FOOD AND DIGESTION 367 



Bile Salts (Chemical Physiology). The most abundant solids 

 are the salts of the bile acids. In man the most important 

 is sodium glycocholate, but in the dog sodium taurocholate is 

 the more abundant. These salts are readily prepared from an 

 alcoholic solution of dried bile by the addition of water-free 

 ether, which makes them separate out as crystals. 



Glyeoeholie acid splits into glycin, amido-acetic acid 

 H 2 N.CH 2 .CO.OH, and a body of unknown constitution, cholalic 

 acid, C 24 H 40 O 5 . 



Taupoeholie acid yields amido-ethane-sulphuric acid or taurin, 

 H 2 N.CH 2 CO.S0 2 OH. a molecule closely resembling amido- 

 acetic acid linked to sulphuric acid and cholalic acid. 



Since both acids contain nitrogen they must be derived from 

 proteins. That they are formed in the liver and not merely 

 excreted by it, is shown by the fact that, while they accumulate 

 in the blood if the bile duct is ligatured, they do not appear 

 if the liver is excluded from the circulation. 



Action of Bile Salts. 1. The bile salts are solvents of fats 

 and fatty acids, and they thus assist in the digestion and 

 absorption of fats. When bile is excluded from the intestines 

 no less than 30 per cent, of the fats of the food may escape 

 absorption and appear in the faeces. When this is the case, 

 as in jaundice from obstruction of the bile duct, the faeces have 

 a characteristic white or grey appearance from the abundance 

 of fat. 



2. These salts keep cholesterin in solution. 



3. They lower the surface tension of solutions, and in this 

 way they may bring the fat particles into more intimate contact 

 with the mucous membrane. 



4. While the salts have no action on proteins, free taurocholic 

 acid precipitates native proteins and acid proteate. In the 

 human intestine this is an action of no importance. 



5. These salts are powerful haemolytic agents, and rapidly 

 dissolve haemoglobin out of the erythrocytes. 



Bile Pigments. These amount to only about "2 per cent, of 

 the bile. In human bile the chief pigment is an orange-brown 

 iron-free substance, bilirubin, C 32 H 36 N 4 6 , while in the bile of 

 herbivora, biliverdin, a green pigment somewhat more oxidised 

 than bilirubin, C 32 H 36 N 4 8 , is more abundant. By further oxida- 

 tion with nitrous acid, other pigments blue, red, and yellow are 



