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PHYSIOLOGY 
If one examines a sagittal cross-section of 
a typical mammalian heart (Figure 1), bound- 
aries between depolarized (stimulated) myo- 
cardium and resting (unstimulated) myocar- 
dium may be established at any instant during 
QRS. This has been accomplished by im- 
paling the heart with numerous multipolar 
electrodes from which characteristic electrical 
potentials are recorded when the wave of de- 
polarization reaches them. 
In animals of category I (carnivores 3.8,9,11 and 
primates/2) ventricular depolarization proceeds 
with 4 general fronts of activity. During the 
initial portion of QRS (boundaries labeled 1), 
the apical-third of the interventricular septum 
is excited from each ventricle toward the middle 
i I 
Figure 1. — Schematic drawings of the sagittal cross-section of the hearts of animals in two major categories. 
Boundaries between depolarized and resting myocardium at 4 instants during ventricular deplorization are 
labeled 1 through 4. To the right of each heart is a schematic drawing of the left ventricular free-wall showing 
the degree of penetration of Purkinje fibers. The bottom free-wall is divided with the top half representing 
Purkinje fibers penetrating the entire endocardial to epicardial distance, while the bottom half shows pene- 
tration only to the subepicardium, Y and Z axis electrocardiograms appear to the right, with arrows and time 
in msec, after onset of QRS corresponding to the boundaries identified in the hearts to the left. Y axis refers 
to the craniocaudal, and Z axis to the dorso-ventral axes of the torso. 
