THE CIRCULATION 289 



eighths of the time; the ventricles work three-eighths of the time and rest 

 five-eighths. Averaging the rest-periods of the auricles and of the 

 ventricles, we see that the heart, as a whole, may be said to rest six- 

 eighths or three-quarters of every beat-period. We can no longer think, 

 therefore, of the heart as "an organ which never rests" it works, 

 indeed, only six hours out of the twenty-four! When the rate increases, 

 the rest-periods are shortened, and the time of the systoles is very little 

 decreased. 



It is not known exactly what proportions of the 0.7 second in the 

 auricles and of 0.5 second in the ventricles (intervals between their 

 respective systoles) are actually occupied in relaxation or diastole, but 

 probably a large part of these periods is so occupied. In any event, 

 practically all of it is a period of rest or anabolism for the muscle- 

 protoplasm, unless (a possibility only) some of the muscle-cells of the 

 auricles are meanwhile working to actively expand them. 



The relations of the valves' actions to the movements of the chambers 

 will be understood if we go over somewhat more in detail the phenomena 

 of the beat. While the auricles are contracting the ventricles are having 

 the last fifth of their rest; while the ventricles are contracting the auricles 

 are expanding; after the contraction of the ventricles the whole heart at 

 once rests until the auricles contract again. This latter contraction 

 begins in the great veins, and extends with great rapidity over both 

 auricles at once toward the auriculo- ventricular groove. This systole 

 very suddenly (0.1 second) empties the auricles in the direction of the 

 least resistance namely, into the now expanded ventricles through the 

 wide-open mitral and tricuspid valves. Upward there is more resistance 

 than below, not so much because of the few valves between the auricles 

 and the veins, as because the veins have previously contracted and so 

 exert the resistance of a column of blood extending backward, against 

 much pressure, even to the capillaries. It is likely that the passive 

 relaxation of the very thick ventricular walls exerts a suction on the 

 auricles of rather more than 23 mm. of mercury. The semilunar 

 valves are meanwhile shut. The very sudden and quick systole of the 

 auricles completely fills the ventricles, and the eddies formed by this 

 sudden torrent pouring in among the thickly set papillary muscles, 

 promptly float together the flaps of the auriculo-ventricular valves. 

 The close apposition of the edges of the flaps is secured by contraction 

 of the papillary muscles which begins (according to Chaveau) in the very 

 brief interval between the two systoles. As the ventricles contract, the 

 auriculo-ventricular openings lessen, also, thus tightening still more 

 these valves for the pressure they are to withstand. Without appre- 

 ciable pause, the ventricles begin to contract, but they occupy thrice as 

 much time in their systole as do the auricles. Meanwhile, this strains the 

 auriculo-ventricular valves and stretches tight the tendinous cords which 

 prevents their flaps from being pushed upward into the auricles. 



As the ventricles contract, the pressure within them rapidly rises and 

 soon reaches such a degree that, despite the pressure in the great arteries 

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