CHAPTER IV.

EXPERIMENTS IN SECTION OF COVERED-EYED MEDUSÆ.

_Amount of Section which the Neuro-muscular Tissues of the Medusæ will endure without suffering Loss of their Physiological Continuity._

The extent to which the neuro-muscular tissues of the Medusæ may be mutilated without undergoing destruction of their physiological continuity is in the highest degree astonishing. For instance, to begin with the covered-eyed Medusæ, I shall briefly state three modes of section, the results of which serve to show in a striking manner the fact in question.

The annexed woodcuts represent the umbrella of Aurelia aurita, with its manubrium cut off at the base, and the under or concave surface of the umbrella exposed to view, shewing in the centre the ovaries, and radiating from them the branched system of nutrient tubes. The umbrella when fully expanded, as here represented, is about the size of a soup plate, and, as previously stated, all the marginal ganglia are aggregated in the eight marginal bodies or lithocysts. Therefore if the reader will imagine the first of the diagrams (Fig. 8) to be overspread with a disc of muslin, the fibres and mesh of which are finer than those of the finest and closest cobweb, and if he will imagine the mesh of these fibres to start from these marginal ganglia, he will gain a tolerably correct idea of the lowest nervous system in the animal kingdom. Now, suppose that seven of these eight ganglia are cut out, the remaining one then continues to supply its rhythmical discharges to the muscular sheet of the bell, the result being, at each discharge, two contraction waves, which start at the same instant, one on each side of the ganglion, and which then course with equal rapidity in opposite directions, and so meet at the point of the disc which is opposite to the ganglion. Suppose, now, a number of radial cuts are made in the disc according to such a plan as this (Fig. 9), wherein every radial cut deeply overlaps those on either side of it. The contraction waves which now originate from the ganglion must either become blocked and cease to pass round the disc, or they must zigzag round and round the tops of these overlapping cuts. Now, remembering that the passage of these contraction waves is presumably dependent on the nervous network progressively distributing the ganglionic impulse to the muscular fibres, surely we should expect that two or three overlapping cuts, by completely severing all the nerve-fibres lying between them, ought to destroy the functional continuity of these fibres, and so to block the passage of the contraction wave. Yet this is not the case; for even in a specimen of Aurelia so severely cut as the one here represented, the contraction waves, starting from the ganglion, continued to zigzag round and round the entire series of sections.

The second mode of section to which I have alluded is as follows (Fig. 10). The central circle (_x_) stands for an open space cut out of the umbrella; the outer circle indicates the margin of the animal, with all lithocysts save one (_l_) removed; and the median circular line represents a cut. It will be seen that the effect of this cut is almost completely to sever the mass of tissue at _z_ from the rest of the umbrella, the only connection between them being the narrow neck of tissue at _z_. Yet, in the case to which I refer, the contraction waves emanating from _l_ passed in the directions represented by the arrows without undergoing any appreciable loss of vigour. Upon completing the circular cut at _z_, the ring of tissue (_y z_) became totally paralyzed, while the outer circle, of course, continued its contractions as before. Now, the neck of tissue at _z_ measured only one-eighth of an inch across, while the ring of tissue (_y z_), when cut through and straightened out upon the table, measured one inch across and sixteen inches in length; that is to say, sixteen square inches of tissue derived its impulse to vigorous contractions through a channel one-eighth of an inch wide, notwithstanding that the latter was situated at the furthest point of the circle from the discharging lithocyst which the form of the section rendered possible.

Lastly, the third mode of section is represented in the next cut. Here seven of the marginal ganglia having been removed as before, the eighth one was made the point of origin of a circumferential section, which was then carried round and round the bell in the form of a continuous spiral--the result, of course, being this long ribbon-shaped strip of tissue with the ganglion at one end and the remainder of the swimming-bell at the other. Well, as before, the contraction-waves always originated at the ganglion; but now they had to course all the way along the strip until they arrived at its other extremity; and, as each wave arrived at that extremity, it delivered its influence into the remainder of the swimming-bell, which thereupon contracted. 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 function. 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 contraction-waves do not depend for their passage on the nervous network at all, but that they are of the nature of the muscle-waves, or of the waves which we see in undifferentiated protoplasm, where all parts of the mass being equally excitable and equally contractile, however severely we cut the mass, as long as we do not actually divide it, contraction-waves will pass throughout the whole mass. But this very reasonable hypothesis of the contraction-waves in the Medusæ being possibly nothing more than muscle-waves is negatived by other facts, which I shall now proceed to state.

In the first place, if a number of experiments be tried in any of the three modes of section above described, it will be found that extreme variations are manifested as regards the degree of tolerance. In the spiral mode of section, for instance, it will sometimes happen that the contraction-wave will become blocked when the contractile strip is only an inch long, while in other cases (as in the one represented) the wave will continue to pass through a strip more than thirty times that length; and between these two extremes there are all possible grades of tolerance. Now it seems to me that if the tissue through which these contraction-waves pass is supposed (as far as they are concerned) to be of a functionally homogeneous nature, no reason can be assigned why there should be such great differences in the endurance of the tissue in different individual cases; while, if we suppose that the passage of the contraction-waves is more or less dependent on the functional activity of the nervous plexus which we know from microscopical examination to be present, we encounter no such difficulty; for it is almost to be expected that in some cases it would happen that important nerves would soon be encountered by the section, while in other cases it would happen that such nerves would escape the section for a longer distance. It is indeed incredible that any one nerve should happen to pursue a spiral course twice or thrice round the umbrella, and at the same time happen to be concentric with the course pursued by the section; but, as we shall presently see, such an hypothesis as this is not necessary to account for the facts.

Again, in the second place, strong evidence that the passage of the contraction-waves is dependent on the functional activity of the nervous plexus, and therefore that they are not merely muscle-waves, is furnished by the fact that at whatever point in a spiral strip which is being progressively elongated by section the contraction-wave becomes blocked, the blocking is sure to take place _completely_ and _exclusively_ at that point. Now, as I have tried this experiment a great number of times, and always tried it by carefully feeling the way round (_i.e._ only making a very short continuation of the cut after the occurrence of each contraction-wave, and so very precisely localizing the spot at which the contraction-wave ceased to pass), I can scarcely doubt that in every case the blocking is caused by the cutting through of nerves.[13]

[13] In a highly interesting paper recently published by Dr. W. H. Gaskell, F.R.S., on "The Innervation of the Heart" (_Journ. of Physiol._, vol. iv. p. 43, _et seq._), it is shown that the experiments in section thus far described yield strikingly similar results when performed upon the heart of the tortoise and the heart of the skate. Dr. Gaskell inclines to the belief that in these cases the contraction-waves are merely muscle-waves. There is one important fact, however, which even here seems to me to indicate that the propagation of the wave is at least in some measure dependent on nervous conduction. This fact is, that after a contraction-wave has been blocked by the activity 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 in 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."

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 Medusæ. For instance, take such a section of Aurelia as this one (Fig. 12), 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 cuts interposed in the parallelogram may, for the present, be disregarded.) Now, if the end marked _a_ 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 passage 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 appeared; for just as in a piece of muslin the constituent threads, although frequently meeting one another, never actually coalesce, so in the nervous network of Aurelia, the constituent fibres, although frequently in contact, never actually unite.

Now, if it is a remarkable fact that in a fully differentiated nervous network the constituent fibres are not improbably capable of vicarious action to almost any extent, much more remarkable does this fact become when we find that no two of these constituent nerve-fibres are histologically continuous with one another. Indeed, it seems to me we have here a fact as startling as it is novel. There can scarcely be any doubt that _some_ influence is communicated from a stimulated fibre _a_ to the adjacent fibre _b_ at the point where these fibres come into close apposition. But what the nature of the process may be whereby a disturbance in the excitable protoplasm of _a_ sets up a sympathetic disturbance in the anatomically separate protoplasm of _b_, supposing it to be really such--this is a question concerning which it would as yet be premature to speculate. But I think it may be well for physiologists to keep awake to the fact that a process of this kind probably takes place in the case of these nerve-fibres. For it thus becomes a possibility which ought not to be overlooked, that in the fibres of the spinal cord, and in ganglia generally, where histologists have hitherto been unable to trace any anatomical or structural continuity between cells and fibres, which must nevertheless be supposed to possess physiological or functional continuity--it thus becomes a possibility that in these cases no such anatomical continuity exists, but that the physiological continuity is maintained by some such process of physiological induction as probably takes place among the nerve-fibres of Aurelia.[14]

[14] That it can scarcely be _electrical induction_ would seem to be shown by the fact that such effects can only be produced on nerves by strong currents, and also by the fact that the saline tissues of the swimming-bell must short-circuit any feeble electrical currents as soon as they are generated.

I have now to detail another fact of a very puzzling nature, but one which is certainly of importance. When the spiral section is performed on Aurelia aurita, and when, as a consequence, the contraction-waves which traverse the elongating strip become at some point suddenly blocked, if the section be stopped at this point it not unfrequently happens that after a time the blocking suddenly ceases, the contraction-waves again passing from the strip into the umbrella as freely as they did before the section reached the point at which the blocking occurred. The time required for this restoration of physiological continuity is very variable, the limits being from a few seconds to an hour or more; usually, however, it is from two to four minutes. This process of re-establishing the physiological connections, although rapid, is not so instantaneous as is that of their destruction by section. In general it requires the passage of several contraction-waves before the barrier to the passage of succeeding waves is completely thrown down. The first wave which effects a passage appears to have nearly all its force expended in overcoming the barrier, the residue being only sufficient to cause a very feeble, and sometimes almost imperceptible, contraction of the umbrella. The next wave, however, passes across the barrier with more facility, so that the resulting contraction of the umbrella is more decided. The third wave, again, causes a still more pronounced contraction of the umbrella; and so on with all succeeding waves, until every trace of the previous blocking has disappeared. When this is the case, it generally happens that the strip will again admit of being elongated for a short distance before a blocking of the contraction-waves again supervenes. Sometimes it will be found that this second blockage will also be overcome, and that the strip will then admit of being still further elongated without the passage of the waves being obstructed; and so on occasionally for three or four stages.

The same series of phenomena may be shown in another way. If a contractile strip of tolerable length be obtained, with the waves passing freely from one end to the other, and if a series of parallel and equidistant cuts be made along one side of the strip, in a direction at right angles to the length, and each cut extending two-thirds of the breadth of the strip, the chances are in favour of the contraction-waves being wholly unaffected by the sections, however numerous these may be. But now, if another series of parallel and equidistant cuts of the same length as the first ones, and alternating with them, be made along the other side of the contractile strip, the result is, of course, a number of interdigitating cuts; and it is easy to see that by beginning with a few such cuts and progressively increasing their number, a point must somewhere be reached at which one portion will become physiologically separated from the rest. The amount of such section, however, which contractile strips will sometimes endure is truly surprising. I have seen such a strip twenty inches long by one and a half inches wide with ten such cuts along each side, and the contraction-waves passing without impediment from end to end. But what I wish more especially to observe just now is, that by progressively increasing the number of such interdigitating cuts up to the point at which the contraction-wave is blocked, and then leaving the tissue to recover itself, in many cases it will be observed that the blocking is sooner or later overcome; that on then adding more interdigitating cuts the blocking again supervenes; but that in time it may again be overcome, and so on. It is, however, comparatively rare to find cases in which blocking is overcome twice or thrice in succession.

Section is not the only way in which blocking of waves may be caused in contractile strips. I find that pressure, even though very gentle, exerted on any part of a strip causes a blocking of the waves at that part, even after the pressure has been removed. If the pressure has been long continued, after its removal the blocking will probably be permanent; but if the pressure has been only of short duration, the blocking will most likely be transitory. Even the slight strains caused by handling a contractile strip in the air are generally followed by a decrease in the rate of the waves, and sometimes by their being completely blocked. Other methods by which the passage of waves in contractile strips admits of being blocked will be alluded to farther on.

Now, in all these cases of temporary blocking we must conclude that when the contraction-waves succeed in at last forcing a passage, some structural change has taken place in the tissue at the region of injury, corresponding with the functional change of the re-establishment of physiological continuity. The waves previously stopped at a certain point of section or otherwise, after beating for a time on the physiological barrier, are at last able to throw down the barrier, and thenceforward to proceed on their way unhindered. What, then, is the nature of the structural change which has taken place?

In the early days of this research, before the presence of a nerve-plexus had been proved histologically, I argued in favour of such a plexus on the grounds furnished by many of the foregoing experiments; and at a lecture given in the Royal Institution I ventured to say that if a careful investigation of the histology of these tissues should fail to show the plexus which the result of those experiments required me to assume, we should still be compelled to suppose that the plexus was present, although not sufficiently differentiated to admit of being seen. I further ventured to suggest that in this event the facts just stated might be taken to substantiate the theory of Mr. Herbert Spencer on the genesis of nerve-tissue in general. This theory is that which supposes incipient conductile tissues, or rudimentary nerve-fibres, to be differentiated from the surrounding contractile tissues, or homogeneous protoplasm, by a process of integration which is due simply to use; so that just as water continually widens and deepens the channel through which it flows, so molecular or nervous waves of stimulation, by always flowing through the same tissue-tracts, tend ever more and more to excavate for themselves functionally differentiated lines of passage.

Such being Mr. Spencer's theory, I applied it hypothetically to the above facts in the words which I may here quote.

"As the successive waves beat rhythmically on the area of obstruction, more or less of the molecular disturbances must every time be equalized through these lines of discharge, which from the first have been almost sufficient to maintain the physiological continuity of the tissue. Therefore, according to the hypothesis, every wave that is blocked imposes upon these particular lines of discharge a much higher degree of functional activity than they were ever before required to exercise; and this greater activity causing in its turn greater permeability, a point will sooner or later arrive at which these lines of discharge, from having been _almost_, become _quite_ able to draft off sufficient molecular motion, or stimulating influence, to carry on the contraction-waves beyond the areas of previous blocking. In such instances, of course, we should expect to find what I always observed to be the case, viz. that the first contraction-wave which passes the barrier is only very feeble, the next stronger, the next still stronger, and so on, according as the new passage becomes more and more permeable by use, until at last the contraction-waves pour over the original barrier without any perceptible diminution of their force. In some cases, by exploring with graduated stimuli and needle-point terminals, I was able to ascertain the precise line through which this eruption of stimulating influence had taken place."

I have now to state the effect upon this hypothesis which in my opinion has been produced by the histological proof that the plexus in question is composed of fully differentiated nerves. Briefly, then, I think that the hypothesis still holds to the extent of being the only one available whereby to explain the facts; but there is this great difference, viz. that the hypothesis need not now be applied to the genesis of nerve-tissue out of comparatively undifferentiated contractile tissue, but rather to the increasing of the functional activity of already well-differentiated nerve-tissue. In other words, we have not now to suppose that nerve-tissue is formed _de novo_ in the region of blocking; but, in my opinion, we still have to suppose that the nerve-fibres which were already there have their functional capabilities so far improved by the greater demand imposed upon them, that whereas at first they were not able, eventually they became able to draft off enough molecular disturbance to carry on a stimulus adequate to cause a muscular contraction. It will be observed that the difference thus expressed is one of considerable importance; for now the facts cease to lend any countenance to Mr. Spencer's theory touching the formation of nerves out of protoplasm, or other contractile material. They continue, however, to countenance his views touching the improvement of functional capacity which nerve-fibres, when already formed, undergo by use; and this, which is in itself an important matter, is the point with which I was mainly concerned in the lecture of the Royal Institution just alluded to. For, as I then observed, in this theory of nerve-fibres becoming more and more functionally developed by use, we probably have a physical explanation, which is as full and complete as such an explanation can ever be, of the genesis of mind. "For from the time that intelligence first dawned upon the scene of life, whenever a new relation had to be established in the region of mind, it could only be so established in virtue of some new line of discharge being excavated through the substance of the brain. The more often this relation had to be repeated in the mind, the more often would this discharge require to take place in the brain, and so the more easy would every repetition of the process become.... Thus it is, according to the theory, that there is always a precise proportion between the constancy with which any relations have been joined together during the history of intelligence, and the difficulty which intelligence now experiences in trying to conceive of such relations as disjoined. Thus it is that, even during the history of an individual intelligence, 'practice makes perfect,' by frequently repeating the needful stimulations along the same lines of cerebral discharge, so rendering the latter ever more and more permeable by use. Thus it is that a child learns its lessons by frequently repeating them; and thus it is that all our knowledge is accumulated."[15]

[15] I have associated the above theory of nerve-genesis with the name of Mr. Spencer, because it occupies so prominent a place in his "Principles of Psychology." But from what I have said in the text, I think it is clear that the theory, as presented by Mr. Spencer, consists of two essentially distinct hypotheses--the one relating to the formation of nerve-tissue out of protoplasm, and the other to the increase of functional capacity in a nerve-fibre by use (a third hypothesis of Mr. Spencer relating to the formation of ganglion-tissue does not here concern us). The latter hypothesis, however, ought not to be associated with Mr. Spencer's name without explaining that it has likewise occurred to other writers, the first of which, so far as I can ascertain, was Lamarck, who says, "Dans toute action, le fluide des nerfs qui la provoque, subit un mouvement de déplacement qui y donne lieu. Or, lorsque cette action a été plusieurs fois répétée, il n'est pas douteux que le fluide qui l'a exécutée, ne se soit frayé une route, qui lui devient alors d'autant plus facile à parcourir, qu'il l'a effectivement plus souvent franchie, et qu'il n'ait lui-même une aptitude plus grand à suivre cette route frayée que celles qui le sont moins." ("Phil. Zool.," tom ii. pp. 318-19.)

_Rate of Transmission of Stimuli._

The rate at which contraction-waves traverse spiral strips of Aurelia is variable. It is largely determined by the length and width of the strip; so that the best form of strip to use for the purpose of ascertaining the _maximum_ rate is one which I shall call the circular strip. A circular strip is obtained by first cutting out the central bodies (_i.e._ manubrium and ovaries), and then, with a single radial cut, converting the animal from the form of an open ring to that of a continuous band. I distinguish this by the name "circular" band or strip, because the two ends tend to preserve their original relative positions, so giving the strip more or less of a circular form. Such a strip has the advantage of presenting all the contractile tissue of the swimming-bell in one continuous band of the greatest possible width, and is therefore the form of strip that yields the _maximum_ rate at which contraction-waves are able to pass. The reason why the _maximum_ rate should be the one sought for is because this is the rate which must most nearly approximate the natural rate of contraction-waves in the unmutilated animal. This rate, at the temperature of the sea and with vigorous specimens, I find to be eighteen inches per second.

In a circular strip the rate of the waves is uniform over the whole extent of the strip; so that the time of their transit from one point to another varies directly as the length of the strip. But on now narrowing such a strip, although the rate is thus slowed, the relation between the narrowing and the slowing is not nearly so precise as to admit of our saying that the rate varies inversely as the width. The following figure will serve to show the proportional extent to which the passage of contraction-waves is retarded by narrowing the area through which they pass:--

In order to ascertain whether certain zones of the circular contractile sheet in all individuals habitually convey more of the contractile influence than do other zones, I tried a number of experiments in the following form of section. Having made a circular strip, I removed all the lithocysts save one, and then cut the strip as represented in Fig. 14. On now stimulating the end _a_, or on watching the lithocyst there discharge, the resulting contraction-wave would be observed to bifurcate at _b_, and then pass on as two separate waves through the zones, _b_ _c_, _b_ _d_. Now, as these two waves were started at the same instant of time, they ran, as it were, a race in the two zones, and in this way the eye could judge with perfect ease which wave occupied the shortest time in reaching its destination. This experiment could be varied by again bisecting each of these two zones, thus making four zones in all, and four waves to run in each race. A number of experiments of this kind showed me that there is no constancy in the relative conductivity of the same zones in different individuals. In some instances, the waves occupy less time in passing through the zone _b_ _c_ than in passing through the zone _b_ _d_; in other instances, the time in the two zones is equal; and, lastly, the converse of the first-mentioned case is of equally frequent occurrence. Very often the waves become blocked in _b_ _c_, while they continue to pass in _b_ _d_, and _vice versâ_. Now, all these various cases are what we might expect to occur, in view of the variable points at which contraction-waves become blocked in spiral strips, etc.; for if the contractile tissues are not functionally homogeneous, and if the relatively pervious conductile tracts are not constant as to their position in different individuals, the results I have just described are the only ones that could be yielded by the experiments in question. Considering, however, that in these experiments the central zones are not so long as the peripheral zones, I think it may fairly be said that the conductile power of the latter is greater than that of the former; for, otherwise, the above experiments ought to yield a large majority of races won by the waves that course through the central zones, and this is not the case. Indeed, it is surprising how often the race is, as it were, neck and neck, thus showing that the relative conductivity of all the zones is precisely adjusted to their relative lengths; and forasmuch as in the unmutilated animal this adjustment must clearly serve the purpose of securing to the contraction-wave a passage of uniform rate over the whole radius of the umbrella, I doubt not that, if it were possible to perform the race-course section without interrupting any of the lines of conduction-tissue, neck and neck races would be of invariable occurrence.

Interdigitating cuts, as might be expected, prolong the time of contraction-waves in their passage through the tissue in which the cuts are interposed. For example, in a spiral strip measuring twenty-six inches in length, the time required for the passage of a contraction-wave from one end to the other is represented by the line _a_ _b_ in the annexed woodcut. But after twenty interdigitating cuts had been interposed, ten on each side of the strip, the time increased to _c_ _d_, the line _e_ _f_ representing one second. And more severe forms of section are, of course, attended with a still more retarding influence.

The effects of temperature on the rate of contraction-waves are very striking. For instance, in a rather narrow strip measuring twenty-eight inches long and one and a half inches wide, the following variations in rate occurred:--

-------------------------+------------------------------- Temperature of water. | Time occupied in passage of | contractile waves. -------------------------+------------------------------- 26° | 4 seconds. 32° | 3 seconds. 42° | 2-2/5 seconds. 65° | 2 seconds. 75° | 1-3/5 seconds. 85° | Blocked.

Or, adopting again the graphic method of statement, these variations may be represented as follows:--

Submitting a contractile strip to slight strains has also the effect of retarding the rate of the waves while they pass through the portions of the strip which have been submitted to strain. The method of straining which I adopted was to pass my finger below the strip, and then, by raising my hand, to bring a portion of the strip slightly above the level of the water. The irritable or contractile surface was kept uppermost, and therefore suffered a gentle strain; for the weight of the tissue on either side of the finger made the upper surface somewhat convex. By passing the finger all the way along the strip in this way, the latter might be gently strained throughout its entire length, the degree of straining being determined by the height out of the water to which the tissue was raised. Of course, if the strip is too greatly strained, the contraction-waves become blocked altogether, as described above; but shortly before this degree of straining was reached, I could generally observe that the rate of the waves was diminished. To give one instance, a contractile strip measuring twenty-two inches had the rate of its waves taken before and after straining of the kind described. The result was as follows:--

Immediately after severe handling of this kind, the retardation of contraction-waves, is sometimes even more marked than here represented; but I think this may be partly due to shock, for on giving the tissue a little while to recover, the rate of the waves becomes slightly increased.

Anæsthetics likewise have the effect of slowing the rate of contraction-waves before blocking them. Taking, for instance, the case of chloroform, a narrow spiral strip between one and two feet long was immersed in sea-water containing a large dose of the anæsthetic; the observations being taken at six seconds' intervals, the following were the results:--

In such experiments, the recovery of the normal rate in unpoisoned water is gradual. Taking, for instance, the case of a spiral strip in morphia (Fig. 19), it will be seen that the original rate did not fully return. Some substances, however, exert a more marked permanent effect of this kind than do weak solutions of morphia. Here, for instance, is an experiment with alcohol (see Fig. 20).

From these experiments, however, it must not be definitely concluded that it is the anæsthesiating property of such substances which exerts this slowing and blocking influence on contraction-waves, for I find that almost any foreign substance, whether or not an anæsthetic, will do the same. That nitrite of amyl, caffein, etc., should do so, one would not be very surprised to hear; but it might not so readily be expected that strychnine, for instance, should block contraction-waves; yet it does so, even in doses so small as only just to taste bitter. Nay, even fresh water completely blocks contraction-waves after the strip has been exposed to its influence for about half an hour, and exerts a permanently slowing effect after the tissue is restored to sea-water. These facts show the extreme sensitiveness of the neuro-muscular tissues of the Medusæ to any change in the character of their surrounding medium, a sensitiveness which we shall again have occasion to comment upon when treating of the effects of poisons.

In conclusion, I may mention an interesting fact which is probably connected with the summation of stimuli before explained. When a contractile strip is allowed to rest for a minute or more, and when a wave is then made to traverse it, careful observation will show that the passage of the first wave is slower than that of its successor, provided the latter follows the former after not too great an interval of time. The difference, however, is exceedingly slight, so that to render it apparent at all the longest possible strips must be used, and even then the experimenter may fail to detect the difference, unless he has been accustomed to signalling, by which method all these observations on rate have to be made.

_Stimulus-waves._

The rate of transmission of tentacular waves is only one-half that of contraction-waves, viz. nine inches a second. This fact appeared to me very remarkable in view of the consideration that the tentacular wave is the optical expression of a stimulus-wave, and that there can be no conceivable use in a stimulus-wave being able to pass through contractile tissue independently of a contraction-wave, unless the former is able to travel more rapidly than the latter; for the only conceivable use of the stimulus-wave is to establish physiological harmony between different parts of the organism, and if this wave cannot travel more rapidly than a contraction-wave which starts from the same point, it would clearly fail to perform this function.

In view of this anomaly, I was led to think that if the rate of the stimulus-wave is dependent in a large degree on the strength of the stimulus that starts it, the slow rate of nine inches a second might be more than doubled, if, instead of using a stimulus so gentle as not to start a contraction-wave, I used a stimulus sufficiently strong to do this. Accordingly I chose a specimen of Aurelia wherein the occurrence of tentacular waves was very conspicuous, and found, as I had hoped, that every time I stimulated too gently to start a contraction-wave, the tentacular wave travelled only at the rate of nine inches a second; whereas, if I stimulated with greater intensity, I could always observe the tentacular wave coursing an inch or two in front of the contraction-wave.

It is remarkable, however, that in this, as in all the other specimens of Aurelia which I experimented upon, the reflex response of the manubrium was equally long, whatever strength of stimulus I applied to the umbrella; or, at any rate, the time was only slightly less when a contraction-wave had passed than when only a tentacular wave had done so. The loss of time, however, appears to take place in the manubrium itself, where the rate of response is astonishingly slow. Thus, if one lobe be irritated, it is usually from four to eight seconds before the other lobes respond. But the time required for such sympathetic response may be even more variable than this--the limits I have observed being as great as from three to ten seconds. In all cases, however, the response, when it does occur, is sudden, as if the distant lobe had then for the first time received the stimulus. Moreover, one lobe--usually one of those adjacent to the lobe directly irritated--responds before the other two, and then a variable time afterwards the latter also respond. This time is, in most cases, comparatively short, the usual limits being from a quarter of a second to two seconds. How much of these enormous intervals is occupied by the period of ganglionic latency, and how much by that of transmission, it is impossible to say; but I have determined that the rate of transmission from the end of a lobe of the manubrium to a lithocyst (deducting a second for the double period of latent stimulation) is the same as the rate of a tentacular wave, viz. nine inches a second. The presumption, therefore, is that the immense lapse of time required for reflex response on the part of the manubrium is required by the lobular ganglia, or whatever element it is that here performs the ganglionic function.

_Exhaustion._

In various modes of section of Aurelia I have several times observed a fact that is worth recording. It sometimes happens that when the connecting isthmus between two almost severed areas of excitable tissue is very narrow, the passage of contraction-waves across the isthmus depends upon the freshness, or freedom from exhaustion, of the tissue which constitutes the isthmus. That is to say, on faradizing one of the two tissue-areas which the isthmus serves to connect, the resulting contraction-waves will at first pass freely across the isthmus; but after a time it may happen in some preparations that every now and then a contraction-wave fails to pass across the isthmus. When this is the case, if the stimulation is still continued, a greater and greater proportion of waves fail to pass across the isthmus, until perhaps only one in every five or six becomes propagated from the one area to the other. If single induction-shocks be then substituted for the faradaic stimulation, it may be found that by leaving an interval of four or five seconds between the successive shocks, every wave that is started in the one area will be propagated across the isthmus to the other area. But if the interval between the successive shocks be reduced to two or three seconds, every now and then a wave will fail to pass across the isthmus; and if the interval be still further reduced to one second, or half a second, comparatively few of the waves will pass across. Now, however, if the tissue be allowed five minutes' rest from stimulation, and the single shocks be thrown in at one second's intervals, all the first six or ten waves will pass across the isthmus, after which they begin to become blocked as before. It may be observed also that when the waves are thus blocked, owing to exhaustion of the connecting isthmus, they may again be made to force a passage by increasing the intensity of the stimulation, and so giving rise to stronger waves having a greater power of penetration. Thus, on re-enforcing the electrical stimulus with the simultaneous application of a drop of spirit, the resulting waves of contraction are almost sure to pass across the isthmus, even though this has been exhausted in the manner just described.

_Ganglia appearing to assert their Influence at a Distance from their own Seat._

Another fact, which I have several times noticed during my sections of Aurelia, also deserves to be recorded. I have observed it under several modes of section, but it will be only necessary to describe one particular case.

In the Aurelia of a portion of which the accompanying woodcut (p. 102) is a representation, seven of the lithocysts were removed, while the remaining one was almost entirely isolated from the general contractile tissue by the incisions _aa_, _bb_, _cc_. The lithocyst continued to animate the tissue-area _xxxx_, and through the connecting passage _y_ the contraction-waves spread over the remainder of the sub-umbrella tissue _zzzz_. So far, of course, the facts were normal; but very frequently it was observed that the contraction-waves did not start from the lithocyst, or from the area _xxxx_, but from the point o in the area _zz_. After this origination of the contraction-waves from the point _o_ had been observed a great number of times, I removed the lithocyst. The effect was not only to prevent the further origination of contraction-waves in the area _xxxx_, but also to prevent their further origination from the point _o_, the entire umbrella thus becoming paralyzed. Hence, before the removal of the lithocyst, the contraction-waves which originated at the point _o_, no less than those which originated at the lithocyst itself, must in some way or other have been due to the ganglionic influence emanating from the lithocyst and asserting itself at the distant point _o_.

This property, which lithocysts sometimes present, of asserting their ganglionic influence at a distance from their own locality, can only, I think, be explained by supposing that at the point where under these circumstances the contractions originate, there are situated some scattered ganglionic cells of considerable functional power, but yet not of power enough to originate contraction-waves unless re-enforced by some stimulating influence, which reaches them from the lithocyst through the nervous plexus.

_Regeneration of Tissues._

The only facts which remain to be stated in the present chapter have reference to the astonishing rapidity with which the excitable tissues of the Medusæ regenerate themselves after injury. In this connection I have mainly experimented on Aurelia aurita, and shall, therefore, confine my remarks to this one species.

If with a sharp scalpel an incision be made through the tenuous contractile sheet of the sub-umbrella of Aurelia, in a marvellously short time the injury is repaired. Thus, for instance, if such an incision be carried across the whole diameter of the sub-umbrella, so as entirely to divide the excitable tissues into two parts while the gelatinous tissues are left intact, the result of course is that physiological continuity is destroyed between the one half of the animal and the other, while the form of the whole animal remains unchanged--the much greater thickness of the uninjured gelatinous tissues serving to preserve the shape of the umbrella. But although the contractile sheet which lines the umbrella is thus completely severed throughout its whole diameter, it again reunites, or heals up, in from four to eight hours after the operation.