42 A MANUAL OF VETERINARY PHYSIOLOGY 



to allow the aortic and pulmonary valves to close. % The fall in 

 pressure which represents the end of the ventricular systole 

 is followed by the pause D, and this is once more succeeded 

 by a contraction of the auricles. 



All that we have attempted to do in the above is to focus 

 attention on the fact that certain positive and negative waves of 

 pressure are constantly occurring in both chambers of the heart. 

 Their shape on a recording surface depends upon the nature 

 of the apparatus employed ; their significance remains, and must 

 now engage our attention. 



There are three ' standard movements ' in the heart to which 

 other cardiac events may be referred in point of time ; in the 



Fig. 18. — Curves of Endocardiac Pressure taken with Cardiac Sounds. 



Aur., Auricular curve; Vent., ventricular curve; AS, period of auricular 

 systole, including relaxation ; VS, of ventricular systole, including relaxation ; 

 D, pause. 



measurement and interpretation of pulse tracings in clinical work 

 these are important landmarks. One is the closure of the semi- 

 lunar valves of the aorta and pulmonary artery, another is the 

 opening of the auriculo-ventricular valves. The first is known 

 for brevity as the ' S.C. period,' the second is known as the 

 ■ A.O. period.' 



In Fig. 18 the S.C. period occurs near the end of the ventricular 

 plateau, just when the pressure in the ventricle becomes less than 

 that in the aorta and pulmonary artery ; the A.O. period occurs 

 at the bottom of the down-stroke. Between these two points, 

 brief as it is, matters within the heart are in rather a peculiar 

 condition : the semilunar valves are closed, but the pressure in 

 the ventricles is still too high to admit of the auriculo-ventricular 



