Page:Popular Science Monthly Volume 14.djvu/325
BEGINNING OF NERVES.
gain nothing on the side of simplicity by assuming that the contractile waves are merely muscle-waves, so long as the undoubtedly nervous waves are equally able to pass round sections interposed in their path. Briefly, then, I find that the nervous waves of stimulation are quite as able to pass round these interposed sections as are the waves of contraction. Thus, for instance, in this specimen (Fig. 5), the tentacular wave of stimulation continued to pass as before, even after I had submitted the parallelogram of tissue to the tremendously severe form of section which is represented in the diagram. And this fact, I am not afraid to say, is one of the most important that has ever been brought to light in the whole range of invertebrate physiology. For what does it prove? It proves that the distinguishing function of nerve, where it first appears upon the scene of life, admits of being performed vicariously to almost any extent by all parts of the same tissue-mass. If we revert to our old illustration of the muslin as representing the nerve-plexus, it is clear that, however much we choose to cut the sheet of muslin with such radial or spiral sections as are represented in the diagram, one could always trace the threads of the muslin with a needle round and round the disk, without once interrupting the continuity of the tracing; for, on coming to the end of a divided thread, one could always double back on it and choose another thread which might be running in the required direction. And this is what we are now compelled to believe takes place in the fibres of this nervous network, if we assume that these visible fibres are the only conductive elements which are present. Whenever a stimulus-wave reaches a cut, we must conclude that it doubles back and passes into the neighboring fibres, and so on, time after time, till it succeeds in passing round and round any number of overlapping cuts.
Now, it was in view of this almost unlimited power of vicarious action on the part of the fibres composing the (then) hypothetical nervous plexus, that I was in the first instance inclined to suppose these nerve-fibres to be of a non-fully differentiated character ; and although the above detailed experiments, and others of a similar kind, proved that an intimate network of such channels was present, I scarcely expected that they would admit of being distinguished by the microscope. But, not to give an inference the value of a fact, I was careful to state in the publication where this inference was adduced — viz., in the printed abstract of a Royal Institution lecture — that this position was only "provisional", and that, until I should have had "time to conduct a systematic inquiry concerning the histology of the Medusæ'' the inference in question must be regarded as premature and uncertain." Such a systematic inquiry has now shown that this provisional inference was
- I guarded the inference in this way, lest the fibres in question should afterward prove to be nerves ; and it will therefore be observed that, supposing them to be nerves, the above inference cannot be negatived until it is shown that there are no other nervous channels present of a less differentiated character.