112 THE HUMAN BODY. 



tion of the walls of the heart, that is in the opening and 

 shutting of its cavities. The ventricles contract simultane- 

 ously, then to this contraction succeeds a period of relaxa- 

 tion, 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 

 cf 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. 



Arterial circulation. The left ventricle, in contracting, 

 pushes the red blood which it contains in the direction of 

 the auriculo-ventricular orifice and toward the orifice of the 

 aorta, but the mitral valve is so placed that it closes under 

 the impulse of the moving blood, which is thus forced into 

 the aorta and from thence into all the arteries, in which its 

 motion is caused by the triple action of ventricular contrac- 

 tion, and of the elasticity and contractility of the arterial 



