150 THE CIRCULATION OF THE BLOOD 



After the aortic valves have been opened, it will be observed that the 

 pressure in the ventricles is just a little above that in the aorta, and that 

 it continues so during the whole of ventricular systole. When diastole 

 sets in, the pressure in the ventricles quickly falls, and a point is soon 

 reached at which equality of pressure in ventricle and aorta is again 

 attained. This corresponds to the moment of the closure of the semi- 

 lunar valves. The pressure in the ventricle, although now rapidly fall- 

 ing, takes a little time before it has fallen low enough to permit the 

 auricular valves to open. Here again, then, the ventricle is a closed cavity, 

 and we have what is known as the postsphygmic period. 



