33 6 A MANUAL OF PHYSIOLOGY 



from bilirubin by oxidation. By the aid of active oxidizing agents, 

 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, 

 to bili-purpurin, which is purple, and finally to choletelin, a yellow 

 substance. It is possible that there are other intermediate bodies. 

 This is the foundation of Gmelin's test for bile-pigments (see Practical 

 Exercises, p. 431). The same substances 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. Starting from biliverdin, the negative pole 

 causes the green to pass through yellowish-green into golden-yellow, 

 and ultimately into pale yellow, indicating a series of bodies formed 

 by reduction of the biliverdin. These reactions can also be used for 

 the detection of bile-pigments. 



By the reducing action of sodium amalgam, or of tin and hydro- 

 chloric acid, on bilirubin, hydrobilirubin is obtained. This is similar 

 to but not identical with the urobilin of urine, or with the urobilin 

 (stercobilin) found in the faeces (partly in the form of its mother- 

 substance or chromogen, urobilinogen) from birth onwards, although 

 not in the meconium (p. 396) . Urobilin is derived from the normal bile- 

 pigment by reduction in the intestine itself, where reducing sub- 

 stances due to the action of micro-organisms are never absent in 

 extra-uterine life. The changes occurring in oxidation and reduc- 

 tion of the bile-pigment may be partially represented as follows : 



(C 32 H 36 N 4 6 ) + 2 = (CaHaeNA), + 2 O 2 - (C 32 H 36 N 4 O 12 ) 



Bilirubin. Biliverdin. Choletelin. 



^(C^H^O,) - 2 + 4 H 2 =2(C 32 H 40 N 4 7 ). 



Bilirubin. Hydrobilirubin. 



The bile of most animals shows no characteristic absorption 

 spectrum. But the fresh bile of certain animals, the ox, for instance, 

 does show bands a strong one over C, and two weaker bands, one 

 of which is just to the left of D, and the other to the right of it, but 

 nearer D than E. The two last bands grow stronger when the bile 

 is allowed to stand for twenty-four hours, and in about three. days, 

 in warm weather, a fourth sharp band may appear between C and B. 

 But none of these bands is 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 cholohaematin, and which 

 does not belong to the bile-pigments proper. Of the derivatives 

 of the bilirubin set, only the lowest and the highest members, 

 hydrobilirubin and choletelin, are described as giving absorption 

 spectra. 



The Bile-salts. These are the sodium salts of two acids, glyco- 

 cholic and taurocholic. 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 ; sometimes 

 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. 



