the following
facts have been established.

The gastric juice attacks the surfaces of bodies, and combines
chemically with their particles. It operates with more energy and
rapidity, the more the food is divided, and its action increased by a
warm temperature. By the action of digestion, the food is not merely
reduced to very minute parts, but its chemical properties become
changed; its sensible properties are destroyed, and it acquires new
and very different ones. This juice does not act as a ferment; so far
from it, it is a powerful antiseptic, and even restores flesh which
is already putrid.

When the alimentary substances have continued a sufficient time in
the stomach, they are pushed into the intestines, where they become
mixed with the bile and pancreatic juice, as was before observed.
What changes are caused by these substances, we have yet to learn;
but there is no doubt, that they serve some important purposes. By
the peristaltic motion of the bowels, the alimentary matters thus
changed are carried along, and applied to the mouths of the lacteal
vessels, which open into the intestines, like a sponge, and by some
power, not well understood, absorb that part which is fitted for
assimilation, while the remainder is rejected as an excrement.

The lacteal vessels are furnished with valves, which allow a free
passage to the chyle from the intestines, but prevent its return. The
most inexplicable thing in this operation, is the power which these
vessels possess of selecting from the intestinal mass, those
substances which are proper for nutrition, and rejecting those which
are not.

These lacteal vessels, as was before observed, pass through the
mesentery, and their contents seem to undergo some important change
in the mesenteric glands. The chyle which passes through vessels,
appears to be an oily liquor, less animalised than milk, and its
particles seem to be held in solution by the intermedium of a
mucilaginous principle. It is conveyed along the thoracic duct in the
manner already described, and enters the blood slowly, and, as it
were, drop by drop, by the subclavian vein; in this way it becomes
intimately mixed with the blood, and combining with oxygen in the
lungs, it acquires a fibrous character, and becomes fit to nourish
the body.

We have now seen how the process of digestion is performed, at least,
so far as we are acquainted with it, and how its products are
conveyed into the blood. But to what p