Glands

Glands. 1- A secreting gland in its simplest form consists of a folding in of a mucous membrane, forming a kind of pocket or tubular depression, lined by a series of epithelial cells (secreting cells) which are continuous with the epithelium of the mucous membrane. Each depression is surrounded by a network of capillary blood-vessels, and the secreting cells abstract from the blood the material which they elaborate, and then pass it on into the tubule to be discharged as the secretion of the gland. The glands of the stomach, the glands of Lieberkuhn in the intestines, and the sweat glands, may be cited as examples of such simple tubular glands. In the compound tubular glands (salivary glands, pancreas, Brunner's glands of the duodenum, and mammary glands) the structure is more complicated; there are a series of main tubes which divide and subdivide, the whole series of tubes being lined as before with epithelium, but the function of secretion is limited to the ultimate divisions of the system of tubes, the main channels serving merely to conduct the secretion to the surface, and being called ducts. A third group, the aggregate or racemose glands, may be alluded to; in these each main duct branches out into a series of vesicles or acini, constituting what is termed a lobule, the whole gland being thus made up of a system of lobules. The meibomian glands of the eyelids may be given as an example of this type of structure. The term gland is sometimes applied to certain structures in no way idlied to the ordinary secreting gland - e.g. the lymphatic glands, thymus gland, pineal gland, etc. (Glands, diseases of, see Lymphatic Glands.) 2. Glands, in mechanical Engineering, are collars of metal surrounding cylindrical pieces, such as piston-rods, for the purpose of keeping them in place. They are frequently made to act as covers for stuffing-boxes, in which oil-soaked asbestos rope, or other lubricative packing, is compressed so as to make an air-tight joint, through which the cylindrical rod may move. [Stuffing-boxes.]