74 
Transactions of the Royal Society of South Africa. 
phases are similar but oppositely directed to those of the variation 
previously described, that is to say the first (positive) phase is upward 
and the second downward. 
When the electrodes from base and apex are transferred to the right 
and left edges of the ventricle respectively, the one variation indicates 
transmission of excitation to the right, the other to the left. 
The details of the curve compounded of the two variations depend 
on the position of entrance of the stimulus relative to the position of 
the leads and to the rate of propagation from it to the respective 
leads. 
The following experiment gives a part of the evidence on v^hich this 
view is based. 
A tortoise was killed, and the ventricle isolated and preserved for an 
hour in Ringer's solution. Non-polarizable electrodes were applied to 
the right and left edges of the quiescent ventricle and a point on the 
dorsal surface between the leads excited by break induction shocks. 
After a time the electrical responses were found to be alternating, the two 
forms a and h following each other as in figure. Response a represents 
one diphasic variation produced by propagation of excitation from the 
point of stimulation to the right, and in this case transmission of excitation 
has not, for some reason, taken place towards the left side of the ventricle. 
Response h represents both diphasic variations summed, and in this case 
excitation has been propagated both to right and left. If curve a is 
subtracted from curve h the ordinate difference c gives the form of 
the other diphasic variation representing propagation of excitation to 
the left. 
