The heart, the principal agent of the circulation, is the source of movements which are not under the control of the will, but which are constantly influenced by moral impressions and by sensations. These movements consist in the alternate contraction and relaxation of the walls of the heart, that is in the opening and shutting of its cavities. The ventricles contract simultaneously, then to this contraction succeeds a period of relaxation, during which the auricles in their turn contract, to relax during a new contraction of the ventricles. The dilating movement is called the diastole, and the contracting the systole. During the diastole the blood flows into the cavities of the heart, to be expelled by the systole; the contraction of the auricles forces it into the ventricles, that of the ventricles throws it into the aorta and the pulmonary artery.

The contraction of the ventricles modifies the form of the heart Its transverse circumference is ellipsoid during the diastole, and becomes circular during the systole; the antero-posterior diameter being thus greater, the point of the heart strikes the anterior wall of the chest, and if the ear be applied to this point, a dull sound will be heard at the moment of the shock, and about half a second after, a clearer sound is heard, coincident with the relaxation or the diastole of the ventricles. The mechanism of these sounds has been variously explained; they seem to be owing, the first to the sudden closing of the tricuspid and mitral valves at the moment when the ventricular systole throws the blood into the aorta and pulmonary artery; the second to the closing of the sigmoid valves during the ventricular diastole, under the influence of the elasticity of the arteries, which tends to cause a reflux of the column of blood.

The alternation of the systole and diastole constitute the rhythm and regularly marked beating of the heart which makes itself heard and felt through the walls of the chest We will follow these movements in their evolution and the blood in its course.