Mr Mines, On Pulsus alternans. 39 
auricles and ventricles, make it easy to imagine that this tissue 
may be. affected differently from the rest of the musculature of 
the heart by a change of external conditions to which the whole 
musculature is exposed. But the ventricular muscle does not, so 
far as we know, present any regular differences in its various parts; 
though it is always to be expected that in any collection of excit- 
able cells some will be above and some below the .average of 
excitability. If the average excitability of the ventricular muscle 
is gradually depressed, some portions of it, those namely below the 
average excitability, will reach the condition in which they assume 
half-rhythm before the main part of the musculature has reached 
this condition. If these fibres are directly connected with each 
other the chances are in favour of their contracting in response to 
the same series of excitations, odd or even. For under these 
circumstances, if we consider the case of two fibres A and B, 
A may respond to excitation, reaching it by way of B or by 
perhaps one or two other routes. If in the even series of excita- 
tions of the ventricle in general, B fails to be excited, it is plain 
that A has a greater chance of receiving a successful stimulus 
during the odd series, when excitation arrives by way of B as well 
as by other routes. And, of course, where a number of contiguous 
fibres are concerned these may surround other fibres which are 
never reached by excitations except those coming by way of the 
affected fibres. In such cases it will be true to speak of heart 
block in the ventricular walls; fibres isolated in this way will of 
necessity follow the rhythm of their neighbours, whether their own 
excitability is depressed or not. 
But there is no justification for the assumption that when the 
excitability of the ventricular muscle is gradually depressed, the 
fibres which are below the average excitability of the muscle at 
the time and in which half-rhythm develops will all be situated 
at a single focus. Much more likely is it that they will be 
distributed in various regions. And whether there are two foci 
or twenty, each one will respond either to the odd or to the even 
series of excitations. The chances are obviously against all of the 
foci responding to the odd or all to the even series. And if there 
are more than two foci, the chances are against their being equally 
divided between the two series—though both of these conditions 
are possible and may be expected to arrive occasionally. 
To put the position tersely we may use symbols. Let V be the 
whole ventricular muscle and v the portion of it which is depressed 
in excitability so as to be capable only of the half-rhythm. 
Then on the view of Muskens and Hering the series of beats 
runs thus, 
Tes We el =a OO (1), 
