THE CHEMISTRY OF THE DIGESTIVE JUICES 359 



such as yellow nitric acid (which contains some nitrous acid), a whole 

 series of. oxidation products of bilirubin is obtained, beginning with 

 biliverdin, and passing through bilicyanin, a blue pigment, and other 

 intermediate bodies, to choletelin, a yellow substance. It is question- 

 able whether these are all definite compounds. This is the foundation 

 of Gmelin's test for bile-pigments (see Practical Exercises, p. 456). The 

 same colours are produced, and in the same order, when a solution of 

 bilirubin in chloroform is treated with a dilute alcoholic solution of iodine. 



The positive pole of a galvanic current causes the same oxidative 

 changes, the same play of colours, while the reducing action of the 

 negative pole reverses the effect, if the action of the positive electrode 

 has not gone too far. These reactions can also be used for the detection 

 of bile -pigments. 



By the reducing action of sodium amalgam on bilirubin, hemi- 

 bilirubin (C33H 44 N 4 O 6 ) is obtained. It gives a beautiful red colour with 

 p-dimethylaminobenzaldehyde (Ehrlich's reaction). Hemibilirubin is 

 identical with the urobilinogen of urine from which urobilin is derived. 

 Urobilinogen and urobilin (often called in this connection stercobilin) 

 are also found in the faeces from birth onwards, although not in the 

 meconium (p. 418). Urobilinogen is derived from the normal bile- 

 pigment by reduction in the intestine itself, where reducing substances 

 due to the action of micro-organisms are never absent in extra-uterine life. 



The bile of most animals shows no characteristic absorption spectrum. 

 But the fresh bile of certain animals, the ox, for instance, does show 

 bands. These, however, are not due to the normal bile -pigment, and 

 they are not essentially changed when this is oxidized or reduced by 

 electrolysis. MacMunn attributes the spectrum of the bile of the ox 

 and sheep to a body which he calls cholohasmatin, and which does not 

 belong to the bile-pigments proper. 



The Bile-Salts. These are the sodium salts of certain acids, of which 

 glycocholic and taurocholic are the chief. In the bile of omnivora, 

 including man, both are in general present, and in various proportions; 

 in human bile there is more glycocholic than taurocholic acid; some- 

 times taurocholic acid is entirely absent. In the bile of many carnivora 

 e.g., the dog and cat only taurocholic acid is found; in that of the 

 carnivora generally it is by far the more important of the two acids. 

 In the bile of most herbivora there is much more glycocholic than 

 taurocholic acid. The bile acids are paired acids: glycocholic acid 

 (better named cholyl-glycin) formed by the union of glycin and cholic 

 acid, and taurocholic acid (or cholyl-taurin), consisting of cholic acid 

 united with taurin. 



The decomposition of the bile-acids into these substances is effected 

 by boiling them with dilute acid or alkali, a molecule of water being 

 taken up; thus 



C 26 H 43 N0 6 + H 2 - CH 2 (NH 2 ) .COOH + C 24 H 40 O 5 ; 



Glycocholic acid. Glycin. Cholic acid. 



C 26 H 45 NS0 7 + H 2 0=CH 2 (NH 2 ).CH 2 .S0 2 .OH +C 24 H 40 5 . 



Taurocholic acid. Taurin. Cholic acid. 



A notable difference between glycocholic and taurocholic acid is that 

 the latter contains sulphur. The" whole of this belongs to the taurin. 



Both glycin and taurin are derived from the disintegration of proteins. 



We have already seen that among the products of protein hydrolysis a 



ystein, which is readily changed into cystin, 

 evidence that taurin is derived from cystein 



