Page:Popular Science Monthly Volume 19.djvu/666

648 forcing pure blood to the body; and a pulmonary, forcing impure blood to the lungs. The pure and impure blood are never mingled, and all the blood has to pass through the lungs and be oxygenated every time it makes the complete circuit. This perfect circulation, with aerial respiration, produces more rapid chemical changes in the blood and tissues, and consequently the higher temperature of the "warm-blooded" animals. In the embryonic stages of the heart, the septum dividing the auricles is slowly formed, and an aperture exists for a time, called the foramen ovale. Cases rarely occur of human subjects in which the opening persists. Such persons are physiologically reduced to the condition of a reptile. It is stated that human infants have lived several days with a circulation as mixed as that of a frog.

To economize space and muscular effort, these two hearts are formed of the same circular muscles, and are inclosed by a lubricating membrane called the pericardium. In the dugong, however, the two ventricles are quite separate, showing a structural distinction corresponding with the functional difference. On account of the structural union, the two hearts contract and dilate in unison, producing the "beating" of the heart. The cause of the first sound in the heart-beat is uncertain, but it occurs at the time of the ventricle contraction. The second sound is produced when the ventricles dilate, by the flapping back of the semilunar valves, those placed at the origin of the arteries to prevent the regurgitation of the blood.

Each half of the heart of birds or mammals is, like the entire heart of the fishes, a double force-pump, with perfection of valves and tubes and surpassing efficiency. The power is enormous. It has been estimated that, while an engine can lift its own weight three thousand feet in an hour, and an active climber can ascend four thousand feet, the human heart performs hourly a labor equal to lifting its own weight twenty thousand feet. Its daily work is also estimated at seventy-five thousand kilogramme-metres. We can otherwise gain an idea of the power of Nature's enginery, by observing what the heart actually performs. The quantity of blood in the human body is at least six quarts. In its course it has to traverse many feet of tubing and two sets of capillaries, and, notwithstanding the friction and loss of power, all the blood completes the circulation in about thirty-two heart-beats. We should further observe that the heart never rests, but is ceaseless from birth to death. Its cessation is death.

The necessity of uninterrupted action of the heart requires that it