ubes and cavities which are connected with
the external surface.

To get from the digestive organs into the blood, the nutrients must pass
through the mucous membrane lining these organs and also the walls of
blood or lymph vessels. Only _liquid materials_ can make this passage. It
is necessary, therefore, to reduce to the liquid state all nutrients not
already in that condition. _This reduction to the liquid state constitutes
the digestive process_.

*How Substances are Liquefied.*--While the reduction of solids to the
liquid state is accomplished in some instances by heating them until they
melt, they are more frequently reduced to this state by subjecting them to
the action of certain liquids, called _solvents_. Through the action of
the solvent the minute particles of the solid separate from each other and
disappear from view. (Shown in dropping salt in water.) At the same time
they mix with the solvent, forming a _solution_, from which they separate
only with great difficulty. For this reason solids in solution can diffuse
through porous partitions along with the solvents in which they are
dissolved (page 73).

By digestion the nutrients are reduced to the form of a solution. _The
process is_, simply speaking, _one of dissolving_. The liquid employed as
_the digestive solvent is water_. The different nutrients dissolve in
water, mixing with it to form a solution which is then passed into the
body proper.

*Digestion not a Simple Process.*--Digestion is by no means a simple
process, such, for instance, as the dissolving of salt or sugar in water.
These, being soluble in water, dissolve at once on being mixed with a
sufficient amount of this liquid. The majority of the nutrients, however,
are insoluble in water and are unaffected by it when acting alone. Fats,
starch, and most of the proteids do not dissolve in water. Before these
can be dissolved they have to be changed chemically and converted into
substances that are _soluble in water_. This complicates the process and
_prevents the use of water alone_ as the digestive solvent.

*A Similar Case.*--If a piece of limestone be placed in water, it does not
dissolve, because it is insoluble in water. If hydrochloric acid is now
added to the water, the limestone is soon dissolved (Fig. 62). (See
Practical Work.) It seems at first thought that the acid dissolves the
limestone, but this is not the case. The acid produces a chemical change
in the limestone (calcium ca