296 COHN— STUDY OF THE [April 23, 



of the mammals, the species which have been studied, the horse, 

 rabbit, cat, dog and man, each shows certain definite characters on the 

 basis of which it can be recognized. Einthoven and Lewis and Gil- 

 der have studied human electrocardiograms and have defined within 

 certain limits the usual form of curves taken from normal persons. 

 But dififerentiation can go further, for the records of individuals have 

 been found to difi^er widely from person to person in health, and 

 more widely still in disease. Whatever form, either normal or ab- 

 normal, the curve assumes, this remains characteristic for the indi- 

 vidual, for a certain length of time at all events, though the records 

 be registered by different instruments and recorded by different in- 

 vestigators. An electrocardiogram may indeed attain a form so 

 peculiarly personal that the suggestion has actually been made that it 

 be employed to serve the purposes of identification in much the same 

 way as the Bertillon system does. We may therefore regard the elec- 

 trocardiogram as a valid and reliable record. 



A number of the factors which can bring about variations from a 

 normal curve are understood. Some of these may now be enumer- 

 ated. The auricular wave, in the first place, is modified by the nature 

 of the derivation or lead used ; in this case the most favorable situ- 

 ations to employ are usually the two upper extremities, but under 

 certain circumstances two points on the chest wall have been found 

 preferable. A rare and not altogether satisfactorily established de- 

 fect in the auricular mechanism is a lack of synchronicity between 

 the contractions of the two auricles. This defect has occasionally 

 been found to split the P wave. But the most significant alteration 

 occurs when the P wave, instead of being directed upward, the direc- 

 tion which in the usual arrangement is associated with primary nega- 

 tivity at the site of the sinus node, is directed downward, an alteration 

 which presumably shows that primary negativity has occurred at a 

 lower level of the auricles. This change takes place when the sinus 

 node is excluded from function, and when another part of the auricle 

 sets the pace instead, but it also occurs spontaneously as the result 

 of causes, the nature of which is not clear. It has been our good 

 fortune to observe on a few occasions a gradual transition from P 

 waves directed upward to P waves directed downward; if the apices 

 of succeding P waves in such curves are joined, a curved line results. 



