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.