156 THE CIRCULATION OF THE BLOOD 



The form and position of the fleshy columns on the internal walls of the 

 ventricles no doubt help to produce the obliteration of the ventricular cavity 

 during contraction. The completeness of the closure may often be observed 

 on making a transverse section of a heart shortly after death in any case in 

 which rigor mortis is very marked, figure 137. In such a case only a central 

 fissure may be discernible to the eye in the place of the cavity of each ventricle. 

 The arrangement of the muscles of the heart, as described on page 148, is 

 such as to expend the whole force of the contraction in diminishing the cavity 

 of the ventricle, or, in other words, in expelling its contents. 



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

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

 ure is set up. This negative or suctional pressure on the left side of the 

 heart is of importance in helping the pulmonary circulation. It is some- 

 what 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 diastole has been variously 

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

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

 and for the diastole 0.4 of a second, with a total of 0.78 of a second. This 

 is equivalent to a rate of 77 per minute. Variations 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 

 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 Auriculo-ventricular Valves. Dur- 

 ing 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 on each side has already been filled and distended 

 by the pressure of blood from the veins. The systole of the auricle com- 

 pletes this filling and slightly overdistends the ventricle. When the force 

 of the auricular contraction is spent, the ventricular walls rebound slightly 

 toward their former position and in so doing exert some pressure upon the 

 ventricular side of the auriculo-ventricular valves which floats them upward 

 toward the auricle. In this connection another force comes into play, viz., 

 vortex or back currents resulting from the flow of blood into the ventricle 

 under the pressure of the auricular systole. These currents aid in floating 

 the valve leaflets into apposition. Thus, the auriculo-ventricular openings 

 are closed at the end of the auricular systole, i.e., the end of the ventricular 

 diastole. The ventricular systole which follows simply serves to place the 



