CIRCULATION OF THE BLOOD 117 



distended; these, then suddenly contracting, fill up the ventricles, as 

 already described (p. 111). 



Cardiac .Revolution. If we suppose a cardiac revolution divided 

 into five parts, one of these will be occupied by the contraction of the 

 auricles, two by that of the ventricles, and two by repose of both auricles 

 and ventricles. 



Contraction of Auricles . . . 1 + Repose of Auricles . . . 4=5 

 Ventricles . . 2 -j- " Ventricles . . 3=5 



Repose (no contraction of either 



auricles or ventricles) . . . 2 -j- Contraction (of either auri- 

 cles or ventricles) . . . 3=5 

 5 



If the speed of the heart be quickened, the time occupied by each 

 cardiac revolution is of course diminished, but the diminution affects only 

 the diastole and pause. The systole of the ventricles occupies very much 

 the same time, about -^ sec., whatever the pulse-rate. 



The periods in which the several valves of the heart are in action may 

 be connected with the foregoing table; for the auriculo- ventricular valves 

 are closed, and the arterial valves are open during the whole time of the 

 ventricular contraction, while, during the dilation and distension of the 

 ventricles the latter valves are shut, the former open. Thus whenever 

 the auriculo-ventricular valves are open, the arterial valves are closed and 

 vice versd. 



SOUNDS OF THE HEAET. 



When the ear 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 sound is dull 

 and prolonged; its commencement coincides with the impulse of the 

 heart, and just precedes the pulse at the wrist. The second is a shorter 

 and sharper sound, with a somewhat flapping character, and follows close 

 after the arterial pulse. The period of time occupied respectively by the 

 two sounds taken together, and by the pause, are almost exactly equal. 

 The relative length of time occupied by each sound, as compared with 

 the other, is a little uncertain. The difference may be best appreci- 

 ated by considering the different forces concerned in the production of 

 the two sounds. In one case there is a strong, comparatively slow, con- 

 traction of a large mass of muscular fibres, urging forward a certain 

 quantity of fluid against considerable resistance; while in the other it is a 

 strong but shorter and sharper recoil of the elastic coat of the large 

 arteries, shorter because there is no resistance to the flapping back of 



