MR, G. J. ROMANES ON THE LOCOMOTOR SYSTEM OF MEDUSAE. 
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tissues, ancl thus occasionally throws them into artificial rhythm. What the precise 
nature of this injury may be is a matter of subordinate interest; but I may state that 
I have never observed artificial rhythm to occur as a result of electrical stimulation 
until the latter has acted for a sufficient length of time to cause an opalescence of the 
tissues in the neighbourhood of the electrodes. This opalescence, in the case of Sarsia, 
is always certain to manifest itself under the influence of an electrical stimulus of 
sufficient duration, and when it does manifest itself it is indistinguishable from that 
which has already been described as the result of chemical action. There can then be 
no doubt that artificial rhythm as an after-effect of electrical stimulation never asserts 
itself until after that stimulation has inflicted a conspicuous injury upon the tissues. 
But in order to assure myself still more effectually that the artificial rhythm in this 
case is due to a constant stimulus supplied by a local injury in the region where the 
electrodes had rested, I tried the following experiment. Having submitted a paralyzed 
bell of Sarsia to a number of successive electrical shocks in the same area of its 
excitable surface, and having then observed, on removing the electrodes and allowing 
the mutilated bell to float freely in the water, that the artificial rhythm was in vigorous 
progress, I suddenly with two snips of the scissors excised the area of tissue which 
had previously been the seat of electrical stimulation. Instantly all movement ceased. 
This experiment, therefore, proved that the rhythmic movements can only have been due 
to a stimulus emanating from the seat of injury, and continuously spreading over all the 
other parts of the excitable surface of the bell. And, such being the case, we can have 
no further doubt that the case of artificial rhythm due to previous electrical stimulation 
belongs to the same category of physiological facts as the case of artificial rhythm due 
to present or previous chemical stimulation. For now, in conclusion, I may observe that 
in order to render the experimental parallel complete, I imitated in the case of chemical 
stimulation the proof of the fact which I have just narrated in the case of electrical 
stimulation, viz.: that the artificial rhythm depends on the continuous reception by the 
excitable tissues of a stimulus which is continuously supplied from a seat, or seats, of 
injury. The method which I employed to prove this fact in the case of chemical 
stimulation was as follows. Having slit open the paralyzed bell of Sarsia along the 
whole of one side from base to apex of the cone, I suspended the now sheet-like mass 
of tissue by one corner in the air, leaving the rest of the sheet to hang vertically 
downwards. By means of a rack-work support I now lowered the sheet of tissue till 
one portion of it dipped into a beaker filled with a solution of glycerine of appropriate 
strength. After allowing this portion to soak in the solution of glycerine until it 
became slightly opalescent, I dropped the entire mutilated bell, or sheet of tissue, into 
another beaker containing ordinary sea-water. If the exposure to the glycerine 
solution had been of sufficient duration, I invariably found that in the normal sea¬ 
water the rhythmic movements were performed by the whole tissue mass quite as 
efficiently as was the case in my other experiments, where the whole tissue-mass, and 
not merely a portion, had been submitted to the influence of the irritant. But on now 
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