816 PHYSIOLOGY OF DIGESTION AND SECRETION. 



cholic acid, in the other by a union of taurin and cholic acid. In 

 this synthesis the reaction takes place between the carboxyl group 

 of the cholic acid and the amino-group of the glycocoll or taurin, 

 as may be represented by the following equation: 



C 23 H 3 9O 3 COOH + NH 2 CH 2 COOH H 2 O = C 23 H 3 9O 3 CONHCH 2 COOH 



Cholic acid. Glycocoll. Glycocholic acid. 



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

 suspected liquids by the well-known Pettenkofer reaction. As 

 usually performed, the test is made by adding to the liquid a few 

 drops of a 10 per cent, solution of cane-sugar and then strong sul- 

 phuric acid. The latter must be added carefully and the tempera- 

 ture be kept below 70 C. If bile acids are present, the liquid as- 

 sumes a red- violet color. It is now known that the reaction consists 

 in the formation of a substance (f urfurol) 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 than in the liver. It is 

 more difficult to ascertain from what substances they are formed. 

 The chemical origin of the cholic acid is not definitely known. The 

 glycocoll is one of the amino-acids formed by the hydrolysis of 

 proteins, and the liver gets a supply of this material after meals 

 through the portal blood, although, in all probability, it can be 

 formed also within the liver cells by autolytic processes. In any 

 case, so far as it is present in the bile as glycocholic acid, it repre- 

 sents a loss or excretion of so much protein nitrogen. The taurin is 

 likewise derived from protein and presumably from the cystein 

 grouping in the protein molecule. By oxidation the cystein may be 

 converted to cysteinic acid: 



CH 2 SHCHNH 2 COOH + 3O = CH 2 SO 2 OHCHNH 2 COOH; 



Cystein. Cysteinic acid. 



and by loss of CO 2 this is converted to taurin : 



CH 2 SO 2 OHCHNH 2 COOH CO 2 = CH 2 SO 2 OHCH 2 NH 2 . 



A circumstance of considerable physiological significance is that 

 these acids or their decomposition products are absorbed in part from 



