424 MANUAL OF PHYSIOLOGY. 



which is absolutely necessary for its own assimilative reintegra- 

 tion. A good supply of nitrogenous food aids in fattening, since 

 it gives vigor to all the protoplasmic metabolisms, and among 

 them fat formation. 



The albuminoid substance, gelatin, which is an important item 

 in the food we ordinarily make use of, is able to effect a saving 

 in the albuminous food stuffs. Although it contains a sufficiently 

 large proportion of nitrogen, it cannot satisfactorily replace albu- 

 min in the food. Indeed, in spite of the great similarity in its 

 chemical composition to albuminous bodies, it is hardly a better 

 substitute for proteids than fat or carbohydrate ; and, although 

 an animal uses up less of its tissue nitrogen on a diet of gelatin 

 and fat than when it is fed on fat alone, it soon dies, as if its diet 

 contained no nitrogenous substance. 



The last case we have to consider is that in which the supply 

 of food material is in excess of the requirements of the economy. 

 This is certainly the commonest case in man. 



Much of the surplus food never really enters the system, but 

 is conveyed away with the faeces. 



In speaking of pancreatic digestion, reference has been made 

 to the possible destiny of excess of nitrogenous food. In the in- 

 testine, some of it is decomposed into leucin and tyrosin, which 

 are absorbed into the intestinal blood vessels. In the body these 

 substances undergo further changes, which probably take place 

 in the liver. As a result of the absorption of leucin, a larger 

 quantity of urea appears in the urine, and hence the leucin formed 

 in the intestine by prolonged pancreatic digestion is a source of 

 urea. (See pp. 170,409.) This view is supported by the almost 

 immediate increase in the quantity of urea eliminated when 

 albuminous food is taken in large quantity. 



From the fact that a considerable amount of fat may be stored 

 up by an animal supplied with a liberal diet of lean meat, we 

 must conclude that part, at least, of the surplus albumin goes to 

 form fat. It has been suggested that after sufficient albumin has 

 been absorbed for the nutritive requirements of the nitrogenous 

 tissues, the rest is split up into two parts, one of which is imme- 



