NUTRITION. 463 



removed, but some birds survive for a time. In them it has 

 been found that the uric acid (which in avian urine has the 

 predominance which urea takes in mammalian) excreted is 

 diminished after extirpation of the liver; and also that leucin 

 which when given to the normal bird reappears in the urine 

 as uric acid, in the bird from which the liver has been removed 

 is excreted unaltered. 



Circulating and Fixed Proteid. When an animal is 

 fed on food deficient in proteids, or containing none of them 

 at all, its urea excretion falls very rapidly during the first day 

 or two, but then much more slowly until death: there is thus 

 indicated a double source of urea, apart resulting from tissue 

 wear and tear, and always present; and a part resulting from 

 the breaking down of proteids not built up into tissue, and 

 ceasing when the amount of this proteid in the Body (in the 

 blood for example) falls below a certain limit as a result of 

 the starvation. As the nitrogen-starved Body wastes, its 

 bulk of proteid tissues is slowly reduced and the urea result- 

 ing from their degradation diminishes also. How well pro- 

 teid built up into a tissue resists removal is shown by the 

 tacts already mentioned as to the relative losses of the pro- 

 teid-rich and proteid-poor tissues during starvation. 



On the other hand, if an animal be taken while starving 

 and losing weight and have a small amount of flesh given it, 

 it will continue to lose weight, and more urea than before 

 will appear in the urine; increased proteid diet increases the 

 proteid metamorphosis, and the animal still loses, though 

 less rapidly than it did. A little more proteid still increases 

 proteid metamorphosis in its body and its urea elimination, 

 and so on for some time; but each increment of proteid in 

 the food increases the nitrogenous metamorphosis in propor- 

 tion to itself somewhat less than the preceding one did, until,, 

 finally, a point is reached at which the nitrogen egesta and 

 ingesta balance: in a dog this occurs when the animal gets 

 daily -^ its weight of lean meat, along with the necessary 

 water. More flesh if then given is at first stored up and the 

 animal increases in weight; but very soon the greater wear 

 and tear of the larger mass of tissues shows itself as increased 

 urea excretion, and again the egesta and ingesta balance, and 

 the animal comes to a new weight equilibrium at the higher 

 level. More meat now causes a repetition of the phenomenon: 

 at first increase of tissue, and nitrogen storage; and then a 



