CYCLE OF THE HEART BEAT. 271 



Or if we assume the human heart to beat some seventy times 

 a minute, each cycle would occupy about ^ of a second, which 

 would be made up as follows : 



Auricular systole, = xV f a second. 



Ventricular systole, = TG 



Pause, = f* " 



The duration of the auricular and ventricular systole varies 

 but little except under abnormal circumstances, but the pause is 

 constantly undergoing slight changes. In fact, the duration of 

 the general diastole depends upon the rate of the heart beat, being 

 less in proportion as the heart beats more quickly. 



If the thorax of a recently killed frog be opened the heart can 

 be observed beating in situ, and the different acts in the cycle 

 studied. 



In mammalians, in order to see the heart in operation, it is 

 necessary to keep up artificial respiration, during which the heart 

 continues to beat regularly, though the thorax be opened. A 

 careful inspection of the beating heart shows that during its cycle 

 of action certain changes take place in the shape and relative 

 position of its cavities. This is owing partly to the change in 

 the amount of their blood contents and partly to the form as- 

 sumed by the muscular wall when contracting. 



During the passive interval the auricles are seen to swell grad- 

 ually on account of the blood flowing into them from the veins; 

 when the auricular cavities are nearly full, a contraction, com- 

 mencing in the great venous trunks near the heart, passes with 

 increasing force over the auricles, and gives rise to their rapid 

 systolic spasm. The auricles appear suddenly to diminish in size, 

 become pale, and empty themselves into the ventricles. 



As the blood is shot through the auriculo-ventricular openings, 

 and the ventricles become distended, their flaccid walls appear to 

 be drawn over the liquid mass by the contracting auricles, just as 

 a stocking is drawn over the foot by the hands, and their walls 

 seem to approach toward the base of the heart. The moment 

 the ventricles have received their full charge of blood from the 

 auricles, they contract, becoming shorter by the movement of the 

 base toward the apex, and thicker by the elongated ventricular 



