On the Interpretation of the Electrocardiogram. 
295 
tions are elicited almost simultaneously, and in the succeeding curves 
again become more and more separate, the left variation in advance. 
In curve 10 of Fig. 13, v^here the point of stimulation is close to the left 
electrode, the left variation leads and is recorded alone for about 0'75' . 
The Q summit of the left variation is not evident. The right variation 
which commences at Q,. is short and can be distinctly seen superposed 
upon the left variation, whose supposed continuation is represented by the 
interrupted line. 
The components of the electrocardiogram bear a close resemblance 
to the electrical variation recorded from the eye on stimulation by a 
flash of light. In Fig. 14 an example of a retinal variation is reproduced.* 
Fig. 15 gives a series of curves obtained from an isolated tortoise 
ventricle. The non-polarizable stimulating electrodes are applied on the 
ventral surface at the left margin, the right leading-off electrode on the 
transverse diameter 2 mm. to the left of the point of stimulation and 
the left electrode at the right margin. The right electrode is moved in 
c 
Fig. 14. 
successive steps of 2 mm. each towards the left electrode, and the response 
to a break induction shock recorded at each position. 
Fig. 16 gives a series of responses from the same tortoise ventricle 
obtained by stimulating the dorsal surface mechanically at the left margin 
by the touch of a glass point. The left electrode is placed on the dorsal 
surface at the right margin. The right electrode is at first applied about 
2 mm. from the point of stimulation at the left margin, and is then moved 
in successive steps of about 3 mm. each along the transverse diameter 
towards the right margin. 
From the results obtained from the isolated tortoise ventricle where I 
have found the rate of conduction to artificial stimulation to be from 9 to 
11 cm. per sec, it is I think possible to analyse the human electrocardiogram 
given in Fig. 1. It represents the summed effect of two variations, that is 
to say, of (1) the changes occurring on activity in the tissue whose potential 
is communicated to the right electrode at the right hand of the subject — 
* Einthoven and Jolly, Quart. Jour, of Exper. Physiol., vol. i, p. 373, 1908. 
