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A MANUAL OF PHYSIOLOGY 



group oi cardiac irregularities. These extra systoles may be either 

 auricular or ventricular, the auricle or the ventricle contracting pre- 

 maturely without waiting for the signal of the sinus rhythm normally 

 originating at the mouths of the great veins. The analysis of pulse- 

 tracings showing these irregularities has led to results of great 

 physiological and clinical interest (Cushny, Mackenzie, etc.), but 

 cannot be dwelt on here. When every second beat is an extra 

 systole, generally weaker than the preceding and the succeeding 

 normal beat, the condition is called pulsus bigeminus. The weaker 

 beat is always followed by a compensatory pause of greater duration 

 than that preceding it. From the pulsus bigeminus must be dis- 

 tinguished that form of alternating pulse termed pulsus alternans, 

 in which every second beat is diminished in size, but the intervals 



separating the 

 beats are of uni- 

 form length. 

 This form of 

 irregularity in- 

 dicates that the 

 power of the 

 heart-muscle to 

 contract is fail- 

 ing. 



The refrac- 

 tory period is 

 shorter for 

 stronger than 

 for weak stim- 

 uli, and is 

 markedly dim- 

 inished by rais- 

 ing the tempe- 

 rature of the 

 heart. So that 

 stimulation of 

 the heated 

 heart with a 



series of strong induction shocks may cause a tetaniform condi- 

 tion, 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. 720) ; 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. 188) . That the cardiac muscle itself, apart from the 

 intrinsic nervous mechanism, shows the phenomenon of ' refrac- 

 tory state ' has been shown in the Limulus heart after extirpa- 

 tion of the ganglion (Carlson). 



Like ordinary skeletal muscle, the cardiac muscle is at first 

 benefited by contraction, perhaps by an ' augmenting ' action of 



FIG. 57. REFRACTORY PERIOD AND COMPENSATORY PAUSE 

 (MAREY). 



A frog's heart was stimulated at a point corresponding to 

 the nick in the horizontal line below each curve. In i and 2 

 there was no response ; in 3 and 4 there was an extra con- 

 traction, succeeded by a compensatory pause. 



