Epithelium (Gr. upon, and a nipple), the layer of cells lining the internal free surfaces of the body, continuous with the epidermis which covers the external surface of the skin. It arises from cells like the epidermis, which are developed and thrown off in the same manner in both structures; the epithelium, however, serves for totally distinct purposes in the animal economy, as from the soft and moist surfaces covered by its cells are elaborated the various secretions of the body. A continuous layer of its cells may be traced the whole length of the alimentary canal, along the other mucous membranes into the glands and follicles, on the serous and synovial membranes, and the coats of the blood vessels and absorbents. The two principal among the numerous forms of epithelial cells are the tessellated or pavement epithelium, and the columnar epithelium. The tessellated epithelium lines the serous and synovial membranes, the blood vessels, the follicles of most of the cutaneous and mucous glands, and many parts of the mucous membranes; the cells are generally flattened and polygonal, forming by their contact a kind of pavement, and the number of layers is usually small.
The columnar epithelium covers the mucous membrane of the alimentary canal, the larger ducts of the glands, the vas deferens, and the urethra; its cells are cylinders, arranged side by side, one end resting on the basement membrane, the other forming the free surface. These two kinds pass into each other at various points, giving rise to various transition forms, and both are often fringed with delicate filaments or cilia, varying in length from 1/13,000 to 1/5000 of an inch. (See Cilia.) Ciliated epithelium is found in the cerebral cavities, the ramifications of the bronchi, the air passages, with their nasal, frontal, maxillary, and lachrymal appendages, the posterior fauces, and Eustachian tube ; their function seems to be to expel the secretions of these various membranes. The epithelial like the epidermic cells are in a state of continual separation and renewal, more rapid according to the activity of the connected function; the introduction of nutrient matters, the separation of effete substances, the various products of secretion, and the development of the reproductive particles, are effected by the agency of epithelium cells.
Cells of Pavement Epithelium.
Ciliated Columnar Epithelium.