300 THE VASCULAR SYSTEM 



costal cartilage at its junction with the sUrnum. This area 

 projects a little to the right of the aorta, but overlaps the 

 vessel at the point where it approaches most nearly to the 

 anterior surface of the body. The tricuspid area, which is 

 placed at the lower extremity of the sternum, is situated over 

 that part of the right ventricle which is most remote from the 

 other orifices. The bicuspid (mitral) area lies over the apex 

 of the heart. In this position the left ventricle is very near 

 the surface of the body, and the area itself is as far as possible 

 from the other orifices. 



The Action of the Heart. The rhythmical contractions 

 of the heart begin in the atria at the orifices of the great veins. 

 During atrial contraction the pressure in the ventricles under 

 normal conditions is less than the pressure in the veins, and 

 so the blood is forced into the ventricles. As the ventricles 

 become filled, the cusps of the atrio-ventricular valves are 

 floated upwards towards the orifices which they guard. Atrial 

 systole is at once followed by ventricular systole and, as the 

 intra-atrial pressure is less than the intra-arterial pressure, the 

 force of the contractions tends to drive the cusps up into the 

 atria. At the same time the papillary muscles contract and, 

 through the chordae tendineae, retain the valve in place at the 

 orifice. A brief period of rest, termed the cardiac diastole, 

 follows the ventricular contraction, and then the cycle begins 

 again. 



The two principal heart sounds which are heard on ausculta- 

 tion are produced by the closure of the atrio-ventricular and 

 the semilunar valves. The atrio-ventricular valves close at the 

 commencement of ventricular systole, and, therefore, sounds 

 which are produced during atrial systole will be heard immedi- 

 ately prior to the first sound, c.g. t the bruit of bicuspid or tri- 

 cuspid obstruction. The semilunar valves, which guard the 

 orifices of the aorta and pulmonary artery, close at the end of 

 ventricular systole, and the bruits produced by regurgitation 

 of blood through the aortic or pulmonary orifices occur at the 

 commencement of diastole. 



