THE FOOD AND DIGESTION 371 



After the nitrogenous portion of the proteid molecule is 

 split up, the liver has the further power of turning the non- 

 nitrogenous part into sugar, and either sending it to the 

 tissues or storing it as glycogen. 



Summary of the Functions of Liver. The functions of the 

 liver may be briefly summarised as follows : (1) It regulates 

 the supply of glucose to the body (a) by manufacturing it 

 from proteids when the supply of carbohydrates is insufficient, 

 and (b) by storing it as glycogen when the supply of carbo- 

 hydrates is in excess, giving it off afterwards as required. 

 (2) Along with the intestinal wall it regulates the supply of 

 proteids to the body, by decomposing any excess, and giving 

 off the nitrogen as urea, &c. (3) It regulates, in many animals 

 at least, the supply of fat to the body by storing any excess. 

 (4) It regulates the number of erythrocytes by getting rid of 

 waste haemoglobin and retaining the iron for further use. 

 ( - 5) From the part it plays in the entero-hepatic circulation, 

 it protects the body against certain poisons by excreting 

 them in the bile. 



V. GENERAL METABOLISM. 



Having considered how the food is digested and absorbed, 

 and how it is then either stored or at once used (a) for 

 building up and repairing the tissues, or (6) as a source of 

 energy, the rate at which the various chemical changes go 

 on and the factors modifying them may be dealt with. 



The changes in the two great constituents of the body 

 proteids and fats have to be separately studied. 



1. Method of Investigating. 



A. Proteid Metabolism. The amount of proteid used in 

 the body is readily calculated from the amount of nitrogen 

 excreted, since, under normal conditions, unless nitrogen 

 in some unusual combination is being taken, it is derived 

 entirely from the proteids in the body. Proteids contain 

 16 per cent, of nitrogen, and hence each grm. of nitrogen 

 excreted is derived from 6'25 grms. of proteid. 



The nitrogen is almost entirely excreted in the urine. 



