552 CIRCULATION OF BLOOD AND LYMPH. 



taneous measurement of the pressure in the aorta, it being evident 

 that the pressure will begin to rise in this latter vessel the moment 

 that the valves open. The different features of the ventricular 

 systole as gathered from these pressure curves are as follows: 

 The systole of the ventricle exhibits three phases or periods: 1. The 

 period of tension or the isometric period, lasting from the begin- 

 ning of the contraction to the time of opening of the semilunar 

 valves (2 to 3 in Fig. 230a). During this period the blood is held 



t\ 



\ -3 f .1 



L 5 V I 



1 



n \ i \ A 



*/ V/ ^Z 



Fig. 230o. Simultaneous curves of intra ventricular pressure and subclavian pressure: X 

 and X' show relative positions. The time curve indicates one-fifth sec.; 2-3, the isometric 

 period; 3, opening of semilunar valves; 3-5, the ejection period; 5, beginning of ventricular 

 diastole; b-e, the ejection period as it affects the subclavian pressure; /, closure of the semi- 

 lunar valves ; e-f, the incisura, the fall of pressure at the beginning of diastole which occasions 

 the closure of the semilunar valves. (From Wiggers.) 



in the ventricle, since both the auriculo-ventricular and the semi- 

 lunar valves are closed, but it is squeezed upon by the contracting 

 muscle and the pressure rises very rapidly. When it exceeds the 

 pressure in the aorta the semilunar valves are thrown open, blood 

 begins to flow from the ventricle, and the cavity of the ventricle 

 diminishes in size. The opening of the semilunar valves occurs at 

 (3), and at this point begins the second period, the period of empty- 

 ing. After the semilunar valves are open pressure in the ventricle 

 continues to rise (3 to 4) owing to the fact that the ventricle is dis- 

 charging blood into the aorta more rapidly than it can escape from 

 the aorta into the peripheral vessels. When this relation is reversed 



