Mr Mines, On Pulsus alternans. on 
stimulus causes a ventricular contraction, it is evident that by 
lowering the excitability of the muscle, the whole cycle of changes 
in excitability of the muscle may be carried out at a lower level 
than before, so that, even without supposing that the process of 
recovery of excitability goes on slower in the depressed than in the 
normal tissue, it is clear that it will have to go on longer before 
the excitability reaches the value at which the stimulus becomes 
liminal. And if the time taken to reach this value is greater than 
the interval between two stimuli, it is evident that every second 
stimulus must fail to excite. The range of muscular excitability 
over which this state of affairs will hold for a given strength of 
stimulus is considerable. In such cases the “half-rhythm” is 
due to the refractory phase of the muscle. In other instances it 
may be attributed to a property possessed by all excitable tissues— 
the power of summation of stimuli. If the excitability of the 
muscle is below a certain grade, a stimulus which is subliminal 
may yet so raise the excitability of the muscle as to render the 
same stimulus on repetition liminal. It appears certain from the 
experiments of von Basch (1880) and others that this is the 
explanation of many cases of “half rhythm.’ For our present 
discussion it matters little which of these explanations holds in 
any particular case, though it is important to note that the exist- 
ence of these two well defined mechanisms, either of which may be 
responsible for “half-rhythm” and both of which are known to 
produce it, increases the chances of the occurrence of this type 
of relation between heart muscle and a rhythmic succession of 
stimuli in any particular instance and extends the range of modi- 
fication of the excitability over which “half-rhythm” may occur. 
That a condition of depressed excitability atfecting part of the 
ventricular muscle is indeed responsible for alternation, is strongly 
supported by the fact that alternation is removed by just those 
methods which improve the excitability of the heart muscle. Thus, 
for example, bathing with Ringer’s solution* or stimulation of 
the vago-sympathetic, as Gaskell showed, produces just the effects 
demanded by the hypothesis. 
Fig. 1 shows an instance in which alternation, here much 
more marked in the electrogram than in the mechanical record, 
was abolished after stimulation of the sinus venosus. 
A peculiar and instructive case is that shown in fig. 2. 
Examination of the two upper lines of the tracing near the 
beginning, the records of the auricular and ventricular contrac- 
tions respectively, would suggest that it is a case of partial 
auriculo-ventricular block. But the electrogram of the ventricle 
* Cf, Mines, Proc. Camb. Phil. Soc. Vol. xvt. (1912), Plate 6. 
