Swallowed whole, or placed directly in the stomach, such food passes
through the body unchanged. Each division of the alimentary canal has its
own distinct digestive juice, and I give them in the order in which they
occur.

First, The saliva; secreted from the glands of the mouth:--alkaline,
glairy, adhesive.

Second, The gastric juice; secreted in the inner or third lining of the
stomach,--an acid, and powerful enough to dissolve all the fiber and
albumen of flesh food.

Third, The pancreatic juice; secreted by the pancreas, which you know in
animals as sweetbreads. This juice has a peculiar influence upon fats,
which remain unchanged by saliva and gastric juice; and not until
dissolved by pancreatic juice, and made into what chemists call an
_emulsion_, can they be absorbed into the system.

Fourth, The bile; which no physiologist as yet thoroughly understands. We
know its action, but hardly _why_ it acts. It is a necessity, however; for
if by disease the supply be cut off, an animal emaciates and soon dies.

Fifth, The intestinal juice; which has some properties like saliva, and is
the last product of the digestive forces.

A meal, then, in its passage downward is first diluted and increased in
bulk by a watery fluid which prepares all the starchy portion for
absorption. Then comes a still more profuse fluid, dissolving all the
meaty part. Then the fat is attended to by the stream of pancreatic juice,
and at the same time the bile pours upon it, doing its own work in its own