38 Mr Mines, On Pulsus alternans. 
(the third line of the tracing) shows that every auricular excitation 
reaches the ventricle, but apparently the alternate excitations 
spread only a very short way into the muscle, affecting so few 
ie that their mechanical response is too slight to move the 
ever. 
After stimulation of the sinus venosus, the electrical response 
of the alternate ventricular excitations which before were very small, 
greatly increases in size—indicating probably that the excitation 
process spreads further into the muscle, while the mechanical 
response makes its appearance. We have thus the appearance 
of a mechanical alternation as a result of stimulating the sinus 
venosus. This condition continues some time after the stimulation 
has ceased and then gradually disappears, the electric response 
returning finally to the condition which obtained at the beginning 
of the tracing. It is obvious that this was an extreme case of 
alternation, in which nearly the whole of the ventricular muscle 
failed to respond to the same alternate series of excitations. The 
stimulation of the intracardiac nerves reduced the grade of the 
alternation, making the alternate beats for a time more nearly 
alike im extent. 
The view propounded by Hering (loc. cit.), by Muskens (1907) 
and by several other authors is that in the condition of alternation 
the whole musculature of the ventricle contracts in one beat, while 
a portion of the musculature fails to contract in the next beat, and 
so forth. 
This view employs Gaskell’s assumption, 
(1) that part of the ventricular muscle has its excitability so 
depressed that it can respond only to every other auricular 
excitation, but it adds the further assumption, which, as I shall 
show, is not only unnecessary, but vicious, namely, 
(2) that the musculature, of which the excitability is depressed, 
responds all of it to the same excitations; ie. all of it to the first, 
third and fifth excitations or all of it to the second, fourth and 
sixth excitations, and so forth. 
The effect on the contraction of the heart as a whole, of 
depression of excitability of a portion of the musculature such 
as to cause half-rhythm im this portion, will depend on the region 
in which this musculature is situated. If it is placed on the sole 
route by which excitations reach a large tract of musculature— 
as for example in the junctional tissue between auricles and 
ventricles—the effect will be to impress the half-rhythm on this 
tract of musculature, the excitability of which is in reality 
normal. 
The differences known to exist in the mammalian heart between 
the musculature of the bundle of Stanley Kent, and that. of the 
