EPITHELIUM is the term applied in anatomy to the cell-tissues which, in layers of various thickness, invests not only the outer surface of the body, and the mucous mem branes connected with it—as, for example, those of the nose, lungs, intestinal canal, • etc.—but also the closed cavities of the body, such as the great serous membranes, the ventricles of the brain, the synovial membranes of joints, the interior of the heart and of the blood-vessels proceeding to and from it, the ducts of glands, etc.
The thickness of this tissue varies extremely with the position in which it occurs. In some parts it consists of numerous of cells, collectively forming a layer of more than a line in thickness; in other parts, it is composed of only a few strata, or often of only a of cells, and can only be detected by the microscope The cells of which the E. is composed are usually soft nucleated cells; they may be rounded, polygonal, fusiform, cylindrical, or conical in shape, and sometimes they pos sess vibratile cilia, the appearance and uses of which will presently be explained.
In his lifanual of Human Histology, K51liker adopts the following arrangement. Iii considers (a) E. in a single stratum, and (b) E. in many layers.
(a) Epithelium in a. single stratum may be composed of 1. Rounded, polygonal cells, constituting the variety known as pavement or tease lated E., and occurring as an investment of the serous membranes of most synovial membranes, of the lining membrane of the heart and of the veins, of the canals of glands, etc.
2. Fusiform, superficially united cells (fusiform E.), as the E. of the arteries and of many veins.
3. Cylindrical cells (cylinder E.), as in the intestine from the stomach to the termina tion of the alimentary canal, in the excretory ducts of all the glands opening into the intestine, etc. Various illustrations of this cylinder E. are given in the article Dins 'PION, ORGANS AND PROCESS OP, 4. Cylindrical or conical ciliated cells, as the E of the more minute bronchial tubes, of the nasal cavities, and of the uterus.
5. Rounded ciliated cells, as the ciliated pavement E. of the ventricles of the brain in the fetus.
(b) Epithelium in many layers may be composed of: 1. Cylindrical or rounded cells below, and more or less flattened cells above. This is
termed laminated pavement E., and occurs in the mouth, lower part of pharynx, esophagus, bladder, etc.
2. Rounded cells below, more elongated ones in the middle, and ciliated conical ones above. This is termed laminated ciliary E., and occurs in the larynx, trachea, and larger bron chial tubes, in the greater part of the nasal cavity, etc. • Iv all the varieties of E., the layer of external cells is being constantly disintegrated mid replaced by the layer immediately beneath.
The uses of the chief varieties of E., especially of ciliated E., require some notice The polygonal or pavement E. mainly acts like the epidermis, as a protecting medium to the soft parts beneath.
The cylindrical E. additionally takes an active part in the process of secretion. Bins trations of the function of the cells forming this variety of E. are given in the articles CELES, ANIMAL: DIGESTION, ORGANS AND PROCESS OF; and the subject will be further noticed under the head SECRETION.
In connection with ciliated E., we must notice ciliary motion generally, in so tar as it occurs in the animal kingdom. Certain surfaces which are lubricated by a fluid, are covered with a multitude of hair-like processes of extreme delicacy and minuteness (their length varying from to of an inch), which from their shape are termed cilia, from cilium, an eyelash. During life, and for a certain period after death, these filaments exhibit a remarkable movement, each cilium bending rapidly in one direction, and rapidly returning to its original position (according to Krause, these movements range from 190 to 23ein a minute). On examining a ciliated surface with a high magni fying power, the motion presents an appearance somewhat resembling that of a corn field agitated by a steady breeze. Any minute objects coming in contact with the free extremities of the cilia are urged onward in the direction of the predominant movement: and the best method of observing the course of the ciliary current is to sprinkle the sur face with a little powdered charcoal, grains of which may speedily be seen to move onwards in a definite direction.