e [Fig 6], just as a large surface of thin cloth can, by
folding, be compressed into a small space. The intestinal canal from
the mouth to the anus is thirty feet long, the circumference varies
greatly, but an average circumference of three inches may safely be
assumed, which would give between seven and eight square feet of
surface, this being many times multiplied by adding the surfaces of
the glands which are connected with it. A diagram of the microscopic
structure of the intestinal wall shows how little appreciation of the
extent of surface the examination with the naked eye gives [Fig. 7].
By means of the intestinal canal food or substances necessary to
provide the energy which the living tissue transforms are introduced.
This food is liquefied and so altered by the action of the various
fluids formed in the glands of the intestine and poured out on the
surface, that it can pass into the interior of the body and become
available for the living cells. Various food residues representing
either excess of material or material incapable of digestion remain in
the intestine, and after undergoing various changes, putrefactive in
character, pass from the anus as feces.

[Illustration: FIG. 7.--A SECTION OF THE SMALL INTESTINE TO SHOW THE
LARGE EXTENT OF SURFACE. (_a_) Internal surface. The small finger-like
projections are the villi, and between these are small depressions
forming tubular glands.]

By means of the lungs, which represent a part of the surface, the
oxygen of the air, which is indispensable for the life of the cells,
is taken into the body and carbonic acid removed. The interchange of
gases is effected by the blood, which, enclosed in innumerable, small,
thin-walled tubes, almost covers the surface, and comes in contact
with the air within the lungs, taking from it oxygen and giving to it
carbonic acid.

The genito-urinary surface is the smallest of the surfaces. In the
male (Fig. 8,--27, 28, 30) this communicates with the general external
surface by the small opening at the extremity of the penis, and in the
female by the opening into the vagina. In its entirety it consists in
a surface of wide extent, comprising in the male the urethra, a long
canal which opens into the bladder, and is continuous with ducts that
lead into the genital glands or testicles. The internal surface of the
bladder is extended by means of two long tubes, the ureters, into the
kidneys, and receives the fluid formed in these organ