former case.

What is true of the digestion of starch by saliva is equally true of the
digestion of other foods in the stomach and intestine. Each of the
digestive juices contains a ferment which brings about a chemical change
in the food. The changes are always chemical changes and are the result
of chemical forces. Apart from the presence of these ferments there is
really little difference between laboratory chemistry and living
chemistry.

_Absorption of food_.--The next function of this machine to attract our
attention is the absorption of food from the intestine into the blood.
The digested food is carried down the alimentary canal in a purely
mechanical fashion by muscular action, and when it reaches the intestine
it begins to pass through its walls into the blood. In this absorption
we find engaged another set of forces, the chief of which appears to be
the physical force of _osmosis_. The force of osmosis has no special
connection with life. If a membrane separates two liquids of different
composition (Fig. i), a force is exerted on the liquids which cause them
to pass through the membrane, each passing through the membrane into
the other compartment. The force which drives these liquids through the
membrane is considerable, and may sometimes be exerted against
considerable pressure. A simple experiment will illustrate this force.
In Fig. 2 is represented a membranous bag tightly fastened to a glass
tube. The bag is filled with a strong solution of sugar, and is immersed
in a vessel containing pure water. Under these conditions some of the
sugar solution passes through the bag into the water, and some of the
water passes from the vessel into the bag. But if the solution of sugar
is inside the bag and the pure water outside, the amount of liquid
passing into the bag is greater than the amount passing out; the bag