218 THE CIRCULATION OF THE BLOOD 



venous or arterial pressure is high, and the cardiac musculature is not 

 sufficiently powerful to empty the ventricle completely; a similar state 

 of affairs will also exist when the aortic valves are incompetent. 



The last statement in the preceding paragraph implies that the state 

 of the ventricular musculature must be an important factor in connection 

 with the law of the heart. Its degree of development, its nutritional con- 

 dition, and the presence or absence of fatigue, must greatly influence the 

 extent to which the ventricle is capable of obeying the law. When the 

 musculature is less powerful than normal, then, in order to increase the 

 output in proportion to a larger venous inflow, the heart must dilate more 

 during diastole, so as to bring about a sufficiently increased contraction; 

 when it is more powerful, as in hypertrophy, a small increase in the length 

 of the muscle fibers (i. e., a slight diastolic dilatation) will call forth a 

 sufficient contraction because of the cumulative effect on the larger number 

 of fibers. 



The Reserve Power of the Heart. These are the principles determining 

 the reserve power of the heart, and the practical question arises as to how 

 we are to know in man when this reserve has become used up. The most 

 useful method is by counting the pulse before and after exercise. When 

 the heart is well developed, as in a trained athlete, it is clear that the 

 increase in venous inflow which occurs during exercise (see page 219) will 

 stimulate the ventricle so that the output per beat exactly corresponds to 

 the inflow, and no increase in the frequency of the beats is required. But 

 when the musculature is feeble, as in a sedentary person, the dilatation 

 must become considerably greater in order to call out sufficient contractile 

 power, the output per beat only moderately increases and quickening be- 

 comes necessary if the minute volume is to be adequate to meet the in- 

 creased demands of the muscles for blood. When the reserve power of the 

 heart is very low, even extreme dilatation during diastole may be insuffi- 

 cient to stimulate contractions that are powerful enough to empty the 

 heart, blood is therefore left over in it at the end of systole, and when the 

 venous blood becomes superadded during diastole, extreme dilatation oc- 

 curs, and the beat becomes very rapid in the attempt to maintain an 

 adequate output. 



The Effect of Alteration in Rate of Heart Beat on Output of Blood 



At this stage it is important to analyze the effect on the output of the 

 heart per minute (the minute-volume) brought about by increase in the 

 rate of beat. When the venous inflow is slow, the ventricle does not dilate 

 to its full capacity in diastole, and acceleration of the beat does not im- 

 prove the output in a unit of time ; when the inflow is more rapid it dilates 



