ELECTRICAL RESPONSE OF CARDIAC MUSCLE. 445 



incidence of the terminal phase almost immediately after the end of the 

 systole, that it marked the duration of the electrical change which is 

 a concomitant of functional activity. To ascertain whether this relation 

 was a constant one, a comparison was made of the relative durations of 

 the systole and of the interval in question at different temperatures, an 

 arrangement being adopted by which the rheotome observations could be 

 repeated at temperatures varying from 12 to 20 C. It was found, as 

 was anticipated, that the duration of the interval of equipotentiality 

 always a little exceeded that of the systole. 



The interpretation applied to these observations eighteen years ago 

 has proved to be correct. As was then indicated, the fact of the 

 existence of a relatively long period of equipotentiality (over 2 seconds 

 in a cooled heart) does not necessarily imply that both contacts are 

 in a state of electrical inactivity, but may mean that the electromotive 

 forces at work at the one electrode happen to balance precisely those 

 that are at work at the other. 1 



Latent period. For the purpose of determining the earliest 

 moment at which an electrical change can be observed, experiments 

 were made in which the exciting electrodes were placed one on either 

 side of the proximal leading-off electrode. With this mode of stimula- 

 tion, the first effect was not observed until more than 0'02 sec. after 

 excitation. The period of closure was always 0*03 second. When it 

 terminated 0'02 sec. after excitation, there was no effect, showing that up 

 to that moment there was no response. 



The injured ventricle. Further proof of the long duration of the 

 excitatory electrical change was obtained by a similar mode of observa- 

 tion, the only difference being that the surface under one of the leading- 

 off electrodes was devitalised. It was already known that in the heart, 

 as in other striped muscle, an injured surface is negative to a sound one, 

 but it was by no means clearly understood that the relative negativity 

 was due, not to any abnormal activity at the injured part, but to 

 unbalanced activity of the living tissue at the opposite electrode. As 

 had already been shown by Engelmann, the resting difference of 

 potential in question (i.e. the relative positivity of the uninjured surface) 

 immediately after the injury is very considerable, amounting to yf^ of 

 a Daniell, or more ; but it soon subsides. In a heart recently injured 

 at the apex, during the whole period of the systole the sound surface 

 at the base becomes relatively negative to the devitalised part at the 

 apex ; so that the previous difference suffers a diminution to about half 

 its initial amount. 



The meaning of the prolonged negativity of the uninjured surface 

 during systole is not difficult to understand, that surface, in common 

 with all other uninjured parts of the excited ventricle, being in a state 

 of functional activity, in which the injured part alone does not partici- 

 pate. Its duration, like that of the systole and of the equipotential 

 period, varies with temperature. Inasmuch as the duration of this 

 "negative variation" can be easily measured, it affords the readiest 

 means of proving the important fact, that the excitatory electrical 

 change lasts about the same time as the systole. 



Effect of local application of heat on the variation. Local 

 inequalities of temperature may be produced in two ways, namely, 

 either by placing the heart upon a support of which one part is warmer 



1 Loc. cit., p. 398. 



