286 THE MECHANICS OF THE HEART 



B. THE ELECTRICAL VARIATIONS 



The Action Current of the Heart. Electrocardiography. — The 



activity of any form of living substance is accompanied by the produc- 

 tion of electrical energy. We have found this to be true in striated 

 as well as in smooth muscle tissue. Cardiac muscle forms no exception 

 to this rule, because, if the heart of a frog or turtle is exposed to the 

 view and the nerve of a gastrocnemius preparation is placed upon it, the 

 muscle is seen to twitch with every systole. In this particular case, 

 the heart acts as a battery, and generates an impulse in the adjoining 

 nerve which then causes the muscle to contract. The electrical current 

 generated by the beating heart may be registered by means of suitable 

 instruments, such as the capillary electrometer, or the galvanometer. 

 Thus, if the two terminals of the former are placed upon the active 

 organ, preferably upon its base and apex, the meniscus of the mercury 

 in the capillary tube moves first in one direction and then in the other 

 in synchronism with the successive periods of activity. The same 

 result may be obtained with the help of the galvanometer, the reflecting 

 mirror of this instrument being doubly deviated with each contraction. 

 The current rendered recognizable by this means is known as the 

 current of action of the heart. It is dependent upon the fact that the 

 active portion of this organ is electronegative to the resting portion. 

 Inasmuch as the cardiac contractions begin at the base, this particular 

 area of the heart is of a lower electrical potential than its still inactive 

 apical portion. A moment thereafter, however, conditions are re- 

 versed. The apical region now having been activated, exhibits a 

 galvanometric negativity, while the basal zone which is in the state 

 of rest at this very time, becomes electropositive. In perfect analogy 

 with skeletal muscle, the action current of the heart exhibits a diphasic 

 character. This is indicated very clearly by the deflections of the in- 

 dicator of the recording instrument which occurs first in one direction 

 and then in the other. It should be added, however, that this current is 

 somewhat different from the ordinary action current of skeletal muscle, 

 its peculiarities being no doubt attributable to the much greater com- 

 plexity of the cardiac musculature. These electrical changes are 

 developed with great rapidity, so that the capillary electrometer and 

 the ordinary type of galvanometer are not sufficiently motile to follow 

 the different phases of this wave with accuracy. This difficulty has 

 been almost entirely overcome by the very sensitive string galvanom- 

 eter, invented by Ader^ and modified by Einthoven.^ The indicator 

 of this instrument is a filament of quartz or platinum covered with a 

 thin coating of silver and suspended between the poles of a powerful 



dilatation of any part of the heart also change its position. The strength of the 

 apex beat is indicative of the condition of the cardiac musculature, but only 

 when the factors previously enumerated have been properly controlled. 



^ Compt. rend., Ac. Sci., Paris, cxxiv, 1897. 



» Ann. der Physik, xii, 1903, and.Pfliiger's Archiv., cxxx, 1909, 287. 



