319 ANIMAL 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 con- 
sist of taurocholate and glycocholate of sodium, and may be 
obtained in bundles of needle-shaped crystals radiating from 
acommon center. These salts are soluble in water and alco- 
hol, with an alkaline reaction, but insoluble in ether. 
Glycocholic acid may be resolved into cholalic (cholic) acid 
and glycin (glycocoll); and taurocholic acid into cholalic acid 
and taurin. Thus: 
Glycocholic acid. Cholalic acid. Glycin. 
CogpHasN Og + H2O = CHO; + C,H;NO>. 
Taurocholic acid. Cholalic acid. Taurin. 
CosHssNSO, + Hey = CorHi005 + C2H,NSOs. 
Glycocoll (glycin) is amido-acetic acid— 
cH, <NE and 
CO.H 
Taurin, amido-isethionic acid, 
C,H, <80:H. and may be made artificially 
NH, 
from isethionic acid. 
It isto be noted that the bile acids both contain nitrogen, 
but that chololic 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 defi- 
nitely known. 
Bile-Pigments.—The yellowish-red color of the bile is owing 
to Bilirubin (CijHisN;O;), 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 (CsHisN20,), the natural 
green pigment of the bile of the herbivora. When a drop of 
nitric acid, containing nitrous acid, is added to bile, it under- 
