TRANSFERENCE OF FOOD MATERIALS 125 



placed in contact with strong nitric acid a series of colours 

 are seen ranging through green, blue, violet, red, yellow, 

 from the bile towards the acid. The colour of bile from 

 different animals varies. Herbivora have a bile which is 

 predominantly greenish due to the green pigment biliverdin 

 and carnivora have an orange bile due to the red pigment, 

 bilirubin. The chemical relation of these pigments to other 

 substances will be described later (p. 189). 



The bile salts are the sodium salts of two acids, glycocholic 

 and taurocholic acids. These are formed by the union of 

 cholalic acid (C 24 H 40 O 5 )* with glycine (CH 2 NH 2 COOH) 

 and with taurin (CH 2 (NH 2 )CH 2 SO 2 OH) respectively. They 

 have the important property of decreasing the surface tension 

 at an air-water surface, the importance of which will be 

 considered later in this chapter. 



The tests for bile acids are first of all a purple colour, 

 produced by acting on the solution with strong sulphuric 

 acid in the presence of cane-sugar. | The strong acid produces 

 furfurol by acting on the cholalic acid and the furfurol con- 

 denses with the cane-sugar to produce the purple colour. The 

 bile acids can also be detected by their action on the surface 

 tension. Flowers of sulphur float on the surface of pure water 

 but the presence of a little bile salt allows the sulphur to break 

 through the surface and sink.J A quantitative measure 

 of the amount of bile salt can be obtained by the stalagmo- 

 metric method, which consists in comparing the number of 

 drops given by a known weight of liquid with the same 

 weight of liquid without bile salt. The drops must be 

 obtained under standard conditions through a standard 

 orifice. 



The bile acids are absorbed from the intestine into the blood, 

 which passing directly back to the liver by the portal circu- 

 lation gives up these substances to be passed into the bile once 

 more. This cycle of changes is termed the circulation of the 

 bile acids. Thus : 



* R. H. A. Plimmer, Practical Organic and Biochemistry (Long- 



/ COOH 



mans), 1915, p. 322, gives the formula as C 26 H 3l =CHOH 



CH 2 OH 

 CH 2 OH 



f M. Pettenkofer, Ann. d. Chem. u. Pharm., 1844, vol. 52, p. 90. 

 J M. Hay, Landois' Physiology, translated by W. Stirling, 1886, note 

 on p. 381. 



O. F. Leyton (Grimbaum), Journ. Physiol., 1904, vol. 30, Proc. 

 p. xxvi. 



