soon becomes distended and the water even rises in the tube to a
considerable height at _a_(Fig. 2). The force here concerned is a force
known as _osmosis_ or _dialysis_, and is always exerted when two
different solutions of certain substances are separated from each other
by a membrane. The substances in solution will, under these conditions,
pass from the dense to the weaker solution. The process is a purely
physical one.

[Illustration: FIG. 1.--To illustrate osmosis. In the vessel _A_ is a
solution of sugar; in _B_, is pure water. The two are separated by the
membrane _C_. The sugar passes through the membrane into _B_.]

[Illustration: FIG. 2.--In the bladder _A_ is a sugar solution. In the
vessel _B_ is pure water. Sugar passes out and water into the bladder
until it rises in the tube to a.]

This process of osmosis lies at the basis of the absorption of food from
the alimentary canal. In the first place, most of the food when
swallowed is not soluble, and therefore not capable of osmosis. But the
process of digestion, as we have seen, changes the chemical nature of
the food. The food, as the result of chemical change, has become
soluble, and after being dissolved it is _dialyzable_--i.e., capable of
osmosis. After digestion, therefore, the food is dissolved in the
liquids in the stomach and intestine, and is in proper condition for
dialysis. Furthermore, the structure of the intestine is such as to
produce conditions adapted for dialysis. This can be understood from
Fig. 3, which represents diagrammatically a cross section through the
intestinal wall. Within the intestinal wall, at _A_, is the food mass in
solution. At _B_ are shown little projections of the intestinal wall,
called _villi_ extending into this food and covered by a membrane. One