504 THE LIVER. [OH. xxxin. 



The taurocholate of soda has the formula C 26 H 4 4NaN0 7 S. 



The colour reaction called Pettenkofer's reaction, is due to 

 the presence of cholalic acid. Small quantities of cane sugar and 

 strong sulphuric acid are added to the bile. The sulphuric acid 

 acting on sugar forms a small quantity of a substance called 

 furfurhldehyde, in addition to other products. The furfuraldehyde 

 gives a brilliant purple colour with cholalic acid. 



The Bile Pigments. The two chief bile pigments are 

 bilirubin and biliverdin. Bile which contains chiefly the former 

 (such as dog's bile) is of a golden or orange-yellow colour, while 

 the bile of many herbivora, which contains chiefly biliverdin, is 

 either green or bluish-green. Human bile is generally described 

 as containing chiefly bilirubin, but there have been some cases 

 described in which biliverdin was in excess. The bile pigments 

 show no absorption bands with the spectroscope ; their origin 

 from the blood pigment has already been stated. 



Bilirubin has the formula C 16 H 1S N 2 3 : it is thus an iron-free 

 derivative of haemoglobin. The iron is apparently stored up in 

 the liver cells, perhaps for future use in the manufacture of new 

 haemoglobin. The bile contains only a trace of iron. 



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

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

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

 atmosphere ; or it may be formed as in Gmelin'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 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 choletelin, 

 Ci6Hi8N 2 6 . 



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 3 2H 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 processes in the 

 intestine, and constitutes stercobilin, the pigment of the fseces. 

 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 frequently disputed, but the 

 recent work of Garrod and Hopkins has confirmed the old 



