CYCLE OF THE HEART-BEAT. 265 



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

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

 would be made up as follows : 



Auricular systole, = -fa of a second. 



Ventricular systole, = -^ " 



Pause, = A 



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, 



