Life in Motion/Lecture VI
LECTURE VI
Electrical phenomena of muscle—
To-day I wish, in the first place, to demonstrate to you certain properties of muscle to which I have not alluded. I will show you that muscle has electrical properties. Let us examine some of the facts that led to the discovery of animal electricity, a discovery of the most momentous consequences to the human race.
At the back of the lecture-theatre we have placed a very sensitive galvanometer, specially constructed for the kind of work to which it is now to be put. It belongs to Professor Dewar, and was presented to him by the late Mr. Warren de la Rue, whose zeal in the cause of science is well known to all connected with this Institution. Professor Dewar has kindly
Fig. 67.—Wiedemann's galvanometer, much employed, especially in Continental schools, by physiologists. The instrument used in the lecture was a Thomson's (Lord Kelvin) galvanometer, and it is shown in Fig. 58, b. Wiedemann's instrument I have often used with great advantage for class demonstration. The two outer coils are of low, and the two inner coils are of high, resistance. The low resistance coils are used for thermal currents, and the high resistance coils for the currents of living tissues. A ring magnet is suspended by a long filament of silk, and hangs in a copper box in the centre of the coils of wire. On the rod carrying the ring-magnet, and in the box above the coils, we have a mirror, which reflects a beam of light on to the scale.
placed it at my service, and I wish to say in a word how much I feel indebted to him for the great interest he has taken in these lectures. This instrument has a resistance of no less than 86,000 ohms, and it is one of the most sensitive of its kind.
Fig. 68.—Mirror and ring-magnet of Wiedemannn's galvanometer. B, mirror; A, ring-magnet.
Now if I brought the wires of this instrument into direct contact with the muscle, we would probably get a current; but that would be no proof that the current really came from the muscle. The copper wires come into contact with the moist surface of the muscle, and this contact would at once, by chemical action, generate a current of electricity. For example, to show you how easily currents can be produced and detected by an instrument of this kind, observe that when I touch the wires the spot of light at once moves. We must have some means, therefore, of leading off from the muscle any current that may be produced by it, without generating currents in the apparatus we employ. The question arises—what contact can be made with a muscle without generating currents at the surface of contact?
After much labour, Professor E. du Bois Reymond of Berlin invented the proper appliances. Let me mention in passing the name of du Bois Reymond with much respect. He has not only laid the foundations, but he has built much of the superstructure of our knowledge of electro-physiology; he, more than most men, has investigated the hidden processes in muscles and nerves, devising and even constructing, in the first instance, with his own hands, much of the apparatus now employed in such investigations; and it is interesting to know that he demonstrated, in 1855, many of his discoveries at a famous lecture given in this Institution. Well, du Bois Reymond found that zinc troughs, carefully rubbed over with mercury, or, as it is termed, amalgamated, and filled with a saturated solution of sulphate of zinc, fulfilled the conditions. Into these troughs we place pads of white blotting-paper (Swedish filter-paper). But if we laid the muscle on these pads, the sulphate of zinc solution would irritate the muscle, and that, we shall see, produces new phenomena. We wish to examine the electrical properties of the muscle at rest. To protect the muscle.
Fig. 69.—Arrangement of du Bois Reymond's troughs, a a, troughs of zinc; b, vulcanite plates for insulation; c c, paper pads; d e, clay pads; f f, connections for wires leading to galvanometer, g.
therefore, we place on the pads of blotting-paper little bits of moist clay: sculptor's clay, moistened with saliva, is usually employed. Here are the troughs just as they are used by their inventor. There are many other forms of these electrodes, or instruments for leading off the electricity, adapted for special use; but I prefer to use the original form, as it is the one I have employed for years, and with which I am familiar.
Our troughs or electrodes are connected with the galvanometer, a key being interposed in the circuit. Notice where the spot of light is at present. I shut the key, and you see there is a very slight movement of the spot of light, showing that the troughs are already producing a certain amount of current. This arises from the fact that somewhere a slight chemical change is going on, quite sufficient to generate a feeble current. Let us remember, however, that the current of the troughs makes the spot of light go to the right. Now we know, or at all events the electricians tell us, that copper is positive to zinc, which is said to be negative; or, in other words, if we placed a copper and zinc plate in a fluid acting on the zinc, a current would travel out by the copper, and if it had a completed circuit, it would return to the zinc. Well, observe when I touch with one hand the copper wire connected with the trough on my right, and with the other hand the zinc trough on my left, the spot of light moves to the right; but when I touch the copper wire on the left and the zinc trough on my right the spot travels to the left; that is to say, when the right-hand trough is positive the light moves to the right, and when the left-hand trough is positive it moves to the left. We must keep this in mind.
I now take this muscle—the gastrocnemius of a frog—and, with a sharp pair of scissors, cut it clean across the fibres. Observe that I lay the muscle on the clay pads, so that the surface of the muscle touches the pad on the right, and the cut surface, that is, the transverse section, touches the pad on the left. I close the key so as to allow any current that may exist to flow to the galvanometer, and you see that at once the spot of light swings to the right. Observe also that the spot of light keeps to the right, showing that a current is flowing through the wire of the galvanometer. You will remember, however, that without the muscle there was a small amount of movement to the right, but the movement you now see is much greater, and cannot be due to the cause that produced the previous small movement. I open the key so as to break the circuit, and at once the spot of light slowly sails back to the original point. You will recollect that when the right-hand trough was positive, the spot travelled to the right, as we have just seen.
Pursuing the experiment farther, I now pick up the muscle with the forceps and place it again on the pads, but in a reversed position; that is to say, the surface now touches the pad on the left, while the section is in contact with the pad on the right. I again close the key, and you now see the spot passes to the left and takes up a position on that side. Open the key, and it again sails back. Remember, once more, that the spot should come to the left when the left pad is positive, as has occurred in this last experiment. These two experiments clearly prove, first, that a muscle at rest gives a current; and, second, that this current travels through the galvanometer circuit from the surface of the muscle to the transverse section. The surface of the muscle is thus positive to the transverse section; or you may get a clearer notion of the statement by supposing the bit of muscle to be a little galvanic element or battery. In that case, the surface of the muscle is the positive pole, find the transverse section is the negative pole. This then is a demonstration of the electrical condition of the muscle at rest.
Now let us go a little farther. Leaving the muscle on the pads, with the longitudinal surface touching the pad on the left, and the transverse section touching the pad on the right, you notice the spot of light has taken up a position well towards the left. The spot will slowly move to the right as the muscle dies, but we need not wait for this. I need hardly say that a dead muscle gives no current. What would happen if we made the muscle contract? Would the contraction increase or diminish the current, or would it have no effect? To answer this question, I shall ask Mr. Brodie to lay the nerve still attached to the muscle over the wires coming from the induction coil, and, as he has a key in the circuit, I ask him by closing it to throw the muscle into tetanus while it remains on the pads. Observe once more the position of the spot of light. Tie now tetanises the muscle, and you see the spot immediately travels to the right, indicating either that the current is less during contraction than it was before, or that a current is now flowing in the opposite direction. This swing backwards will carry the galvanometer needle to the point from which it started, or even to the opposite side. It is known technically as the negative variation of the uscle current; and careful experiment has shown that it is not due to a mere diminution of the current from the muscle while the muscle is at rest, but that it is really a current in the opposite direction; that is to say, when a muscle having a cross section contracts, the surface of the cross section becomes positive instead of negative, and the longitudinal surface becomes negative instead of positive. As a current always travels from positive to negative, it follows that the current in a contracting muscle travels in the opposite direction from its course in a resting muscle.
Other tissues than muscle show so-called resting currents, and when the tissue acts or responds to a stimulus, there are strong indications that a negative variation current is produced. Thus a nerve shows the same phenomena as a muscle. Currents have also been observed in glands.
One of the most striking demonstrations of these currents is the one I shall now show you. The frog's heart beats for a long time after the death of the animal. Here is a little heart still beating. I cut off with the scissors a small portion of the apex, and I place the heart on the pads, so that the surface touches one pad and the cut apex the other. I now close the key, and you observe a swing of the galvanometer with each beat of the heart. You observe the spot of light is unsteady; it swings to the right with the current of the heart at rest, and to the left with the current of the heart when it contracts. The latter is the negative variation current. Sometimes we even get a double swing of the galvanometer. These beautiful results are shown in another way by my friend Dr. Augustus Waller. He uses an instrument called a capillary electrometer, invented by Lippmann, and he can demonstrate the electrical variations of the human heart.
The current of the cut muscle at rest is not of so much importance as the current of the same muscle in action. There are strong reasons for holding that the resting current is due to the death or dying of the layer of muscle-substance laid bare by the transverse section. Dying muscle-substance, it would appear, becomes negative to living substance. But the negative variation current, or the action current, as we may well term it, indicates changes happening in the muscle, changes that are somehow connected with the phenomena of contraction. It may be detected by special methods without even injuring the muscle.
It was supposed, until recently, that the negative variation occurred solely in the period of latent stimulation, a period perhaps as short as the one-two-hundredth of a second; but by special photographic methods of exquisite delicacy, Professor Burdon Sanderson has shown that this is not the case, and that it continues into the time of the contraction of the muscle. Nor have we any definite proof that these electrical changes are dependent on the chemical changes discussed in last lecture, although it is highly probable that the two are intimately connected.
May not the negative variation change in one set of muscular fibres do something in the way of irritating adjacent fibres? There is an old experiment devised by Matteucci that favours this view. I have two of the usual nerve and muscle preparations connected with these two telegraphs. Call this one A and the other B. I stretch the nerve of A over B, and the nerve of B is placed on the wires of the induction coil. I now irritate the nerve of B, and of course its muscle B contracts;
Fig. 70.—Arrangement to show Matteucci's induced contraction. A, muscle, the nerve of which is placed on B ; G, galvanic element; K, key; P primary and S secondary coil of induction machine.
but you observe the muscle A also contracts. We explain this by supposing that the negative variation change in the muscle B is sufficient to stimulate the nerve of A, and therefore A contracts as well as B. Matteucci, in a similar way, found that a number of muscles might be thus connected, the nerve of the one lying on the muscle preceding it, so that when the nerve of the first member of the chain was stimulated all the muscles contracted.
If this be the case, may we not suppose, as was suggested by Professor Kühne of Heidelberg some time ago, that those parts of certain muscles which are destitute of nerves may be
Fig. 71.—Diagram showing arrangement of apparatus used in demonstrating the action of light on the retina of a frog's eye. A, the eye, having one electrode, b, touching the centre of the cornea, and the other, c, touching the transverse section of the optic nerve; G, galvanometer; D, key. The arrows show the direction of the current.
thrown into action by the stimulus of the negative change happening in adjacent portions of the muscle supplied with nerves? I think this is highly probable.
I shall now endeavour to show you the electrical change produced by the action of light on the frog's eye, a subject on which Professor Dewar and I worked nearly twenty years ago. The eye of a frog has been carefully dissected out (the animal of course being dead, although the tissues of its eye still live), and it has been placed on the clay pads so that one pad touches the back of the eye while the other is in contact with the front of the eye—the cornea. We now place the eye in darkness by covering the troughs over with a bandbox, in which, however, we have left a small window which we can open and shut at pleasure, and the position of the window is such that if we place a light before it, the light will shine on the cornea of the little eye. Now I shut the key so as to allow any current that there may be to flow to the galvanometer. You see at once there is a very considerable current. That is the resting current of the eye in the dark. Mr. Brodie will now allow light to fall on the eye. You see at once the spot of light on the scale moves and indicates an increase in the first current. As light continues to act the current begins to diminish; but I now ask Mr. Brodie to take away the light and leave the eye in darkness. You see the moment the light was removed that the current again suddenly increased and then fell to a point lower than it had hitherto been. We repeat the experiment and you observe we get the same results. A sudden influx of light causes an increase in the current of the living eye; under the continued action of light the retina becomes fatigued, and, when light is taken off, there is another increase and then a great falling off. You see how sensitive the eye is—even a flash or striking a match near it produces the effect. If I let the light pass through this bit of red glass we get an effect almost equal to what we got with yellow light from the taper; but if it passes through this dark blue glass, you notice the effect is much less. This is perhaps the most delicate experiment in the range of physiological science.
We have now examined the muscle-current, the nerve-current, the heart-current, and the eye-current. Let me next endeavour to show you the man- current. I have here two flat vulcanite troughs into which we have poured a three-quarters per cent solution of common salt. Mr. Brodie has placed one zinc trough by the side of each flat trough, and he has dipped the points of the clay pads into the salt solution. We first of all connect the vulccanite troughs together by a bit of wet blotting-paper, so as to put them in circuit, and I now close the key. If any current came from the troughs themselves, we would see a movement of the spot of light on our scale. You observe there is scarcely any movement. I now place my hands in the troughs, laying them in the salt solution. At first there is a considerable movement of the spot of light on the scale, but it soon comes to rest. If, however, I contract the muscles of my right arm, the spot moves to the right with a great swing, and if I contract the muscles of my left arm the swing is in the opposite direction. You observe by alternately contracting the muscles of the right and of the left arm I can cause the spot to swing in either direction. Notice how sensitive the effect is, even when I place only a forefinger in each vessel containing the salt solution. This is the man-current. It may be explained by supposing that when the muscles of both sides of the body are at rest there is no difference of electrical potential between the one side and the other side; but if the muscles of one side, say of one arm, are contracted, this at once produces a disturbance, and a current flows through the galvanometer. It is not an entirely satisfactory explanation, but it is the best that can at present be given.
I shall not discuss the theories that have been propounded to explain these remarkable phenomena, the investigation of which clearly demonstrates the existence of a true animal electricity. In 1791, Galvani, who was Professor of Anatomy and Physiology in Bologna, first announced that electricity, to use his own phrase, was secreted in, or originated from, the animal tissues. The great controversy that then arose, more especially between Galvani and Volta, who was Professor of Natural Philosophy in Pavia, led to the invention of the Voltaic pile in 1799, and still more to the discovery of the production of electric currents by the contact of dissimilar metals, more especially when one is acted on chemically by certain fluids. For a long time the brilliancy of the results flowing from investigations into Voltaic electricity threw the discoveries of Galvani into the shade; but by and by, as methods of observation became more refined, it was found that there is in truth an animal electricity, and that Galvani was right in many of his views.
One branch of science often helps another. By the discovery of the influence of a current of electricity on a magnetic needle, made, in 1820, by Oersted, the galvanometer or multiplier became possible. Nobili, about 1825, constructed such an instrument for physiological purposes, and again demonstrated the muscle-current. In 1837, Matteucci enriched the subject by many beautiful investigations, and, in 1841, du Bois Reymond took it up with rare enthusiasm, and from that year to the present year has laboured on it with much success. One feels, after reading du Bois Reymond's monographs, that he has left little for the gleaners in this great harvest.
How striking is it, my young friends, that the splendid results of modern electrical science, with which we are familiar every day, flowed, in no small degree, from the first physiological experiments of Galvani. Electric lighting, the application of electricity to- the construction of motors, and the thousand ways in which this mysterious thing is becoming the servant of man, sprang from discoveries that date from the time when the Italian philosopher noticed the twitches of a frog's leg near his electrical machine. Truly there is nothing small and insignificant in science. An observation of a phenomenon obscure in character and not striking to the senses may be the key by which we open new stores of knowledge.
It has often been remarked that many of man's inventions have their counterpart in nature. In the subject we are discussing we have a proof of the truth of the remark. In nature we find living electrical machines. When one considers how potent electricity is, a kind of "vril" (to use the word coined by the author of The Coming Race), armed with which a being might become a terrible antagonist, it is remarkable that electric organs have as yet been found only in a few species of fishes. No doubt the explanation may be offered that the conditions of the environment of living things are not favourable to the general evolution of electric organs, and while this explanation is probably true, it does not lessen, I think, the sense of astonishment. Some fifty fishes possess electrical organs, and of these only five or six have been investigated. These remarkable creatures are of interest to us at present because the electrical
Fig. 72.—Torpedo Galvani, showing the prisms of the electric organ as seen from the dorsal surface. Each organ contains about 800 prisms, and each prism is divided by delicate membranous plates, separated from each other by a jelly-like fluid. Each prism has about 600 plates, and as there are 800 prisms in each electric organ, the organ contains about half a million electric plates, each of which is supplied by a nerve filament. The figure shows the large nerve trunks, b, ending in the smaller nerves, a, distributed to the prisms. See next figure.
organs of many of them are, in a sense, modified muscular structures. Thus in the ray called the torpedo (Torpedo Galvani) of the Mediterranean the electric organ takes the place of the outer gill muscles of the fifth gill arch. In ordinary rays, and their distant cousins the sharks, these muscles are powerful organs for moving the lower jaw, but in the torpedo for
Fig. 73.—Attachment of the electric plates of torpedo to the sheath of the prism. d, sheath of prism v, ventral or nervous layer or plate; d (to the left), dorsal plate; e, fine layer of connective tissue; b, intermediate layer; n, nuclei of this layer; a, a portion reflected from the plate.
these muscles electric organs have been substituted. At an early stage in the development of the torpedo the tissue of the electric organ is like that of an embryonic muscle, showing numerous nuclei, and even a distinct longitudinal and a more faint transverse striation may be seen. Somewhat later, the striations, both longitudinal and transverse, disappear, the nuclei become larger and more numerous, and
Fig. 74.—The electric eel, Gymnotus electricus.
the disc-like arrangement of plates begins to appear. The process goes on until the slight resemblance to muscle is entirely lost. I show you here a specimen of a large torpedo from the Hunterian Museum of the University of Glasgow, and in this other jar you see the electric organ dissected out of the specimen. This is an interesting preparation. It is very old, and may have been put up by the hands of John Hunter, the brother of William, who founded the museum in my own university.
Again, in the more formidable electric eel (Gymnotus electricus) of the region of the Orinoco, in South America, we find huge electric organs running almost from head to tail, which occupy the same positions as are filled by muscles of eels of allied species, and a study of their development shows that they originate from the same kind of substance.
Several species of skates from our own seas have an electric organ in the tail, a fusiform body, about half way up the tail of the fish, in contact with the skin, and partly enveloped in a well-known muscle—the sacrolumbalis muscle. This organ shows a disc-like structure, somewhat similar to that found in the electrical organs of the torpedo and gymnotus, but more resembling the latter than the former. There can be no doubt again that we may view the electrical organ of the skate as a modified muscular apparatus.
I shall now ask Mr. Brodie to show you sections of these organs by the electric microscope. [This was done, and a demonstration was given, which is partially illustrated by the Figs. 73, 76, 77, 80, and 81.]
But nature shows often a remarkable power of modifying different parts for the same purpose, or of similar parts for different purposes. This is seen both in plant and animal life. Thus the tendrils by which a plant clings to other structures may be modified leaves, stipules, or branches; and, on the other hand, similar parts in many of the
Fig. 76.—Portion of electric organ of gymuotus magnified 400 diameters. p, Pacini's line; a a, anterior papillæ; b b, posterior papillæ, sometimes called thorn papillæ. They contain cells that move (amœboid movements). n, nerve-fibres entering papillæ; x x, connective tissue. The plates sometimes cleave transversely along Pacini's line, a division analogous to the cleavage of muscle into Bowman's discs.
Fig. 77.—Semi-diagrammatic view of a disc from the electric organ of the skate (Raia batis). a a, connective tissue with capillaries; b, nerve layer, nerve endings branching; c, striated layer; d d, processes of transparent structureless material containing numerous nuclei corresponding to the thorn papillæ of gymnotus.
Crustacea (crabs, lobsters, etc.) may become either gills (organs for breathing) or feet (organs for locomotion). We might expect, then, that electrical organs might be found that were not muscular in their origin.
Fig. 78.—The Raʺ̄́āsh or thunderer fish of the Arabs. Malapterurus electricus, Var. affinis.
Accordingly we find that the electrical organ of the raasch or thunderer fish of the Arabs (Malapterurus electricus), an inhabitant of the Nile, is not muscular, but is a modification of peculiar glandular structures found below the skin of allied species.
Electrical organs, in their physiological behaviour, present many striking resemblances to muscle. Thus they are all richly supplied with nerves. When the nerve is irritated the electrical organ discharges electricity, not as a current, but in a number of short sudden shocks, like the quivers of a muscle in tetanus. The battery, however, does not go off at once. There is a latent period preceding the discharge. The electrical organ is connected with the central nervous system, the nerves
Fig. 79—Section of malapterurus, showing the electric organ surrounding the body between a and b.
Fig. 80.—Electric cell from the middle region of the spinal cord of gymnotus, magnified 314 diameters. a, sheath of neuroglia, the peculiar connective tissue found in the central nervous organs; b, smaller nerve processes or fibres, forming a network with those of adjoining cells; c, chief nerve process passing into the axis-cylinder of a nerve-fibre. The nerve-fibre ends in an electric plate, as shown in Fig. 76, n.
springing from special nerve-cells, so that it is under the control of the will; but, at the same time, it may be excited to discharge by drugs that act on the nervous centres. Thus strychnia throws the muscles of an animal into terrible tetanic convulsions by acting on its nervous centres; but it causes a torpedo to discharge a quick succession of shocks till the creature is exhausted. Again, an electric organ shows fatigue, and it needs time to rest and recover. Lastly, the organ is the seat of chemical changes.
There are many other phenomena that time will not permit me to mention. If we could get a supply of live torpedoes or of electric eels, and have some means of keeping them alive, I can conceive a course of Christmas lectures of surpassing interest; or we might get over the difficulty of having the live animals in the lecture-room by delivering the lectures in Cairo instead of in London, where no doubt the thunderer would be willing to show his powers. May we, however, hazard an explanation of the nature of these organs and of their relation to muscle and gland? In the case of torpedo, gymnotus, and the skate, the nerve ending is evidently analogous to a motor end-plate in muscle, and in malapterurus to the terminations of nerves in the cells of glands. The molecular disturbance transmitted along a nerve causes changes in its end organ, and these are propagated to the surrounding substance. These changes are associated with a change of potential, and the part becomes negative. A wave of negativity passes through the organ, and there may be a result, the nature of which will, depend on the kind of organ in which the change may take place. If it be a muscle, the chief expression of the change is a variation in form or contraction; if it be a gland-cell, the change is the formation, or disintegration, or modification of certain matters of the secretion; and if it be an electrical organ, it is an electrical discharge. In all three, however, similar phenomena occur, but to varying amounts. Thus, call contraction a, electrical phenomena b, and glandular changes c. In a muscle a is large and b and c small; in a gland a may not occur as an active movement at all, although the cell may change slowly in volume; b is also small, but c is large; and in an electrical organ a is no doubt small (if it exist at all), b is very large, and c is small.
Fig. 81.—One of the lozenge shaped spaces in the electric organ of malapterurus, magnified 200 diameters. s, space filled with fluid. To the left of a observe the electric tissue darkly tinted. Notice that it occurs on two sides of the lozenge-shaped space. See to the right of b the connective tissue wall of the space. n n, nuclei in electric tissue; n, nerve-fibre passing into electric tissue. The electric discs in this fish are epithelial and not muscular. The total number of discs, each of which is supplied with a nerve, is 2,000,000. All the nerves for the electric organs spring from two gigantic nerve-cells in the spinal cord, one for each lateral half. One nerve-process issues from each cell and, by dividing and subdividing, supplies each of the one million discs on one side of the body with a distinct nerve-fibre. The sum of the diameters of these nerve-fibres is very much greater than the diameter of the fibre that issues from the cell. It is evident, therefore, that the conducting matter of the nerve-fibres must increase in amount as we pass to the periphery of the body (p. 98).
Thus, in my opinion, all these phenomena are but manifestations of the same essential process; they are all linked together, and as our knowledge of the nature of the molecular processes connected with life advances, we will be better able to explain and correlate such phenomena as contraction, secretion, and electrical action.
I have now the pleasure of showing you a large live gymnotus which has been kindly lent for demonstration by Mr. P. L. Sclater, the Secretary of the Royal Zoological Society. The fish, in charge of his keeper, is in this large tank. He is about four feet in length, and it is satisfactory to know that he has lived for seven years in the "Zoo," far away from his own Amazon, and that, with good feeding, he has nearly doubled in size. It is quite proper that the fish should first of all give a shock to a physiologist who is endeavouring to demonstrate his properties, so I seize hold of these handles while the keeper touches the fish with the ends of insulated wires. Ah! I have got a pretty smart shock, felt up to the elbow, like the discharge of a Leyden jar. Next we shall lead off a little to the galvanometer, making the instrument as insensitive as possible by pushing down the magnet; but you see how wildly the spot careers about on the scale when the keeper touches the fish. If we had time, we might cause the fish to stimulate a muscle in the frog-interrupter, and thus ring the bell. This is the method adopted by Professor du Bois Reymond in his investigations on electric fishes. If the keeper wishes it, we can easily fit up an arrangement in the Zoological Gardens by which the fish can ring him up at any time, say when he wants his dinner! I see by the keeper's face that this does not meet with his approbation: a bell at one end of a wire and a gymnotus at the other might almost be as troublesome an arrangement as a telephone in one's bedroom. We are much obliged to Mr. Sclater for lending the fish, and we hope the gymnotus will have a safe journey to his warm tank in the "Zoo."
Time warns me, however, that I must be drawing to a close. We have been trying in these lectures to get an insight into the hidden machinery connected with animal motion. Up to this point, we have only been discussing the mechanism of each individual wheel and pinion, and we have not considered the machine as a whole. It is impossible to discourse, with any degree of fulness, on this subject in the present course of lectures; but I shall content myself with alluding to one or two points of surpassing interest.
In the first place, these—the muscular mechanisms we have been considering—are controlled and regulated by the central nervous system. Each muscle is supplied by one or more nerves, and these originate in central nervous organs of great complexity, and regarding which much of our knowledge is singularly indefinite and unsatisfactory. We know, however, that there are two classes of movements—those that we make voluntarily and consciously, and those that we make involuntarily, and of which we may be either conscious or unconscious. We cannot make a voluntary movement without being conscious of so doing. An effort of will is always a conscious effort, and to speak of unconscious will, as some writers do, is, in my opinion, a very misleading mode of expression. What they mean, no doubt, is that certain movements may be made which are so purpose-like as to lead one to suppose that they are voluntary, and yet they may be made without the person or animal being conscious of making them.
Now, whether movements are voluntary or involuntary, they always require a nervous mechanism having the same structural type, although it may be more or less complicated by the necessities of the act to be performed. The simplest form of this mechanism is what we term in physiology a reflex mechanism. It consists of a centre, a sensory or afferent nerve, carrying impressions to the centre, and a motor or efferent nerve, carrying impressions from the centre to something in the circumference or periphery of the body. Thus, if we pinch the toe of a decapitated frog, it draws the leg away. The pinching irritates the sensory nerve, something, as you now know, travels along it to the nerve-centre, which, in this case, is in the frog's spinal marrow, and from the spinal marrow, after the lapse of a little time (the latent period in the marrow) a new impulse starts outwards along motor nerves to the muscles, reaches these, and causes them to contract. There are many varieties of these reflex acts. We may be quite unconscious of them, or we may feel the stimulus, and we may feel that we make a movement, and yet we may be unable to restrain the movement. Many reflex movements are beyond the control of the will when they have once been fairly set agoing. Thus, we cannot stop swallowing when the food has gone far enough back in the mouth and throat.
Now suppose we make the motions voluntarily: I wish to point out to you that the mechanism is still essentially of the same character. We usually speak as if we were free to make any movement we like, and when we know a little physiology we say the impulse begins somewhere in the brain and travels down nerves to the appropriate muscles. In a sense this is true, and yet it is not wholly true. We seem to act as if the mandate started in the brain, but this is because we miss the influences and impulses that called this mandate into activity. To start a mechanism that will produce well-ordered movement, as when I lift this book from the table, impulses or messages, whatever you like to call them, must first be transmitted from the body itself to the brain. Such messages may come to the brain by nerve-fibres from some organ of sense, it may be from the skin, or from the eyes, or from the muscles themselves, but they must be sent to the brain before the brain will send out messages along motor nerves to groups of muscles which are called into action, so as to perform a definite movement. If, from a disorder of the nerves, or of the nerve strands in the spinal cord that carry messages up to the brain, these messages cannot reach the brain, movements will either not be made at all, or if they are made, they are irregular, spasmodic, wanting in adjustment for a definite, purposive-like action. Thus sensory impressions come before and determine even so-called voluntary movements.
If this be the case, you will naturally inquire as to the mechanism by which certain messages sent to the brain are so arranged or transmitted as to call forth and transmit nerve-currents along certain specific nerve-fibres to certain specific muscles. We now get into a misty region in which we have only to grope our way. Analogies may help us a little. Is there something that answers the purpose of a telephonic exchange, in which a presiding genius, by putting in and taking out pegs, puts one part into connection with another? Or is there some kind of shunting-place, a sort of Clapham Junction, to which lines from all parts converge, and from which currents are sent here and there, according to the necessities that arise? Or is there at work something like the card of a Jacquard loom, by which all the threads are collected and arranged and transmitted, so that each takes its place in the complicated pattern of the woven web? All these analogies fail in giving a notion of the intricate phenomena that occur, and it must not be supposed that the nervous system works in the least like any one of the mechanisms I have alluded to. Still such arranging of the impulses does take place,—some think in the spinal cord itself, others in the cerebellum, others in the cerebrum, others in the nervous system as a whole,—and the result is exquisitely harmonised movement.
All these phenomena are undoubtedly connected with molecular movement. Such movements occur even in the brain itself, and there is little doubt they are also associated with all mental phenomena. It does not follow, however, that mental phenomena are the result of such movements alone. Wider knowledge strengthens the view that behind mental phenomena, and indeed behind all phenomena, there is something more than movements of matter and transformations of energy.
My task is now at an end. I have had great pleasure in delivering these lectures, because I felt that in endeavouring to interest you I was instructing myself You have got a glimpse into the world of science, and I hope the glimpse will induce many of you to ask for more knowledge. Science is simply the truth about natural phenomena, so far as we can reach it. Some of you may become men of science, and you will probably advance much farther than we can do at present, and you will add to science, I hope, by your own work. The majority, however, will not follow scientific pursuits, but I trust this course of lectures will lead you always to keep a mind open for the reception of truth, from whatever quarter it may come, and that you will always cherish a lively sympathy with scientific men and with scientific progress. In conclusion, let me thank Mr. Broclie of King's College and Mr. Heath of the Royal Institution for their valuable assistance; and let me also thank the Directors of the Royal Institution for giving the opportunity to a physiologist to represent his science in this lecture-theatre, as it may show how physiologists work and reason on the difficult problems with which they have to deal.