136 APPLIED PHYSIOLOGY 



creased resistance to the outflow from the heart, as in 

 aortic stenosis, or it may be unusually short from 

 diminution of the resistance to be overcome, or from 

 incomplete contraction of a dilated ventricle. 



In the former condition the time between the first and 

 second sounds will be prolonged, and the heart assumes 

 on auscultation a pendulum rhythm. In the latter 

 state of affairs the interval between the two sounds is 

 unusually brief. 



(b) Prolongation of the diastole occurs in bradycardia, 

 shortening of it in palpitation with increased frequency 

 of action, and in dilatation of the ventricle. In the 

 latter case one gets a tic-tac or foetal rhythm on auscul- 

 tation. Shortening of the diastole is always of serious 

 significance for the heart, as it is only during that phase 

 of the cycle that rest and restoration of the fibres can 

 take place. 



8. Variations in the regularity of the beats are 



also frequently met with, and occur, indeed, in many 

 persons throughout life without affecting the circulation. 

 As we have already seen, the regular beat of the heart 

 depends upon a proper degree of excitability in the 

 auricle which initiates the systole, the wave of contrac- 

 tility so started being propagated to the ventricles by 

 the conductivity of the muscle fibres, the ventricles in 

 their turn then passing into systolic contraction. Now, 

 it must not be supposed that the muscle substance of 

 the ventricles is equally excitable and responsive to the 

 auricular stimulus at all periods of the cardiac cycle. 

 During their systole, and for a short period after it, the 

 ventricles are irresponsive to stimuli, this constituting 



