Section of an artery. 8. Section of a lymphatic. The magnification is
too low to show the smaller blood vessels. 5. One of the glands
alongside of the hair which furnishes an oily secretion. 6. A sweat
gland. 7. The fat of the skin. Notice that hair, hair glands and sweat
glands are continuous with the surface and represent a downward
extension of this. All the tissue below 2 and 3 is the corium from
which leather is made.]

[Illustration: FIG. 4.--DIAGRAMMATIC SECTION OF A SURFACE SHOWING THE
RELATION OF GLANDS TO THE SURFACE. (_a_) Simple or tubular gland,
(_b_) compound or racemose gland.]

All of the causes of disease act on the body from without, and it is
important to understand the relations which 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