THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 287 



electromagnet. The deviations which the string of this instrument 

 suffers in the course of each cardiac cycle, may be projected and photo- 

 graphed upon sensitive paper moved with proper rapidity. The 

 record so obtained is known as the electrocardiogram, and the complex 

 apparatus necessary to take these \racii^s, as the electrocardiograph. 

 This method of studying the chaM^- of the cardiac contractions 

 has attracted much attention in recSit years; in fact, it has been so 

 highly developed that it may be employed as an important diagnostic 

 aid in ascertaining the functional 

 capacity of even the human heart. 

 While the currents produced by 

 this organ are of very moderate 

 strength, the modern type of elec- 

 trocardiograph has been renderied 

 sufficiently sensitive to detect them 

 with ease. As Waller^ has shown 

 years ago, it is quite unnecessary 

 to expose the heart to the view, 

 because the current generated by it 

 can be led off to the galvanometer 

 by simply applying the terminals of 

 this instrument to the integument. 

 In the human subject, the elec- 

 trodes are usually connected with 

 the body in three ways, designated 

 as leads, namely: 



Lead I. — Right arm and left arm. 

 Lead IL — Right arm and left leg. 

 Lead III. — ^Left arm and left leg. 



Fig. 145. — Distribution of Poten- 

 tial Differences Due to Electrical 

 Variations in the Beating Heart. 

 (Waller.) 



To record the variations any of the 

 points a may be led ofiE, together with 

 any of the points b. 



In the first case in which the two 

 hands are connected with the poles 

 of the string galvanometer, the 

 right one may be regarded as the 



conductor which leads off from the base, and the left one, as the con- 

 ductor which leads off from the apex of the heart. 



Regarding the general outline of the normal electrocardiogram and 

 the causes of its different minor phases, some uncertainty still prevails. 

 Figure 146 represents the electrocardiogram most commonly obtained 

 from normal human subjects. It is readily observed in the curve of 

 Lead I that each cardiac cycle begins with a slight wave which has been 

 designated by Einthoven as the P-wave (presystolic). Subsequent 

 to this point, the string either retains its position of zero or is deviated 

 somewhat below the base line. This primary deflection is due to the 

 contraction of the auricles and is spoken of, inclusive of the presystolic, 



1 Philos. Transact., 1889, 180. 



