36 Mr Mines, On Pulsus alternans. 
(2) that it could be abolished temporarily by stimulation of the 
vago-sympathetic trunk ; 
(3) that during the course of alternation there was a relation 
between the beats such that if the even series got smaller, the odd 
series got larger, and vice versa. 
After describing and illustrating the last pomt Gaskell con- 
tinued as follows: 
“Now we know from the experiments of Bowditch that the 
force of the ventricular contractions is independent of the strength 
of the stimulus. The explanation, therefore, of this alternation in 
the force of the contractions must be sought for in the muscular 
tissue itself, and it seems to me that the most probable explanation 
is that a larger amount of tissue contracts when the beats are 
large than when they are small, and that, therefore, in all prob- 
ability, certain portions of the ventricle respond only to every 
second impulse, while other portions respond to every impulse. 
Tke observations of Aubert show that by the direct action of 
a blow a circumscribed area of the ventricular muscle can be 
made to remain quiescent, while the rest of the ventricle is 
contracting rhythmically. 
“JT am inclined, therefore, to suggest that, owmg to some 
cause in the manipulation, such as cutting open the ventricle, 
or some other cause which affects the ventricle unequally, the 
excitability of the ventricular muscle is at the time not absolutely 
the same throughout, so that, although the impulses remain the 
same in strength, yet certain parts which possess a lower excitability 
are able to respond only to every second impulse, while the rest of 
the tissue responds to every impulse. In this way, if the strength 
of the contractions depends upon the amount of tissue contracting, 
we see not only that every second beat must be larger, but also 
that the size of each strong contraction must vary inversely as the 
size of each corresponding weaker contraction.” 
The quantitative expression for the excitability of an irritable 
tissue towards a particular variety of stimulus is the inverse of the 
strength of stimulus needed to excite it. If a piece of ventricular 
muscle is acted upon by stimuli, whether these be electric or 
whether they be auricular excitations, the muscle will respond 
to every stimulus, provided that a certain relation exists between 
the strength of each stimulus and the degree of excitability of the 
muscle when it arrives. But since the excitability of the muscle 
is greatly depressed after the beginning of each excitation, and 
increases after a time gradually, at first rapidly and then more 
slowly, it is evident that if the stimuli arrive too frequently only 
every alternate stimulus will find the heart muscle in a condition 
in which it can be excited (Hofmann, 1901). In a similar way, if 
stimuli of a certain strength arrive at a frequency such that each 
