THE CIRCULATION. 241 



of the moment of closing or opening of either auriculo-ventricular 

 or semilunar valves. These instances can, however, be marked 

 upon it after they have been obtained in an indirect manner. 



In Fig. 72 the ordinate 1 indicates the closing, and ordinate 4 

 the opening of the mitral valve; ordinate 2 indicates the opening, 

 and ordinate 3 the closing of the aortic valve. In the arterial 

 curve, 2 marks the beginning of the systolic rise and 3 the begin- 

 ning of the dicrotic wave, which corresponds closely to the closure 

 of the aortic valve. 



During the period when the bicuspid valve is open, the pres- 

 sure is lower in the ventricle than in the artery, the aortic valve is 

 shut, and blood is entering the ventricle, this being the "period of 

 the reception of blood." During the greater part of the period 

 when the bicuspid valve is shut, the aortic valve is open, the pres- 

 sure is higher in the ventricle than in the artery, the ejection of 

 blood is taking place, this being the "period of ejection," which lies 

 between the ordinates 2 and 3 (Fig. 72). 



There are two brief periods, during each of which both valves 

 are shut and the ventricle is a closed cavity; one immediately pre- 

 cedes the period of ejection, the other immediately follows it. The 

 explanation of these periods is that it takes a brief time for the 

 cardiac muscle, contracting upon the blood in the closed ventricle, 

 to raise the pressure to the high point required to overcome the 

 opposing pressure within the artery and to open the aortic valve. 

 Hence the ventricular cycle is composed of four periods: the first, 

 the period of complete closure with strongly rising pressure; the 

 second, the period of ejection, relatively long; the third, a period 

 of complete closure, with swiftly falling pressure; the fourth is the 

 period "when the pressure is low and blood is entering the ventricle. 



Persistence of the Heart Movement. The heart may continue 

 to beat for some time after its removal from the body. This is 

 particularly noticeable in cold-blooded animals like the turtle, whose 

 heart movements have been known to continue, even for days. 



When the heart dies the ventricles stop first, but the right 

 auricle is the last to be arrested; hence it is called the "ultimum 

 moriens." 



W. Koch observed the dying heart in the human foetus and the 

 last part to cease beating was not the root of the superior vena cava 

 but in the wall of the coronary vein of the heart which he thinks 

 corresponds embryologically to the sinus venosus. 



