The entire series of chemical changes that take place in the living body, beginning with assimilation and ending with excretion, is included in one word, metabolism.  The process of building up living material, or the change by which complex substances (including the living matter itself) are built up from simpler materials, is called anabolism.  The breaking down of material into simple products, or the changes in which complex materials (including the living substance) are broken down into comparatively simple products, is known as katabolism.  This reduction of complex substances to simple, results in the production of animal force and energy.  Thus a complex substance, like a piece of beef-steak, is built up of a large number of molecules which required the expenditure of force or energy to store up.  Now when this material is reduced by the process of digestion to simpler bodies with fewer molecules, such as carbon dioxid, urea, and water, the force stored up in the meat as potential energy becomes manifest and is used as active life-force known as kinetic energy.

16.  Epithelium.  Cells are associated and combined in many ways to form a simple tissue.  Such a simple tissue is called an epithelium or surface-limiting tissue, and the cells are known as epithelial cells.  These are united by a very small amount of a cement substance which belongs to the proteid class of material.  The epithelial cells, from their shape, are known as squamous, columnar, glandular, or ciliated.  Again, the cells may be arranged in only a single layer, or they may be several layers deep.  In the former case the epithelium is said to be simple; in the latter, stratified.  No blood-vessels pass into these tissues; the cells derive their nourishment by the imbibition of the plasma of the blood exuded into the subjacent tissue.

[Illustration:  Fig. 4.—­Nerve Cells from the Gray Matter of the Cerebellum. (Magnified 260 diameters.)]

17.  Varieties of Epithelium.  The squamous or pavement epithelium consists of very thin, flattened scales, usually with a small nucleus in the center.  When the nucleus has disappeared, they become mere horny plates, easily detached.  Such cells will be described as forming the outer layer of the skin, the lining of the mouth and the lower part of the nostrils.

The columnar epithelium consists of pear-shaped or elongated cells, frequently as a single layer of cells on the surface of a mucous membrane, as on the lining of the stomach and intestines, and the free surface of the windpipe and large air-tubes.

The glandular or spheroidal epithelium is composed of round cells or such as become angular by mutual pressure.  This kind forms the lining of glands such as the liver, pancreas, and the glands of the skin.

The ciliated epithelium is marked by the presence of very fine hair-like processes called cilia, which develop from the free end of the cell and exhibit a rapid whip-like movement as long as the cell is alive.  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.