THE HEART-BEAT IN ITS PHYSIOLOGICAL RELATIONS 155 



is also possessed by the skeletal muscle- fibre. When a whole skeletal 

 muscle is excited either directly or through its motor nerve, it is 

 true that throughout a considerable range increase of stimulus is 

 accompanied by an apparent increase in the strength of contraction. 

 But there is reason to believe that this is because a larger and larger 

 number of fibres become involved in the excitation as the stimulus 

 is increased, and not because each fibre responds more and more 

 strongly (Lucas). In skeletal muscle the fibres are completely 

 isolated from each other, and the excitation does not spread from 

 fibre to fibre, as happens in the heart. 



Refractory Period and Extra Contraction of Hear' Muscle. A 

 more characteristic property of the cardiac muscle tha .1 the ' all or 

 nothing ' law is that a true tetanus of the heart cannot be obtained 

 at all, or only under 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 stimu- 

 lated 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. 68). 



In man, extra systoles followed by compensatory pauses may occur 

 under pathological conditions, giving rise to an important group of 

 cardiac irregularities. These extra systoles may be either auricular or 

 ventricular, the auricle or the ventricle contracting prematurely without 

 waiting for the signal of the sinus rhythm. The analysis of pulse- 

 tracings showing these irregularities has led to results of great physio- 

 logical 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 condi- 

 tion 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 distinguished that form of alterna- 

 ting pulse termed pulsus alternans, in which every second beat is dimin- 

 shed in size, but the intervals separating the beats are of uniform length. 



