THE CIRCULATION OF THE BLOOD AND LYMPH 141 



In addition to its marked power of rhythmical contraction, 

 the cardiac muscle is distinguished from ordinary skeletal muscle 

 by other peculiarities. It used to be considered the most striking 

 of these peculiarities that ' it is everything or nothing with the 

 heart '; in other words, that the heart muscle, when it contracts, 

 makes the best effort of which it is capable at the time ; a weak 

 stimulus, if it can just produce a beat, causing as great a con- 

 traction as a strong stimulus. Recent work, however, has indi- 

 cated that this property 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 con- 

 siderable 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. 



A more characteristic property of the cardiac muscle than 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 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 in- 

 capable 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. 57). 



In man, extra systoles followed by compensatory pauses may 

 occur under pathological conditions, giving rise to an important 



