CHAP, iv.] THE VASCULAR MECHANISM. 



155 



cardiac events. It will be understood of course that the diagram is 

 intended to shew merely the general relations of the several events 

 and not to represent exact measurements. 



Fio. 26. 



DIAGRAMMATIC KEPBESENTATION OF THE MOVEMENTS AND SOUNDS OP TUB 

 HEART DURING A CARDIAC PERIOD. (After Dr SHARPET.) 



We may repeat that the details given above are at the best 

 approximative only, and, we may add, to a certain extent hypo- 

 thetical. We have given them at such length not on account 

 of their intrinsic importance, or because they are trustworthy data 

 for further calculations, but because the study of them may help 

 the reader in forming a more vivid image in his mind of what is 

 taking place in the heart during a beat. Moreover it must be 

 remembered that the figures quoted are those belonging to what 

 may be considered a normal rate of heart beat. The rate how- 

 ever at which the heart beats varies, as we shall see, under the 

 influence of circumstances, within very wide limits. With regard 

 to the duration of the several phases at different rates of heart 

 beat, the most important fact is perhaps that the pause varies 

 much more than does the systole of the ventricles. A quickly 

 beating heart differs from a slowly beating heart by reason of the 

 pause being shortened, much more than by each systole being of 

 less duration. 



We may briefly recapitulate the main facts connected with 

 the passage of blood through the heart as follows. The right 

 auricle during its diastole, by the relaxation of its muscular fibres, 

 and by the fact that all pressure from the ventricle is removed by 

 the tension of the tricuspid valves, offers but little resistance to 

 the ingress of blood from the veins. On the other hand, the blood 

 in the trunks, of both the superior and inferior vena cava, is under a 

 pressure, which diminishing towards the heart and becoming within 



