George John Romanes

“ 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. ”

“ Turning next to experiments in stimulation with reference to the spines, I may observe that we have found these organs to be, physiologically considered, highly remarkable and interesting, from the fact that they display co-ordinated action in a degree which entitles them to be regarded as a vast multitude of limbs. Thus, for instance, if an Echinus be taken out of the water and placed upon a table, it is no longer able to use its feet for the purpose of locomotion, as their suckers are only adapted to be used under water. ”

“ Whether one is sailing about upon the sunny sea, fishing with muslin nets for the surface fauna, or steaming away far from shore to dredge for other material, or, again, carrying on observations in the cool sea-water tanks and bell-jars of a neat 4 little wooden workshop thrown open to the sea-breezes, it alike requires some effort to persuade one's self that the occupation is really something more than that of finding amusement. ”

“ Whether or not there is a dawning consciousness in any nerve-centres considerably lower in the scale of nervous evolution, is a question which we cannot answer; but we may be quite certain that, if such is the case, the consciousness which is present must be of a commensurately dim and unsuffering kind. Consequently, even on this positive aspect of the question, we may be quite sure that by the time we come to the jelly-fish—where the object of the experiments in the first instance was to obtain evidence of the very existence of nerve-tissue—all question of pain must have vanished. ”

“ As the animal has a geometrical form of perfect symmetry, we might suppose that when it is placed upon its pole, all the 281 five rows of feet would act in antagonism to one another; for there seems nothing more to determine either the action or the inaction of one row rather than another. Indeed, if there were any moral philosophers among the Echinoderms, they might point with triumph to the fact of their being able to right themselves as an irrefutable argument in favour of the freedom of the Echinoderm will. ”

“ When, for instance, I contrast my own work in a town laboratory on vertebrated animals with that which I am now about to describe upon the invertebrated in a laboratory set up upon the sea-beach, it is impossible not to feel that the contrast in point of enjoyment is considerable. In the latter case, a summer's work resembles the pleasure-making of a picnic prolonged for months, with the sense of feeling all the while that no time is being profitlessly spent. ”

“ Among the most beautiful, as well as the most common, of the marine animals which are to be met with upon our coasts are the jelly-fish and the star-fish. Scarcely any one is so devoid of the instincts either of the artist or of the naturalist as not to have watched these animals with blended emotions of the æsthetic and the scientific—feeling the beauty while wondering at the organization. ”

“ With no species of Medusa on of artificial rhythm by immersing the paralyzed animals in heated water. I can only explain this fact by supposing that the stimulus which is supplied by the heated medium is of too uniform a character over the whole extent of the excitable tissues; it would seem that in order to produce artificial rhythm there must be a differential intensity of stimulation in different parts of the responding tissue, for no doubt even the excitatory influence of acidulated water is not of nearly so uniform an intensity over the whole of the tissue-area as is that of heated water. ”

“ If the manubrium, for instance, while in situ is continually supplying a gentle stimulus to the marginal ganglia, when it is suddenly cut off, the nerve-tracts through which this stimulating influence had previously been conveyed must be cut through; and as it is well known how irritable nerve-fibres are at their points of section, it is to be expected that the irritation caused by cutting these nerve-tracts, and probably also by the action of the sea-water on their cut extremities, would cause them to stimulate the ganglia more powerfully than they did before their mutilation. ”

“ On reaching the surface, the animal does not wish to leave its native element—in fact, cannot do so, because its sucking feet can only act under water—and neither does it wish again to descend into the levels from which it has just ascended. It, therefore, begins to feel about for rocks or sea-weeds at the surface, by crawling along the side of the tank, and every now and then throwing back its uppermost ray or rays along the surface of the water to feel for any solid support that may be within reach. ”

“ The manubrium is always the first to respond to the stimulation, its retraction preceding the first movements of the bell by an interval of several seconds, so that by the time the bell begins its rhythmic response the manubrium is usually retracted to its utmost. The initial response of the manubrium is also rhythmic, and the rhythm which it manifests—especially if the glycerine solution be not over-strong—is of the same slow character which has already been described as manifested by this organ when under the influence of mechanical stimulation. ”

“ Now, in the first place, I have here to show that there is, as I have said, a fundamental error touching a well-known principle of physiology. So far as there is any difference between the excitability of nerve and muscle with respect to mechanical and electrical stimulation, it is the precise converse of that which Dr. Krukenberg supposes; it is not muscle, but nerve, which is the more sensitive to electrical stimulation—by which I understand him to mean the induction shock. ”

“ Nevertheless, as just stated, other modes of constant stimulation certainly have a more or less marked effect in producing rhythmic response. The constant current, during the whole time of its passage, frequently has this effect in the case of the paralyzed nectocalyx of Sarsia; and dilute spirit, or other irritant, when dropped on the paralyzed swimming organ of Aurelia aurita, often gives rise to a whole series of rhythmical pulsations, the systoles and diastoles following one another at about the same rate as is observable in the normal swimming motions of the unmutilated animal. ”

“ To explain the function of nerve-tissue, it is necessary to begin by explaining what physiologists mean by the term "excitability." Suppose that a 24 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. ”

“ In the case of Sarsia, for instance, a flash of light let fall upon a living specimen almost invariably causes it to respond with one or more contractions. If the animal is vigorous and swimming freely in water, 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, or if it be removed from the water and spread flat upon an object-glass, it usually gives only one contraction in response to every flash. ”

“ It is somewhat difficult, in the case of paralyzed swimming organs, to prove the occurrence of a contraction in response to thermal stimulation, from the fact that while these tissues are not nearly so sensitive to this mode of excitation as might be anticipated, they are, as just observed, extraordinarily sensitive to mechanical excitation. It therefore becomes difficult to administer the appropriate thermal stimulus without at the same time causing a sufficient mechanical disturbance to render it doubtful to which of the stimuli the response is due. ”

“ If the stimulation is persistent, the manubrium will every now and then pause for a few seconds, as if trying to decide from which direction the stimulation is proceeding, and will then suddenly move over and apply its extremity, perhaps to the point that is opposite to the one which it is endeavouring to find. ”

“ The manubrium, like the umbrella, varies much in size and shape in different species, as a glance at any figures of these animals will show. Both the manubrium and umbrella are almost entirely composed of a thick, transparent, and non-contractile jelly; but the whole surface of the manubrium and the whole concave surface of the umbrella are overlayed by a thin layer or sheet of contractile tissue. This tissue constitutes the earliest appearance in the animal kingdom of true muscular fibres, and its thickness, which is pretty uniform, is nowhere greater than that of very thin paper. ”

“ I shall now conclude this chapter by giving a brief account of those general principles of the physiology of nerve and muscle with which it is necessary to be fully acquainted, in order to understand the course of the following experiments.
Nerve-tissue, then, 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. ”

“ By-and-by a state of continuous quiescence comes on, during which the animal is not responsive to tentacular irritation, but remains so to direct muscular irritation, giving one response to each direct stimulus. The tentacles and manubrium are much relaxed. In a sea-water solution just strong enough to taste bitter, this phase may continue for hours; in fact, till a certain opalescence of the contractile tissues—which it is a property of strychnia, as of most other reagents, to produce—has advanced so far as to place the tissues beyond recovery. ”

“ Moreover, in some cases it is interesting to observe that when the equator position has been reached with difficulty, the Echinus, as it were, gives itself a breathing space before beginning the movement of descent—drawing in all its pedicels save those which hold it securely in the position to which it has attained, and remaining in a state of absolute quiescence for a prolonged time. It then suddenly begins to protrude all its feet again, and to continue its manœuvre. At any time during such a period of rest, a stimulus of any kind will immediately determine a recommencement of the manœuvre. ”

“ Thus, for instance, when the nervous continuity of the spinal cord is interrupted, so that a stimulus applied to the lower extremities is unable to pass upwards to the brain, the feet will be actively drawn away from a source of irritation without the man being conscious of any pain; the lower nervous centres in the spinal cord respond to the stimulation, but they do so without feeling the stimulus. ”

“ I entertain this supposition because, as explained in my Royal Society papers, I see reason to believe that the natural swimming movements of Sarsia are probably in part due to an intermittent discharge of the ganglia. I think, therefore, that in this particular case the ganglia supply a tolerably constant stimulation to the manubrium, while it is only at intervals that their energy overflows into the bell, and that the higher degree at irritability on the part of the manubrium ensures the tonic response of this organ at a small cost of nervous energy. ”

“ The suckers, moreover, in being protruded from all sides of a globe instead of from the under side of a flat organism, are of much more use as feelers than they are in the Star-fish. Therefore, while advancing, the feet facing the direction of advance are always kept extended to their fullest length, in order to feel for any object which the animal may possibly be approaching. When a perpendicular surface is reached, the Echinus may either ascend it or not, as in the case of the Star-fish. While walking, the animal keeps pretty persistently in one direction of advance. ”

“ In the first place, we must remember that in the Medusæ there are no nervous centres of such vital importance to the organism that any temporary suspension of their functions is followed by immediate death. Therefore, in these animals, the various central nerve-poisons are at liberty, so to speak, to exert their full influence on all the excitable tissues without having the course of their action interrupted by premature death of the organism, which in higher animals necessarily follows the early attack of the poison on a vital nerve-centre. ”

“ That 7 is to say, before convincing me of sin, it must be shown that there is some reasonable ground for supposing that a jelly-fish or a star-fish is capable of feeling pain. I submit that there is no such ground. The mere fact that the animals are alive constitutes no such ground; for the insectivorous plants are also alive, and exhibit even more physiological "sensitiveness" and capability of rapid response to stimulation than is the case with the animals which we are about to consider. ”

“ 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. ”

“ In specimens mutilated by severing the nerves at the base of each of the five rays, or by dividing the nerve-ring between all the rays, the animal loses all power of co-ordination among its rays. When a common Star-fish is so mutilated it does not crawl in the same determinate manner as an unmutilated animal, but, if it moves at all, it moves slowly and in various directions. When inverted, the power of effecting the righting manœuvre is seen to be gravely impaired, although eventually success is always achieved. ”

“ 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. ”

“ In the tissue, as in the cotton, every fibre appears to be endowed with the capacity of liberating energy in either of two very different ways; and whenever one part of the mass is made, by the appropriate stimulus, to liberate its energy in one of these two ways, all other parts of the mass do the same, and this no matter how far through the mass the liberating process may have to extend. ”

“ But, by experimenting on other species of Medusæ, I have been able to obtain, in response to mechanical and chemical stimulation, artificial rhythm of a much more sustained character than that which, under such modes of stimulation, occurs in Aurelia. I have no explanation to offer why it is that some species, or some tissues, present so much more readiness to manifest sustained rhythm under certain modes of stimulation, and less readiness to manifest it under other modes, than do other species or tissues. ”

“ If the strength of the solution be now again increased, a stage of deeper anæsthesiation may be produced, in which the Medusa will only respond to each stimulation by a single spasm. In still stronger solutions, the only response is a single feeble contraction; while in a nearly saturated solution the animal does not respond at all. But even from a saturated solution Tiaropsis diademata will recover when transferred to unpoisoned water. ”