Page:Popular Science Monthly Volume 14.djvu/323

Rh Now, in this experiment, when the spiral strip is only made about half an inch broad, it may be made more than a yard long before all the bell is used up in making the strip; and as nothing can well be imagined as more destructive of the continuity of a nerve-plexus than this spiral mode of section must be, we cannot but regard it as a very remarkable fact that the nerve-plexus should still continue to discharge its functions. Indeed, so remarkable does this fact appear, that to avoid accepting it



we may well feel inclined to resort to another hypothesis—namely, that these contractile waves do not depend for their passage on the nervous network at all, but that they are of the nature of muscle-waves, or of the waves which we see in indifferentiated protoplasm, where all parts of the mass being equally excitable and equally contractile, however severely we cut the mass, so long as we do not actually divide it, contractile waves will pass throughout the whole mass. But this very reasonable hypothesis of the contractile waves in the Medusæ being possibly nothing other than muscle-waves, is negatived by another fact of a most extraordinary nature. At the beginning of this article I stated that the distinguishing function of nerve consists in its power of conducting stimuli to a distance, irrespective of the passage of a contractile wave; and I may here add that, when a stimulus so conducted reaches a ganglion or nerve-centre, it causes the ganglion to discharge by so-called "reflex action." Now, this distinguishing function of nerve can plainly be proved to be present in the Medusæ. For instance, take such a section of Aurelia as this one (Fig. 5), wherein the bell has been cut into the form of a continuous parallelogram of tissue with the polypite and