SOUNDS OF THE HEART. 647 



immediately arrested, a condition which is thus guarded against. It 

 is certain that the quantity of blood delivered by the right auricle into 

 the right ventricle, is subject to variation ; whilst that propelled from 

 the left auricle into the left ventricle is probably uniform ; on that side 

 of the heart, the mitral valves close accurately, and no regurgitation 

 takes place. Vierordt, however, doubts the normal occurrence of any 

 reflux, even on the right side, because no such regurgitation takes place 

 during the auricular systole, nor yet any backward pressure in the 

 venous trunks. 



The semilunar valves at the orifices of the pulmonary artery and 

 aorta, have no chordae tendineae, but they meet accurately across the 

 orifices of those vessels, the little corpora Arantii assisting in the com- 

 plete closure of the thre^ segments in the centre. These valves, 

 opened by the blood projected from the ventricles, would be closely 

 applied to the walls of the arteries, were it not for the presence of the 

 pouches of Yalsalva behind them ; but the blood retained in those 

 pouches, facilitates the separation of the valves from the sides of the 

 vessels, and their subsequent opening out across those vessels, by the 

 pressure of the blood acted upon by the distended and resilient arte- 

 ries. The shock of the columns of blood in the arteries, is sustained 

 mainly by the stronger and more tendinous part of the valves, their 

 thinner marginal lunules being applied against each other, so as to form 

 three lines radiating from the centre of the vessel between the con- 

 tiguous borders of the valves. The greater the resilient force of the 

 arteries, the more accurate and close is the apposition of the valves. 



The closure of the auriculo-ventricular and the semilunar valves, is 

 necessarily accompanied by a tightening out of their respective seg- 

 ments ; this is the chief cause of certain sounds, which, on listening 

 over the region of the heart, are heard, at stated intervals, in the 

 period of each single cardiac beat. These sounds of the heart are two 

 in number, in each beat. They are audible to the ear placed on the 

 thorax ; but they are commonly examined by the tubular instrument, 

 named the stethoscope. In certain circumstances, they may be heard 

 by the individual himself. The two sounds occur in quick succession, 

 after which there is a period of silence, often named the pause. The 

 first sound is deep, dull, and long ; the second is higher in tone, sharp, 

 and short. The first closely precedes the pulse at the wrist, and co- 

 incides nearly with the impulse of the heart ; the second follows im- 

 mediately after the pulse. Supposing the entire period of the heart's 

 beat to be divided into eight equal parts, the first sound and the brief 

 period which elapses between it and the second, have been said to oc- 

 cupy four parts; the second sound, to take rather less than two; and 

 the silent interval, or pause, between it and the recurrence of the first 

 sound of the next beat, to occupy rather more than two parts (Laeri- 

 nec) ; hence the period of the sounds would be to the period of silence, 

 as 3 to 1. But the proportions have been perhaps more correctly, 

 stated to be 2 to 1 (Williams), or nearly 1 to 1 (Volkmann). 



From observations on living animals, it has been found that the 

 first sound coincides with the systole of the ventricles, the closure of 

 the auriculo-ventricular valves, the opening of the semilunar valves, 



