CHAP, iv.] THE VASCULAR MECHANISM. 185 



tricuspid valves, so here, while the pressure of the blood is borne 

 by the tougher bodies of the several valves, each two thin, adjacent 

 lunulae, pressed together by the blood acting on both sides of 

 them, are kept in complete contact, without any strain being 

 put upon them ; in this way the orifice is closed in a most efficient 

 manner. 



As the ventricular systole passes off, the muscular walls relax- 

 ing, 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 in the main like the tricuspid valve, and the action of the 

 semilunar 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. 



110. 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. 



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

 that the side-to-side diameter is much lessened during the systole. 

 There is also evidence that the front-to-back diameter is greater 

 during the systole than during the diastole, the increase taking 

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

 be placed so as to press very gently 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. 36 

 may be 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 ventricle, 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 



