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HEART ; a hollow, muscular organ, the function of which is to maintain the circulation of the blood, and which is of different formations in different animals. The organs of circulation are the heart, the arteries, the veins (see Blood Vessels), and the capillary vessels. The blood (q. v.) is divided into the arterial blood and the venous blood. The object of the circulation (see Harvey) is to carry the venous blood, which has returned from the body, into the lungs, where, by the influence of the air, it is converted into arterial blood, which is then again sent out into the system, to nourish it and repair its losses. The heart in men, quadrupeds and birds (see Animal) is composed of four cavities, two auricles and two ventricles (thence called double). It is enveloped in a membrane called the pericardium, situated toward the left of the cavity of the chest, between the Jungs, and resting on the diaphragm. Its form is that of a cone flattened on its inferior and superior faces, the latter formed principally by the right, the former by the left auricle and ventricle. The right auricle communicates with the right ventricle, besides which there are in it three openings, that of the vena cava inferior, that of the vena cava superior, and that of the coronary vein. The communication between this auricle and ventricle is closed by a valve when the heart contracts. The right or pulmonary ventricle communicates with the pulmonary artery, which is provided with three valves. When these valves are brought together, they interrupt the^ communication between the ventricle and the artery. The left auricle communicates with the left ventricle, and contains also the orifices of the four pulmonary veins. The left ventricle, besides the communication with the left auricle, contains the orifice of the aorta, (q. v.) The ventricles are divided from each other by a fleshy wall, called the septum cordis. The valves at the openings of the arteries are called semilunar; that at the orifice of the right auricle, tricuspid; that at the orifice of the left auricle, mitral; and that at the orifice of the vena cava inferior, the Eustachian valve. The heart is formed of a firm, thick, muscular tissue, composed of fibres, interlacing with each other. It is also composed of nerves, membranes and vessels. The coronary arteries arise from the aorta, and are distributed on the heart The coronary veins return the blood of the heart into the right auricle. The arteries (from the Greek fop, air, and to preserve, because they were thought to contain air) are the vessels which serve to carry the blood from the heart to all parts of the body. They terminate in the cap illary vessels (q. v.)a series of extremely minute vessels, which pass over into the veins. The veins are the channels by which the blood passes back from the body into the auricles of the heart. The blood which is returned from the veins is black, and is called venous; that which leaves the heart is red, and is called arterial. The red blood, possessing nourishing and vital properties, rises in the capillary system of the lungs, flows into the pulmonary veins, thence is received into the left cavities of the heart, from which it passes into the aortg, and is transmitted to all parts of the body, to the capillary system. It there loses two degrees of temperature, and undergoes other changes, by the loss of some of its elements in the important functions of nutrition, calorification, and the secretions. It is now become black, passes through the veins, from the extremities of the body towards the heart, receives the chyle and the lymph, and is emptied into the right cavities of that organ, which returns it, through the pulmonary artery, to the capillary vessels of the lungs, where it is subjected to the influence of the air, resumes the qualities of red or arterial blood, and is ready for a new course. Having thus described the route of the blood through the different parts of the system, we will now explain the mechanism of the sanguineous system. The blood contained in the two vena cava is poured into the right auricle, which contracts, and thus forces the fluid to escape ; but the vena cava superior opposes to its passage the column of blood which it contains, the other veins are closed by valves, and it must therefore pass into the right ventricle. The ventricle then contracts, and the tricuspid valve closing the passage through which the liquid entered, it is forced forward into the pulmonary artery, which contracts, and its orifice being closed by the semilunar valve, propels the blood still forward into the capillary system of the lungs, whence it passes into the pulmonary veins, which pour it into the left auricle by their four orifices. The contraction of the auricle impels it into the left ventricle, by which it is, in the same manner, driven forward into the, aorta (the mitral valve preventing its return into the auricle), and thence into the general circulation as above described. The two auricles contract and dilate simultaneously with each other, as do also the two ventricles. The dilatation is called diastole; the contraction, systole. It is difficult to determine what quantity of blood the heart projects at each systole. It is generally estimated at two ounces. The causes of the alternate contraction and dilatation of the heart are not legs difficult to decide. They are entirely involuntary and dependent on the nervous system. The force of its contractions is likewise unknown. The systole of the ventricles is the cause of the motion of the blood in the arteries, which also dilate with each wave driven into them by the motion of the heart. (See Pulse.) By what means the blood is made to penetrate the thousand windings of the capillary system, and what causes impel it to flow back through the veins, are yet subjects of dispute among physiologists. The time in which a drop of blood completes its circle of motion, has been differently estimated, at from 24 hours to 2 minutes. Among the lower orders of animals, the organization of the circulating system is very different. The infusoria, polypi and intestinal worms have no distinct vessels, much less a heart; the echinodermata have distinct organs of circulation, but no part resembling a heart. Insects have a small cylindrical vessel, running along the back, which is rather the rudiment of a vascular system, than of a heart. The first traces of a heart are found in some worms, in which some expansions are perceptible in a part of the vessel which runs the whole length of the body. In the spiders, lateral vessels are given off from the main vessel, and a pulsation is perceptible. The crustacea have a heart composed of one fleshy ventricle. In the mollusca, the heart appears completely formed; some of them have three cavities. The four classes of vertebral animals have red blood, but fishes and reptiles have only what is called a single heart, that is, composed of one auricle and one ventricle.