CH. xx.] SOUNDS OF THE HEART. 235 



this ring is tough, and does not admit of extension under such 

 pressure as it is commonly exposed to ; the valves are equally inex- 

 tensile, being formed of fibrous tissue. Hence, when the ventricle 

 propels blood through the orifice into the canal of the artery, 

 the lateral pressure which it exercises is sufficient to dilate the 

 walls of the artery, but not enough to stretch in an equal degree, 

 if at all, the unyielding valves and the ring to which their lower 

 borders are attached. The effect, therefore, of each such propul- 

 sion of blood from the ventricle is, that the wall of the first por- 

 tion of the artery is dilated into three pouches behind the valves 

 while the free margins of the valves are drawn inward towards 

 its centre. Their positions may be explained by the diagrams 

 in fig. 234, in which the continuous lines represent a transverse 

 section of the arterial walls, the dotted ones the edges of the 

 valves, first, when the valves are nearest to the walls (A), as in 

 the dead heart, and, secondly, when the walls are dilated, and the 

 valves are drawn away from them (B). 



This position of the valves and arterial walls is retained so long 

 as the ventricle continues in contraction ; but as soon as it 

 relaxes, and the dilated arterial walls can recoil by their elasticity, 

 the blood is forced backwards towards the ventricles and onwards 

 in the course of the circulation. Part of the blood thus forced 

 back lies in the pouches (sinuses of Valsalva) (a, fig. 234, u) 

 between the valves and the arterial walls ; and the valves are by 

 it pressed together till their thin lunated margins meet in three 

 lines radiating from the centre to the circumference of the artery 

 (7 and 8, fig. 209). 



The Sounds of the Heart. 



When the car is placed over the region of the heart, two sounds 

 may be heard at every beat of the heart, which follow in quick 

 succession, and are succeeded by a pause or period of silence. 

 The first or systolic sound is dull and prolonged ; its commence- 

 ment coincides with the impulse of the heart against the chest 

 wall, and just precedes the pulse at the wrist. The second or 

 diastolic sound is shorter and sharper, with a somewhat flapping 

 character, and follows close after the arterial pulse. The periods 

 of time occupied respectively by the two sounds taken together 

 and by the pause, are nearly equal. Thus, according to Walshe, 

 if the cardiac cycle be divided into tenths, the first sound occu- 

 pies -j* v ; the second sound y 2 ^ ; the first pause (almost imper- 

 ceptible) ^\; ; and the second pause ^\. The sounds are often 

 but somewhat inaptly compared to the syllables, lubb d&p. 



