320 ESSENTIALS OF PHYSIOLOGY. 



Bile thus becomes more concentrated by the absorption of water 

 during the time it remains in the gall-bladder. It is a viscid fluid, 

 golden brown in carnivora, green in herbivora, and possesses a bitter 

 taste. The viscidity is due to the presence of mucin in human bile, 

 and of a nucleo-protein in that of the ox and some other animals. The 

 colour depends upon the presence of the bile pigments, bilirubin and 

 biliverdin. The former is more, abundant in the bile of carnivora, the 

 latter in the bile of herbivora. The proportion appears to vary in 

 human bile according to the nature of the diet, the brown tint of 

 bilirubin predominating when a flesh diet is taken, and the green of 

 biliverdin when the diet is largely vegetarian. The bitter taste of bile 

 is due to the bile salts, glycocholate and taurocholate of sodium. In 

 dog's bile the latter only is present. 



The Properties, Source, and Fate of the Chief Constituents of the 

 Bile. The mucin of human bile gives a stringy precipitate on the addi- 

 tion of acetic acid, the precipitate being insoluble in excess of the acid. 

 The nucleo-protein of ox bile gives a similar precipitate with acetic acid, 

 but as in the case of nucleo-proteins generally, the precipitate is dissolved 

 by excess of the acid. The mucin or nucleo-protein of the bile is derived 

 from the mucus-secreting cells which line the bile ducts and gall- 

 bladder. 



The bile salts are compounds of sodium with glycocholic and 

 taurocholic acid respectively. These acids may be split up by hydrolysis, 

 glycocholic into glycine (amino-acetic acid) and cholic acid, and 

 taurocholic into taurine (amino-ethylsulphonic acid) and cholic acid. 



C 26 H 43 N0 6 + H 2 = Crf 2 . NH 2 . COOH + C 24 H 40 5 



Glycocholic acid. Glycine. Cholic acid, 



C 26 H 45 N0 7 S + H 2 = C 2 H 4 . NH 2 . HS0 3 + C 24 H 40 5 



Taurocholic acid. Taurine. Cholic acid. 



If a little syrup of cane sugar be added to a solution of bile salts 

 in a test tube, and strong sulphuric acid be poured down the side of 

 the tube so as to lie below the solution, a cherry-red colour appears at 

 the junction of the two fluids. The colour is due to a reaction between 

 cholic acid and furfuraldehyde, the latter being formed by the action 

 of the sulphuric acid on the cane sugar. The bile salts have the 

 property of reducing the surface tension of the fluid in which they are 

 dissolved. This can be shown by comparing the effect of scattering 

 flowers of sulphur on water and on a solution of bile salts. The sulphur 

 floats on the water, but sinks immediately in the bile salt solution 

 (Hay's test). Further, watery solutions of bile salts readily dissolve 

 fatty acids. Bile acids are derived from protein sources and are formed 



