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MR. G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
principle of the summation of stimuli, enormous variations in the force of contraction 
may often be observed under the influence of stimuli of varying intensities. This 
relation, within certain limits, between the intensity of a stimulus and the force of the 
responding contraction, is, of course, to be observed more or less in the case of all 
excitable tissues; but in the case of the excitable tissues of Aurelia the limits of 
variation in this respect are extreme. As it will be necessary to discuss this subject 
at length in the next division of the present paper, it will be sufficient in this place 
merely to state the principal fact, viz. : that the excitable tissues of Aurelia admit of 
giving either a very strong or a very weak response according to the intensity of the 
stimulus in relation to the excitability of the tissue. In view of this fact it occurred 
to me that a valuable means was afforded of testing the hypothesis whereby I had 
sought to explain the phenomena of artificial rhythm. For, if this hypothesis were sound, 
we might expect that by employing single stimuli of uniform and slightly more than 
minimal intensity, and by throwing them in at a rate slightly more rapid than is 
required to produce a strong response to every stimulus, the strong responses would 
alternate with weak responses. For after the occurrence of each strong response the 
restoration of excitability (or recovery from exhaustion) must be a gradual process, so 
that if the stimulus is thrown in when this restoration has proceeded to a certain, 
though not to its full extent, the stimulus ought to be strong enough to evoke a feeble 
response. This feeble response, however, would not greatly exhaust the tissue, so that 
when the next stimulus is thrown in it would encounter a tissue more excitable than 
did its predecessor, and consequently would give rise not to a weak contraction, but to 
a strong one. In order to test this idea I selected a specimen of Aurelia which 
exhibited marked differences in the strength of its contractile waves under the influence 
of stimuli of different intensities. Employing single induction shocks of slightly more 
than minimal intensity, and throwing them in at twice the rate that was required to 
produce a strong response to every shock, I found that midway between every two 
strong responses there was a weak response. In other words, a stimulus of uniform 
intensity gives rise alternately to a strong and to a weak contraction, as shown in the 
appended tracing. It will be observed that in this tracing each large curve represents 
Fig. 4. 
the whole time occupied by the strong contraction, the latter beginning at the highest 
point of the curve on the left hand side in each case. The effect of the weak con¬ 
traction is that of momentarily interrupting the even sweep of diastole after the 
