HEART-MUSCLE 505 



This is the "compensatory pause" In other words, the rhythm of the 

 beat is not disturbable during systole, but when disturbed at any other 

 time the inherent rhythm is maintained by a pause of just the right 

 length so that the next beat occurs in the regular rhythm. 



All the metabolic processes of cardiac muscle probably take place 

 more slowly than do those in cross-striated muscle. It is on this account 

 perhaps that the heart's refractory period is so long; that the viscus can- 

 not easily be tetanized; and that there is no relation between the power 

 of the beat and the strength of the stimulus (Gaskell). The cause of the 

 refractory period is probably, in fine, then, the absence during that time 

 of energy sufficient to allow of a contraction, the immediate supply of 

 power having been used up by the systole. Indeed, according to Engle- 

 mann, the power of responding to very weak stimuli is regained only at 

 the end of the following rest (diastole). Cross-striated muscle, however, 

 has a refractory period, although it is much shorter than the heart's. 



The compensatory pause means that when the rhythm is disturbed by 

 an extra beat the heart cannot contract for a long period again because 

 the energy is used up; it takes up, however, the rhythm inherent in the 

 muscle and beats at the time this rhythm requires. 



The extra contraction during the systolic period is nowise unlike 

 that occasioned by a stimulus applied to a cross-striated muscle, there 

 being now nothing to prevent this contraction, for the heart has now 

 again an adequate supply of energy. 



Expt. 69. The Action-current Precedes the Beat. (Apparatus: 

 Heart-lever, kymograph, etc., myograph, counterpoise for heart-lever.) 

 Smoke a drum lightly, and weight the short arm of the heart-lever so 

 that it is nearly as heavy as the long arm. Set the cardiograph on the 

 base of the stand-rod and arrange the gastrocnemius muscle in the femur- 

 clamp, etc., above it in the ordinary way save that the lever is above the 

 femur-clamp. (Use the pulley on the muscle-lever so that on contraction 

 the muscle pulls the lever upward as usual, and not downward, although 

 below the lever.) Adjust the femur-clamp to the cardiograph so that the 

 nerve of the nerve-muscle preparation shall rest longitudinally on the 

 heart. Arrange the two levers to write exactly over each other on the 

 slowly rotating drum. At each beat of the heart the sciatic nerve will 

 be stimulated and the gastrocnemius will contract. Note that the cross- 

 striated muscle's record begins at each beat before the heart's record 

 begins. Apply a tuning-fork and try to measure the length of time by 

 which the action-current anticipates the actual action of the heart. (To 

 insure prompt success in this experiment the gastrocnemius and sciatic 

 must be freshly prepared and very sensitive. The result, when obtained, 

 is striking.) 



It has been already shown (Expt. 50) that when a skeletal muscle 

 contracts its action-current is an adequate stimulus to muscle like any 

 other electric current of sufficient energy. It is easier to show the pre- 

 cedence of this action current on the heart which beats "automatically." 



The impulse occasioning the beat starts at the central end of the large 



