THE CIRCULATION OF THE BLOOD. 121 



tion, are situated upon the outer edge of the thick upper border of the 

 ventricles, and that consequently the portion of each seinilunar valve 

 adjacent to the vessel passes over and rests upon the muscular substance 

 being thus supported, as it were, on a kind of muscular floor formed 

 by the upper border of the ventricle. The result of this arrangement is 

 that the reflux of the blood is most efficiently sustained by the ventricu- 

 lar wall. 



As soon as the auricles have completed their contraction they begin 

 again to dilate, and to be refilled with blood, which flows into them in a 

 steady stream through the the great venous trunks. Indeed, a chief func- 

 tion of the auricles is to form a receptacle for the on-streaming blood 

 during the ventricular contraction. They are thus filling during all the 

 time in which the ventricles are contracting ; and the contraction of the 

 ventricles being ended, these also again dilate, and receive again the 

 blood that flows into them from the auricles. By the time that the ven- 

 tricles are thus from one-third to two-thirds full, the auricles are dis- 

 tended : these, then suddenly contracting, fill up the ventricles, as already 

 described (p. 116). 



Cardiac Cycle. If we suppose a cardiac cycle 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 + Eepose of Auricles, . 4 = 5 

 " Ventricles, . . 2 + " " Ventricles, . 3 = 5 

 Eepose (no contraction of either 



auricles or ventricles), . . . 2 + Contraction (of either au- 

 ricles 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 T 4 F 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 distention 

 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 versa. 



The Sounds of the Heart. 



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 



