ELECTRICAL RESPONSE OF CARDIAC MUSCLE. 443 



Certain points in respect of which cardiac and skeletal muscle seem to 

 differ, will be referred to in the concluding paragraph of this section. In the 

 meantime we are justified by the analogies indicated above relating to the 

 mechanical phenomena of the response, in stating that, for their elucidation, 

 cardiac muscle is quite as available as skeletal. There are, indeed, several 

 reasons why the former can be referred to with greater advantage ; the long 

 duration of the ventricular systole, and the facility with which the organ can 

 be preserved in a normal state after removal from the animal, being conditions 

 extremely favourable to exact observation. If, for example, the theory that is 

 now entertained by many physiologists, that the character of the second part 

 of a muscular contraction is affected by the stimulating influence of resistance 

 or other conditions present during the first part, we should expect to be able 

 to observe this more readily in the deliberate systole of the heart than in the 

 almost instantaneous twitch of a muscle. So far as I am aware ; 110 suggestion 

 of the kind has been offered. 



RATE OF PROPAGATION. 



It was discovered by Engelmann in 1875, 1 that the change by which 

 each muscular element of the ventricle passes from the unexcited state 

 into that of excitation is propagated from element to element with 

 equal facility in every direction. This he showed to be the case by 

 the following experiment : The ventricle of a freshly-killed frog is 

 separated into a number of small pieces, united to one another by 

 bridges of muscular substance not more than a millimetre wide, and is 

 left for half an hour or so in a moist chamber, in order that the 

 living tissue may recover from the immediate effects of the injury. It 

 is then found that a contraction wave proceeds with equal facility in any 

 direction, from any portion which has been stimulated. In a ventricle 

 so injured, the rate of conduction was not more than 20 min. per second ; 

 but Engelmann recognised that in uninjured tissue the rate must be 

 much greater. By recently published experiments, he has now shown 

 that, when the circulation is going on under normal conditions, it may 

 be more than 150 mm. per second in the auricle, and rather less than 

 this in the ventricle. 2 



In other experiments with the "suspended" heart, in which the 

 organ was removed from the body, the rate was much less. By remov- 

 ing it, however, Engelmann was able to show that cardiac muscle 

 deprived of its power of visibly contracting by immersing it in 0'2 

 per cent. NaCl, still retained its power of propagating the excitatory 

 state. 



THE ELECTRICAL PHENOMENA OF THE RESPONSE. 



Historical. In 1856, Kolliker and H. Muller 3 made the discovery 

 that the "negative variation" can be observed in the beating heart. They 

 applied the nerve of a rheoscopic limb to the pulsating ventricle, and 

 found that whether the nerve was laid across the ventricle, or in the 

 direction of its length, the muscle of the preparation contracted a 

 "scarcely preceptible time before the systole." In certain cases a 

 second feebler twitch was observed in the beginning of the diastole. 

 This they considered to be dependent on a " positive variation " of the 



1 Engelmann, Arch.f. d. ges. PhysioL, Bonn, Bd. xi. S. 465. 



* Ibid., 1894, Bd. Ivi. S. 149. 



3 Verhandl. d. phys.-med. Gesellsch. in Wurzbury, Bd. vi. S. 528. 



