Page:Popular Science Monthly Volume 14.djvu/320

306 pulsations of the bell instantly and forever ceased. On the other hand, the severed margin continued its pulsations with vigor and pertinacity, notwithstanding its severance from the main organism. For hours and even for days after its removal the severed margin would continue its rhythmical contractions; so that the contrast between the death-like quiescence of the mutilated bell and the active movements of the thread-like portion which had just been removed from its margin was a contrast as striking as it is possible to conceive.

I may here add that, although excision of the margin of the bell thus completely destroys the spontaneity of the bell, it does not at all diminish the excitability of the bell; so that, although the mushroom-shaped mass will never move of its own accord after having been thus mutilated, it will give any number of locomotor contractions in response to an equal number of artificial stimulations, just in the same way as a frog with its head (nerve-centres of spontaneity) removed will give any number of hops in response to successive stimulations.

These experiments, therefore, prove conclusively that, in the extreme marginal rim of all the numerous species of Medusæ which I examined, there is situated an intensely localized system of nervous centres, to the functional activity of which the rhythmical contractions of the swimming-bell are exclusively due. And as the Medusæ are thus the lowest animals in which a nervous system has yet been or probably ever will be discovered, we have in them the animals upon which we may experiment with the best hope of being able to elucidate all questions concerning the origin and endowments of primitive nervous tissues. I may here add that these experiments were independently made by Dr. Eimer, of Würzburg.

After I had made the observation which I have described, it seemed to me desirable to follow it up with a number of other physiological, as distinguished from histological, researches. For I was much struck by the certainty and precision of the results which I had obtained by experiment, as distinguished from the uncertainty and disagreement of the results which had previously been obtained by the histological methods. Accordingly, I decided, in the first instance, to feel my way in the direction of physiological experiment before beginning that systematic histological research which, sooner or later, it was manifestly imperative to make. Study of function having so far guided the study of structure as to show that it was in the margin of the Medusæ, that we must look for the principal if not the exclusive supply of central nervous tissue, it seemed desirable to ascertain how much light a further study of function might throw on the character and the distribution of the peripheral nervous tissue. Accordingly, I began my physiological work chiefly with the view of guiding my subsequent