_Effects of Temperature on Excitability._
I shall now conclude this chapter with a brief statement of the effects of temperature on the excitability of the Medusæ; and before stating my results, I may observe that in all my experiments in this connection I changed the temperature of the Medusæ by drawing off the water in which they floated with a siphon, while at the same time I substituted water of a different temperature from that which I thus abstracted. In this way, without modifying any of the other conditions to which the animals were exposed, I was able to observe the effects of changing the temperature alone.
With regard to the effect of temperature on the latent period of stimulation, the following table, setting forth the results of one among several experiments, explains itself.
Period of latent stimulation of the deganglionated tissues of Aurelia aurita as affected by temperature:--
-------------------------------+------------------------------- Temperature of water (Fahr.). | Period of latent stimulation. -------------------------------+------------------------------- 70° | 1/5 second 50° | 1/3 second 35° | 2/5 second 20° | 1/2 second -------------------------------+-------------------------------
In the case of each observation, several shocks were administered before the latent period was taken, in order to decrease this period to its _minimum_ by the staircase action. When this is not done, the latent period at 20° may be as long as 1-1/5 seconds; but soon after this irritability disappears.
The extraordinary sluggishness of the latent period at very low temperatures is fully equalled by the no less extraordinary sluggishness of the contraction.
In order to render apparent the degree in which both these effects are produced, I here append a pair of tracings which were procured from the same piece of tissue when exposed to the different temperatures named. (N.B.--The seconds are wrongly marked in Fig. 7; they ought to be the same as in Fig. 6.)
I may as well state here that in water at all temperatures, within the limits where responses to stimuli are given at all, the staircase action admits of being equally well produced; but in order to procure the _maximum_ effect for any given temperature, the rate at which the successive stimuli are thrown in must be quicker in warm than in cold water.