436 THE HUMAN BODY. 



Proteid Starvation and Overfeeding. When an ani- 

 mal 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, a part 

 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 resulting from their degrad- 

 ation diminishes also. How well proteid built up into a 

 tissue resists removal is shown by the facts already men- 

 tioned (p. 432) as to the relative losses of the proteid-ricli 

 and proteid-poor tissues in 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 in- 

 creases the proteid metamorphosis, and the animal still 

 loses, though less rapidly than it did. A little more proteid 

 still increases proteid metamorphosis in the body and 

 the urea elimination, and so on for some time; but each 

 increment of proteid in the food increases the nitrogenous 

 metamorphosis somewhat less than the last did, until r 

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

 ingesta balance: in a dog this occurs when it gets daily 

 2*5- its weight of meat, and no other solid food. More food 

 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 ex- 

 cretion; again the egesta and ingesta balance, and the ani- 

 mal 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 

 cessation of the gain in weight, and an excretion in twenty- 

 four hours of all the nitrogen taken. And so on, until th& 

 animal refuses to eat any more. 



