Page:The American Cyclopædia (1879) Volume IV.djvu/619

 CIRCULATION 607 this blood, which is thence carried into the left ventricle. By a large blood vessel called the aorta the blood expelled from the left ventricle passes into the arterial system, and is thence distributed to all the parts of the body. So that, as may be now easily understood, there is apparently a double circulation, one through the whole body, and one through the lungs and the heart. The blood from all parts of the body comes to the heart through the veins ; it passes from the trunks of these vessels, the venae cavce, into the right auricle, thence iato the right ventricle, which sends it to the lungs through the pulmonary artery; thence it re- turns to the heart by the pulmonary veins, and enters the left auricle, which impels it into the left ventricle, from which it goes to the whole body through the aorta and its ramifications, the various arteries ; finally, from the arteries it passes into the veins through the very mi- nute blood vessels called capillaries. In wri- ting the names of the parts through which the blood circulates, we may begin with either, as these parts form a circuit ; thus : 10. General capillaries. 1. Veins. 9. Aorta and arteries. Eight (2. Right auricle. 8. Left ventricle. (Left heart. | 3. Eight ventricle. 7. Left auricle. ) heart 4 Pulmonary artery 6. Pulmonary veins. 5. Capillaries of the lungs. If, instead of beginning with number 1, we begin with number 7, the blood then passes from 7 to 8, to 9, to 10, to 1, to 2, to 3, to 4, to 5, to 6, and thence reaches again number 7. The principal facts discovered or clearly proved for the first time by Harvey are : 1, that the movements of the heart are similar to those of the muscles of the limbs, &c., as re- gards the parts which produce them ; 2, that the arteries become full at the time the ventri- cles of the heart expel the blood they contain ; 3, that the pulmonary artery receives blood at the same time the aorta and other arteries receive it, and therefore that the two ventri- cles contract and expel the blood at the same time ; 4, that the two auricles contract simul- taneously, and that their contraction precedes that of the two ventricles ; 5, that after a liga- ture has been applied on an artery this vessel becomes quite distended with blood between the ligature and the heart, and empty in the other parts, showing that the blood comes from the heart into the arteries ; 6, that after the application of a ligature on a vein the blood disappears above the ligature, i. e., in the direction toward the heart, while it accu- mulates in the vein below the ligature, *. e., in the part which seems to be separated from the heart ; 7, that the valves in the veins prevent the blood from going in a wrong direction. From these facts and the natural conclusions that may be drawn from them, and also from many other facts and reasonings, Harvey de- duced the theory of the circulation of the blood, which we have stated, and which is now universally admitted. Harvey, in giving 193 VOL. iv. 39 this complete demonstration of the circulation of the blood, achieved the most important dis- covery yet made in physiology. We pass now to the examination of the most interesting questions concerning the circulation of the blood, omitting all those relating to the fre- quency, strength, and causes of the move- ments of the heart, for which see HEART and PULSE. There are four principal members or parts of the circulatory apparatus, each of which has a special office to perform in the function as a whole. These are : 1, the heart ; 2, the arteries; 3, the capillaries; and 4, the veins. We shall describe the action of these different organs or groups of organs in succes- sion. The heart is the main cause of the movement of the blood in the circulation. It is, as Harvey first demonstrated, a muscular organ, which contracts upon the blood con- tained in its cavities and drives it out into the arterial trunks, exactly like an animated force pump. The left ventricle, which is by far the thickest and strongest of all the chambers of the heart, nearly empties itself at each con- traction, and the blood, being prevented from regurgitating into the auricle by the shutting back of the ventricular valves, is forced on- ward by a vigorous impulse into the aorta. The elastic resistance of the arterial system, being inferior in force to the muscular con- traction of the heart, yields before it, and the blood which previously filled the left ventricle is thus suddenly transferred to the cavity of the aorta. The contraction of the heart is then at once followed by a condition of relax- ation, and the ventricle begins to be again pas- sively filled by the blood flowing into it from behind, from the veins and through the auri- cle. Thus the circulation through the heart is intermittent. The blood is at one instant driven out of the ventricle in a powerful stream, and is then temporarily arrested until another muscular contraction takes place, to repeat the same phenomenon. Movement of the blood in the arteries. The most marked physical property of the arteries, as a whole, is their elasticity. In this respect they are like a series of India-rubber tubes, owing to the abundant development of elastic fibres in their middle coat. Accordingly they are distensible, but offer nevertheless a certain amount of resistance to any distending force. This resistance, as we have remarked above, being less than the impulse with which the blood is injected into them from the heart, the arteries expand at the instant of the heart's contraction, and are thus made to contain a larger quantity of blood than before. This ex- pansion of the arteries under the heart's action, which is nearly simultaneous all over the body, can easily be perceived by the finger placed above an artery of moderate size, and is known as the arterial pulse. As each stroke of the pulse is caused by and is synchronous with a cardiac pulsation, its frequency becomes a valuable and convenient means by which the