HEART, SOUNDS OF TUE. On applying the ear to the cardiac region of a living man or mammal, in a state of health, two successive sounds are heard, each pair of which corresponds with one pulsation. These are known as the first and the second sound. There is scarcely any interval between these two different sounds, the second one follow ing immediately upon the conclusion of the first; but after the second sound there is a perceptible pause before the first sound is again heard. The first sound is dull and pro longed, while the second is short and sharp, and the difference between them is well expressed (as Dr. C. J. B. Williams has remarked) by articulating the syllables lubb p dri.
The cause of the first of these sounds has been a subject of much discussion, at least 30 explanations of its mode of production having been offered. During the first sound, seventh distinct actions are taking place, to each of which it has been ascribed by differ ent physiologists. Thus we have (1) the impulse of the apex of the heart against the side of the chest; (2) the contraction of the muscular walls of the ventricles; (3) the tension of the auriculo-ventricular (tricuspid and mitral) valves (see (4) the rush of blood through the narrowed openings of the aorta and pulmonary artery; and (5) the collision of the particles of blood with one another, and their friction against the sides of the heart's cavities.
The hearts of mammals being constructed like our own, give out sounds different in degree, but not in character, from the sounds heard in man. In birds (if we except the ostrich and the apterix, whose hearts approximate to the mammalian type), there is no perceptible difference between the first and second sound; and Dr. Halford has ingeni ously explained why this should be in his essay on Tim Action and Sounds of the Heart. The action of the heart in reptiles (the alligator, python, and turtle) seems to be accom panied with no definite sounds.
When the valves are changed by disease, the sounds undergo special alterations, which are of the highest importance in diagnosis.