THE ACTION OF THE HEART.
the lungs from the right ventricle by the pul monary artery, the same blood that previously entered the right auricle from the superior and inferior verve cavre. From the left ventricle one large arterial trunk arises—the aorta, the largest arterial trunk in the body (c, Fig. 136). It passes upwards in the chest for a short dis tance towards the root of the neck, where it gives off branches for both right and left arms, and right and left sides of the head. (See Fig. 136.) It then turns backwards and downwards and passes down along the back-bone, giving off branches on its way, and ending by dividing into branches for the lower limbs. It is the vessel from which branches arise which convey the mood to all parts of the body.
The heart being the chief instrument in main taining a steady flow of blood through the body, its action must be methodical and regular. It does not contract as a whole. The two auricles contract at the same instant, and the contrac tion of the two ventricles immediately follows. While the ventricles are contracting, the auricles begin to relax, and after the ventricles have contracted they also relax. There is a period, following the ventricular contraction, when the whole heart is at rest, till the auricles again contract. The order of events is thus—contrac tion of auricles, contraction of ventricles, pause, contraction of auricles, contraction of ventricles, pause, and so on. The contraction is called systole, and the relaxation diastole, so that the order of events might be stated as systole of the auricles, immediately followed by systole of the ventricles, and thereafter a period during which the whole heart is in diastole. The occurrences follow one another so regularly as to be called rhythmic, and, on that account, the rhythm of the heart's action is spoken of. Suppose the heart to be beating 65 to 75 times a minute, which is the average, the time occupied from the instant the auricles began to contract till, after the contraction of the ventricles and the pause, they began to contract again, would be less than a second. Of this time kth is occupied by the contraction of the auricles iths by the contraction of the ventricles, and the time during which the whole heart is at rest is iths of the period. We know, from what has been said in previous paragraphs, that (luring the contraction of the auricles the blood is being poured down into the lower chambers, that immediately after they have emptied themselves they begin to relax and to be refilled from the veins, that during the ventricular contraction the blood is being forced into the arteries, the pulmonary artery on the right side, and the aorta on the left, and that during the relaxation both auricles and ventricles are refilling. We
know also that during the ventricular contrac tion the tricuspid and mitral valves are closed to prevent the return of blood to the auricles, while the valves of the arteries are open to permit of the blood passing into them, and that during the relaxation the tricuspid and mitral valves are open, while the valves of the arteries are closed to prevent the return of blood to the ventricles.
The Beat of the occurrences are attended by various others worthy of note. The first of these is the beat of the heart. If the hand be laid flat on the chest over the region of the left nipple, the heart will be felt beating against the chest wall. This is due to the fact that, by its sudden and vigorous contraction, the point of the heart is jerked forwards against the chest wall. The impulse should be felt in the space between the fifth and sixth ribs, an inch below and a little to the right of the left nipple. It is important to know this position, for the heart is sometimes displaCed by disease, and the beat indicates its new position.
Sounds also accompany the heart's action. If the ear be applied over the heart, two sounds will be heard following one another with per fect regularity. They have been imitated by uttering the syllables Cupp, depp. One is heard immediately after the other, then there is a pause, and then the two sounds again; and so on. They are distinguished by being called the first sound and the second sound. The second sound is short and sharp as compared with the first. It is not certain what the first sound is due to, but while it is being produced the ventricles are contracting, the tricuspid and mitral valves are closing, the blood is rushing into the arteries, and the heart is driven against the chest. The second sound has been conclusively shown to be caused by the closing of the semilunar valves of the pulmonary artery and aorta, and while it is being produced the ventricles are relaxing, and the blood is entering both upper and lower chambers. These sounds afford indications of the greatest importance in thedetection of heart disease. For, if the valves of the heart are dis eased, the sounds will be either accompanied or replaced by blowing murmurs, owing to the blood rushing past roughened surfaces, and if one can detect which sound is associated with the murmur, the valve affected may be deter mined and the exact position of the disease fixed.