ACTION - OF THE EXCISED MAMMALIAN HEART. 
225 
we could roughly estimate excitability by the greater or less energy with which a 
mechanical stimulus is required to be applied to provoke contraction ; with what we 
have termed “ acute ” excitability, the lightest superficial touch was sufficient to 
discharge a beat; with “obtuse” excitability, pricking or scratching with a needle 
was required to produce any effect; between these two extremes we could and did 
test gradations which justified our statement. To the second feature, viz., length of 
latent period, we paid particular attention ; a summary of our results is given in 
Table E, from which it appears that the latent period is prolonged with prolongation 
of the heart’s contraction. There is a rough relationship between the two magnitudes, 
the former being to the latter about one-tenth; but the relationship is only a rough 
one, owing to the fact that when the contraction is very prolonged it is impossible to 
determine where it ends on the tracing, and its true time-value cannot, therefore, be 
exactly given. But the chief point is unmistakably clear : the latent period of 
stimulation may be extraordinarily prolonged, a very usual length being about '5 sec., 
and am exceptional maximum being as great as 2 secs, (see Tracing 8). This, as far as 
Tracing 8. 
Rabbit’s heart; effects of cold and heat on latent period ; in the cooled heart the latent period is about 
2 seconds, the ventricular contraction rises slowly to its maximum; in the case of the warmed heart 
the latent period is very short, and the ventricular contraction very rapidly reaches its maximum; 
the decline is in both cases gradual, most so in the case of the cooled heart. The heart was placed 
in a porcelain capsule and cooled or warmed by being surrounded by ice or with water at 40° C. 
X denotes instant of stimulation. 
we know, exceeds any value that has been observed for the Frog’s heart, in which the 
latent period is usually between T and ‘2 sec. 
The above remarks are applicable to the ventricular beat. We have also made one 
or two observations on the auricular beat with analogous results, viz., prolongation of 
the latent period and of the duration of contraction. 
That lowered temperature is the chief factor in the prolongation of the latent period 
as in that of the duration of the contraction, we learned by experimentally varying the 
temperature, alternately cooling and warming the heart. 
The effects as regards the latent period are especially striking; the same heart may 
respond to a stimulus only after a period of one second or more while it is in a 
relatively cold medium (12° to 0° C.), and when this is replaced by a relatively warm 
medium (38° to 40° C.) response becomes almost immediate* 
* We use this indefinite expression because our tracings were taken on a slow-rate cylinder, and 
MDCCCLXXX VII. — B. 2 G 
