210 CIRCULATION. 



duced by the sudden closure of the auriculo-ventricular 

 valves at the beginning of the ventricular systole ; to which 

 are superadded the muscular sound, due to the contraction 

 of the muscular fibres of the heart, and the impulsion sound, 

 due to the shock of the organ against the walls of the thorax. 



The second sound is simple. It is produced by the sud- 

 den closure of the aortic and pulmonary semilunar valves, 

 immediately following the ventricular systole. 



It is of the greatest importance, with reference to pathol- 

 ogy, to have a clear idea of the currents of blood through the 

 heart, with their exact relation to the sounds and intervals. 



At the commencement of the first sound, the blood is 

 forcibly thrown from the ventricles into the pulmonary 

 artery on the right side and the aorta on the left, and the 

 auriculo-ventricular valves are suddenly closed. During the 

 entire period occupied by this sound, the blood is flowing 

 rapidly through the arterial orifices, and the auricles are re- 

 ceiving blood slowly from the venae cavse and the pulmonary 

 veins. 



"While the second sound is produced, the ventricles hav- 

 ing become suddenly relaxed, the recoil of the arterial walls, 

 acting upon the column of blood, immediately closes the 

 semilunar valves upon the two sides. The auricles continue 

 to dilate, and the ventricles are slowly receiving blood. 



Immediately following the second sound, during the first 

 part of the interval the auricles become fully dilated ; and 

 in the last part of the interval immediately preceding the 

 first sound, the auricles contract, and the ventricles are fully 

 dilated. This completes a single revolution of the heart. 



