CII. XXXIII.] 



BILE. 



505 



statement that they are the same substance with different 

 names. Hydrobilin differs from urobilin in containing much 

 more nitrogen in its molecule (9-2 instead of 4*1 per cent.). 



Cholesterin. This substance is contained not only in bile, but 

 very largely in nervous tissues. Like lecithin, it is an abundant 

 constituent of the white substance of Schwann. It is found also 

 in blood corpuscles. In bile it is normally present in small 

 quantities only, but it may occur in excess, and form the concre- 

 tions known as gall-stones, which are usually more or less tinged 

 with bilirubin. 



Though its solubilities remind one of a fat, cholesterin is not a 

 fat. It is, in fact, chemically speaking, a monatomic alcohol. Its 

 formula is C^H^.HO. 



From alcohol or ether cont lining water it crystallises in the 

 form of rhombic tables, which contain one molecule of water of 

 crystallisation : these are easily 

 recognised under the microscope 

 (see fig. 411). 



It gives the following colour 

 tests : 



1. With iodine and concen- 

 trated sulphuric acid the crystals 

 give a play of red, blue, and 

 green. 



2. Heated with sulphuric acid 

 and water (5:1) the edges of the 

 crystals turn red. 



3. A solution of cholesterin 

 in chloroform, shaken with an 



equal amount of concentrated sulphuric acid, turns red, and ulti- 

 mately purple, the subjacent acid acquiring a green fluorescence. 

 (Salkowski's reaction.) 



" The mode of origin of cholesterin in the body has not been 

 clearly made out. Whether it is formed in the tissues generally, 

 in the blood, or in the liver, is not known ; nor has it been 

 determined conclusively that it is derived from albuminous or 

 nervous matter. It is also doubtful if we are to regard it as a 

 waste substance of no use to the body, as its presence in the 

 blood- corpuscles, in nervous matter, in the egg, and in vegetable 

 grains, points to a possible function of a histogeuetic or tissue- 

 forming character." (McKendrick.) 



A substance called iso-c/wlesterin, isommc with ordinary chole- 

 sterin, is found in the fatty secretion of the skin (sebum) ; it is 

 largely contained in the preparation called lanoline made from 



Fig. 41 1. Crystalline scales of cholesterin. 



