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TEXT-BOOK OF PHYSIOLOGY 



action of the heart and the inertia of the mercury, the exact changes of the 

 electric potential recorded are difficult of interpretation. 



By means of the highly sensitive and quickly responsive string-galvan- 

 ometer devised by Einthoven (see appendix) not only can the variations of the 

 electric potential of the heart be detected, but the deviations of the string 

 caused by the passage of the most delicate current, can be magnified and a 

 shadow projected on a sensitive surface and photographed. The record 

 thus obtained is termed 1 



The Electrocardiogram. An electrocardiogram obtained by photo- 

 graphing the oscillations of the galvanometer string during the cardiac cycle, 

 when different portions of the body surface, a and b, are connected with the 

 terminals of the string, is shown in Fig. 133, the different features of which 

 are designated by the letters P, Q, R, S, T. This curve, which represents 

 the relative strength, duration, succession and direction of the currents during 

 the cycle, it is believed furnishes a correct picture of the strength, duration, 



succession and direction of the physio- 

 logic processes the excitation and con- 

 traction processes as they arise in the 

 heart muscle during its activity. 



The interpretations of the electrocar- 

 diogram and the significance of its various 

 features are not, in some respects, in as 

 complete accord as seems desirable. In 

 general, it may be said that P corres- 

 ponds to the auricular systole and that 

 Q, R, S, T correspond to the ventricular 

 systole. R and T are always present in 

 physiological conditions at least, while Q 

 and S, may, either one or the other or 

 both, be wanting. 



Einthoven is of the opinion that the general form of the electrocardio- 

 gram indicates the path and propagation of the stimulus or the excitation 

 process through the heart, as well as the order of contraction of different 

 portions of the ventricular walls, both of which are in harmony with the 

 origin, course and ultimate distribution, of the auriculo-ventricular con- 

 duction system as determined by Tawara (see page 272). 



The record presents first a horizontal portion the image of the gal- 

 vanometer string which indicates that the string is at rest by reason of 

 the fact that all portions of the heart, during the diastolic pause are isoelectric 

 or of equal electropotential. 



P is the result of the contraction of the auricle, indicating a condition 

 of negativity toward the ventricle which is now electropositive in conse- 

 quence of which, the electric currents pass down the heart, thence to the 

 body surface, thence through the galvanometer circuit and back to the heart, 

 causing the string as they pass through it, to move outward. With the 

 cessation of the auricular contraction the string returns to its position of 

 rest. The more or less horizontal line between P and Q indicates that both 

 auricles and ventricles are now at rest and in the same state of electric 

 potential. The passage of the excitation process through the conduction 

 system, which is now taking place, from its beginning to its terminations, 



FIG. 133. SCHEME OF THE ELECTRO- 

 CARDIOGRAM. (Einthoven.) 



