d
from the blood. The blood is thus continually being impoverished by having
its nourishment drained away. How, then, is the efficiency of the blood
maintained? The answer is that while the ultimate purpose of the food is
for the repair of the waste, its immediate destination is the blood.[19]

129. Absorption of Food by the Blood. How does the food pass from the
cavity of the stomach and intestinal canal into the blood-vessels? There
are no visible openings which permit communication. It is done by what in
physics is known as _endosmotic_ and _exosmotic_ action. That is, whenever
there are two solutions of different densities, separated only by an
animal membrane, an interchange will take place between them through the
membrane.

To illustrate: in the walls of the stomach and intestines there is a
network of minute vessels filled with blood,--a liquid containing many
substances in solution. The stomach and intestinal canal also contain
liquid food, holding many substances in solution. A membrane, made up of
the extremely thin walls of the blood-vessels and intestines, separates
the liquids. An exchange takes place between the blood and the contents of
the stomach and bowels, by which the dissolved substances of food pass
through the separating membranes into the blood.

[Illustration: Fig. 46.--Cavities of the Mouth, Pharynx, etc. (Section in
the middle line designed to show the mouth in its relations to the nasal
fossae, the pharynx, and the larynx.)

A, sphenoidal sinus;
B, internal orifice of Eustachian tube;
C, velum palati;
D, anterior pillar of soft palate;
E, posterior pillar of soft palate;
F, tonsil;
H, lingual portion of the pharynx;
K, lower portion of the pharynx;
L, larynx;
M, section of hyoid bone;
N, epiglottis;
O, palatine arch
]

This change, by which food is made ready to pass into the blood,
constitutes food-digestion, and the organs concerned in bringing
about this change in the food are the digestive organs.

130. The General Plan of Digestion. It is evident that the digestive
organs will be simple or complex, according to the amount of change which
is necessary to prepare the food to be taken up by the blood. If the
requisite change is slight, the digestive organs will be few, and their
structure simple. But if the food is varied and complex in composition,
the digestive apparatus will be complex. This condition applies to the
food and the digestion of man.