92 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



practical exercises at the head of this lesson, is due to the presence 

 of cholalic acid. The sulphuric acid acting on sugar forms a small 

 quantity of furfuraldehyde, in addition to other products. It is the 

 furfuraldehyde which gives the purple colour with cholalic acid. 



The Bile Pigments. The two chief bile pigments are hilirubin 

 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 bili- 

 rubin, 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 (p. 90). 



Bilirubin has the formula C 1G H 18 N 2 O 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 haemo- 

 globin. 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 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 choletelin, C 16 H 18 N 2 O 6 . 



Hydrobilirubin. 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, C tV2 H 44 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. 



Urobilin. Hydrobilirubin 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 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 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 F line, and when partially precipitated from 

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



