CHEMISTRY OF DIGESTION AND NUTRITION. 263 



iron-free compound, bilirubin or biliverdin. It is very significant to find that 

 the iron separated by this means from the haemoglobin is for the most part 

 retained in the liver, a small portion only being secreted in the bile. It seems 

 probable that the iron held back in the liver is again used in some way to 

 make new hemoglobin in the haematopoietic organs. The bile-pigments are 

 carried in the bile to the duodenum and are mixed with the food in its long 

 passage through the intestine. Under normal conditions neither bilirubin nor 

 biliverdin is found in the feces, but in their place is found a reduction pro- 

 duct, hydrobilirubin. Moreover, it is believed that some of the bile-pigment is 

 reabsorbed as it passes along the intestine, is carried to the liver in the portal 

 blood, and is again eliminated. That this action occurs, or may occur, has 

 been made probable by experiments of Wertheimer * on dogs. It happens that 

 sheep's bile contains a pigment (choloha3matin) which gives a characteristic 

 spectrum. If some of this pigment is injected into the mesenteric veins of a 

 dog, it is eliminated while passing through the liver, and can be recognized 

 unchanged in the bile. The value of this " circulation of the bile," so far as 

 the pigments are concerned, is not apparent. 



Bile-acids. " Bile-acids " is the name given to two organic acids, glyco- 

 choliG and taurocholic, which are always present in bile, and, indeed, form 

 very important constituents of that secretion ; they occur in the form of their 

 respective sodium salts, and not as uncombined acids, as the term "bile-acids" 

 might lead one to believe. In human bile both acids are usually found, but 

 the proportion of taurocholate is variable, and in some cases this latter acid 

 may be absent altogether. Among herbivora the glycocholate predominates 

 as a rule, although there are some exceptions ; among the carnivora, on the 

 other hand, taurocholate occurs usually in greater quantities, and in the dog's 

 bile it is present alone. Glycocholic acid has the formula C^H^NO,., and 

 taurocholic acid has the formula C^H^NSO^ Each of them can be obtained 

 in the form of crystals. When boiled with acids or alkalies these acids take 

 up water and undergo hydrolytic cleavage, the reaction being represented by 

 the following equations: 



C M H 43 N0 6 + H 2 = C 21 H <0 5 + CH,(NH 2 )COOH. 



Glycocholic acid. Cholic acid. Glycocoll (amido-acetic acid). 



C^NSO^ H 2 = C 21 H 40 5 + C 2 H 7 NS0 3 . 



Taurocholic acid. Cholic acid. Taurin (amido-ethyl- 



sul phonic acid). 



These reactions are interesting not only in that they throw light ou the structure 

 of the acids, but also because similar reactions doubtless take place in the intes- 

 tine, cholic acid having been detected in the intestinal contents. As the for- 

 mulas show, cholic acid is formed in the decomposition of each acid, and we 

 may regard the bile-acids as compounds produced by the synthetic union of 

 cholic acid with glycocoll in the one case and with taurin in the other. 

 Cholic acid or its compounds, the bile-acids, are usually detected in suspected 

 1 Archives de Physiologie normale et pathologigue, 1892, p. 577. 



