202 UTILISATION OF FOOD BY THE ANIMAL [chap. 



tain no nitrogen they cannot do the special work of 

 the proteins. We have already briefly indicated 

 the metabolism suffered by the proteins in the 

 diet; they are broken down by the enzymes of the 

 stomach and the intestine into comparatively simple 

 amino-acids and amides, and in this form are ab- 

 sorbed by the walls of the stomach and intestines. 

 It is still in doubt whether they are there recon- 

 verted into proteins which pass into the blood, or 

 whether the blood is given the simple split products 

 resulting from the enzyme action. The end result is 

 the same in either case, the nitrogenous compounds are 

 carried always in the blood-stream and so reach succes- 

 sively every cell in the body where such materials . are 

 absorbed as are required to maintain the structure of 

 the cell ; at the same time the cell hands over to the 

 blood the waste nitrogen products excreted by the cells. 

 The amount of nitrogen compounds thus taken up for 

 tissue repairs and renewals is only a small portion of 

 the nitrogenous compounds entering the blood from the 

 food, unless the animal has been placed on a minimal 

 ration which barely maintains a nitrogen equilibrium. 

 As a rule, the animal is receiving more protein than is 

 absolutely necessary, but when the blood containing the 

 digested protein products passes through the kidneys 

 the nitrogen part is split off in the form of urea and 

 only non-nitrogenous compounds are passed on, to 

 be used for generating energy or forming fat as need 

 may be. This accounts for the rapid excretion of urea 

 after a meal, when it cannot be supposed that time 

 enough has elapsed to give rise to an amount of tissue 

 waste equivalent to the urea excreted. Thus we must 

 distinguish between the excreted nitrogen compounds, 

 which are due to tissue waste and represent the 

 indispensable nitrogen requirements of the body if 



