. Biology and man. Biology; Human beings. Open; Closed ^Semiiuna^ valves If icuspid valve THE HEART A DOUBLE ORGAN The two auricles receive blood at the same time from veins. Blood passes from the auricles to the ventricles, through valves that prevent flow in the opposite direction. The two ventricles discharge blood at the same time into the main arteries, through the semilunar valves, which keep blood from returning when the ventricles expand ing all parts of the body. The smallest branches, the capillaries, form a network and combine into larger and larger tubes—the veins. The capil- larie
. Biology and man. Biology; Human beings. Open; Closed ^Semiiuna^ valves If icuspid valve THE HEART A DOUBLE ORGAN The two auricles receive blood at the same time from veins. Blood passes from the auricles to the ventricles, through valves that prevent flow in the opposite direction. The two ventricles discharge blood at the same time into the main arteries, through the semilunar valves, which keep blood from returning when the ventricles expand ing all parts of the body. The smallest branches, the capillaries, form a network and combine into larger and larger tubes—the veins. The capil- laries thus carry the blood over from the arteries to the veins. The capillaries were first seen by the Italian Marcello Malpighi (1628-1694), who was born in the very year that Harvey published his book on the circulation of the blood, and who solved Harvey's puzzle—How does the blood complete its circuit? Among warm-blooded animals (birds and mammals) the heart is a double organ. Each half of the heart consists of an auricle, or receiving chamber, and a ventricle, or pumping chamber (see illustration above). Blood cannot pass directly from either side to the other. The lejt heart is somewhat larger and stronger than the right heart. Its ventricle contracts at fairly regular intervals, forcing the contained blood into the largest artery of the body, the aorta. Branches of the aorta carry the blood on to the various organs and tissues of the whole body. The auricle of the left heart receives blood from a large vein into which blood gathers from the capillaries oj the lungs. A set of valves between the auricle and the ventricle keeps the blood from flowing back when the ventricle contracts. Another set of valves prevents the blood from flowing back from the aorta when the ventricle expands again. The left heart thus pumps blood received from the capillaries of the lungs into arteries reaching to all parts of the body. The auricle of the right heart receives blood from two large v
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