of these _villi_ is shown more highly magnified in Fig. 4, in which _B_
shows this membrane. Inside of these villi are blood-vessels, _C_, and
it will be thus seen that the membrane, _B_, separates two liquids, one
containing the dissolved food outside the villus, and the other
containing blood inside the villus. Here are proper conditions for
osmosis, and this process of dialysis will take place whenever the
intestinal contents holds more dialyzable material than the blood.
Under these conditions, which will always occur after food has been
digested by the digestive juices, the food will begin to pass through
this membranous wall of the intestine into the blood under the influence
of the physical force of osmosis. Thus the primary factor in food
absorption is a physical one.

We must notice, however, that the physical force of osmosis is not the
only factor concerned in absorption. In the first place, it is found
that the food during its passage through the intestinal wall, or shortly
afterwards, undergoes a further change, so that by the time it has
fairly reached the blood it has again changed its chemical nature. These
changes are, however, of a chemical nature, and, while we do not yet
know very much about them, they are of the same sort as those of
digestion, and involve probably nothing more than chemical processes.

[Illustration: FIG. 3--Diagram of the intestinal walls. _A_, lumen of
intestine filled with digested food. _B_, villi, containing blood
vessels. _C_, larger blood vessel, which carries blood with absorbed
food away from the intestine.]


Secondly, we notice that there is one phase of absorption which is still
obscure. Part of the food is composed of fat, and this fat, as the