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.