maintained, and the other 

 nitrogen compounds (urea only, probably) split off 

 from proteins which are being used as mere sources 

 of energy, though both kinds are excreted together 

 in the urine. The former quantity of nitrogen, the 

 minimum required to repair tissue waste, can be 

 measured either by putting the animal on a purely non- 

 nitrogenous diet and seeing how much nitrogen is 

 excreted under such conditions of nitrogen starvation, 

 or, more accurately, by gradually reducing the protein in 

 a ration supplying the proper amount of energy until a 

 limiting condition of nitrogen equilibrium is reached 

 when any further diminution in the protein fed results 

 in the excretion remaining greater than the intake, 

 after the first disturbance due to the change of diet has 

 passed away. In this way the minimum requirements 

 of protein for oxen or horses on a maintenance diet 

 appear to be about \ lb. per day per 1000 lb. live 

 weight ; for sheep this quantity must be nearly doubled, 

 because of the comparatively large draught on protein 

 for the growth of wool, which cannot be brought to a 

 standstill like the formation of flesh. It is, however, 

 never safe in feeding animals to get down to this 

 minimal limit ; the digestibility of the carbohydrates in 

 the ration is reduced if the proteins are low, especially 

 when the rations are large, because the animal is then 

 called upon to secrete an increased amount of enzymes 

 which are themselves nitrogenous bodies. Moreover, it 

 has been shown that respiration is quickened, the 

 circulation is more vigorous, and the temperature is 

 raised a trifle, if the supply of proteins is above the 

 absolute minimum. There is another point of view 

 also to be considered : though the proteins form a great 

 natural group of substances having many characters 

 in common, which can be further grouped into smaller 



