Page:First course in biology (IA firstcourseinbio00bailrich).pdf/476

 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.

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.

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