392 VETERINARY PHYSIOLOGY 



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

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

 from proteins when the supply of carbohydrates is insufficient, 

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

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

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

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

 off the nitrogen as urea, etc. (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 

 (see p. 213). (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- 

 proteins and fats have to be separately studied. 



1. Method of Investigating 



A. Protein Metabolism. The amount of protein 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 proteins in the body. Proteins contain 16 per cent, 

 of nitrogen, and hence each grm. of nitrogen excreted is derived 

 from 6 '25 grms. of protein. 



The nitrogen is almost entirely excreted in the urine. Only 

 a small amount escapes by the bowels and skin, and hence only 

 when very accurate observations are desired is it necessary to 

 analyse the fteces and sweat. 



B. Metabolism of Fats. Proteins contain nearly three and a 



