1002 
DR. W. H. GASKELL ON THE RHYTHM OF THE HEART OF 
the half-rhythm when it is clearly due to a diminution in the excitability of the ven¬ 
tricular muscle, points unmistakably to the conclusion that the half-rhythm caused by 
clamping, in which, as has been shown, the ventricle contracts with greater force than 
before the clamp was tightened, is not due to any diminution of muscular excitability 
caused by the action of the clamp. This is further shown by the following facts :— 
1. The effect of tightening the clamp upon the rhythm of the ventricle is the same 
in kind, whether the clamp be near the ventricle, as in the auriculo-ventricular groove, 
or far from, it, as at the junction of the sinus and auricles ; i.e., whether the clamp be 
m a position where it might possibly lower the excitability of the muscle by direct 
injury, or so far removed as to make it impossible to affect the muscle directly. 
2. When the clamp is placed across the middle of the ventricle the base may continue 
to beat with the same rhythm as the auricles, while the apex beats synchronously with 
every second, third, fourth, or more beats of the base; and it seems impossible to 
imagine that a direct mechanical injury across the middle of a muscle should so 
markedly lower the excitability of one side without perceptibly diminishing that of 
the other. 
3. It frequently happens, as already mentioned, that a standstill of the ventricle 
occurs immediately upon the clamp being tightened, but that after an interval the 
ventricle commences to beat again ; if, therefore, the clamp acts by diminishing the 
excitability of the ventricular muscle, this recommencement of the beats must be due 
to a gradual recovery of that excitability from the first depressing effect of the 
increased compression; this recovery of the excitability ought therefore to be capable 
of measurement by the method of sending single induction shocks through the muscle 
at regular intervals, after it has been rendered quiescent by tightening the clamp in 
the auriculo-ventricular groove. I have made this experiment, and with single shocks 
sent through every 3 or 5 seconds with the secondary coil carefully regulated so as just 
to give a contraction at every shock, I could find no evidence whatever that a weaker 
stimulus was sufficient to produce contractions synchronous with the stimuli, even 
after a length of time much greater than is ever seen between the time of tightening 
the clamp and the recommencement of spontaneous beats. 
For these different reasons, then, it is most probable that the compression of the 
clamp does not alter the rhythm of the ventricle by lowering the excitability of its 
muscular tissue, and we may infer that the same conclusion holds good in reference to 
the effect of heating the auricles and sinus. Another explanation must therefore be 
found. 
Since the experiments of Marey and others it is recognised as a fact that every 
contraction of the ventricle is accompanied by a marked diminution of the excitability 
of the muscle, so that if minimal stimuli be applied to the ventricle at too rapid a rate 
the muscle will not be able to contract to each stimulus, because it has not had time 
to recover sufficiently from the loss of excitability caused by the previous contraction. 
For this reason, then, we can conceive that any influence like heating the auricles and 
