GLANDS, in anatomy, are composite masses of tissue vary ing widely in complexity and size but agreeing in the formation of a secretion. Every gland is formed by an ingrowth from an epithelial surface. This ingrowth may from the beginning possess a tubular structure, but in other instances may start as a solid column of cells which subsequently comes tubulated. As growth proceeds, the column of cells may divide or give off shoots, in which case a compound gland is formed. In many glands the number of branches is limited, in others (salivary, pancreas) a very large structure is finally formed by repeated growth and sion. As a rule the branches do not unite with one another, but in one instance, the liver, this does occur when a reticulated compound gland is produced. In pound glands the more typical or secretory epithelium forms the terminal portion of each branch, and the uniting portions form ducts and are lined with a less modified type of epithelial cell.
Glands are classified according to their shape. If the gland retains its shape as a tube throughout it is termed a tubular gland, simple tubular if there is no division (large intestine), com pound tubular (fig. 1) if branching occurs (pyloric glands of stom ach). In the simple tubular glands the gland may be coiled without losing its tubular form, e.g., in sweat glands. In the second main variety of gland the secretory portion is enlarged and the lumen variously increased in size. These are termed alveolar or saccular glands. They are again subdi vided into simple or compound alveolar glands, as in the case of the tubular glands (fig. 2) . A further complication in the case of the alveolar glands may occur in the form of still smaller saccular diverticula growing out from the main sacculi (fig. 3). These are termed alveoli.
The typical secretory cells of the glands are found lining the terminal portions of the ramifications and extend upwards to varying degrees. Thus in a typical acinous gland the cells are restricted to the final alveoli. The remain ing tubes are to be considered mainly as ducts. In tubulo-alveolar glands the secret ing epithelium lines the alveolus as well as the terminal tubule.
The gland cells are all placed upon a basement membrane. In many instances this membrane is formed of very thin flattened cells, in other instances it is parently a homogeneous membrane, and according to some observers is simply a modified part of the basal surface of the cell, while according to others it is a definite structure distinct from the epithelium. In the secretory portion of the gland and in the smaller ducts the epithelial layer is one cell thick only. In the larger ducts there are two layers of cells, but even here the surface cell usually extends by a out stalk down to the basement membrane. The detailed acters of the epithelium of the different glands of the body are given in a separate article (see ALIMENTARY CANAL, etc.) . It will be sufficient here to give the more general characters possessed by these cells. They are cubical or conical cells with distinct oval nuclei and granular protoplasm. Within the protoplasm is ac cumulated a large number of spherical granules arranged in diverse manners in different cells. The granules vary much in size in different glands, and in chemical composition, but in all cases represent a store of material ready to be discharged from the cell as its secretion. Hence the general appearance of the cell is found to vary according to the previous degree of activity of the cell. If it has been at rest for some time the cell contains very many granules which swell it out and increase its size. The nucleus is then largely hidden by the granules. In the opposite condition, i.e., when the cell has been actively secreting, the pro toplasm is much clearer, the nucleus obvious and the cell shrunken in size, all these changes being due to the extrusion of the granules.