ROBERT L. HAMLIN AND C. ROGER SMITH 
611 
of the septum ; but the contribution of the left 
ventricle is greater than that for the right. This 
activity generates Q-waves in leads Y and Z, 
since the exploring electrode of each lead "sees" 
a boundary of negativity "leaving" it. 
Next, as the activation process is expedited 
to the end-points of Purkinje fibers penetrating 
from 14 to 1/3 of the endocardial to epicardial 
distance into both ventricular free-walls, the 
boundary of activity (labeled 2) proceeds in a 
general endocardial to epicardial direction 
through the entire ventricular mass, as well as 
ascending the interventricular septum in an 
apico-basilar direction. This activity produces 
little electrical disturbance in the Y axis lead, 
but a large negative deflection in the Z axis lead 
since it is "seen" as a large negative boundary 
"leaving" that electrode. 
During the instant when the deflection of 
greatest magnitude is inscribed on the electro- 
cardiogram, a large boundary of activity (la- 
beled 3) traverses the thick- walled left ven- 
tricle in a subepicardial to epicardial direction, 
as well as continuing in an apico-basilar direc- 
tion up the middle to basilar third of the inter- 
ventricular septum. This activity produces a 
large R-wave in the Y axis lead and a continuing 
negative deflection in the Z axis lead. 
Lastly, the base of the interventricular sep- 
tum and bases of both ventricles are activated 
in an apico-basilar direction (labeled boundary 
4). This activity is responsible for generating a 
R-wave in the Z axis lead ; and, depending upon 
its spatial orientation to the Y axis lead, either 
a S-wave, a r-prime, or merely a "sloppy" de- 
scending limb of the R-wave. 
Animals classified in this group all have QRS 
complexes in the Z axis lead of the Qr contour. 
Animals in category II have a Z axis QRS 
of the qR pattern. Animals in category IIA 
have a Y axis QRS of the QS or rarely qrS; 
while animals in category IIB have a Y axis 
QRS of usually a qRS. 
In animals of category IIA (ruminants 
and birds,^^) ventricular activation proceeds with 
only 2 general fronts of depolarization. During 
the initial portion of QRS, the apical third of 
the interventricular septum is activated in pre- 
cisely the same manner as that in animals of 
category I, and q-waves are generated in both 
Y and Z axis electrocardiograms. 
During the remainder of QRS, a boundary 
ascends the interventricular septum in an apico- 
basilar direction. This activity generates a 
R-wave in the Z axis lead and usually a S-wave 
in the Y axis lead. 
Both ventricular free-walls are excited with 
a burst of activity in species of this category 
because of the complete penetration of the Pur- 
kinje system from endocardium to epicardium.^^ 
Because of anastomoses between first or second 
generation branches of both main bundles in the 
apex, little or no alteration in ventricular acti- 
vation occurs even when one sections either 
main bundle.* In birds, Davis describes pene- 
tration of Purkinje fibers from epicardium to 
endocardium following major intramural cor- 
onary arteries into the transmural depths. 
However, because of the complete transmural 
penetration, even though from epicardium to 
endocardium, free-wall activation in birds sim- 
ulates that in ruminants. 
In animals of category IIB (equidae,^^ ce- 
taceae,^^ pinnapeds,^ possibly swinej^") ventricu- 
lar activation proceeds with the same two gen- 
eral fronts as in species of category IIA; but, 
after the initial septal activation and at the time 
the apical to middle third of the septum is being 
activated, an intermediate boundary excites the 
epicardum of the left ventricular free-wall. This 
activity generates a R-wave in the Y axis elec- 
trocardiogram, usually of low magnitude and 
short duration. 
The genesis of this activity is equivocal, and 
we propose three possibilities. If the thin epi- 
cardial layer of the left ventricular free-wall 
were void of Purkinje tissue ;2i'22 or, if Purkinje 
tissue were present but conducted at a slower 
velocity because of it being of small diameter,^^ 
then a front of sufficient magnitude to contrib- 
ute to genesis of QRS might be produced by 
activity of this zone. 
The alternative explanation is that even 
though the Purkinje fibers do penetrate the en- 
tire endocardial to epicardial transmural di- 
mension, because they conduct at a velocity of 
from 2 to 5 M./second,^^ time skewing in activa- 
tion between endocardial and epicardial layers 
might produce a boundary between these layers. 
