GLANDS arc divided by anatomists into two great classes, viz., true secreting glands and ductless elands.
The first class constitute special organs which are destined for the production of the chief secretions; as, for example, the lachrymal, mammary, and salivary glands, the liver, pancreas, kidneys, etc.; while the suprarenal capsules, the spleen, the thym'us, and the thyroid belong to the second class.
An ordinary secreting gland consists of an aggregation of follicles, all of which open into a common duct, by which the glandular product is discharged. The follicles con tain in their interior cells (q.v.), which are the active agents in the secreting process; while their exterior is surrounded by a net-work of capillaries, from whose contents the materials of secretion are extracted.
The simplest form of a gland is the inversion of the surface of a secreting membrane into follicles, which discharge their contents upon it by separate mouths. Of this we have examples in the gastric glands and follicles of Lieberkithn described and figured in the article DIGESTION. Dr. Carpenter very well exhibits the commencement of thepro gressive complication which is observed in most of the glandular structures occurring in man and the higher animals.
The articulate (for example, insects) present glandular structures which can be unraveled much easier than the glands of vertebrate animals; and the forms, in all of which a large amount of secreting surface is presented in comparatively little space, are often very graceful.
To understand the structure.of a complex gland like the lirer or kidney, it must be followed from the simplest form in which it is known to occur through its various degrees of complication. In this way the liver may be traced, from the lowest mollusea (where it exists as simple follicles, lodged in the walls of the stomach, and their product into its cavity by separate orifices) up to man, in whom it is an organ of extreme intricacy; and similarly in the early fetal state of the higher animals, the liver and other secreting organs more or less resemble the persistent state of those parts in animals lower in the animal scale. In the same way, the mammary gland (q.v.), which is a
structure of considerable complexity in the higher mammals, presents a very simple arrangement in the lowest type of this class, the ornithorhynchus, being merely a cluster of ctecal follicles, each of which discharges its contents by its own orifice.
Sometimes a gland has several ducts (as, for example, the lachrymal gland), but, as a general rule, the most important glands have only a single canal, formed by the union of the individual ducts, which conveys away the product of the secreting action of the whole mass. ' Whatever be the complexity in the general arrangement of the elements of a gland in the higher animals, these elemepts arc always found to resolve themselves into follicles or tubuli, which inclose the true secreting cells.
The second class of glands resemble the secreting glands in external conformation, and in the possession of a solid parenchymatous tissue, but differ from them in the absence of a duct or opening for the removal of the products of secretion; and indeed, except in the case of the thymus, no material resembling a secreted product is yielded by any of them. In all of them, the tissue mainly consists of cells and nuclei, with a great abundance of blood-vessels. They may probably be regarded as .appendages to the vascular system; and from the absence of any excretory duct, they have received the name of vascular ductless glands.
The lymphatic glands belong to a different class of structures, and will be described in the article on the LY?PUATIC SYSTEM.