232 THE CHANGE OF FORM OF THE HEART. [BOOK i. 



The ventricular systole now passes off, the muscular walls relax, 

 the ventricle returns to its previous form and position, and the 

 cycle is once more ended. 



What thus takes place in the right side takes place in the left 

 side also. There is the same sudden sharp auricular systole 

 beginning at the roots of the pulmonary veins, the same systole of 

 the ventricle but, as we shall see, one much more powerful and 

 exerting much more force ; the mitral valve with its two flaps 

 acts exactly like the tricuspid valve, and the action of the semi- 

 lunar valves of the aorta simply repeats that of the valves of the 

 pulmonary artery. 



We may now proceed to study some of the cardiac events in 

 detail. 



128. The change of form. The exact determination of the 

 changes in form and position of the heart, especially of the ven- 

 tricles, during a cardiac cycle is attended with difficulties. 



The ventricles for instance are continually changing their form ; 

 they change while their cavities are being filled from the auricles, 

 they change while the contraction of their walls is getting up 

 the pressure on their contents, they change while under the 

 influence of that pressure their contents are being discharged into 

 the arteries, and they change when, their cavities having been 

 emptied, their muscular walls relax. 



We may take it for granted that the internal cavities are 

 obliterated by the systole, for it is probable that practically the 

 whole contents are driven out at each stroke, and probably also 

 each cavity is emptied from its apex towards the mouth of the 

 artery. 



With regard to changes in external form, there seems no doubt 

 that-khe side-to-side diameter is much lessened. It seems also 

 clear that the front-to-back diameter is greater during the whole 

 time of the systole than during the diastole, the increase taking- 

 place during the first part of the systole. If a light lever be 

 placed on the surface of the heart of a mammal, the chest having 

 been opened and artificial respiration being kept up, some such 

 curve as that represented in Fig. 35 is obtained. The rise of 

 the lever in describing such a curve is due to the elevation of 

 the part of the front surface of the heart on which the lever is 

 resting. Such an elevation might be caused, especially if the 

 lever were placed near the apex, by the heart being " tilted " 

 upwards during the systole, but only a small portion at most of 

 the rise can be attributed to this cause ; the rise is perhaps best 

 seen when the lever is placed in the middle portion of the ven- 

 tricle, and must be chiefly due to an increase in the front-to-back 

 diameter of the ventricle during the beat. We shall discuss this 

 curve later on in connection with other curves and may here 

 simply say that the part of the curve from b' to d probably corre- 

 sponds to the actual systole of the ventricle, that is to the time 



