In some cases the surface involutions are uniform in diameter, or nearly so, throughout (B, Fig. 40). Such glands are known as tubular; examples are found in the lining coat of the stomach (Fig. 48); also in the skin (Fig. 76), where they form the sweat-glands. In other cases the involution swells out at its deeper end and becomes more or less sacculated (E); such glands are named racemose or acinous. The small glands of the skin which form the oily matter poured out on the hairs (p. 303) are of this type. In both kinds the lining cells near the deeper end are commonly different in character from the rest; and around that part of the gland the finest and thinnest walled blood-vessels form a closer network. These deeper cells form the true secreting tissue of the gland, and the tube, lined with different cells, leading from the secreting recesses to the surface on which the secretion is poured out, and serving merely to drain it off, is known as the duct of the gland. When the duct is undivided the gland is simple; but when, as is more usual, it is branched and each branch has a true secreting chamber at its end we get a compound gland, tubular (0) or racemose (F, H) as the case may be. In many cases the chief duct, in which the smaller ducts unite, is of considerable length, so that the secretion is poured out at some distance from the main mass of the gland.

What is a tubular gland? Examples? A racemose gland? Example? Where do we find the closest network of blood-vessels in a gland? Which cells of a gland make its secretion? What is meant by the " duct" of a gland? What is a simple gland?

A fully formed gland, H, is, therefore, a complex structure, consisting primarily of a duct, c, ductules, dd, and secreting recesses, ee. The ducts and ductules are lined with cells which are merely protective, and differ in character from the secreting cells which line the deepest parts. The cells lining the ultimate recesses differ in different glands, and produce different liquids; consequently, though all glands are built on much the same plan, they make very varied secretions, the nature of the secretion of any gland depending on the properties of its cells.