288 



THE CIRCULATION OF THE BLOOD 



pression x is an indication of the fall in intraauricular pressure, refer- 

 ence to the intraauricular pressure curve (Fig. 97- A) to which, as already 

 explained, the venous pulse tracing is qualitatively similar, will make 

 this clear. The factors producing this fall in pressure are as follows: 

 (1) relaxation of the auricular wall, (2) dragging down of the auricular 

 floor by the contracting ventricles (compare the jugular and apex curves 

 in Fig. 95), and (3) reduction of intrathoracic pressure consequent 



Fig. 96. Polysphygmograph. This instrument records in ink on glazed paper two simul- 

 taneous tracings, i. e., radial pulse and one other, such as carotid, jugular, apex beat, etc., in addi- 

 tion to the time tracing. The ink tracings are both more convenient and permanent than smoked 

 paper tracings. The clockwork operates at variable speeds, permitting the taking of protracted 

 records at different speeds. 



Fig. 97. Normal jugular tracing. The spacing below shows the duration of the a-c interval. 



(From E. P. Carter.) 



upon the ejection of blood from the ventricle into the arterial system 

 (see page 215). 



The slope from x to y is due to the rising intraauricular pressure as the 

 blood, flowing into the auricle from the great veins, is dammed back 

 by the closed auriculo-ventricular valves. A small wave, sometimes, is 

 seen on the upstroke of the "v" wave at the point where the latter is 

 intersected by line 5 ; this wave coincides with the closure of the semi- 

 lunar valves. At the summit of the "v" wave the auriculp^astricular 

 valves open and the fall in intraauricular pressure which occurs, as 



