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TEXT-BOOK OF PHYSIOLOGY 



suits 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 necessitated for the restoration of the normal rhythm 

 is therefore not tenable. 



The explanation assigned and generally accepted at present for the 

 production of a compensatory pause is as follows: In a spontaneously 

 beating heart the ventricular systole is evoked by the arrival of an excitation 

 process coming from the auricles. When the extra systole is induced by an 

 artificial stimulus, the next succeeding excitation from the auricle falls into 

 the refractory period and 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 



FIG. 136. DIAGRAM SHOWING THE VARIATIONS OF IRRITABILITY DURING THE SYSTOLE AND THE 

 DIASTOLE. (Modified from Waller.) 



is coincidently an impairment in the irritability and contractility of the heart- 

 muscle. Extra systoles, however, may have their origin in the auricular 

 walls as well. 



If a series of successive stimuli be thrown into the heart-muscle the 

 effect will vary in accordance with their time intervals. Should this be less 

 than about three seconds there will be a gradual increase in the height for 

 some half dozen contractions, a result to which the term "staircase" or 

 "treppe" has been given. This increase in the height of the contraction 

 is attributed to an increase in the irritability and contractility of the muscle 

 the result of the primary stimulating action of fatigue products. 



THE NERVE MECHANISM OF THE HEART 



By this term is meant a combination of nerves and nerve centers which 

 cooperate to increase or decrease either the rate or force of the heart's con- 

 traction, and hence the proper distribution of the blood, in accordance with 

 the activities of different organs of the body. That the heart is normally 



