234 PEACTICAL PHYSIOLOGY 



to be lost in the faeces. In the blood they seem to act as solvents for 

 cholesterin, and in the intestine they assist in the absorption of fat. 



A small amount (one eighth), however, escapes in the faeces, and, to 

 make good the loss, more must be produced. 1 The substance which 

 yields them is proteid, both glycin and taurin being undoubtedly 

 derived from this. The evidence that glycin results in the decomposi- 

 tion of proteid we have already obtained in studying tryptic digestion, 

 and the presence of sulphur, as well as of nitrogen, in taurin, betrays its 

 derivation from the same source. Nothing certain is known of the 

 derivation of cholalic acid. 



The Bile Pigments. These are bili-rubin and bili-verdin. The former 

 occurs most plentifully in the bile of carnivorous, the latter in that of 

 herbivorous animals. Their presence can be detected by oxidising a 

 mixture containing them with nitrous acid, when a play of colours 

 green, blue, purple, and then yellow is produced. This is called 

 Gmelin's test. 2 



EXPERIMENT V. Dilute some ox bile with an equal amount of water. 

 Hold the test tube as nearly horizontal as possible, and allow some 

 fuming nitric acid to run down it, so that this forms a layer under the 

 bile. Where the two fluids are in contact, a play of colours is produced. 



Bili-rubin is the least oxidised pigment, and its empirical formula is 

 C 32 H 3G N 4 6 . If we compare this with the formula of haematin 

 C 32 H 32 N 4 4 Fe we see that it must be from this body that it is derived, 

 the change being the abstraction of iron and the addition of two mole- 

 cules of water. This is also the formula of iron-free haematin or 

 haematoporphyrin, and of haematoidin, a pigment which crystallises 

 out in old blood clots in the tissues. Although the same empirically, 

 these bodies vary somewhat in their physical behaviour, so that we may 

 assume that they have different constitutional formulae. 



When it reaches the intestine, the bile pigment is reduced by the 

 nascent hydrogen generated by bacterial growth, to another pigment 

 called stercobilin. Most of this is absorbed into the portal blood along 

 with the bile salt. This reabsorbed stercobilin is partially re-excreted 

 in the bile, and partially excreted in the urine, where it goes by the 

 name of urobilin (see Urine). The stercobilin which is not reabsorbed 

 forms the colouring matter of the faeces. 



1 Increased proteid metabolism in the tissues is not accompanied by an increase 

 in the nitrogen and sulphur excreted in the bile. This shows that the bile salts 

 cannot be excretory products, but that they must be useful secretory bodies. 



2 This test depends on the various colours of the oxidation products of bili-rubin. 

 The first oxidation product is bili-verdin, which is green ; the next is bili-cyanin, 

 which is blue ; the next is bili-purpurin, which is purple ; and the last is choletelin, 

 which is yellow. 



