THE VASCULAR MECHANISM. 213 



and a second shock is sent into the heart during the systole evoked by 

 the first stimulus, the second stimulus produces no visible effect on the 

 heart, which is said to be "refractory." This "refractory period" 

 extends from the instant when the first stimulus is applied until the 

 end of the systolic phase. Owing to the length of the refractory period 

 of cardiac muscle as compared with that of skeletal muscle, it is 

 impossible to tetanise the heart, since only those stimuli which fall 

 during the diastolic period are effective. 



The refractory period also accounts for the effect observed in the 

 normally beating heart when a single shock is sent into the ventricle 

 at the beginning of diastole. In this case the ventricle responds with 

 an extra beat, and the stimulus coming down from the sinus to produce 

 the next ventricular systole falls within the refractory period of this 



FIG. 78. Normally beating frog's heart tracing. A single induction shock, 

 applied to the ventricle at A, caused an extra systole, followed by a com- 

 pensatory pause. Downstroke = systole. 



extra beat and is ineffective. There is consequently a long pause, which 

 is known as the "compensatory pause," between the extra beat and the 

 next beat originating from the sinus (fig. 78). 



(4) The force of the contraction is greatly influenced, as in the case 

 of skeletal muscle, by the tension to which the fibres are exposed, that 

 is, by the length of its fibres. If the heart is isolated and perfused 

 with saline solution (0'65 per cent. NaCl), the force of the ventricular 

 contraction will vary with the pressure under which the fluid is allowed 

 to enter the heart. The higher the pressure, the greater will be the 

 amount of saline solution entering the ventricle during diastole ; and 

 the pressure of the fluid will put tension on, and thus increase the 

 length of, the muscle fibres. The heart muscle has the power of 

 responding to the increased tension by contracting more strongly, 

 with the result that it empties itself as completely as it did when 

 the pressure of the perfusing fluid was low. Increase of internal 

 tension is thus a stimulus to contraction, and it is for this reason that 

 the apical half of the frog's heart contracts rhythmically when it is 



