298 Transactions of the Boyal Society of South Africa. 
the right variation, and (2) the changes in the tissue whose potential is that 
of the left electrode, connected with the left foot of the subject — the left 
variation. Each variation commences with a positive deflection succeeded 
by a negative deflection. The downward direction of the summit Q of 
Einthoven is to be attributed to the positive preliminary deflection of the 
right variation — in my nomenclature ; and is due to the commencing 
electrical change in the tissue at the base and right side of the ventricles. 
B is the combined effect of the negative deflection of the right variation 
and the positive preliminary deflection of the left variation {B^ + Qi). 
S is the summit of the negative deflection of the left variation (Bi) due 
to negative electrical change at the apex and left side of the ventricles. 
The horizontal stretch following S indicates compensation of the two 
variations, and T is due to the right variation, which persists longer or 
passes off less steeply than the left variation. 
In comparing the forms of the human electrocardiogram with the 
curves in Fig. 13, it is evident that the point at which the excitation 
transmitted from the auricles reaches the general ventricular muscu- 
lature would correspond to the point of artificial stimulation of the 
isolated tortoise ventricle.* The position of this point relative to the 
right and left lead respectively will thus determine the form of 
the electrocardiogram, since it will determine the time relations of the 
two variations which are summed. The human electrocardiogram is 
not always of the same form even in normal individuals, and varies 
very greatly under pathological conditions. The form of the typical 
normal curve indicates that the two variations or phases are occurring 
almost simultaneously, but with the right variation slightly before the 
left. We would therefore conclude that the excitation reaches the 
ventricular musculature almost at the middle point but slightly nearer 
the base. Atypical curves will be recorded when the excitation begins 
in the ventricles at an abnormal point, and the two phases will be 
separated by a greater time interval. 
I do not propose to discuss here the various forms of the patho- 
logical electrocardiogram. A very large number of atypical curves have 
been recorded in the literature. In Fig. Ih there is an atypical curve 
from a human electrocardiogram which I have recently recorded in 
this Laboratory, in which it seems possible to make out the various 
summits that can be identified in the tortoise response. The curve 
has been recorded by derivation II, that is to say the right electrode 
is connected with the right hand, the left with the left foot. The left 
variation or phase is an advance of the right, which indicates that the 
excitation has begun in the ventricular muscle at a point nearer the 
apex than in the normal case. 
* Cf. Einthoven, PMger's Archiv., Bd. 149, s. 65, 1912. 
