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 corpuscles transport the oxygen from the lungs to the tissues. The white corpuscles devour and destroy irritating particles, such as drugs, poisons, and germs. They are of great importance in purifying the blood and as a protection against disease. One is shown in Fig. 60.

The sounds of the heart beat may be heard by applying the ear to the chest. They are two, a long, dull sound and a short, clear one. The first comes from the vibration of the bicuspid valve together with an unexplained tone arising from large contracting muscles, in this case the walls of the ventricles. The second, or short, clear sound, is produced by the sudden closing and vibration of the semi-*lunar valves.

Changes in the Composition of the Blood as it passes through the Various Organs.—When the blood is forced out by the heart, part of it goes to the stomach and intestines through arteries which divide into capillaries. These capillaries absorb all kinds of food from the alimentary canal except the fats (see p. 64), and unite to form the portal vein, which takes the absorbed food to the liver. In the liver some of the impurities of the blood are burned up and changed into bile. The blood, purified and laden with food, is carried from the liver to the heart, where it reënters the general blood stream. The blood flow from the food tube through portal vein and liver to the heart, as just described, is called the Portal circulation.

Renal circulation. Two branches from the aorta carry blood to the kidneys. There the urea and a large amount of water are taken out, and the purified blood is emptied into the large vein that leads up to the heart.

Pulmonary circulation (Fig. 67). This is the circulation through the lungs. During this circulation carbon dioxid gas is removed from the blood and oxygen is added to it.