Some one has said that the blood is the life; and certainly the blood is both the symbol and the instrument of the body's unity. The blood penetrates to all parts of the body and maintains and renews them. If the blood is normal, the work of renewal does not often fail. If there is a failure of renewal--that is, a disease--we shall generally find an abnormal condition of the blood. The distribution of the blood is controlled by the heart, a great four-chambered pump. One chamber drives the blood to the lungs, a mass of fine porous membranes, where it comes into contact with the air, and gives off the poisons which it has accumulated in its course through the body, and takes up a fresh supply of oxygen. By another chamber of the heart the blood is then sucked out of the lungs, and by the next chamber it is driven to every corner of the body. It takes to every cell of the body the protein materials which are necessary for the body's renewal, and also the fuel materials which are to be burned to supply the body's energy; also it takes some thirty million millions of microscopic red corpuscles which are the carriers of oxygen, and an even greater number of the white corpuscles, which are the body's scavengers, its defenders from invasion by outside germs. There are certain outer portions of the body, such as nails and the scales of the skin, which are dead matter, produced by the body and pushed out from it and no longer nourished by the blood. But all the still living parts of the body are fed at every instant by the stream of life. Each cell in the body takes the fuel which it needs for its activities, and combines it with the oxygen brought by the red corpuscles; and when the task of power-production has been achieved, the cell puts back into the blood-stream, not merely the carbon dioxide, but many complex chemical products--ammonia, uric acid, and the "fatigue poisons," indol, phenol and skatol. The blood-stream bears these along, and delivers some to the sweat glands to be thrown out, and some to the kidneys, and the rest to the lungs. All of this complicated mass of activities is in normal health perfectly regulated and timed by the nervous system. You lie down to sleep, and your muscles rest, and the vital activities slow up, your heart beats only faintly; but let something frighten you, and you sit up, and these faculties leap into activity, your heart begins to pound, driving a fresh supply of blood and vital energy. You jump up and run, and these organs all set to work at top speed. If they did not do so, your muscles would have no fresh energy; they would become paralyzed by the fatigue poisons, and you would be, as we say, exhausted.