=Fatty (Adipose) Tissue.=--Fatty tissue is formed by _the deposit of oil in connective tissue cells_ (see Fig. 14). Fat is held in meshes of connective tissue fibers. That fatty tissue consists not alone of fat, but of fibers also, is shown when hog fat is rendered into lard, certain tough parts called “cracklings” being left. What is the difference between beef fat and tallow? [Illustration: FIG. 14.--FATTY TISSUE. Five fat cells, held in bundles of connective tissue fibers. _a_ is a large oil drop; _m_, cell wall; nucleus (_n_) and protoplasm (_p_) have been pushed aside by oil drop (_a_).] =Epithelial tissue= consists of one or more _layers of distinct cells packed close together_ (see Fig. 15). It contains no connective tissue or other fibers, and is the simplest of the tissues. Epithelial tissue forms the outer layer of the skin, called the _epidermis_, and the mucous membrane lining the interior of the body. It contains no blood vessels, the _epithelial cells obtaining their nourishment from the watery portion_ of the blood which soaks through the 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. [Illustration: FIG. 15.--EPITHELIAL TISSUE (epidermis of skin, magnified).] [Illustration: FIG. 16.--EPITHELIAL TISSUE; cells forming two glands in wall of stomach.] [Illustration: FIG. 17.--SIX GLAND CELLS: at left, shrunken after activity; at right, rested, full of granules.] 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_. =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.