274 



THE CIRCULATION OF THE BLOOD 



Although such comparisons give us considerable insight into the cause 

 of several of the waves, there yet remain certain peculiarities of the 

 electrocardiogram to be considered. These are: (1) the cause of the 

 slight downward deflection, Q; (2) the cause of S; (3) the cause for the 

 period of equal potential during ventricular systole indicated by the 

 portion of the curve between S and T; (4) the cause for the wave, T. To 

 solve these problems it is necessary to compare electrocardiograms taken 

 from the surface of the body with those from a series of electrodes placed 

 directly on the ventricle of the exposed heart. 



I i : : 



Fig. 83. Electrocardiogram (dog) taken simultaneously with curves from auricle and ven- 

 tricle. It will be observed that wave P slightly precedes auricular systole and that wave R occurs 

 just before the presphygmic period starts in the ventricle. (From Lewis.) 



The Ventricular Complex 



The researches of Lewis 64 have shown that the spread of the excita- 

 tion wave in the ventricles is not primarily along muscular pathways 

 but occurs through the divisions of the auriculo-ventricular bundle and 

 the subendocardial arborizations of Purkinje. Since there is no con- 

 nection between the arborization of one ventricle with those of another, 

 the course of the wave is distinct in the two chambers. It travels at 

 first downwards over the septum in each ventricle to the apex and sub- 

 sequently ascends upon the endocardial surface of the free wall to ter- 

 minate at the basal attachment of the latter. The spread through the 

 specialized conducting tissue is very rapid (about 5000 mm. per sec.) 

 the wave taking about .04 seconds to complete its journey over the en- 

 docardial surface. The wave travels more slowly (400 mm. per sec.) 

 through the ventricular muscle, its spread being through the muscle 

 wall from endocardial to pericardial surface and at right angles to its 

 course along the subendocardial network. 



