THE HEART BEAT. 539 



The Electrical Variation. — The contraction of the heart mus- 

 cle, Hke that of skeletal muscle, is accompanied or preceded by 

 an electrical change. That is, where the muscle substance is in 

 contraction its electrical potential is different from that of the 

 resting muscle. The advancing wave of contraction causes a 

 corresponding electrical change or, to be more accurate, the ad- 

 vancing wave of excitation, which precedes the actual contraction, 

 is accompanied by an electrical change. If two points of the heart 

 are connected with an electrometer an electrical current will be 

 shown, since the electrical change will affect the electrodes at 

 different times. This electrical variation of the contracting heart 

 muscle may be shown easily by means of the rheoscopic muscle- 

 nerve preparation (see p. 106). If the heart is exposed and the 

 nerve of the preparation is laid over its surface each ventricular 

 systole is accompanied by a kick of the muscle, since the nerve by 

 connecting separated points acts as a conducting wire for the 

 current generated, and is stimulated, therefore, at each systole. 

 Since the muscle-nerve preparation gives only a simple contrac- 

 tion for each ventricular systole, we may assume that this latter 

 contraction is itself simple, — that is, due to a single stimulus. 

 The electrical variation may be obtained also by means of the 

 capillary electrometer or the string-galvanometer (p. 101), and 

 since the movement of the mercury or of the string in these in- 

 struments may be photographed, the results can be studied in 

 detail. Owing to the sensitiveness of the instrument, the beat 

 of the human heart may be registered in this way (Waller) when 

 the right hand, giving the potential changes of the base of the 

 heart, is connected with one electrode, and the left hand (apex 

 of heart) is connected with the other. The electrocnrdiograms 

 thus obtained photographically show that, in the ventricle at 

 least, the electrical variation exhibits several phases, and the char- 

 acter of these phases, that is, whether the base or the apex first 

 shows a negative potential, has been used in discussions upon 

 the direction of the wave of contraction. In Fig. 228 is given 

 an illustration of a human electro-cardiogram obtained by con- 

 necting the right and left hands with the electrodes of a string 

 galvanometer. With such an arrangement or "lead" the elec- 

 trode in the right hand may be regarded as leading off from the 

 auricular end of the heart, while that in the left hand leads off 

 from the apex of the ventricle.* As the galvanometer is arranged, 

 a negativity (indicative of excitation) toward the auricular end 

 is shown by a movement .above the horizontal base line, while 



* For a description of the Electrocardiogram and the htcrature consult 

 James and Williams, "American Journal of the Medical Sciences," Nov., 1910, 

 or Ivraus and Nicolai, "Das Elektrokardiogramm," Leipzig, 1910. 



