BILE 



73 



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

 finally yellow, 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 clioletelin, C.6HjgN206. 



flydrobilimbin. — If a solution of bilirubin or biliverdin in dilute 

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

 ish pigment is formed called hydrobilirubin, C32H44N4O7. With the 

 spectroscope it shows a dark absorption band between h and F, and a 

 fainter band in the region of the D liue. 



Urobilin. — Hydrobilirubin is interesting because a similar substance 

 is formed from the bile pigment by reduction processes in the intestine, 

 and constitutes stercohilin, 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 urobiUn and stercohilin 

 has been frequently disputed, but the recent work of Garrod and 

 Hopkins has confirmed the old statement that they are the same 

 substance with different names. Urobilin has a well-marked absorption 

 band in the region of the P line, and when partially precipitated from 

 an alkaline solution by acidification, it also shows an absorption band 

 in the region of the E line. Hydrobilirubin differs from urobilin in 

 containing much more nitrogen in its molecule (9'2 instead of 4*1 per 

 cent.), and is probably a product of less complete reduction than 

 urobilin. (See further Lesson XXVI.) 



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 cor- 

 puscles. In bile it is normally present in 

 small quantities only, but it may occur in 

 excess, and so form the concretions known 

 as gallstones, which are generally more or 

 less tinged with biUrubin. 



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 C27H45.HO. 



Prom alcohol or ether containing water 

 it crystallises in the form of rhombic tables, which contain a molecule 

 of water of ciystallisation : these are easily recognised under the 

 microscope (fig. 25). It gives the following tests : — 



Fig. 25. — Cholesterin crystals. 



