the body of a highly
developed organism such as man has with the world external to him.
This relation is effected by means of the various surfaces of the
body. On the outside is the skin [Fig. 3], which surface is many times
increased by the existence of glands and such appendages to the skin
as the hair and nails. A gland, however complicated its structure, is
nothing more than an extension of the surface into the tissue beneath
[Fig. 4]. In the course of embryonic development all glands are formed
by an ingrowth of the surface. The cells which line the gland surface
undergo a differentiation in structure which enables them to perform
certain definite functions, to take up substances from the same source
of supply and transform them. The largest gland on the external
surface of the body is the mammary gland [Fig. 5] in which milk is
produced; there are two million small, tubular glands, the sweat
glands, which produce a watery fluid which serves the purpose of
cooling the body by evaporation; there are glands at the openings of
the hairs which produce a fatty secretion which lubricates the hair
and prevents drying, and many others.

[Illustration: FIG. 5.--A SECTION OF THE MAMMARY GLAND. (_a_) The
ducts of the gland, by which the milk secreted by the cells which line
all the small openings, is conveyed to the nipple. All these openings
are continuous with the surface of the skin. On each side of the large
ducts is a vein filled with blood corpuscles.]

[Illustration: FIG. 6.--PHOTOGRAPH OF A SECTION OF THE LUNG OF A MOUSE.
_x x_ are the air tubes or bronchi which communicate with all of
the small spaces. On the walls of the partitions there is a close
network of blood vessels which are separated from the air in the
spaces by a thin membrane.]

The external surface passes into the interior of the body forming two
surfaces, one of which, the intestinal canal, communicates in two
places, at the mouth and anus, with the external surface; and the
other, the genito-urinary surface, which communicates with the
external surface at one place only. The surface of the intestinal
canal is much greater in extent than the surface on the exterior, and
finds enormous extensions in the lungs and in the great glands such as
the liver and pancreas, which communicate with it by means of their
ducts. The extent of surface within the lungs is estimated at
ninety-eight square yards, which is due to the extensive infoldings of
the surfac