THE CIRCULATION OF THE BLOOD. 193 



of the arterial orifice are secured by the peculiar construction of their 

 borders before mentioned. Among the cords which are interwoven in 

 the substance of the valve are two of greater strength and prominence 

 than the rest; of which one extends along the free border of each valve, 

 and the other forms a double curve or festoon just below the free 

 border. Each of these cords is attached by its outer extremities to the 

 outer end of the free margin of its valve, and in the middle to the 

 corpus Arantii; they thus enclose a lunated space from a line to a line 

 and a half in width, in which space the substance of the valve is much 

 thinner and more pliant than elsewhere. When the valves are pressed 

 down, all these parts or spaces of their surfaces come into contact, and 

 the closure of the arterial orifice is thus secured by the apposition not 

 of the mere edges of the valves, but of all those thin lunated parts of 

 each which lie between the free edges and the cords next below them. 

 These parts are firmly pressed together, and the greater the pressure 

 that falls on them the closer and more secure is their apposition. The 

 corpora Arantii meet at the centre of the arterial orifice when the valves 

 are down, and they probably assist in the closure; but they are not 

 essential to it, for, not unfrequently, they are wanting in the valves of 

 the pulmonary artery, which are then extended in larger, thin, flapping 

 margins. In valves of this form, also, the inlaid cords are less distinct 

 than in those with corpora Arantii; yet the closure by contact of their 

 surfaces is not less secure. 



Cardiac Cycle. Taking 72 as the average number of cardiac evolu- 

 tions per minute, each revolution may be considered to occupy | of a 

 second, or about .8, which may be approximately distributed in the 

 following way : , 



Auricular systole, about . 1 + Auricular diastole . . . . 7 = . 8 

 Ventricular systole " .3 -(- Ventricular diastole . . .5 = .8 

 Period of joint auricular 

 and ventricular diastole .4 -j- Period of systole of 



auricles or ventricles . . . 4 = . 8 



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

 cardiac revolution is of course diminished, but the diminution aifects 

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

 much the same time, whatever the pulse-rate. 



The exact period in which the several valves of the heart are in 

 action is a matter of some uncertainty; the auriculo-ventricular valves 

 are probably closed during the whole time of the ventricular contrac- 

 tion, while, during the dilatation and distention of the ventricles, they 

 are open. The semilunar valves are only certainly open during the 

 middle period of the ventricular contraction. 



'3 



