=Bony (Osseous) Tissue.=--Solid bone is seen under the microscope to contain many minute cavities (Fig. 19). _In these cavities the bone cells lie_ self-imprisoned in walls of stone; for these cells have formed the bone by depositing limestone and phosphate of lime around themselves. There are _minute canals_ (3, Fig. 19), however, through which nourishment comes to the cells. The watery portion of the blood passes through these small canals from the blood vessels that flow through _the larger canals_ (1, Fig. 19). Bone cells may live for years, although some of the other cells of the body live only a few hours. [Illustration: FIG. 18.--CARTILAGINOUS TISSUE. A thin slice highly magnified. _a_, _b_, _c_, groups of cells; _m_, intercellular substance.] New cells to repair the tissues are formed by subdivision of the cells, as with the ameba. Unlike protozoans, many-celled animals are mortal because the outer cells prevent the deeper cells from purifying themselves perfectly and obtaining pure food and oxygen. Even the arteries of an old man become hardened by the deposit of mineral matter which the body has been unable to excrete. [Illustration: FIG. 19.--BONY TISSUE. Thin slice across bone, as viewed through microscope. Larger blood tubes pass through the large holes (1); the cavities containing bone cells lie in circles, and are connected by fine tubes (3) with the larger tubes.] =The body is kept alive and warm= by burning, or oxidation. One fifth of the air is oxygen gas. We breathe it during every minute of our existence. It is carried by the blood to all the tissues. Not one of the cells could work without oxygen. Without it the body would soon be cold and dead, for oxygen keeps the body alive and warm by uniting in the cells with sugar, fat, and all other substances in the body except water and salt. Oxygen burns or consumes the substances with which it unites, and the process is called _oxidation_. Hence the cells have to be continually growing and multiplying to repair the tissue and replace the material used up by oxidation. Sugar and flour and fat oxidize, or burn, outside of the body, as well as in it, as can be proved by throwing them into a fire. Water and salt are two foods that do not burn. Hence they can furnish no heat or energy to the body. Water puts out a fire instead of helping it, and so does salt. Throw salt into a fire or on a stove; it will pop like sand, but will not burn. The cells need the oxygen of fresh air; they need food for the oxygen to unite with, but _they are injured by many substances called poisons_. Arsenic destroys the red blood cells. Strychnine attacks the nerve cells in the spinal cord. Alcohol attacks the epithelial cells lining the stomach and, when it is absorbed, attacks the nerve cells and other cells. Morphine attacks the nerve cells.