156 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tractions by alterations in the conductivity or irritability of the several parts 

 of the heart successively traversed by the excitation wave. For example, 

 a Lessening of the normal conductivity at the auriculo-ventricular junction 

 might permit only every second sino-auricular impulse to reach the ventricle; 

 in this ease the ventricle would drop every second beat. The same inter- 

 mittence would result if the irritability ol* the ventricle were so far reduced 

 that it could not respond to the normal excitation. 1 Engelmann has recently 

 found that ventricular systole lowers the conductivity of the ventricle for a 

 time. 2 



Refractory Period and Compensatory Pause. — Schiff found in 1850 

 that the heart which contracted to each stimulus of a series of slowly repeated 

 mechanical stimuli would not contract to the same stimuli if they followed each 

 other in too rapid succession. Kronecker got a similar result with induction 

 shucks. The heart contracted to every stimulus only when the interval between 

 them was not too brief. The following year Marey published a systematic 

 study of the phenomenon. He observed that the irritability of the heart sank 

 during a part of the systole, but returned during the remainder of the systole 

 and the following diastole. The stimulus which fell between the beginning of 

 the systole and its maximum produced no extra contraction, whilst that which 

 fell between the maximum of one systole and the beginning of the next 

 called forth an extra contraction. During a part of the cardiac cycle therefore 

 the heart is " refractory " toward stimuli. The irritability of the heart is 

 removed for a time by an adequate stimulus. 



Kronecker and Marey noticed further that stimulation with the induction 

 shock during the non-refractory period did not influence the total number of 

 systoles. The extra systole called forth by the artificial stimulus was followed 

 by a pause the length of which was that of the normal pause plus the interval 

 between the appearance of the extra systole and what would have been the end 

 of the cardiac cycle in which the extra systole fell. The extra length of this 

 pause restored the normal frequency or rhythm. It was called the compensa- 

 tory pause (see Fig. 28). 3 



The systole following the extra contraction and its compensatory pause is 

 of marked strength, at least in the surviving mammalian heart (cat). The 

 weaker the extra systole the stronger the first subsequent contraction. 

 The unusual force of this "compensatory systole" may serve to compensate 

 the loss in the output of the heart incident to the disturbance in its rhthym. 4 



If the heart, or the isolated apex, is beating at a rate so slow that an extra 

 contraction falling in the interval between two normal contractions has time to 

 complete its entire phase before the next normal contraction is due, there will 

 be no compensatory pause. 5 



1 Oehrwall: Skandinavisches Archiv fiir Physiologie, 1898, viii. p. 1. 

 2 Engelmann: Archiv fur <U> geswnmte Plu/xiologie, 1896, Ixii. p. 543. 

 ' ( lourtade: Archives de Physioloffie, 1897, p. 69. 



* Langendurfl": Archiv fiir >li> gr.mminti' f'ln/siolagie, 1898, lxx. p. 473. 

 5 Kaiser: Zeituclirifl fiir Biologic, 1895, xxxii. p. 449. 



