But, lastly, the strongest evidence in favour of this view as afforded by the following observations. At the beginning of this treatise I stated that the distinguishing function of nerve consists in its power of conducting stimuli to a distance, irrespective of the passage of a contraction-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 Medusa. For instance, take such a section of Aurelia as this one (Fig. 12),

that the propagation of the wave is at least in some measure dependent on nervons conduction. This fact is, that after a contraction-wave has been blocked by the severity of a spiral or other form of section, it may again be made to force a passage under the influence of vagus stimulation.

Moreover, in a paper still more recently published by Drs. Brunton and Cash on "Electrical Stimulation of the Frog's Heart" (Proc. Roy. Soc., vol. xxxv., No. 227, p. 455, et seq.) it is remarked, "Another interesting consideration is, whether the stimulus which each cavity of the heart transmits to the succeeding one consists in the propagation of an actual muscular wave, or in the propagation of an impulse along the nerves. The observations of Gaskell have given very great importance to the muscular wave occurring in each cavity of the heart of cold-blooded animals as a stimulus to the contraction of the next succeeding cavity. Our observations appear to us to show that, while this is an important factor, it is not the only one in the transmission of stimuli. . . . We consider that stimuli are also propagated from one chamber of the heart to another through nervous channels: thus we find that irritation of the venus sinus will sometimes produce simultaneous contractions of the auricle and ventricle, instead of the ventricular beat succeeding the auricular in the ordinary way. This we think is hardly consistent with the hypothesis, that a stimulus consists of the propagation of a muscular wave only from the auricle to the ventricle."

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wherein the bell has been cut into the form of a continuous parallelogram of tissue with the ovaries and a single remaining ganglion at one end.

(The

a

Fig. 12.

of the neuro-muscular sheet most remote from the ganglion be gently brushed with a camel's hair brush-i.e. too gently to start a responsive contraction-wave-the ganglion at the other end will shortly afterwards discharge, as shown by its starting a contraction-wave at its own end of the parallelogram, b; thus proving that the stimulus caused by brushing the tissue at the other end, a, must have been conducted all the way along the parallelogram to the terminal ganglion, b, so causing the terminal ganglion to discharge by reflex action. Indeed, in many cases, the passage of this nervous wave of stimulation admits of being seen. For the numberless tentacles which fringe the margin of Aurelia are more highly excitable than is the general contractile tissue of the bell; so that on brushing the end a of the parallelogram remote from the ganglion, the tentacles at this end respond to the stimulus by a contraction, then those next in the series do the same, and so on-a wave of contraction being thus set up in the tentacular fringe, the passage of which is determined by the passage of the nervous wave of stimulation in the superjacent nervous network. This tentacular wave is in the illustration represented as having traversed nearly half the whole distance to the terminal ganglion, and when it reaches that ganglion it will cause it to discharge by reflex action, so giving rise to a visible wave of muscular contraction passing in the direction b a, opposite to that which the nervous or tentacular wave had previously pursued. Now this tentacular wave, being an optical expression of a pas

sage of a wave of stimulation, is a sight as beautiful as it is unique; and it affords a first-rate opportunity of settling this all-important question, namely, Will this conductile or nervous function prove itself as tolerant towards a section of the tissue as the contractile or muscular function has already proved itself to be? For, if so, we shall gain nothing on the side of simplicity by assuming that the contraction-waves are merely muscle-waves, so long as the conduction or 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. 12), 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 illustration; and this fact, in my opinion, is one of the most important that has 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 illustrations, one could always trace the threads of the muslin with a needle round and round the

disc, 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 conductile elements which are present. Whenever a stimulus wave reaches a cut, we must conclude that it doubles back and passes into the neighbouring fibres, and so on, time after time, till it succeeds in passing round and round any number of overlapping cuts.

This is, no doubt, as I have already observed, a very remarkable fact; but it becomes still more so when we have regard to the histological researches of Professor Schäfer on the structural character of this nerve-plexus. For these researches have shown that the nerve-fibres which so thickly overspread the muscular sheet of Aurelia do not constitute a true plexus, but that each fibre is comparatively short and nowhere joins with any of the other fibres; that is to say, although the constituent fibres of the network cross and recross one another in all directions-sometimes, indeed, twisting round one another like the strands of a rope-they can never be actually seen to join, but remain anatomically insulated throughout their length. So that the simile by which I have represented this nervous network-the simile, namely, of a sheet of muslin overspreading the whole of the muscular sheet-is, as a simile, even more accurate than has hitherto