E. LEPESCHKIN 
653 
in humans. The mouse and rat also have the 
advantage of low cost and upkeep and are com- 
parable to humans as far as excitation of the 
heart is concerned but the repolarization process 
and the T wave as well as the excitation-con- 
traction coupling seem to depend on a different 
ionic mechanism than in the human heart and 
are not comparable to the latter. The rabbit and 
guinea pig show the greatest similarity to hu- 
mans in these respects. The cat and dog are also 
fairly similar to man with regard to these char- 
acteristics, but may show considerable differ- 
ences as far as repolarization sequence and T 
waves are concerned. The ungulates have the 
advantage of a larger heart size but their se- 
quence of ventricular activation is completely 
different, and the results concerning intraven- 
tricular conduction disturbances in these ani- 
mals do not apply to man. In all these animals 
the position of the heart is different from man, 
and findings in the limb lead electrocardiogram 
cannot be compared to those in humans. The 
only animals which are completely similar to 
man in all respects are those primates which 
normally have an upright body position, 
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