354 A MANUAL OF VETERINARY PHYSIOLOGY 



The modern view does not differ greatly from that held by 

 Voit. The split products of protein digestion, the amino-acids, 

 now no longer protein, are reconstructed by the process ol 

 synthesis, and a protein formed allied to the body of the animal 

 in which the change is taking place. This synthesis occurs either 

 in the wall of the intestine or the liver, and serum albumin and 

 globulin are thus formed. The observations on nitrogenous 

 metabolism indicate that the whole of the split products are not 

 so converted, only perhaps a small portion, while the balance 

 not converted into tissue protein takes the short cut to urea after 

 having been utilised as a source of energy. If the energy re- 

 quirements of the body are less than the available energy, the 

 non-nitrogenous portion of the protein surplus is stored up either 

 as glycogen or fat. It has been shown experimentally that a dog- 

 may maintain its tissue nourishment, and even add to its body 

 weight, when fed on the split products of pancreatic digestion, 

 and this is a very important fact as bearing out the above views. 

 We have seen that the urea is increased or diminished by 

 increasing or reducing the protein supply in the food, while, 

 on the other hand, the purin bodies excreted are not reduced 

 by diminishing the protein in the diet. It has accordingly 

 been suggested that the urea represents that portion of the 

 protein which is not incorporated with the tissues, while krea- 

 tinin, xanthin, hypoxanthin, and uric acid represent the changes 

 occurring in the body protein. 



The difficulty in establishing nitrogenous metabolism on a 

 purely protein diet, and the cause of that difficulty, has already 

 been dealt with. We are clearly shown from it that protein in 

 the body splits up into a nitrogenous and non-nitrogenous 

 portion, the former, as we have seen, being quickly got rid of, 

 mainly as urea, while the latter is gradually oxidised. The 

 percentage composition of protein is about C 53 ,H 7 ,0 22 ,N 16 . The 

 nitrogen having been cleared off, the carbon, hydrogen, and 

 oxygen represent the non-nitrogenous moiety available after the 

 formation of urea for the production of glycogen and perhaps of 

 fat. Physiologists are not, however, agreed on the question of 

 the formation of fat from protein, though the formation of 

 glycogen is generally admitted. Most proteins possess what is 

 known as a carbohydrate group, the exception being casein, 

 which yields a reducing sugar on decomposition. 



Nitrogen is a most expensive food-stuff, and economists in 

 watching dietaries have repeatedly drawn attention to the un- 

 necessarily nitrogenous, and therefore wasteful, nature of many 

 diets for animals. We shall see that the bulk of the nitrogen in 

 fattening animals passes away with the excreta, and only a small 

 proportion of it stored up, but though the bulk has been excreted, 



