ABSORPTION 361 



process of protein catabolism ma}" be conducted in the liver 

 cells. When haemoglobin is set free from the corpuscles in 

 moderate amounts, the nitrogen of its protein part is changed 

 to urea, while the pigment part is deprived of its iron and 

 excreted as bilirubin. 



The process of urea formation from proteins may be 

 divided into four stages — (Ij The liberation of the amino- 

 acids. (2) The de-aminisation of the amino-acids. This is 

 probably effected by de-aminising enzymes. (3) The 

 ammonia set free is probably linked to carbonic acid; and (4) 

 the carbonate of ammonia is then dehydrated by other 

 enzymes and so changed into urea (p. 5 59). 



The nitrogen excreted is not all in the form of urea. 

 The other nitrogen-containing waste products are dealt with 

 on p. 559 et seq. 



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 muscles (a) by manu- 

 facturing it from proteins wlien the supply of carbohydrates 

 is insufficient, and (6) by storing it as glycogen when the 

 supply of carbohydrates is in excess, and giving it off after- 

 wards as required. (2) Along with the intestinal wall, it 

 regulates the supply of proteins to the body by de-aminising 

 any excess, conserving/ the non-nitrogenous part by convert- 

 ing it into glucose for use by the muscles, and giving off the 

 nitrogen as urea, etc. (8) It regulates, in many animals at 

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

 and it probably plays an important part in de-saturating the 

 fatty acids, and thus making them more available for combus- 

 tion in the tissues. (4) It breaks down the haemoglobin of old 

 erythrocytes, and retains the iron for farther use (see p. 491). 

 (5) From the part it plays in the enterohepatic circulation, 

 it protects the body against certain poisons by excreting 

 them in the bile (see p. 831). 



The Faeces, 



The unabsorbed contents of the alimentarv tract, whether 

 originally derived from the food or from the tract itself, are 



