688 



PHYSIOLOGY 



tissues which have resisted the digestive processes of the alimentary 

 canal. There is in addition a certain amount derived from the 

 intestine itself. During complete starvation faecal masses are formed 

 in the intestine, and it has been calculated that in a normal individual 

 about one gramme of nitrogen a day is excreted by the mucous 

 membrane of the gut and contributes to the formation of the faeces. 

 It is usual therefore to regard one gramme of the nitrogen of the 

 faeces as belonging to the output of the body and representing the 

 result of nitrogenous metabolism, while the balance is taken as 

 belonging to undigested food-stuffs, and is subtracted from the total 

 nitrogen of the latter in reckoning the real income of the body. A 

 small amount of nitrogen is also lost by sweat, but this can be dis- 

 regarded unless the sweating is profuse, when the loss of nitrogen 

 by this channel may rise to as much as 4 per cent, of the total nitro- 

 genous output of the body. Although a trace of ammonia has been 

 described as occurring in the expired air, the amount is so minute 

 that any loss of nitrogen by the lungs can be neglected. That the 

 loss both by lungs and skin under ordinary circumstances can be 

 disregarded is shown by the fact that it is possible to account directly 

 for the whole nitrogen of the body by a comparison of the composition 

 of food with that in the urine and faeces. If, for instance, an animal 

 is kept on a sufficient diet which contains a perfectly regular amount 

 of nitrogen, after a few days a condition known as nitrogenous equili- 

 brium is set up, i.e. the total nitrogen of faeces and urine is exactly 

 equal to the total nitrogen of the food. The same thing applies 

 to the sulphur, as is shown in the following Table (quoted by 

 Tigerstedt) : 



In order to express the nitrogenous metabolism in terms of 

 protein, we use the factor employed in estimating the amount of 

 protein in the food, i.e. we multiply the total nitrogen of the excreta 

 by 6-25. This will give the total protein which has been broken 

 down during the period of the experiment. Much more important 

 from the energy standpoint is the determination of the total processes 

 of oxidation of the body, information on which is given by a com- 

 parison of the oxygen intake with the output of carbon dioxide and 

 water. The estimation of these substances presents much greater 

 difficulties than the investigation of the nitrogenous exchange and 



