BLOOD VESSELS 



445 



checked, and consequently the veins distend, causing a crest 

 (^.>S^.). At the moment of auricular diastole the outflow is 

 again free, a rush of blood takes place into the distending 

 auricles, and thus the pressure in the veins falls, (ii.) But, as 

 this is occurring, two things happen — (a.) blood is shot from 

 the ventricles into the arteries, and the carotid, lying behind 

 the jugular vein, transmits its pulse through the vein ; (6) the 

 auriculo-ventricular valves are closed and pressed upon from the 

 ventricular side, and thus a wave of pressure is sent back 

 through the auricles. These two together cause a second 

 wave early in ventricular systole, (iii.) While the ventricle is 



Fig. 187. — Tracings of the Pulse in the Great Veins in Relationship to the 



Cardiac Cycle. normal venous pulse, on which is shown the 



fourth crest which is often absent. — -a and h, venous pulse in 

 tricuspid incompetence. 



contracted, blood cannot pass on from the auricles, and hence 

 it accumulates in the great veins and makes a third crest at 

 the end of ventricular systole. At the moment when the 

 ventricles dilate, a sudden rush of blood takes place from the 

 veins and auricles into the ventricles, and thus a sudden fall 

 in the pressure is produced, (iv.) This may be interrupted by 

 the rebound of the auriculo-ventricular ring, which was pulled 

 downwards during ventricular systole, and this may cause a 

 fourth crest on the pulse. Gradually, as the ventricles fill, 

 the pressure in the auricles and veins increases till the next 

 auricular systole. 



