ACTION OF THE VALVES l8l 



On the conclusion of the systole the ventricular diastole begins. The 

 muscular walls relax and, by virtue of their elasticity, a slight negative 

 pressure may be set up. This negative or suctional pressure on the left 

 side of the heart may be of importance in helping the pulmonary circula- 

 tion. It is somewhat inconstant in appearance, but has been found to be 

 equal to as much as 20 mm. of mercury, and is said to be quite independent 

 of the aspiratory power of the thorax itself, which will be described in a 

 later chapter. The ventricles now remain in a state of relaxation or rest 

 until the next systole begins. 



The duration of the ventricular systole and the diastole has been variously 

 estimated. A computation of the time of these two phases, for man, in 

 figure 153, reproduced from Hiirthle, gives for the systole 0.38 of a second 

 and for the diastole 0.4 of a second, with a total of o. 78 of a second. This 

 is equivalent to a rate of 77 per minute. Variation in the time of the systole 

 and the diastole of the ventricle falls chiefly on the pause of the diastole. 



The ventricles undergo little or no change of shape in the unopened chest. 

 At the moment in the systole when the ventricles begin to discharge their 



Conus arteriosus 



Left posterior cusp of 

 pulmonary valve 



Left posterior cusp of 

 aortic valve 



Right coronary 

 artery 



Anterior cusp of 

 aortic valve 



Right posterior cusp 



Right (marginal) 

 cuspoUncusp,d 



Posterior (septal) 



Posterior cusp of v -\,v ^ ^ \ M^M CUS P ^ tricuspid 



mitral valve ? : ' ValvC 



Left ventricle 



Right ventncle 



FIG. 151. The Bases of the Ventricles of the Heart, showing the auriculo-ventricular, 

 aortic, and pulmonary orifices and their valves. (Cunningham.) 



contents into the aorta and pulmonary arteries, respectively, there is a sharp 

 decrease in size of the ventricles. This decrease takes place in all 

 dimensions. 



Action of the Valves. The Tricuspid Valve. During the diastole 

 of both auricles and ventricles blood flows directly through the auricles into 

 the ventricles, the auricles during this period acting as continuations of the 

 large veins which empty into them. At the end of the period the ventricle 



