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underlying tissues. Epithelial cells are usually transparent; for instance, the blood is visible beneath the mucous membrane of the lips. The finger nails are made of epithelial cells, and they are nearly transparent.

Fig. 15.—Epithelial Tissue (epidermis of skin, magnified).

There are two classes of epithelial cells; one class forms protective coverings (Fig. 15); the other class forms the lining of glands (Fig. 16). Glands are cavities whose lining of epithelial cells (Fig. 17) form either useful fluids called secretions to aid the body in its work, or harmful fluids called excretions to be cast out, or excreted. Most glands empty their fluids through tubes called ducts.

Fig. 16.—Epithelial Tissue; cells forming two glands in wall of stomach.

Cartilag'inous tissue is tough, yet elastic. Cartilage or gristle may be readily felt in the ears, the windpipe, and the lower half of the nose. This tissue consists of cartilage cells embedded in an intercellular substance through which run connective tissue fibers (see Fig. 18). If yellow fibers predominate, the cartilage is yellow and very elastic, as in the ear; if white fibers predominate, it is white and less elastic, as in the pads of gristle between the bones of the spinal column. Cartilage is to prevent jars, and, in movable joints, to lessen friction.

Fig. 17.—Six Gland Cells: at left, shrunken after activity; at right, rested, full of granules.

Bony (Osseous) Tissue.—Solid bone is seen under the microscope to contain