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TEXT-BOOK OF PHYSIOLOGY. 



fact that the restoration of the energy-yielding material and the return of 

 the irritability gradually increases from the beginning of the relaxation to 

 the end of the diastole (Fig. 138). For this reason weak stimuli are more 

 effective in the later than in the earlier period of the relaxation and the 

 diastole. 



After the development and disappearance of the extra systole a consider- 

 able pause in the heart's action occurs to which the term compensatory pause 



FIG. 138. DIAGRAM SHOWING THE VARIATIONS OF IRRITABILITY DURING THE SYSTOLE AND THE 

 DIASTOLE. (Modified from Waller.) 



has been given (Fig. 139), on the assumption that it was necessary on the 

 part of the heart to compensate for the disturbance of the rhythm by remain- 

 ing at rest until the time of the next beat and thus restore the rhythm. This 

 was thought to be a special property of the heart-muscle. This view, how- 

 ever, is no longer entertained. For if an isolated ventricle of a frog heart be 

 employed and made to contract rhythmically by an artificial stimulus, or if 

 a spontaneously beating portion of the dog's heart be employed for experi- 

 mentation instead of the whole heart, the results of the same methods of 

 stimulation are different. Though an extra systole is called forth as usual, 

 there is no compensatory pause; indeed, if anything the pause is shorter 

 than the regular pause. The theory that a compensatory pause is necessi- 

 tated for the restoration of the normal rhythm is therefore not tenable. 



The explanation assigned and generally ac- 

 cepted at present for the production of a com- 

 pensatory pause is as follows: In a spontaneously 

 beating heart the ventricular systole is evoked by fc the 

 arrival of an excitation process coming from the 

 auricles. When the extra systole is induced by an 



FIG. 139. THE EXTRA 

 SYSTOLE AND THE COMPEN- 

 SATORY PAUSE. The break 

 in the horizontal line indicates artificial stimulus, the next succeeding excitation 

 the moment the electric cur- f rom t h e auricle falls into the refractory period and 

 sthroughthe heart ' hence the ventricle is not stimulated. It, therefore, 

 simply waits for the arrival of the second succeeding excitation, when it 

 responds and takes up the regular rhythm. 



This fact is of great interest clinically for it frequently happens that 

 extra systoles of the ventricle arise in the human heart in conditions of the 

 circulation characterized by a high blood-pressure and especially when there 



