It seems desirable, however, in this connection again to mention a fact briefly stated in a former chapter, namely, that section conclusively proves a contraction-wave to have the power, when it reaches a lithocyst, of stimulating the latter into activity; for it is not difficult to obtain a series of lithocysts connected in such a manner that the resistance offered to the passage of the waves by a certain width of the junction-tissue, is such as just to allow the residuum of the contraction-wave which emanates from one lithocyst to reach the adjacent lithocyst, thus causing it to originate another wave, which, in turn, is just able to pass to the next lithocyst in the series, and so on, each lithocyst in turn acting like a reinforcing battery to the passage of the contraction-wave. Now this fact, I think, sufficiently explains the mechanism of ganglionic action in those cases where one or more lithocysts are prepotent over the others; that is to say, the prepotent lithocyst first originates a contraction-wave, which is then successively reinforced by all the other lithocysts during its passage round the swimming-bell. In this way the passage of a contraction-wave is no doubt somewhat accelerated; for I found, in marginal strips, that the rate of transit from a terminal lithocyst to the other end of the strip was somewhat lowered by excising the seven intermediate lithocysts.
Fig. 24.
I may here state, in passing, a point of some little interest in connection with this reinforcing action of lithocysts. When I first observed this action, it appeared to me a mysterious thing why its result was always to propagate the contraction-wave in only one direction—the direction, namely, in which the wave happened to be passing before it reached the lithocyst. For instance, suppose we have a strip A D, with a lithocyst at each of the equidistant points A, B, C, D; and suppose that the lithocyst B originates a stimulus: the resulting contraction-wave passes, of course, with equal rapidity in the two opposite directions, B A, B C (arrows b a, b c): the contraction-wave b a therefore reaches the lithocyst A at the same time as the contraction-wave b c reaches the lithocyst C, and so both A and C discharge simultaneously. What, then, should we expect to be the result? I think we should expect the wave b c to continue on its course to D, after having been strengthened at C, and a reflex wave a´ b´ to start from A (owing to the discharge at A), which would reach B at the same time as a similar reflex wave c´ b´ starting from C (owing to the discharge at C); so that by the time the original wave b c d had reached D, the point B would be the seat of a collision between the two reflex waves a´ b´ and c´ d´. And, not to push the supposed case further, it is evident that if such reflex waves were to occur, the resulting confusion would very soon require to end in tetanus. As a matter of fact, these reflex waves do not occur; and the question is, why do they not? Why is it that a wave is only reinforced in the direction in which it happens to be travelling—so that if, for instance, it happens to start from A in the above series, it is successively propagated by B C in the direction A, B, C, D, and in that direction only; whereas, if it happens to start from D, it is propagated by the same lithocysts in the opposite direction, D, C, B, A, and in that direction only—the wave in the one case terminating at the lithocyst D, and in the other case at the lithocyst A? Now, although this absence of reflex waves appears at first sight mysterious, it admits of an exceedingly simple explanation. I find that the contractile tissues of the covered-eyed Medusæ cannot be made to respond to two successive stimuli of minimal, or but slightly more than minimal intensity, unless such stimuli are separated from one another by a certain considerable interval of time. Now, when in the above illustration the contraction-wave starts from A, by the time it reaches B the portion of tissue included between A and B has just been in contraction in response to the stimulus from A, while the portion of tissue included between B and C has not been in contraction. Consequently, the stimulus resulting from a ganglionic discharge being presumably of minimal, or but slightly more than minimal intensity, the tissue included between A and B will not respond to the discharge of B; while the tissue included between B and C, not having been just previously in contraction, will respond. And conversely, of course, if the contraction-wave had been travelling in the opposite direction.
Seeing that this explanation is the only one possible, and that it moreover follows as a deductive necessity from my experiments on stimulation, I think there is no need to detail any of the further experiments which I made with the view of confirming it. But the following experiment, devised to confirm this explanation, is of interest in itself, and on this account I shall state it. Having prepared a contractile strip with a single remaining lithocyst at one end, I noted the rhythm exhibited by this lithocyst, and then imitated that rhythm by means of single induced shocks thrown in with a key at the other end of the strip. The effect of these shocks was, of course, to cause the contraction-waves to pass in the direction opposite to that in which they passed when originated by the lithocyst. Now I found, as I had expected, that so long as I continued exactly to imitate the rate of ganglionic rhythm, so long did the waves always pass in the direction B A—A being the lithocyst, and B the other end of the strip. I also found that if I allowed the rate of the artificially caused rhythm to sink slightly below that of the natural rhythm, after every one to six waves (the number depending on the degree in which the rate of succession of my induction shocks approximated to the rate of the natural rhythm) which passed from B to A, one would pass from A to B.[18]
Of course the only interpretation to be put on these facts is that every time an artificially started wave reached the terminal ganglion it caused the latter to discharge; but that the occurrence of a discharge could not in this case be rendered apparent, because of the inadequacy of that discharge to start a reflex wave. But that such discharges always took place was manifest, both à priori because from analogy we may be sure that if there had happened to be any contractile tissue of appropriate width on the other side of the ganglion, the discharge of the latter would have been rendered apparent, and à posteriori because, after the arrival of every artificially started wave, the time required for the ganglion to originate another wave was precisely the same as if it had itself originated the previous wave.
In view of these results, it occurred to me as an interesting experiment to try the effect on the natural rhythm of exhausting a ganglion thus situated, by throwing in a great number of shocks at the other end of the strip. I found that after five hundred single shocks had been thrown in with a rapidity almost sufficient to tetanize the strip, immediately after the stimulation ceased, the natural rhythm of the ganglion, which had previously been twenty in the minute, fell to fourteen for the first minute, eighteen for the second, and the original rate of twenty for the third. In such experiments the diminution of rate is most conspicuous during the first fifteen or thirty seconds of the first minute. Sometimes there are no contractions at all for the first fifteen seconds after cessation of the stimulating process, and in such cases the natural rhythm, when it first begins, may be as slow as one-half or even one-quarter its normal rate. All these effects admit of being produced equally well, and with less trouble, by faradizing the strip, when it may be even better observed how prolonged may be the stimulation, without causing anything further than such slight exhaustion of the ganglion as the above results imply.[19]
Naked-eyed Medusæ.
It would be impossible to imagine movements on the part of so simple an organism more indicative of physiological harmony than are the movements of Sarsia. One may watch several hundreds of these animals while they are swimming about in the same bell-jar and never perceive, as in the covered-eyed Medusæ, the slightest want of ganglionic co-ordination exhibited by any of the specimens. Moreover, that the ganglionic co-ordination is in this case wonderfully far advanced is proved by the fact of members of this genus being able to steer themselves while following a light, as previously described.[20]
In the discophorous species of naked-eyed Medusæ, however, perfectly co-ordinated action is by no means of such invariable occurrence as it is in Sarsia; for although in perfectly healthy and vigorous specimens systole and diastole occur at the same instant over the whole nectocalyx, this harmoniously acting mechanism is very liable to be thrown out of gear, so that when the animals are suffering in the least degree from any injurious conditions, often too slight and obscure to admit of discernment, the swimming movements are no longer synchronous over the whole nectocalyx; but now one part is in systole while another part is in diastole, and now several parts may be in diastole while other parts are in systole. And as in these animals very slight causes seem sufficient thus to impair the ganglionic co-ordination, it generally happens that in a bell-jar containing a number of specimens belonging to different species, numerous examples of more or less irregular swimming movements are observable.