DIGESTION OF FOOD. 313 
goes a series of color changes in a certain tolerably constant 
order, becoming green, greenish-blue, blue, violet, a brick red, 
and finally yellow; though the green is the most characteristic 
and permanent. Each one of these represents a distinct stage 
of the oxidation of bilirubin, the green answering to biliverdin. 
Such is Gmelin’s test for bile-pigments, by which they may be 
detected in urine or other fluids. The absence of proteids in 
bile is to be noted. 
The Digestive Action of Bile-——1. So far as known, its action 
on proteids is nil. When bile is added to the products of an 
artificial gastric digestion, bile-salts, peptone, pepsin, and para- 
peptone are precipitated and redissolved by excess. 2. It is 
slightly solvent of fats, though an emulsion made with bile is 
very feeble. But it is likely helpful to pancreatic juice, or 
more efficient itself when the latter is present. With free fatty 
acids it forms soaps, which themselves help in emulsifying fat. 
3. Membranes wet with bile allow fats to pass more readily; 
hence it is inferred that bile assists in absorption. 4. When 
bile is not poured out into the alimentary canal the feeces 
become clay-colored and ill-smelling, foul gases being secreted 
in abundance, so that it would seem that bile exercises an anti- 
septic influence. It may limit the quantity of indol formed. 
It is to be understood that these various properties of bile are 
to be traced almost entirely to its salts; though its alkaline 
reaction is favorable to digestion in the intestines, apart from 
its helpfulness in soap-forming, etc. 5. It is thought by some 
that the bile acts as a stimulant to the intestinal tract, giving 
rise to peristaltic movements, and also, mechanically, as a lubri- 
cant of the feces. In the opinion of many, an excess of bile 
naturally poured out causes diarrhea, and it is well known 
that bile given by the mouth acts asa purgative. However, 
we must distinguish between the action of an excess and that 
of the quantity secreted by a healthy individual. The acid of 
the stomach has probably no effect allied to that produced by 
giving acids medicinally, which warns us that too much must 
not be made out of the argument from bilious diarrhcea. 6. As 
before intimated, a great part of the bile must be regarded as 
excrementitious. It looks as though much of the effete hemo- 
globin of the blood and of the cholesterin, which represents 
possibly some of the waste of nervous metabolism, were expelled 
from the body by the bile. The cholalic acid of the feeces is 
derived from the decomposition of the bile acids. Part of their 
mucus must also be referred to the bile, the quantity originally 
