THE SIGNIFICANCE OF THE FOOD-STUFFS 719 



explains the rapid excretion of nitrogen which follows the ingestion 

 of a protein meal by the special avidity of the animal cell for protein. 

 When enough of this is presented to it, it feeds upon nothing else, 

 and only when there is a comparative lack of protein will it make 

 use of carbohydrate or fat for its needs. Thus while a dog is fed on 

 a rich mixed diet it lives practically on protein alone, storing up 

 the fats and carbohydrates of the food as fat. If food be now with- 

 drawn the animal must live either at the expense of its own living 

 tissue (proteins) or must attack the stored-up fats in its body. The 

 latter alternative, as a matter of fact, takes place. The animal 

 spares the precious protein and lives on the fat of its own body. 

 Hence comes the great fall in the excretion of urea that is observed 

 in starvation, the consumption of proteins sinking to the indispensable 

 minimum. If now a protein meal be given, the cells of the body 

 return to their former way of living, and satisfy as much of their 

 needs as possible at the expense of protein, so that the urea excretion 

 rises almost in proportion to the food given. In order to attain nitro- 

 genous equilibrium on a purely protein diet, it is necessary to give 

 the cells enough protein for their total requirements, i.e. three to five 

 times as much as would correspond to the nitrogenous excretion during 

 hunger. If a larger amount of protein be given than is necessary for the 

 maintenance of nitrogenous equilibrium, a certain amount of nitrogen 

 is retained in the body, probably as protein, giving rise to an increase 

 in the total living material of the body, and the animal increases 

 in weight. The amount of urea excreted by an animal is propor- 

 tional not only to the quantity of protein taken in with the food 

 but also to the weight of the animal ; so the animal which has grown 

 heavier in consequence of increased supply of nitrogenous food will 

 need a larger amount of protein to maintain its nitrogenous equili- 

 brium, which will be produced with the same amount of protein 

 as soon as the animal has increased in weight to a certain extent. 

 In order therefore to maintain the increase in weight, it is necessary 

 to give ever-increasing quantities of protein, and the stuffing process 

 is finally put an end to by the refusal of the digestive organs to digest 

 any more. 



In support of these views considerable stress has been laid by Pfliiger on 

 some experiments carried out in his laboratory by Schondorff. According to 

 Voit the greater excretion of urea in a protein-fed animal is due to the fact 

 that there is an increased circulation of a fluid that is rich in proteins round 

 the cells. According to Pfliiger's views, however, the presence of a greater 

 or less amount of protein in the nourishing medium would not be the deter- 

 mining factor for the amount of urea formed, which would be regulated simply 

 and solely by the condition of the cells themselves. To decide this point, 

 defibrinated dog's blood was led alternately through the hind limbs and the liver 

 of another dog, in order to get the products of metabolism of the limb tissues 

 and then convert them into urea by passing the blood through the liver. 



