138 



APPLIED PHYSIOLOGY 



the appearance of a feeble first sound immediately after 

 the second (Fig. 4). On the other hand, the excita- 

 bility of the ventricles may be lowered, so that they fail 

 to respond to all of the auricular contractions, again 

 with the result that some beats are dropped. 



FIG. 4. 



a = Normal heart sounds ; b and c = sounds heard when extra-systoles 

 are occurring. 



Now, inasmuch as the variations in the excitability 

 and conductivity of the heart upon which those irregu- 

 larities of rhythm depend may be brought about either 

 through disease of the heart muscle or from nervous 

 influences, it will readily be understood that the clinical 

 interpretation of irregularities of the pulse is often a 



73 ' 76 ' 7* ' 76 



152 



77 



' 76 ' 72 ' 73 



FIG. 5. PULSE TRACING, SHOWING VENTRICULAR INTERMISSION. 

 (AFTER CUSHNY.) (THE FIGURES REPRESENT ONE-HUNDREDTHS 

 OF A SECOND.) 



The intermission, 152, is exactly twice the length of the preceding 

 pulse, 76. 



matter of no small difficulty. We are aided, however, 

 in arriving at an idea of the nature of the irregularity in 

 any particular case by the fact that ventricular inter- 

 missions during which the auricle continues to beat 



