THE FROG, AND ON THE ACTION OF THE VAGUS NERVE. 
1001 
synchronously with every second beat of the auricles, and. upon loosening the clamp 
the original sequence of beats returns. 
Finally, in some cases heating the auricles and sinus causes the ventricle more or 
less suddenly to cease beating and remain quiescent in the relaxed condition, although, 
as will be shown hereafter, we have reason to think that the motor impulses are still 
passing to the muscle; similarly, when the clamp in the auriculo-ventricular groove is 
tightened sufficiently the ventricle remains quiescent, even although there is proof that 
the impulses have not been entirely prevented from reaching the muscle (see Plate 69, 
figs. 17 and 18). 
Again, it is to be noticed that when the ventricle is made to beat synchronously 
with every second beat of the auricles, either by heating the auricles and sinus as in 
Plate 67, fig. 1), or by tightening the clamp as in Plate 67, fig. 3, the slower contrac¬ 
tions are always larger than the previous ones, and this greater force of the contraction 
is only partly, at all events, due to the fact that with a slower beat the muscle con¬ 
tracts from a position of greater relaxation than with a quicker one. The figures, it is 
true, show that the lever does fall lower between the contractions when the ventricle 
is beating slowly than when it is beating quickly—a fact which proves that, with the 
ordinary rate of beating, the ventricle has not time to relax to its full extent before 
another contraction commences—but they also show that the lever rises above the level 
of the previous quicker beats ; in other words, the slower beats are accompanied by an 
increase in the force of the contraction as well as by an increase in the relaxation of 
the muscle between the beats. 
The similarity between the effects of clamping and those of heating the auricles and 
sinus upon the rhythm of the ventricle is so marked and consistent, that we may fairly 
conclude that any explanation which is sufficient for the one phenomenon must also 
resolve the other. 
What factors, then, can be conceived as common to these two methods by which 
such similar results may be produced ? 
In the first place, it may be argued that the excitability of the ventricular muscle 
has been diminished by the clamping, and therefore the ventricle can only respond once 
to every two impulses. The possibility of such a causation for the production of the 
half-rhythm is shown by the fact that curare or muscarin, when applied to the ventricle 
alone, does ultimately cause that muscle to beat synchronously with every second 
auricular beat, without any alteration of the rate of the auricular contractions. 
It is, however, to be noticed that these poisons never produce this effect until they 
have acted so powerfully upon the muscular tissue of the ventricle as markedly to lower 
the strength of its contractions; that, in fact, as one would expect, any method by 
which the excitability of the muscle is lowered to such an extent that it is only able to 
respond to every second impulse, must at the same time greatly diminish the strength 
of the contractions of that muscle. 
This fact that a diminution of the contraction force accompanies the production of 
6 M 2 
