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Some of this fluid finds its way through Bowman's membrane, apparently by way of the minute channels which permit the passage of small nerve twigs, and enters the epithelial cell layer. The fluid finds its way between the epithelial cells in the deeper layers, apparently being taken into some of the superficial cells by imbibition.
The eyes of insects, simple and compound, are at first simply epithelial cells, modified for a special purpose, and even the egg is but a modified epithelial cell attached to the walls of the ovary, which in turn is morphologically but a gland.
The histology of ainhum shows it to be a direct ingrowth of epithelium, with a corresponding depression of surface due to a rapid hyperplasia that pushes down and strangles the papillae, thus cutting off the blood supply from the epithelial cells, causing them to undergo a horny change. The disease is not usually symmetric, as formerly stated, nor is it simultaneous in different toes.
These are the thymus and thyroid glands, apparently vestigial structures. The thymus gland attains a considerable development in the embryo and shrinks away to the merest vestige in the adult. It begins to form early in the embryo life as an epithelial ingrowth from the throat, and extends from the neck into the chest.
This motion is always in the same direction, and serves to carry away mucus and even foreign particles in contact with the membrane on which the cells are placed. This epithelium is especially common in the air passages, where it serves to keep a free passage for the entrance and exit of air. In other canals a similar office is filled by this kind of epithelium. Functions of Epithelial Tissues.
The thin wall of the capillaries within each capsule separates the blood from the cavity of the tubule. The blood-pressure on the delicate capillary walls causes the exudation of the watery portions of the blood through the cell walls into the capsule. The epithelial cell membrane allows the water of the blood with certain salts in solution to pass, but rejects the albumen.
Thus a granulating surface may be covered entirely by the ingrowing of the cutaneous epithelium from the margins; or islets, originating in surviving cells of sebaceous glands or sweat glands, or of hair follicles, may spring up in the centre of the raw area. Such islets may also be due to the accidental transference of loose epithelial cells from the edges.
The absorption of the chyle through the walls of the intestine seemed to be a mechanically intelligible process of osmosis and diffusion. But in reality it proves to be rather a process of selection on the part of the epithelial cells of the intestine, analogous to the selection and rejection exercised elsewhere by unicellular organisms.
It also receives lymphatic vessels, and minute twigs of the pulmonary plexus of nerves. The walls of the air cells are of extreme thinness, consisting of delicate elastic and connective tissue, and lined inside by a single layer of thin epithelial cells. In the connective tissue run capillary vessels belonging to the pulmonary artery and veins.
Melanin pigment is formed in relation to the cells and stroma of certain epithelial tumours, giving rise to melanotic cancer, one of the most malignant of all new growths. Cyst-like spaces may form in the tumour by the accumulation of the secretion of the epithelial cells, or as a result of their degeneration cystic carcinoma.
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