224 
DR. A. D. WALLER AND MR, E. W. REID ON THE 
the excitability of the heart is great, and its decline rapid, at high temperature, while 
excitability is small, and decline slow, at low temperature. 
§ IV. The latent 'period of stimulation. 
Our observations show that there is a general correspondence between the duration 
of contraction of the ventricle, and its excitability, and the length of the latent period 
of stimulation. 
In general, the correspondence is such that, with the lengthening of the contraction, 
excitability decreases, and vice versd. That these effects are in the main dependent 
on temperature is shown by the fact that they can be altered at will by variations of 
the temperature of the surrounding medium. Thus we may alternately obtain with 
the same heart, 1st, long contraction, long latent period, and obtuse excitability, or 
2nd, short contraction, short latent period, and acute excitability, at (1st) lowered or 
(2nd) heightened temperature respectively. 
A heart removed from the body, and examined at 15° to 18° C., shows the 
gradually increasing changes characteristic of lowered temperature; independently 
of this factor, however, quite similar changes supervene which are to be referred to 
the natural decline of action from the life normal to the death zero. This decline is, 
as for other tissues, not instantaneous, but gradual, and characterised by gradually 
increasing sluggishness, first, of spontaneous action, and secondly, of responsive action. 
The complete elimination of the temperature factor did not enter into the plan of our 
observations, and we give no experiments in support of the above general proposition, 
although we have observations sufficient to justify our assertion of it as of an 
experienced fact, not merely of an untested truism. The fact which we desired to 
clearly demonstrate is the great sluggishness of action to which the Mammalian heart 
may be reduced before action is extinguished ; so that its processes, normally far more 
rapid than those of the cold-blooded heart, become so protracted as not only to equal, 
but to exceed, the latter in slowness of accomplishment. Our observations furnished 
the demonstration, and clearly show that the Mammalian heart is more susceptible to 
differences of temperature than is that of the Frog.'" 
Of the statements regarding the three features referred to above, viz., length of 
systole, length of latent period, degree of excitability, the first has been considered in 
§ 1 ; with regard to the third, we have no evidence to give beyond the statement that 
27°—0-9 1 
24°—1 
21°—1-3 
18 "— 1-6 
Normal duration of Frog’s systole at 15°-—1'9 
12°—2T 
Frog’s systole ten times as long at 3 n to 7° as at 18° to 20°. Hermann, vol. 4, p. 372, 
Burdon Sanderson, 
>■ ‘ Journal of Physiology,’ 
vol. 2, p. 384. 
