h, as already shown, are
simple liquids; but they consist of water and different substances
dissolved in the water. They belong to a class of substances called
_solutions_. The chief point of interest about substances in solution is
that they are very finely divided and that their little particles are free
to move about in the liquid that contains them. Both the motion and the
finely divided condition of the dissolved substances are necessary to the
process of osmosis. All substances, however, that appear to be in solution
are not able to penetrate membranes, or take part in osmosis.

*Kinds of Solutions in the Body.*--The substances in solution in the body
liquids are of two general kinds known as _colloids_ and _crystalloids_.
The crystalloids are able to pass through membranous partitions, while the
colloids are not. An example of a colloid is found in the albumin of an
egg, which is unable to penetrate the membrane which surrounds it.
Examples of crystalloids are found in solutions of salt and sugar in
water. The inability of a colloid to penetrate a membrane is due to the
fact that it does not form a true solution. Its particles (molecules),
instead of being completely separated, still cling together, forming
little masses that are too large to penetrate the membrane. Since,
however, it has the appearance, on being mixed with water, of being
dissolved, it is called a _colloidal solution_. The crystalloid substance,
on the other hand, completely separates in the water and forms a _true
solution_--one which is able to penetrate the partition or membrane.

*Osmosis not a Sufficient Cause.*--The passage of materials through animal
membranes, according to the principle of osmosis, is limited to
crystalloid substances. But colloid substances are also known to pass
through the various partitions of the body. An example of such is found in
the proteids of the blood which, as a colloidal solution, pass through the
capillary walls to become a part of the lymph. Perhaps the best
explanation offered as yet for this passage is that the colloidal
substances are changed by the cells lining the capillaries into substances
that form true solutions and that after the passage they are changed back
again to the colloidal condition.

*Summary.*--Between the cells and the capillaries is a liquid, known as the
lymph, which is similar in composition and physical properties to the
blood. It consists chiefly of escaped plasma. The vessels