DIGESTION. 



117 



out of solution to form concretions around any foreign particles 

 or previously hardened concretions, forming a gall-stone in man. 

 Another kind of gall-stone is bilirubinate of calcium, rare in man, 

 but frequent in the ox. Besides its characteristic crystals, choles- 

 terin is also detected by various color-reactions in the presence of 

 iodine and sulphuric acid. 



The general presence of cholesterin in so many parts and cells 

 of the body leads to the impression that it is a cleavage product of 

 metabolism, being one of the waste-elements in the life of the cell, 

 especially the nerve-cell. Being absorbed by the blood, it finds its 

 way to the liver, there to be elaborated and to appear in the bile. 

 The cholesterin of the bile is now generally admitted to be drawn 

 from the cells lining the bile passages and gall-bladder. In catarrh 



Fig. 32. Crystals of Cholesterin. ( DUVAL. ) 



of these passages the amount of cholesterin is increased so much 

 that bile salts cannot keep it in solution, and the formation of gall- 

 stones is accelerated. Being an excrement, it is not reabsorbed, but 

 is expelled from the economy as a part of the faeces. Pathological 

 changes in tissues are always marked by an increased quantity, 

 which may be accounted for by loss of vitality in the diseased cells 

 so that they are unable to break down the cholesterin. 



Cholesterin is not poisonous to animals. 



Lecithin is found chiefly in nervous tissues, red corpuscles, and 

 the bile. It is most abundant in the nervous system. This is a 

 compound of a nitrogen base, cholin, with glycero-phosphoric acid 

 with fatty acid radical. Combined with a carbohydrate residue, it 

 is found in the liver, and is then called jecorin. 



When lecithin is taken by the mouth it is broken up in the 



