192. The Rhythmic Action of the Heart. To maintain a steady flow of blood throughout the body the action of the heart must be regular and methodical. The heart does not contract as a whole. The two auricles contract at the same time, and this is followed at once by the contraction of the two ventricles. While the ventricles are contracting, the auricles begin to relax, and after the ventricles contract they also relax. Now comes a pause, or rest, after which the auricles and ventricles contract again in the same order as before, and their contractions are followed by the same pause as before. These contractions and relaxations of the various parts of the heart follow one another so regularly that the result is called the rhythmic action of the heart. The average number of beats of the heart, under normal conditions, is from 65 to 75 per minute. Now the time occupied from the instant the auricles begin to contract until after the contraction of the ventricles and the pause, is less than a second. Of this time one-fifth is occupied by the contraction of the auricles, two-fifths by the contraction of the ventricles, and the time during which the whole heart is at rest is two-fifths of the period. 193. Impulse and Sounds of the Heart. The rhythmic action of the heart is attended with various occurrences worthy of note. If the hand be laid flat over the chest wall on the left, between the fifth and sixth ribs, the heart will be felt beating. This movement is known as the beat or impulse of the heart, and can be both seen and felt on the left side. The heart-beat is unusually strong during active bodily exertion, and under mental excitement. The impulse of the heart is due to the striking of the lower, tense part of the ventricles—the apex of the heart—against the chest wall at the moment of their vigorous contraction. It is important for the physician to know the exact place where the heart-beat should be felt, for the heart may be displaced by disease, and its impulse would indicate its new position. Sounds also accompany the heart’s action. If the ear be applied over the region of the heart, two distinct sounds will be heard following one another with perfect regularity. Their character may be tolerably imitated by pronouncing the syllables _lubb_, _dŭp_. One sound is heard immediately after the other, then there is a pause, then come the two sounds again. The first is a dull, muffled sound, known as the “first sound,” followed at once by a short and sharper sound, known as the “second sound” of the heart. The precise cause of the first sound is still doubtful, but it is made at the moment the ventricles contract. The second sound is, without doubt, caused by the sudden closure of the semilunar valves of the pulmonary artery and the aorta, at the moment when the contraction of the ventricles is completed. Illustration: Fig. 76.—Muscular Fibers of the Ventricles.