CH. XXXIII.] 



THE BILE 



511 



Biliverdin has the formula C 16 H 18 N 2 4 (i.e., one atom of oxygen 

 more than in bilirubin): it may oocur as such in bile; it may be 

 formsd by simply exposing red bile to the oxidising action of the 

 atmosphere ; or it may ba formed as in G-melin's test by the more 

 vigorous oxidation produced by fuming nitric acid. 



Gmelin's test consists in a play of colours green, blue, red, and 

 finajjy r ellow, produced by the oxidising action of fuming nitric acid 

 (that is, nitric acid containing nitrous acid in solution). The end or 

 yellow product is called choletelin, C 16 H 18 N.,0 G . 



Hydrobilirubin. If a solution of bilirubin or biliverdin in dilute 

 alkali is treated with sodium amalgam or allowed to putrefy, a 

 brownish pigment, which is a reduction product, is formed called 

 hydrobilirubin, C 32 H 40 N 4 7 . With the spectroscope it shows a dark 

 absorption band between b and F, and a fainter band in the region 

 of the D line. 



This substance is interesting because a similar substance is formed 

 from the bile pigment by reduction pro- 

 cesses in the intestine, and constitutes 

 stercobilin, the pigment of the faeces. 

 Some of this is absorbed and ultimately 

 leaves the body in the urine as one of 

 its pigments called urobilin. A small 

 quantity of urobilin is sometimes found 

 preformed in the bile. The identity of 

 urobilin and stercobilin has been fre- 

 quently disputed, but the recent work 

 of Garrod and Hopkins has confirmed 

 the old statement that they are the 

 same substance with different names. 

 Hydrobilirubin differs from urobilin 

 in containing more nitrogen in its molecule (9*2 instead of 41 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 concretions 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 27 H 45 .HO. 



From alcohol or ether containing water it crystallises in the form 

 of rhombic tables, which contain one molecule of water of crystal- 

 lisation : these are easily recognised under the microscope (see 

 fig. 409). 



FIG. 409. Crystalline scales of cholesterin. 



