THE ACTION OF THE HEART 



149 



therefore shut off from the ventricle. The exact moment in diastole at 

 which the two cavities are again brought into communication i.e., the 

 ventricular valves open is indicated by the curves coming together. 

 Having thus determined the -exact moments of opening and closing 

 of the auriculoventricular valve, we may now proceed to compare the 

 intraventricular pressure curve with that taken from the aorta. After the 

 necessary calibration corrections, this curve has been placed in Fig. 34 

 in its true relationship to the ventricular curve. Beginning again at the 

 end of diastole, we find that the aortic pressure is very considerably 

 above that of the ventricles, indicating that the semilunar valves must 

 be closed; and it will be observed that the intraventricular pressure at 



Fig. 34. Pressure curves after being graduated have been superimposed. The presphygmic, 

 sphygmic and postsphygmic periods of ventricular systole are shown by the vertical lines. The 

 A-V valves close at the first line. The aortic valves open at the second line and close again at 

 the third line. The A-V valves open at the fourth line. The position of the two main heart 

 sounds is also indicated. 



the beginning of systole does not rise sufficiently to open them until an 

 appreciable interval (0.02 to 0.04 second) after the closure of the auric- 

 uloventricular valves; that is to say, there is a period at the beginning 

 of ventricular systole during which the ventricle is a closed cavity. It 

 is a period during which the ventricle by its contraction is getting up a 

 sufficient amount of pressure in the fluid contained in it to force open 

 the semilunar valves against the resistance of the pressure in the aorta, 

 and it has been popularly called "the period of getting up steam," or, 

 in physiological language, the isometric, or the presphygmic, period. We 

 shall use the last-mentioned term in our further discussion here. 



