132 A MANUAL OF PHYSIOLOGY 



very special conditions. When the ventricle of a normally 

 beating frog's heart is stimulated by a rapid series of induction 

 shocks, its rate is generally increased, but there is no definite 

 relation between the number of stimuli and the number of 

 beats. Many of the stimuli are ineffective. In the same 

 way a portion of the heart, such as the apex of the ventricle, 

 when stimulated in the quiescent condition by an interrupted 

 current, responds by a rhythmical series of beats, and not by 

 a tetanus. It is evident that the cardiac muscle, like ordinary 

 striped muscle, is for some time after excitation incapable 

 of responding to a fresh stimulus, i.e., there is a refractory 

 period. But this is immensely longer in cardiac than in 

 skeletal muscle. When the phenomenon is analyzed, it is 

 found that a stimulus falling into the heart muscle between 

 the moment at which the contraction begins and the 

 moment at which it reaches its maximum, produces no effect 

 is, so to speak, ignored. When the stimulus is thrown in 

 at any point between the maximum of the systole and the 

 beginning of the next contraction, it causes what is called 

 an extra contraction. The extra contraction is followed by 

 a longer pause than usual a so-called compensatory pause 

 which just restores the rhythm, so that the succeeding 

 systole falls in the curve where it would have fallen had there 

 been no extra contraction (Fig. 44). The refractory period 

 is shorter for strong than for weak stimuli, and is markedly 

 diminished by raising the temperature of the heart. So 

 that stimulation of the heated heart with a series of strong 

 induction shocks may cause a tetaniform condition, if not a 

 typical tetanus. The contraction of the normally beating 

 heart is really a simple contraction, and not a tetanus. The 

 capillary electrometer shows only the electrical changes 

 corresponding to a single contraction (p. 644) ; and when 

 the nerve of a nerve-muscle preparation is laid on the heart, 

 the muscle responds to each beat by a simple twitch, and 

 not by tetanus (p. 179). 



Like ordinary skeletal muscle, the cardiac muscle is at first 

 benefited by contraction, so that when the apex is stimulated 

 at regular intervals, each contraction is somewhat stronger 

 than the preceding one. To this phenomenon the name of 



