THE CIRCULATION 



221 



abdomen tend to cause a retardation in the rate of the 

 heart. 



The sequence of events making up the cardiac cycle is 

 simpler than in the frog. 



The contraction starts in the great veins which enter the 

 auricles, and spreads down along them to these chambers. 

 This corresponds to the contraction of the sinus in the frog's 

 heart. It is followed by a short sharp contraction of the 

 auricles, which become smaller in all directions and seem to 

 be pulled down towards the ventricles. The contraction of 

 the auricles in mammals is not accompanied by so marked a 

 dilatation of the ventricles as in the frog. 



After the auricles have fully contracted, the contraction of 

 the ventricles begins, and immediately the auricles relax and 

 resume their original size. 



The ventricular contraction develops suddenly, lasts for 

 some time, and then suddenly passes of. 



The contraction of the ventricle is followed by a period 

 during which both auricles and ventricles remain relaxed. 

 This is called the pause of the cardiac cycle. 



The cardiac cycle in mammals may be represented as 

 in Fig. Ill : 



2. Duration of the Phases. Ventricular systole lasts three 

 times as long as auricular systole ; the auricles contract for 

 about O'l of a second, 

 the ventricles for 0*3 

 of a second. 



The duration of 

 these two phases in 

 relationship to the 

 pause varies very 



greatly. Whatever FIG. 111. Scheme of the Cardiac Cycle in the 



<-"U^ M 4^ ^f Human Heart. A.S. , Auricular Systole ; V.S., 



may be the rate Of Ventricular Systole; P., Pause. 



the heart, the auri- 

 cular and ventricular systoles do not vary, but in a rapidly 

 acting heart the pause is short, in a slowly acting heart it 

 is long. Taking the ordinary heart rate of 72 per minute, 

 the auricular systole lasts for |th of the whole cardiac 

 cycle, the ventricular for f ths, and the pause for f ths. 



AWICLES 



VENTRICLES 



