CHAP, iv.] THE VASCULAR MECHANISM. 



253 



inflow of blood from the auricle and veins ; but it too is not 

 always present. Both the steady 

 though slight rise of the lever 

 throughout the diastole, with a 

 sudden increase at the end, coin- 

 cident with the auricular systole are 

 often seen in cardiograms ; see the 

 diagrammatic curve in Fig. 55. The 

 ventricle as a whole enlarges under 

 the venous inflow, and is more sud- 

 denly enlarged by the auricular 

 systole. 



The feature on which we wish to 

 insist is the rapid rise of the intra- 

 ventricular pressure, and the sudden 

 change at the commencement of the 

 systolic plateau. What does this 

 sudden change mean ? To answer 

 this question we must ascertain what 

 is taking place at the same time in 

 the aorta. 



134. If two catheters be in- 

 troduced at the same time into the 

 left side of the heart of a dog, being 

 so arranged that while the end of 

 one catheter lies in the left ventricle, 

 Fig. 49, V, that of the other lies in 

 the aorta A above the semilunar 

 valves, and if each catheter be con- 

 nected with a membrane-manometer, 

 the two manometers recording on 

 the same surface, one below the 

 other, we obtain some such result 

 as that shewn in Fig. 50. 



An examination of the two curves thus obtained shews us the 

 following. At o, the beginning of the ventricular systole, or rather 

 the time when the contraction of the ventricular fibres is beginning 

 to raise the pressure within the ventricle, no effect is being produced 

 in the aorta ; the blood in the aorta is completely sheltered by 

 the closed aortic valves. A little later however, at 1, the pressure 

 in the aorta begins to rise. This means that the semilunar valves 

 are now' opened, so that the force of the ventricular systole can 

 make itself felt in the aorta. Up to 1, the pressure in the 

 ventricle, though increasing, is still less than that remaining in the 

 aorta after the last beat, but at 1 the pressure in the ventricle 

 becomes equal to or rather slightly greater than that in the aorta, 

 and the valves are thrown open. 



This is also shewn by comparing, as may be done by means 



FlG. 49. DlAGBAM ILLUSTBATING 

 THE METHOD OF EECOBDING SI- 

 MULTANEOUSLY THE PBESSUBE IN 

 THE LEFT VENTBICLE AND AT THE 



BOOT OF THE AOBTA. HijBTHLE. 



