soda_. Both are crystalline, resinous substances, and, although
resembling each other in many respects, the chemist may distinguish them
by their reaction, for both yield a precipitate if treated with
subacetate of lead, but only the glycocholate will give a precipitate
with acetate of lead. In testing for biliary substances, the most
satisfactory method is the one proposed by Pettenkoffer. A solution of
cane-sugar, one part of sugar to four parts of water, is mixed with the
suspected substance. Dilute sulphuric acid is then added until a white
precipitate falls, which is re-dissolved in an excess of the acid. On
the addition of more sulphuric acid, it becomes opalescent, and passes
through the successive hues of scarlet, lake, and a rich purple. Careful
experiments have proved that it is a _constant_ secretion; but its flow
is mere abundant during digestion. During the passage through the
intestines it disappears. It is not eliminated, and Pettenkoffer's test
has failed to detect its existence in the portal vein. These facts lead
physiologists to the conclusion, that it undergoes some transformation
in the intestines and is re-absorbed.

After digestion has been going on in the stomach for some time, the
semi-digested food, in the form of chyme, begins to pass through the
_pyloric orifice_ of the stomach into the duodenum, or upper portion of
the small intestine. Here it encounters the intestinal juice, pancreatic
juice, and the bile, the secretion of all of which is stimulated by the
presence of food in the alimentary tract. These fluids, mingling with
the chyme, give it an alkaline reaction, and convert it into chyle. The
transformation of starch into sugar, which is almost, if not entirely,
suspended while the food remains in the stomach, owing to the acidity of
the chyme, is resumed in the duodenum, the acid of the chyme, being
neutralized by the alkaline secretions there encountered.