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20 during twenty-four hours, in the form of carbonic acid, is fairly represented by a piece of pure charcoal weighing eight ounces, while the quantity of water amounts to about nine ounces, or half a pint, on an average; but it may be less, and is sometimes doubled or trebled. The oxygen which passes into the capillaries combines chemically with the carbon it finds in the blood, which carbon is a product of the digested food, and as a result of this combination heat is given off. The venous blood which comes to the lungs is not in a condition to nourish the tissues, but the purified bright red blood is ready to perform that duty, and is carried by the pulmonary veins to the left side of the heart. This part of the circulation, by which the blood is purified, has been called the lesser circulation. In the course of the greater circulation the blood is pumped from the left side of the heart into the aorta, the chief artery; this divides into other large arteries, and each large artery breaks up into smaller ones, a process which is continued until the tubes become capillaries. These capillaries are found in countless numbers all over the body, and through their thin walls the nutritive parts of the blood pass out to the tissues, while the waste matters of the tissues pass into the capillaries. The capillaries widen into small veins, the veins into larger ones, and finally into the great hollow veins, the superior and inferior vena cava, which bring back the dark blood to the right side of the heart, to start again on its journey to the lungs for purification. The processes which