ME. a. J. ROMANES ON THE LOCOMOTOR SYSTEM OE MEDUSAS. 
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larity of the first of the above tracings with those published in his paper. It will be 
remembered that he experimented on the physiologically severed apex of the heart, 
and therefore on a tissue which, in the absence of any active ganglionic element, 
resembles the paralyzed umbrella of Aurelia. His tracings were obtained in exactly 
the same way as my own, so that the fact of tissues separated from one another in the 
animal scale so widely as are the muscle-fibres of the heart and those of the Medusae, 
nevertheless behaving towards stimulation in so peculiar and yet so similar a manner, is 
to my mind a fact of great interest. Dr. Bowditch does not appear to have deter- 
mined the effect of what he conveniently terms the staircase action (from the appear- 
ance of the tracings to a staircase) upon the period of latent stimulation ; but from the 
fact that in other respects the case of the heart and that of the Medusae are so similar, 
there can be little doubt that in the former, as in the latter, the latent period will be 
found to be greatly influenced by a series of stimuli. 
But although the case of the heart and the Medusae are so wonderfully parallel in 
the particulars we are considering, there are one or two points of difference between 
them which must here be noted. In the case of Aurelia , after a staircase has been 
built up by means of a series of stimuli, if a pause of ten seconds be allowed to elapse 
and the stimulation be then again commenced, I find that the first response given is 
not quite of maximum intensity, but corresponds with perhaps the second or third step 
from the top of the previously completed staircase. Again, if a pause of fifteen seconds 
be allowed to elapse, the first step of the next staircase corresponds with the third or 
fourth step from the top of the standard one. If, again, a pause of half a minute be 
allowed to elapse, the first step of the next staircase will correspond in height only 
with the second or third step from the bottom of the first staircase. Lastly, if a whole 
minute be allowed to elapse between the maximum effect of one series of stimulations 
and the first stimulus of another series, it is observable that the tissue has, as it were, 
completely forgotten the occurrence of the previous series, so that the next staircase has 
to begin anew from the first step. Now Dr. Bowditch has found, in the case of the 
heart, that an interval of five minutes must be allowed to intervene between two series 
of stimuli before the effect of the first on the second is thus totally abolished ; so that, 
returning to the metaphor first employed, we may say that the memory of the cardiac 
tissue is about five times as long as that of the medusoid tissue. But in the case of 
exhausted medusoid tissue the difference may be even greater than this ; for in this case 
I have observed all memory to fade in the course of half a minute. There is one other 
point of difference between the heart and the Medusae analogous to the one first stated. 
From Dr. Bowditch’s tracings it appears that the maximum staircase effect is produced 
on the heart-apex when the successive shocks are thrown in at six seconds’ intervals ; 
whereas, in the case of the Medusae, I found that the more rapidly the shocks are thrown 
in, the more marked is the beneficial influence of each contraction on its successor, — that 
is to say, up to the time when the interval between the successive shocks is not so short 
as to give rise to incipient tetanus by summation of contractions (when, of course, the 
