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PHYSIOLOGY 
Figure 3. — Electrocardiogram lead II, monitoring the cranio-caudal axis, from a normal horse. Notice the changing 
configuration of the QRS complex in 3rd, 4th, and 5th complexes from the left. The R-wave becomes progres- 
sively lower in amplitude, changing from that observed in a dog to that observed in a goat. 
Differences in forms of body surface poten- 
tials among the various species may be caused 
by differences in pathwiiys of cardiac activation 
or differences in orientation of ^he heart within 
the torso. Another possible cause, might be 
alterations in the characteristics of the body as 
a volume conductor. It is interesting to specu- 
late about the significance of voluminous air 
sacs in birds and the pendulous rumen in goats 
and cattle and the cecum in horses. In any case, 
we feel that we can explain electrocardiographic 
differences on the basis of only the activation 
process and heart position. 
SUMMARY 
The electrocardiogram has serious limitations 
in studying abnormalities of the heart in certain 
species in which inordinate degrees of cancella- 
tion occur during ventricular depolarization. 
If, in the pathogenesis of disease, alterations 
in ventricular activation are important, then 
animals with complete penetration of Purkinje 
fibers throughout both ventricular free-walls 
should not be used as models. No matter how 
severe the heart lesion, ventricular activation 
may be changed only minimally. However, these 
species may serve as useful models to study the 
effects of lesions independent from changes in 
ventricular activation. 
Larger species are of interest because of their 
propensity for developing atrial arrhythmias. 
Another interesting aspect of comparative 
electrocardiography lies with phenomena con- 
