290 



THE CIRCULATION 



holding the semilunar valves shut, these valves are forcibly burst open 

 and the torrent of blood pours outward into the aorta and the pulmonary 

 arteries. The opening of these valves could occur only when the pressure 

 below them had come to exceed that in the arteries above them namely, 

 about 200 mm. of mercury. As the great arteries are distended with 

 blood, the little pouches (sinuses of Valsalva) behind the cups of the 

 semilunar valves become filled. At the instant when ventricular systole 

 is complete and the pressure in the ventricles therefore stops rising, more 

 blood is forced into these sinuses by the instantaneous passive recoil 

 of the distended arterial walls, and thus the cups of these semilunar 

 valves are pushed together. With the aid of the corpora Arantii they 

 quite close their openings. The semilunar valves then are open only 

 during the latter part of ventricular systole, say for 0.2 second, just long 



FIG. 155 



\ 



Traces showing O, the auricular pressure; V, the ventricular pressure; and P, the beat of 

 the heart, together with their time-relations, in the horse. (Chauveau and Marey.) 



enough for the blood to be crowded through them into the already dis- 

 tended arteries. The auriculo-ventricular (mitral and tricuspid) valves 

 are closed for a somewhat longer period namely, during practically 

 the whole of ventricular systole and for a brief interval afterward, for 

 0.4 second perhaps altogether. These two sets of valves are open alter- 

 nately, but never at the same time save for a specious instant at the 

 beginning of ventricular systole when the semilunars are already open, 

 but the auriculo-ventriculars are not yet closed. This is a condition that 

 in practice may or may not be present, depending on the pressure in the 

 great arteries which determines the time of opening of the semilunars. 



At once after ventricular systole, ventricular diastole begins and lasts 

 'about 0.5 second, until the conclusion of the auricular systole of the next 



