•°,24 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Tlnse 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 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 

 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 sulphuric acid. The latter must be added carefully 

 and the temperature be kept below 70° C. If bile-acids arc present, the liquid 

 assumes a beautiful red-violet color. It is now known that the reaction con- 

 sists 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 com- 

 pletely 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 ascer- 

 tain from what substances they arc formed. The fact that glycocoll and 

 taurin contain nitrogen, and that the latter contains sulphur, indicates that 

 some proteid or albuminoid constituent is broken down during their pro- 

 duction. 



A circumstance of considerable physiological significance is that these acids 

 or their decomposition products are absorbed in part from the intestine and 

 are again secreted by the liver: as in the case of the pigments, there is an 

 intestinal-hepatic circulation. The value of this reabsorption may lie in the 

 fact that the bile-acids constitute a very efficient stimulus to the bile-secreting 

 activity of the cells, being one of the best of cholagogues, or it may be that it 

 economizes material. From what we know of the history of the bile-acids 

 it is evident that they are not to be considered as excreta: they have some 

 important function to fulfil. The following suggestions as to their value have 

 been made: In the first place, they serve as a menstruum for dissolving the 

 cholesterin which is constantly preseut in the bile and which is an excretion 

 to be removed ; secondly, they facilitate the absorption of fats from the intes- 

 tine. The value of bile in fat -absorption will presently be referred to more 

 in detail. It is an undoubted fact that when bile is shut off from the intes- 

 tine the absorption of fats is very much diminished, and it has been shown 

 that this action of the bile in fat absorption is owing to the presence of the 

 bile-acids. 



Cholesterin. — Cholesterin is a non-nitrogenous substance of the formula 

 C 26 H 44 G or CyE^OH). It is a constant constituent of the bile, although it 



