1018 
DR. W. H. GASKELL ON THE RHYTHM OF THE HEART OF 
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 probability, certain portions of the ventricle respond only to 
every second impulse, while other portions respond to every impulse. The observa¬ 
tions 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 con¬ 
tracting rhythmically. I am inclined, therefore, to suggest that, owing 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 con¬ 
tracting, 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. 
Upon this assumption the action of the vagus admits of easy explanation, for it is 
evident that the alternation disappears during the time when we have reason to sup¬ 
pose that the maximum rise of excitability caused by vagus stimulation is produced, and 
reappears when the normal excitability is regained, or when the excitability is lowered 
below the normal by the action of the vagus. If, therefore, we look upon this alterna¬ 
tion in the strength of the contractions as due to a local lowering of excitability, so 
that certain portions contract once only to every two contractions of the rest of the 
ventricle, then it is clear that as soon as the excitability of these parts is raised suf¬ 
ficiently for them to respond to every impulse, every contraction must become as 
strong as every other contraction, for every part of the ventricle will contract to 
every impulse. On the other hand, if the general excitability is lowered there must be 
a greater tendency for this alternation to occur ; thus, it sometimes happens that when 
the ventricle is beating with contractions of equal height, stimulation of the vagus 
causes not only a lowering of the height of the contractions, but also a temporary 
appearance of alternately strong and weak beats. 
Also, when there is a great difference in size between the consecutive contractions, 
it is possible for the stimulation of the vagus to fail in increasing the excitability of all 
portions of the ventricle to an extent sufficient to cause them to respond synchronously 
with every impulse, so that the alternation does not entirely disappear in any part of 
the curve, although the difference in height between consecutive beats is not so great 
during the period of augmentation of the force of the contractions as before the stimu¬ 
lation ; in other words, the nerve stimulation has sufficiently increased the excitability 
of some portions of the affected area, though not of the whole of it. 
* Pfluger’s* Archiv, Bd. xxiv., S. 357. 
