302 COMPARATIVE PHYSIOLOGY. 



latter two especially being absent from all other fluids of the 

 body. Sodium chloride is the most abundant salt. Bile must 

 be regarded as an excretion as well as a secretion ; the pig- 

 ments, copper, manganese, and perhaps the iron and the cho- 

 lesterin being of little or no use in the digestive processes, so 

 far as known. 



The bile-salts are the essential constituents of bile as a 

 digestive fluid. In man and many other animals, they consist 

 of taurocholate and glycocholate of sodium, and may be ob- 

 tained in bundles of needle-shaped crystals radiating from a 

 common center. These salts are soluble ia water and alcohol, 

 with an alkaline reaction, but insoluble in ether. 



Glycocholic acid may be resolved into cholalic (cholic) acid 

 and glycin (glycocol) ; and taurocholic acid into cholalic acid 

 and taurin. Thus : 



Glycocholic acid. Cholalic acid. Glycin, 



Cs.HisNOa + H,0 = CwHioOe -1- CaHsNOa. 



Taurocholic acid. Cholalic acid. Taurin. 



Ca.H«NSO, -I- Hao = CaiHwOs -I- C»H,NS08. 

 Grlycocol (glycin) is amido-acetic acid — 



Taurin, amido-isethionic acid, 



CiiH4<-vrT| , and may be made artifi- 

 cially from isethionic acid. 



It is to be noted that both the bile acids contain nitrogen, 

 but that cholalic acid does not. The decomposition of the bile 

 acids takes place in the alimentary canal, and the glycin and 

 taurin are restored to the blood, and are possibly used afresh in 

 the construction of the bile acids, though this is not definitely 

 known. 



Bile-Pigfments.— The yellowish-red color of the bile is owing 

 to Bilirubin (CieHisNaOs), which may be separated either as 

 an amorphous yellow powder or in tablets and prisms. It is 

 soluble in chloroform, insoluble in water, and but partially 

 soluble in alcohol and ether. It makes up a large part of 

 gall-stones, which contain, besides cholesterin, earthy salts in 

 abundance. 



It may be oxidized to Biliverdin (C18H18N2O4), the natural 

 green pigment of the bile of the herbivora. When a drop of 



