758 



PHYSIOLOGY 



amount, we find evidence of a qualitative difference between the two 

 kinds of metabolism. The difference is well brought out in the Tables just 

 quoted. On a large diet the greater part of the nitrogen can be regarded 

 as derived directly from the food, whereas on a small diet a relatively larger 

 proportion of it must come from protein which has been previously built up 

 into the tissues. Folin distinguishes these two sources of the nitrogen 

 of the urine as exogenous, i.e. that from the food, and endogenous, i.e. derived 

 from the tissues. Two facts stand out in comparing these two urinary 



20 



24 Hours 



FIG. 353. The hourly variation in the excretion of nitrogen after a meal. 

 The meal was given at 0. The thick line represents the average ab- 

 sorption of the food from the alimentary canal. The thin-lined curves 

 represent the N. excretion (1) after a meal of 1000 grm.meat ; (2) after 500 

 grm. meat and 150 grm. fat ; (3) after a meal of 500 grm. meat ; (4) and 

 (5) both represent the excretion in a fasting animal. (From TIGERSTEDT 

 after FBDEB.) 



analyses. In the first place, on a normal protein diet the urea accounts for 

 87 per cent, of the total nitrogen of the urine. On an excessive protein diet 

 this percentage may rise to 90 or 95. On the low protein diet the percentage 

 of nitrogen appearing as urea is reduced to 60. On the other hand, practi- 

 cally identical amounts of creatinine are obtained under the two conditions, 

 so that whereas on the full diet it amounts only to 3-6 per cent., on the low 

 protein diet it forms as much as 17 per cent, of the total nitrogen output. 

 We are therefore justified in regarding urea as to a large extent exogenous in 

 origin, and as derived directly from the nitrogenous moiety of the protein 

 molecule, which may not at any time have formed part of the living tissues 

 of the body. 



On giving a large protein meal to a dog the urea in the urine rapidly 

 rises, and at the end of four or five hours 50 per cent, of the total nitrogen 

 taken in with the food has appeared in the urine as urea (Fig. 353). If we 

 take into account that the digestion of a meat meal in this animal may go 

 on for eight hours, we are justified in the statement that by far the greater 



