288 Transactions of the Royal Society of South Africa. 
The curves in this series are exphcable on the assumption that the two 
components have each a form similar to diagram 20 of Fig, 6, but may 
also present irregularities or oscillations, such, for example, that there 
may be a rise following B and exceeding it in height. 
Fig. 7 gives a series of responses obtained from the ventral surface of 
the same ventricle. The stimulating electrodes are applied in this case at 
the right margin of the ventricle and the right electrode is placed first close 
to that point and then moved in successive steps along the transverse 
diameter to the left margin. The left electrode remains at the middle 
point. The reversal of T is well seen in this series as the variations 
change in time relations. 
In Fig. 8 we have a series of records from the same ventricle with the 
following arrangement. The non-polarizable stimulating electrodes are 
applied at the left margin. The left leading-off electrode is at the 
right margin, and the right electrode is moved by successive steps from 
a position close to the point of stimulation towards the right margin. 
In curve 1 Q,., the positive deflection of the right variation is not evident. 
It is present in curve 2, and as the right electrode is removed farther from 
the point of stimulation this deflection increases in breadth and amplitude. 
It would appear that the positive change becomes more evident the farther 
the wave of excitation has to travel before reaching the proximal electrode, 
and it may be suggested that this is due to a difference in the rates of 
conduction of the positive and negative changes. 
In Fig. 9 the records are obtained by placing the non-polarizable stimu- 
lating electrodes on the dorsal surface of the ventricle at the middle of the 
base and the left leading-olf electrode on the dorsal surface at the apex. 
The distance between the point of stimulation and the left electrode 
is 34 mm. The right electrode is placed on the dorsal surface 2-6 mm. 
from the point of stimulation on the line joining the middle of the base 
with the apex. It is then shifted in successive steps of 2-6 mm. each 
towards the apex, and the response to a break induction shock is 
recorded at each position. 
In Fig. 10 the stimulation is given on the dorsal surface at the apex, 
and the left electrode is placed at the middle of the base. The right 
electrode is applied 3 mm. from the point of stimulation on the line 
joining apex and base, and is then moved by successive steps of 3 mm. 
each towards the middle point of the base. It will be observed that 
in the earlier records of this series, where the right electrode is applied 
near the apex, the curve does not show a downward summit as its initial 
deflection, but the initial movement of the string is upward. As the right 
electrode is moved farther from the apex the summit becomes evident, 
and in curve 7 it forms the initial deflection. This may be compared with 
responses obtained from other tortoise hearts. In Fig. 11 the non-polar- 
