Popular Science Monthly/Volume 14/January 1879/The Beginning of Nerves in the Animal Kingdom
|THE BEGINNING OF NERVES IN THE ANIMAL KINGDOM.|
NERVE-TISSUE universally consists of two elementary structures, viz., very minute nerve-cells and very minute nerve-fibres. The fibres proceed to and from the cells, so in some cases serving to unite the cells with one another, and in other cases with distant parts of the animal body. Nerve-cells are usually found collected together in aggregates, which are called nerve-centres or ganglia, to and from which large bundles of nerve-fibres come and go.
To explain the function of nerve-tissue, it is necessary to begin by explaining what physiologists mean by the term "excitability." Suppose that a muscle has been cut from the body of a freshly-killed animal; so long as it is not interfered with in any way, so long will it remain quite passive. But every time a stimulus is supplied to it, either by means of a pinch, a burn, an electrical shock, or a chemical irritant, the muscle will give a single contraction in response to every stimulation. And it is this readiness of organic tissues to respond to a suitable stimulus that physiologists designate by the term "excitability."
Nerves, no less than muscles, present the property of being excitable. If, together with the excised muscle, there had been removed from the animal's body an attached nerve, every time any part of this nerve is stimulated the attached muscle will contract as before. But it must be carefully observed that there is this great difference between these two cases of response on the part of the muscle—that, while in the former case the muscle responded to a stimulus applied directly to its own substance, in the latter case the muscle responded to a stimulus applied at a distance from its own substance, which stimulus was then conducted to the muscle by the nerve. And in this we perceive the characteristic function of nerve-fibres, viz., that of conducting stimuli to a distance. The function of nerve-cells is different, viz., that of accumulating nervous energy ; and, at fitting times, of discharging this energy into the attached nerve-fibres. The nervous energy, when thus discharged, acts as a stimulus to the nerve-fibre ; so that, if a muscle is attached to the end of a fibre, it contracts on receiving this stimulus. I may add that, when nerve-cells are collected into ganglia, they often appear to discharge their energy spontaneously ; so that in all but the very lowest animals, whenever we see apparently spontaneous action, we infer that ganglia are probably present. Lastly, another important distinction must be borne in mind—the distinction, namely, which I now draw between muscle and nerve. A stimulus applied to a nerveless muscle can only course through the muscle by giving rise to a visible wave of contraction, which spreads in all directions from the seat of disturbance as from a centre. A nerve, on the other hand, conducts the stimulus without undergoing any change of shape. Now, in order not to forget this distinction, I shall always speak of muscle-fibres as conveying a visible wave of contraction, and of nerve-fibres as conveying an invisible, or molecular, wave of stimulation. Nerve-fibres, then, are functionally distinguished from muscle-fibres—and also from protoplasm—by displaying the property of conducting invisible, or molecular, waves of stimulation from one part of an organism to another, so establishing physiological continuity between such parts, without the necessary passage of contractile waves.
Such being the structure and the function of nerve-tissue in its fully evolved form, I will now proceed to give the results of my researches on the structure and function of nerve-tissue where this tissue is first found to occur in the ascending series of animal life. The animals in which it so occurs are the Medusæ or jelly-fishes, which must be familiar to all who frequent the seaside. These animals present the general form of a mushroom. The organ which occupies the same position as the stalk does in the mushroom is the mouth and stomach of the medusa, and is called the polypite ; while the organ which resembles in shape the dome of the mushroom non-contractile jelly; but the whole surface of the polypite, and the whole concave surface of the bell, are overlaid by a thin layer, or sheet, ofthe main bulk of the animal, and is called the swimming-bell. Both the polypite and the swimming-bell are almost entirely composed of a thick, transparent, and
contractile tissue. This tissue constitutes the earliest appearance in the animal kingdom of true muscular fibres. The thickness of this continuous layer of incipient muscle is pretty uniform, and is nowhere greater than that of very thin paper. The margin of the bell supports a series of highly contractile tentacles, and also another series of bodies which are of great importance in the following researches. These are the so-called marginal bodies, which are here represented, but the structure of which I need not describe. Lastly, it may not be superfluous to add that all the Medusæ are locomotive. The mechanism of their locomotion is very simple, consisting merely of an alternate contraction and relaxation of the entire muscular sheet which lines the cavity of the bell. At each contraction of this muscular sheet the gelatinous walls of the bell are drawn together; the capacity of the bell being thus diminished, water is ejected from the open mouth of the bell backward, and the consequent reaction propels the animal forward. In these swimming movements systole and diastole follow one another with as perfect a rhythm as they do in the beating of a heart.
Previous to my researches, the question as to whether or not the Medusæ, possess a nervous system was one of the most vexed questions in biology — some eminent naturalists maintaining that they could detect microscopical indications of nervous tissues, and others maintaining that these indications were delusive — the deliquescent nature of the gelatinous tissues rendering microscopical observation in their case a matter of great difficulty. But amid all this controversy no one appears to have thought of testing the question by means of physiological experiments as distinguished from microscopical observations. Accordingly, I made the experiment of cutting off now one part and now another part of a jelly-fish, in order to see whether by so doing I could alter the character of its movements in such a way as to show that I had removed nerve-centres or ganglia. The results which I obtained were in the highest degree astonishing. For, on removing the extreme margin of the swimming-bell, I invariably found that the operation caused immediate, total, and permanent paralysis of the entire organ. That is to say, if, with a pair of scissors, I cut off the whole marginal rim of the bell, carrying the cut round just above the insertion of the tentacles, the moment the last atom of the margin was removed, the pulsations of the bell instantly and forever ceased. On the other hand, the severed margin continued its pulsations with vigor and pertinacity, notwithstanding its severance from the main organism. For hours and even for days after its removal the severed margin would continue its rhythmical contractions; so that the contrast between the death-like quiescence of the mutilated bell and the active movements of the thread-like portion which had just been removed from its margin was a contrast as striking as it is possible to conceive.
I may here add that, although excision of the margin of the bell thus completely destroys the spontaneity of the bell, it does not at all diminish the excitability of the bell; so that, although the mushroom-shaped mass will never move of its own accord after having been thus mutilated, it will give any number of locomotor contractions in response to an equal number of artificial stimulations, just in the same way as a frog with its head (nerve-centres of spontaneity) removed will give any number of hops in response to successive stimulations.
These experiments, therefore, prove conclusively that, in the extreme marginal rim of all the numerous species of Medusæ which I examined, there is situated an intensely localized system of nervous centres, to the functional activity of which the rhythmical contractions of the swimming-bell are exclusively due. And as the Medusæ are thus the lowest animals in which a nervous system has yet been or probably ever will be discovered, we have in them the animals upon which we may experiment with the best hope of being able to elucidate all questions concerning the origin and endowments of primitive nervous tissues. I may here add that these experiments were independently made by Dr. Eimer, of Würzburg.
After I had made the observation which I have described, it seemed to me desirable to follow it up with a number of other physiological, as distinguished from histological, researches. For I was much struck by the certainty and precision of the results which I had obtained by experiment, as distinguished from the uncertainty and disagreement of the results which had previously been obtained by the histological methods. Accordingly, I decided, in the first instance, to feel my way in the direction of physiological experiment before beginning that systematic histological research which, sooner or later, it was manifestly imperative to make. Study of function having so far guided the study of structure as to show that it was in the margin of the Medusæ, that we must look for the principal if not the exclusive supply of central nervous tissue, it seemed desirable to ascertain how much light a further study of function might throw on the character and the distribution of the peripheral nervous tissue. Accordingly, I began my physiological work chiefly with the view of guiding my subsequent histological work. But, as the physiology of the subject continued to open up in the wonderful way in which it did, I felt it was undesirable either, on the one hand, to suspend this part of the inquiry, or, on the other hand, any longer to defer a thorough investigation of the histological part. I therefore represented the case to my friend Mr. Schäfer, who very kindly consented to join me in Scotland with the view of coöperating with me in the research. The histological results which he has obtained from a most skillful and painstaking investigation are in the highest degree interesting. He worked chiefly with Aurelia aurita and found that the tissue which performs the ganglionic function in the marginal bodies is of the nature of modified epithelium-cells, the ganglionic function of which could scarcely have been suspected but for the paralyzing effects which are produced by their excision. From these marginal ganglia there radiate what he regards as delicate pale nerve-fibres, which sometimes present the appearance of fibrillation.
These fibres spread over the entire expanse of the muscular sheet in
Fig. 2. great numbers. It will thus be seen that these microscopical researches of Mr. Schäfer fully bear out my inference from the result of physiological experiments, which was previously published at the Royal Society—the inference, namely, that the entire muscular sheet of the Medusæ is overspread by a dense plexus of nervous channels. But these researches of Mr. Schäfer tend to negative another inference which was published at the Royal Institution—the inference, namely, as to the degree in which these channels are differentiated. As the facts on which this inference was based have not been previously published in the Fortnightly Review, and as, apart from the dubious inference, they are facts of the first importance, it is necessary that I should here very briefly restate them. The annexed woodcut (Fig. 3) represents a specimen of Aurelia aurita with its polypite cut off at the base, and the under or concave surface of the bell exposed to view. The bell, when fully expanded, as here represented, is about the size of a soup-plate, and in it all the ganglia are collected into these eight marginal bodies, as proved by the fact that on cutting out all the eight marginal bodies paralysis of the bell ensues. Therefore, if the reader will imagine this diagram to be overspread with a disk of muslin, the fibres of which start from one or other of 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 contractile 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
Fig. 3. the point of the disk which is opposite to the ganglion. Suppose now a number of radial cuts are made in the disk, according to such a plan as this, wherein every radial cut deeply overlaps those on either side of it. The contractile waves which now originate from the ganglion must either become blocked and cease to pass round the disk, or they must zigzag round and round the tops of these overlapping cuts. Now, remembering that the passage of these contractile 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 contractile wave. Yet this is not the case; for, even in a specimen of Aurelia so severely cut as the one here represented, the contractile waves, starting from the ganglion, continued to zigzag round and round the entire series of sections.
The same result attends other forms of section. Here, for instance, 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 contractile 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 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 a single remaining ganglion at one end. (The cuts interposed in the parallelogram may for the present be neglected.) Now, if the end-mark a of the nervo-muscular sheet most remote from the ganglion be gently brushed with a camel's-hair brush — i. e., too gently to start a responsive contractile wave — the ganglion at the other end will shortly afterward discharge, as shown by its starting a contractile 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 actually 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 here represented as having traversed 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 the 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 toward 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 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 perhaps erroneous, and that, in any case, when stained with gold, some of the nervous channels show themselves in the form of fully differentiated nerves. Now this fact, it is needless to say, greatly enhances the interest of the previous experiments. If, as I formerly said, the proof of vicarious action being possible to an almost unlimited extent in these incipient nerve-fibres appeared to me one of the most interesting among the additions to our knowledge of invertebrate physiology, much more interesting does this proof become if we further learn that these incipient nerve-fibres are only incipient in the sense of constituting the earliest appearance of nerve-fibres in the animal kingdom. For if these true nerve-fibres admit, from the peculiarly favorable plan of their anatomical distribution, of being proved to be not improbably capable of vicarious action to so extraordinary a degree, we may become the more prepared to believe that nerve-fibres elsewhere are similarly capable of vicarious action. But the interest does not end here, for Mr. Schäfer's numerous preparations all show the highly remarkable fact 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 around one another like the strands of a rope—they can never be actually seen to join, but remain anatomically isolated 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 that 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 if, for the sake of a name, we call this process, whatever it may be, a process of physiological induction, we may apply a similar name to a process which seems closely analogous to, if it is not really identical with, the process we are now considering. I refer to some highly remarkable observations which were published a year or two ago in Mr. Darwin's work on "Insectivorous Plants". It is there stated that, while looking at a linear series of excitable cells with the microscope, Mr. Darwin could observe the passage of a stimulus along the series, the protoplasm in the cells immediately stimulated first undergoing aggregation, then the protoplasm in those next adjacent doing the same, and so on. Now, the protoplasm in each cell was separated from the protoplasm in the adjacent cell by the walls of both the cells ; yet, notwithstanding there was no observable anatomical continuity between these masses of protoplasm, a disturbance set up in any one of the series of masses immediately set up, by some process of physiological induction, a sympathetic disturbance in the immediately adjacent masses.
This, then, is one case that seems to be comparable with the case of physiological induction in the nerve-fibres of Aurelia, and 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.
Before quitting the histological part of the subject, it is desirable to state that at about the same time as Mr. Schäfer's work was communicated to the Royal Society, two other papers were published in Germany on the same subject. One of these papers was by Messrs. Hertwig, and the other by Dr. Eimer. Both memoirs display a large amount of patient research, and describe the character and distribution of the nervous tissues in various species of Medusæ. These authors, however, do not describe the nervous network which has been described by Mr. Schäfer. I may add the interesting fact that the nervous tissues in Medusæ appear to be exclusively restricted to the body-layer which is called the ectoderm, and which is the structural homologue of that body-layer in which the nervous tissues of all the higher animals are known to have their origin during the life-history of the embryo.
Proceeding now to state some further results of various physiological experiments, I shall begin with the department Stimulation. And first to take the case of a physiological principle which I observed in the jelly-fish, and which has also been found to run through all excitable tissues. If a single stimulation is supplied to a paralyzed jelly-fish, a short period, called the period of latency, will elapse, and then the jelly-fish will give a single weak contraction. If, as soon as the tissue has relaxed, the stimulation is again repeated, the period of latency will be somewhat shorter, and will be followed by a somewhat stronger contraction. Similarly, if the stimulation is repeated a third time, the period of latency will be still shorter, and the ensuing contraction still stronger. And so on up to nine or ten times, when the period of latency will be reduced to its minimum, while the force of the contraction will be raised to its maximum. So that in the jelly-fish the effect of a series of excitations supplied at short intervals from one another, is that of both arousing the tissue into a state of increased activity, and also of producing in it a state of greater expectancy. Now, effects very similar to these have been found to occur in the case of the excitable plants by Dr. Burdon-Sanderson; in the case of the frog's heart by Dr. Bowditch; and in the case of reflex action of the spinal cord by Dr. Sterling. Indeed, the only difference in this respect between these four tissues, so widely separated from one another in the biological scale, consists in the time, which may be allowed to elapse between the occurrence of the successive stimuli, in order to produce this so-called summating effect of one stimulus upon its successor: the memory, so to speak, of the heart-tissue, for the occurrence of a former stimulus being longer than the memory of the jelly-fish tissue; while the memory of the latter is longer than that of the plant-tissue. And I may here add that even in our own organization we may often observe the action of this principle of the summation of stimuli. For instance, we can tolerate for a time the irritation caused by a crumb in our throats; but very rapidly the sense of irritation accumulates to a point at which it becomes impossible to avoid coughing. And similarly with tickling generally, the convulsive reflex movements to which it gives rise become more and more in controllable the longer the stimulation is continued, until they reach a maximum point, where, in persons susceptible of this kind of stimulation, the muscular action passes completely beyond the power of the will. Lastly, I may further observe, what I do not think has ever been observed before, that even in the domain of psychology the action of this principle admits of being clearly traced. Who, for instance, has not felt it in the case of the ludicrous? We can endure for a short time, without giving any visible response, the psychological stimulation which is supplied by a comical spectacle; but if the latter continues sufficiently long in a sufficiently ludicrous manner, our appropriate emotion very rapidly runs up to a point at which it becomes incontrollable, and we burst into an explosion of ill-timed laughter. But in this case of psychological tickling, as in the previous case of physiological tickling, some persons are much more susceptible than others. Nevertheless, there can be no doubt that, from the excitable tissues of a plant, through those of a jelly-fish and a frog, up even to the most complex of our psychological processes, we have in this recently discovered principle of the summation of stimuli a very remarkable uniformity of occurrence.
Hitherto light has never been actually proved to act as a direct stimulus to ganglionic matter. It is therefore of interest to note that it thus acts in the case of some species of Meduscæ. Sarsicæ, for instance, almost invariably respond to a single flash by giving one or more contractions. If the animal is vigorous, the effect of a momentary flash thrown upon it during one of the natural pauses is immediately to originate a bout of swimming; but if the animal is non-vigorous, it usually gives only one contraction in response to every flash. That it is light per se, and not the sudden transition from darkness to light, which here acts as the stimulus, is proved by the result of the converse experiment, viz., placing a vigorous specimen in sunlight, waiting till the middle of one of the natural pauses, and then suddenly darkening. In no case did I thus obtain any response. Indeed, the effect of this converse experiment is rather that of inhibiting contractions; for if the sunlight be suddenly shut off during the occurrence of a swimming-bout, it frequently happens that the quiescent stage immediately sets in. Again, in a general way, it is observable that Sarsiæ are more active in the light than they are in the dark: it appears as though light acts toward these animals as a constant stimulus. Nevertheless, when the flashing method of experimentation is employed, it is observable that the stimulating effect of the flashes progressively declines with their repetition. The time during which the deleterious effect of one such stimulus on its successor lasts appears to be about a quarter of a minute. The period of latent stimulation is, judging by the eye, as short in the case of luminous as in that of other stimulation; but when the efficacy of luminous stimulation is being diminished by frequent repetition, the period of latency is very much prolonged.
The question as to what part of the organism it is which is thus susceptible of luminous stimulation, was easily determined by detaching various parts of the organism and experimenting with them separately. I thus found that it is the marginal bodies alone which are thus affected by light; for, when these are removed, the swimming-bell, though still able (in the case of Sarsia) to contract spontaneously, no longer responds to luminous stimulation; whereas, if only one marginal body be left in situ, or if the severed margin, or even a single excised marginal body, be experimented on, unfailing response to this mode of stimulation may be obtained.
Responses to luminous stimulation occur in all cases equally well, whether the light employed be direct sunlight, diffused daylight, polarized light, or any of the luminous rays of the spectrum employed separately. On the other hand, neither the non-luminous rays beyond the red, nor those beyond the violet, appear to exert the smallest degree of stimulating effect. Hence, in all respects, the rudimentary eye of Sarsia appears to be affected by the same qualities of light as are our own.
Not so, however, in the case of another species of medusa, which I have called Tiaropsis polydiademata. This jelly-fish responds to luminous stimulation in the same peculiar manner as it responds to all other artificial — as distinguished from natural ganglionic — stimulation; that is to say, instead of giving a locomotor contraction of the bell, it throws the bell into a violent contraction of a long-sustained character, resembling cramp or tonic spasm. Now, in the case of this medusa, the luminous stimulation requires to act for a comparatively long time in order to produce a response. For, while in Sarsia the period of latent stimulation appears to be as short in the case of luminous as it is in the case of other modes of stimulation, in the ease of Tiaropsis this is not so, although, as regards all modes of stimulation other than luminous, the latent period is as brief in the case of Tiaropsis as it is in the case of Sarsia. In other words, while this period is quite as instantaneous in the case of Tiaropsis as it is in the case of Sarsia when the stimulus employed is other than luminous, in response to light the characteristic spasm does not take place till slightly more than a second has elapsed after the first occurrence of the stimulus. Now, as my experiments on Sarsia proved that the only respect in which luminous stimulation differs from other modes of stimulation consists in its being exclusively a stimulation of ganglionic matter, we have evidence, in the case of Tiaropsis, of an enormous difference between the rapidity of response to stimuli by the contractile and by the ganglionic tissues respectively. The next question, therefore, is as to whether the enormous length of time occupied by the process of stimulation in the ganglia is due to any necessity on the part of the latter to accumulate the stimulating influence of light prior to originating a discharge, or to an immensely lengthened period of latent stimulation manifested by the ganglia under the influence of light. To answer this question, I first allowed a continuous flood of light to fall on the medusid, and then noted the time at which the responsive spasm first began. This time, as already stated, was slightly more than one second. I next threw in single flashes of light of measured duration, and found that, unless the flash was of slightly more than one second's duration, no response was given. That is to say, the minimal duration of a flash required to produce a responsive spasm was just the same as the time during which a continuous flood of light required to operate in order to produce a similar spasm. From this, therefore, I conclude that the enormously long period of latent excitation in the case of luminous stimuli is not, properly speaking, a period of latent excitation at all ; but that it represents the time during which a certain summation of stimulating influence is taking place in the ganglia, which requires somewhat more than a second to accumulate, and which then causes the ganglia to originate an abnormally powerful discharge. So that in the action of light upon the ganglionic matter of this medusid we have some analogy to its action on certain chemical compounds in this respect—that, just as in the case of those compounds which light is able to split up, a more or less lengthened exposure to its influence is necessary in order to admit of the summating influence of its vibrations on the molecules; so in the case of this ganglionic material, the decomposition which is effected in it by light, and which terminates in an explosion of nervous energy, can only be effected by a prolonged exposure of the unstable material to the summating influence of the luminous vibrations. Probably, therefore, we have here the most rudimentary type of a visual organ that is possible; for it is evident that, if the ganglionic matter were a very little more stable than it is, it would either altogether fail to be thrown down by the luminous vibrations, or would occupy so long a time in the process that the visual sense would be of no use to its possessor. How great is the contrast between the excitability of such a sense-organ and that of a fully evolved eye, which is able to effect the needful molecular changes in response to a flash as instantaneous as that of lightning!
Before leaving the case of luminous stimulation, I may observe that some of the Medusæ appear to be very fond of light. For, on placing a number of Sarsiæ in a large bell-jar in a dark room, and then throwing a beam of light through a part of the water in the bell-jar, the Medusæ all crowded into the path of the beam, and dashed themselves against the glass nearest to the light, very much as moths might do under the influence of similar stimulation. On moving the lamp round the jar, a cluster of Medusæ always followed it. This latter experiment is important, because it proves that the marginal ganglia are so far coordinated in their action that they can steer the animal in any particular direction.
Staurophora laciniata is a large species of naked-eyed medusa, which responds to stimulation in two very different ways, according as the stimulation is applied to the nervo-muscular sheet, or to the marginal ganglia. For, if the stimulation is applied to the nervo-muscular sheet, the response is an ordinary locomotor contraction; whereas, if the stimulation is applied to the marginal ganglia, the response is a tonic spasm of the same kind as that already alluded to in the case of Tiaropsis polydiademata. Now, it is a remarkable fact that into whatever form the bell of this medusa is cut — say, for instance, into the form of a long ribbon — whenever a locomotor contraction is started by stimulating any part of the general nervo-muscular sheet, it will pass all through that sheet, from end to end of the ribbon, in the form of an ordinary or gentle contractile wave. On the other hand, whenever a spasmodic contraction is started in the nervo-muscular sheet by stimulating any of the marginal ganglia, it will pass all through that sheet, from end to end of the ribbon, in the form of a spasmodic or violent contractile wave. Hence the muscular fibres of this medusa are capable of liberating this energy in either of two very different ways; and whenever some of them liberate their energy in one of these two ways, they determine that all the other fibres in the nervo-muscular sheet shall do the same. So that we may adopt a far-fetched but convenient simile, and liken the muscular fibres in this medusa to the fibres in a mass of gun-cotton. For in a mass of gun-cotton the fibres are likewise able to liberate their energy in either of two very different ways — viz., either by burning in quiet flame when they are simply ignited, or by exploding in a violent manner when they are detonated, as by a percussion-cap. And both in the case of the muscle-fibres of Staurophora and the cotton-fibres of gun-cotton, whenever any one of the whole number is made by appropriate stimulation (i. e., muscular stimulation or ignition) to liberate its energy in a quiet manner, then all the other fibres in the mass do the same ; whereas, if any one of the whole number is made by another appropriate stimulation (i. e., ganglionic stimulation or detonation) to liberate its energy in a violent manner, then all the other fibres in the mass do the same. Now why the ganglia of this medusa should thus act as detonators to the muscular fibres, and why, if they do, the muscular fibres should be capable of two such different kinds of response — these are questions quite novel in physiology, and as such I will not endeavor to answer them.
Poisons. — As my space is now very nearly exhausted, I will conclude this article by very briefly stating the general results of a large number of observations concerning the action of various nerve-poisons on the Medusæ. It is easy to see that this is an important branch of the inquiry on which I am engaged; for in the nerve-poisons we have, as it were, so many tests whereby to ascertain whether nerve-tissue, where it first appears upon the scene of life, is of the same essential character, as to its various functions, as is the nerve-tissue of higher animals.
Chloroform, ether, morphia, etc., all exert their anæsthesiating influence on the Medusæ quite as decidedly as they do on the higher animals. Soon after a few drops of the anæsthetic have been added to the water in which the Medusæ are contained, the swimming motions of the latter become progressively slower and feebler, until in a minute or two they cease altogether, the animals remaining at the bottom of the water, apparently quite dead. No form or degree of stimulation will now elicit the slightest response; and this fact, it must be remembered, is quite as remarkable in the case of the Medusæ as in that of any other animal. Recovery in normal sea-water is exceedingly rapid, especially in the case of chloroform and ether.
The effects of strychnia may be best observed on a species called Gyancæ capillata, from the fact that, in water kept at a constant temperature, the ordinary swimming motions of this animal are as regular and sustained as the beating of a heart. But soon after the water has been poisoned with strychnia, unmistakable signs of irregularity in the swimming motions begin to show themselves. Gradually these signs of irregularity become more and more pronounced, until at last they develop into well-marked convulsions. The convulsions show themselves in the form of extreme deviations from the natural rhythm of this animal's motion. Instead of the heart-like regularity with which systole and diastole follow one another in the unpoisoned animal, we may now observe prolonged periods of violent contraction, amounting in fact to tonic spasm; and even when this spasm is momentarily relieved, the relaxation has no time to assert itself properly before another spasm supervenes. Moreover, these convulsions are very plainly of a paroxysmal nature; for after they have lasted from five to ten minutes, a short period of absolute repose comes on, during which the jelly-fish expands to its full dimensions, falls to the bottom of the water in which it is contained, and looks in every way like a dead animal. Very soon, however, another paroxysm sets in, and so on — prolonged periods of convulsion alternating with shorter periods of repose for several hours, until finally death puts an end to all these symptoms so characteristic of strychnine-poisoning in the higher animals.
Similarly, without going into tedious details, I may say in general terms that I have tried caffein, nitrite of anyl, nicotin, veratrium, digitalin, atropin, curare, cyanide of potassium, alcohol, as well as other poisons; and almost without any exception I find them to produce the same effects on the Medusæ as they severally produce on the higher animals. The case of alcohol is particularly interesting, not only because an intoxicated jelly-fish is a ludicrous object to observe, but also because the experiments with alcohol show how precisely the specific gravity of the Medusæ is adjusted to that of the sea-water. For if, after a jelly-fish has become tolerably well drunk by immersion in a mixture of alcohol-and-water, it is transferred to normal sea-water, the exceedingly small amount of alcohol which it has imbibed is sufficient to make the animal remain permanently floating at the surface of the water until it again gets rid of the alcohol by osmosis.
As my space is now at an end, I must postpone for the present my account of a number of other experiments which, in point of interest, though not in point of systematic arrangement, have a better claim to statement than some of those which I have now detailed. It is impossible, however, in one article to treat of all the new facts which have been yielded by this research; so that by making the present article dovetail with the one which was previously published in Nature, and also with future articles on the same subject, I shall hope eventually to lay all the results before the general public.—Fortnightly Review.
- Although it sounds somewhat paradoxical to speak of the central nervous tissue as distributed on the periphery of a circular animal, and of the peripheral nervous tissue as occupying all the more centrally situated parts, the paradox is unavoidable.
- I may here state that, previous to Mr. Schäfer's researches, I had observed both the tissue-elements which he describes; but I hesitated to pronounce upon their nervous character. It will thus be understood that even now, without wishing to dispute the accuracy of his judgment in this matter, I do wish it to be known that the responsibility of this judgment rests entirely with my friend.
- 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.
- That it can scarcely be an electrically inductive effect 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.
- In all the naked-eyed division of Meduscæ, to which Sarsia belongs, total paralysis of the bell can only be obtained by removing the entire margin; but in all the covered-eyed division, to which Aurelia belongs, paralysis of the bell ensues on removing the marginal bodies alone.
- The period of latent stimulation merely means the time after the occurrence of an excitation during which a series of physiological processes are taking place which terminate in a contraction ; so that, whether the excitation is of a strong or of a weak intensity, the period of latent stimulation is not much affected. The above question, therefore, was simply this: Does the prolonged delay on the part of these ganglia, in responding to light, represent the time during which the series of physiological processes are taking place in response to an adequate stimulus, or does it represent the time during which light requires to act before it becomes an adequate stimulus?