746 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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, 

 glycocholic 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. In human bile 

 both acids are usually found, but the proportion of taurocholate 

 is variable, and in some cases it 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 26 H 43 NO 6 , 

 and taurocholic acid the formula C 26 H 45 NSO 7 . Each of them can 

 be obtained in the form of crystals. When boiled with acids or alka- 

 lies these acids take up water and undergo hydrolytic cleavage, the 

 reaction being represented by the following equations: 



C^H.-NOe + H 2 O = C M HO A + CH ? (NH 2 )COOH. 



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



C 26 H 46 NS0 7 + H 2 = C.,H 40 0. + C^NH^OH 



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



sulphonic acid). 



These reactions are interesting not only in that they throw light on 

 the structure of the acids, but also because similar reactions doubtless 

 take place in the intestine, cholic acid having been detected in the 

 intestinal contents. As the formulas 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 

 liquids by the well-known Pettenkofer reaction. As usually per- 

 formed, the test is made by adding to the liquid a few drops of a 10 

 per cent, solution of cane-sugar and then strong sulphuric acid. The 

 latter must be added carefully and the temperature be kept below 

 70 C. If bile acids are present, the liquid assumes a red-violet 

 color. It is now known that the reaction consists in the formation 

 of a substance (furfurol) by the action of the acid on sugar, which 

 then reacts with the bile acids. The bile acids are formed directly 

 in the liver cells. This fact, which was for a long time the subject of 

 discussion, has been demonstrated in recent years by an important 

 series of researches made upon birds. It has been shown that if the 

 bile-duct is ligated in these animals, the bile formed is reabsorbed and 

 bile acids and pigments may be detected in the urine and the blood. 

 If, however, the liver is completely extirpated, then no trace of either 

 bile acids or bile pigments can be found in the blood or the urine, 

 showing that these substances are not formed elsewhere in the body 



