CIRCULATION. 133 



tricular above the arterial pressure, and therefore to the opening <>t' the semi- 

 lunar valve. But other forces beside the arterial pressure must be overcome 

 by the contracting muscle; and the ventricular pressure mounts higher vet, 

 and either stays high for a while, producing the plateau, or, in a peaked curve, 

 at once descends. In either ease, not long after the beginning of the sharp 

 descent, the point occurs at which the ventricular pressure falls below the arte- 

 rial, and the semilunar valve is closed. Beyond this point the curve continues 

 steeply downward, but it is not till a point is reached not far above, or possibly 

 even below, the atmospheric pressure that the pressure in the ventricle falls 

 below that in the auricle, and the cuspid valve is opened. 



The Period of Reception, the Period of Ejection, and the Two Periods 

 of Complete Closure of the Ventricle. — During the whole of the period 

 when the cuspid valve is open, the pressure is lower in the ventricle than in 

 the artery ; the arterial valve is shut ; and blood is entering the ventricle. 

 This may be called the " period of reception of blood." During the greater 

 part of the period when the cuspid valve is shut, the arterial valve is open ; 

 the pressure is higher in the ventricle than in the artery; and the ejection 

 of blood from the former is taking place. This may be called the "period 

 of ejection," and lies in Figures 23 and 24 between the ordinates 2 and 

 3. The careful work which has enabled us to mark the valve-play upon 

 the ventricular curve has demonstrated the interesting fact that there occur 

 two brief periods during each of which both valves are shut, and the ven- 

 tricle is a closed cavity. Of these two periods, one immediately precedes the 

 period of ejection, and the other immediately follows it. The first lies, in 

 Figures 23 and 24, between the ordinates 1 and 2 ; the second, between 3 

 and 4. The explanation of these two periods is simple. It takes a brief but 

 measurable time for the cardiac muscle, forcibly contracting upon the impris- 

 oned liquid contents of the closed ventricle, to raise the pressure to the high 

 point required to overcome the opposing pressure within the artery and to open 

 the semilunar valve. Again, it takes a measurable time, probably seldom 

 quite so brief as the period just discussed, for the cardiac muscle to relax suffi- 

 ciently to permit the pressure in the closed ventricle to fall to the low point 

 required for the opening of the cuspid valve. The ventricular cycle, thus 

 studied, falls into four periods: the first is a brief period of complete closure 

 with swiftly rising pressure; the second is the period of ejection, relatively 

 long, and but little variable ; the third is a period of complete closure, with 

 swiftly falling pressure; the fourth is the period when the pressure is low and 

 blood is entering the ventricle. This last period is very variable in length, 

 but at the average pulse-rate it is the longest period of all. 



Phenomena of the Period of Reception of Blood. — We have already 

 followed the course of the pressure within the ventricle from the moment of 

 opening of the auriculo-ventrieular valve to that of its closing (p. 128). 

 During this time the ventricle is receiving its charge of blood, the flaccidity of 

 the wall rendering expansion easy and keeping the pressure low. The blood 

 which ent<r- first has been accumulating in the auricle since the closing of the 



