XIII. THE ANIMAL 277 



of food rich in carbohydrates, sometimes rising to 10 or 12 per cent ; 

 normally, however, it is probably between 1 and 4 per cent. It dis- 

 appears during starvation. It is an amorphous, white, tasteless powder. 



Bile is a reddish yellow (especially in carnivorous animals) or green 

 (in herbivora) fluid, with faint alkaline reaction and bitter taste. That 

 found in the gall bladder is more concentrated than that obtained 

 directly from the liver. Bile contains sodium and potassium salts of the 

 so-called bile acids, mucin, or other albumin (to which its viscidity is 

 due), bile pigments, lecithin, cholesterol, fats, soaps and mineral matter. 



The bile acids are chiefly glycocholic acid, C 26 H 43 NO 6 , and tauro- 

 cholic acid, C 26 H 45 N0 7 S. These are both monobasic acids and give, 

 with strong sulphuric acid and cane sugar (or better with a minute 

 trace of furfurol), characteristic colour reactions (Pettenkofer's reaction). 

 The liquid becomes successively cherry red, dark red and finally 

 purple violet. The acids and their alkaline salts have a strong bitter 

 taste. Solutions of sodium or potassium salts of both acids have sol- 

 vent powers for soaps, lecithin and cholesterol. 



The acids undergo hydrolysis, taurocholic acid the more readily, 

 yielding, in the one case, glycocoll and cholalic acid, and in the other 

 taurine and cholalic acid, thus : 



C 26 H 43 N0 6 + H 2 = CH 2 (NH 2 )COOH + C 24 H 40 5 

 and C 26 H 45 N0 7 S + H 2 = CH 2 (NH 2 ).CH 2 .SO 2 .OH + C 24 H 40 5 . 



Glycocoll glycine amino-acetic acid CH 2 (NH 2 ).COOH, is a 

 crystalline body, of sweet taste, very soluble in water, and of acid 

 reaction. It is also produced by the decomposition of hippuric acid, 

 C 6 H 5 CO.NH.CH 2 .COOH, and in the pancreatic digestion of gelatine. 



Taurine, amino-ethyl sulphonic acid, NH 2 .CH 2 .CH 2 .S0 2 .OH, is a 

 crystalline, tasteless body, soluble in water, insoluble in alcohol, and 

 of neutral reaction. 



Cholalic acid, C 24 H 40 O 5 , a monobasic acid, is very slightly soluble 

 in cold water. It has the characteristic bitter taste of the bile acids. 



The bile pigments are chiefly bilirubin and biliverdin. 



These pigments appear to be formed from haemoglobin, or rather 

 haematin, but contain no iron. 



Bilirubin, C 32 H 36 N 4 6 , is a reddish-yellow substance, insoluble in 

 water but soluble in alcohol or chloroform. It is also soluble in 

 alkalies. It is found in bile, especially in that of the carnivora, and 

 also occurs, in combination with calcium, in gall-stone. On exposure 

 to air, alkaline solutions absorb oxygen and become green (biliverdin). 



Biliverdin, C 32 H 36 N 4 8 , is an amorphous green substance, insoluble 

 in water but soluble in alcohol and in alkalies. It is found in bile, in 

 shells of birds' eggs, and sometimes in gall-stones. Other colouring 

 substances, e.g., bilipurpurin, C 32 H 34 N 4 5 , have been found in bile. 



The mineral matters include potassium, sodium, calcium, mag- 

 nesium, iron and often traces of copper and zinc ; phosphates and 

 chlorides are also present. 



Bile undoubtedly plays an important part in the digestion of fats. 

 If the bile be prevented from entering the alimentary canal, the faeces 



