DIGESTION 311 



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, but not apparently by electrolysis, hydro- 

 bilirubin is obtained. This is identical with the 'febrile' urobiTi 

 of some pathological urines, and with stercobilin, a pigment found 

 in the faeces from birth onwards, although not in the meconium 

 (pp. 359, 390), and therefore probably 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 reduction 

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



(C 32 H 36 N 4 6 ) + 2 = (C 3: H 36 N 4 8 ), + 20 2 = (C 32 H 36 N 4 O 12 ) ; 



Bilirubin. L.iliverdin. Choletehn. 



2 (C 32 H 36 N 4 6 ) - 2 + 4 H 2 = 2 (C 32 H 3f) N 4 5 . 2 H 2 0). 



Bilirubin. Hydrobilirubin. 



Judging from the analogy of the blood-pigment from which, as 

 we shall see, the bile-pigment is derived, and the changes in which, 

 through oxidation and reduction, have a certain superficial resem- 

 blance to those which bilirubin undergoes when it is converted into 

 biliverdin, and which biliverdin undergoes when it passes back again 

 to bilirubin we might have expected bile to possess a characteristic 

 spectrum. 



This, however, is not the case. The bile of most animals shows no 

 bands at all. 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 are 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, hydro- 

 bilirubin and choletelin, are described as giving absorption spectra. 



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

 cholic and taurocholic. In human bile both are present, but the 

 former in greater quantity than the latter; sometimes taurocholic 

 acid is entirely absent. In the bile of the dog and cat only tauro- 

 cholic 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. 



Both acids are made up of a non-nitrogenous body, cholic or 



