Popular Science Monthly/Volume 2/January 1873/Velocity of the Will< Popular Science Monthly | Volume 2 | January 1873
|THE VELOCITY OF THE WILL.|
TRANSLATED BY A. R. MACDONOUGH.
IT is a common idea that the saying "as quick as thought" expresses the ne plus ultra of speed—an unapproachable rapidity, instantaneous and lightning-like. The phrase seems used, indeed, as an hyperbole; but in one sense, at least, this is a mistake. Thought, it is true, can transport us afar without taking note of distances, because there is no more difficulty in bringing up, in fancy, a remote object, than one that is close to us, and in this view it may be allowable to say that space creates no obstacle to thought—not impeding nor changing it in the least. But thought never springs instantaneously under the influence of an external cause; an appreciable time elapses, one or two tenths of a second, before an idea is aroused in the mind in consequence of an impression received by the brain, and before it will respond to that idea by the movement of a limb. So the nervous current which transmits sensations to the brain, and bears the commands of the will to the extremities of the body, requires a certain time to finish its course. Impressions coming to us from without are not perceived at the very instant of their production; they travel along the nerves with a speed of from 60 to 90 feet a second, equal to that of the carrier-pigeon, or the hurricane, or of a locomotive under full steam, but very much less than the swiftness of a cannon-ball. For instance, we are conscious of an injury in the feet only after a half-tenth of a second has elapsed. The commands of the will pass from the centre to the circumference with no greater rapidity; the limbs do not instantaneously obey the motive thought. When a movement is provoked by a shock received in any part whatever of the body, the stimulus at first travels as far as the brain; there a thought is developed, the will determines to send out an order; this order runs along the nerves to the limb which is bidden to act, and, at last, the limb begins movement. All this takes place in three times, of quite an appreciable duration.
In the human body, this time lost is a mere trifle, some hundredths of a second; but let us suppose one of a great cetacea, a whale, for instance, in which the telegraphic network of the will controls a wider range. A boat attacks it in the rear, the harpoon strikes the monster's tail. Then the pain sets out on its course to demand revenge; but the journey is long—it must travel over 90 feet before reaching the headquarters of the will. Here is one second lost. What takes place then? How much time does reflection need? That depends on circumstances; but it is certain that the will does require a measurable time to make a decision. Then it acts; the order is sent out to the tail to thrash the boat to bits. Another second elapses before the message reaches its destination: total, two seconds gone, during which the boat and sailors get off clear by vigorous rowing.
It will be asked, How have philosophers succeeded in measuring the rapidity of the onward movement of nervous stimulus? Several methods of calculating it have been devised. A doctor of the middle ages, cited by Haller, gave some thought to it long ago. He conceived—a singular notion—that the speed of the nervous fluid might be deduced from that of the blood in the aorta; these two rates, he fancied, must be in the inverse ratio of the sizes of the aorta and the nerve-tubes. That calculation assigned, as the speed of the nervous fluid, 600,000,000,000 of yards a minute—600 times the rapidity of the motion of light.
Haller himself undertook the task in a different way. Reading the "Æneid" aloud, he counted the number of letters he could pronounce in a minute with a very rapid utterance. He found 1,500 the extreme limit, or one fifteen-hundredth part of a minute for each letter. Now the letter r requires, Haller says, ten successive contractions of the muscle that gives the tongue vibration, and from that, he adds, we may conclude that in one minute this muscle can contract and relax 15,000 times, which represents 30,000 simple motions. The distance from the brain to the muscle in question is a little over three inches. If the nervous fluid travels it 30,000 times, that makes more than 9,000 feet, and 9,000 feet a minute represents a speed of 154 feet in a second. This reasoning is a mere sequence of mistakes, and the approximation to the right view that Haller gained is the more astonishing because his method was not in the least likely to ascertain it. The "Æneid" justifies, in this instance, its ancient pretensions as a book of oracles.
Not until 1850 were these researches resumed by a new method that led to the solution of the problem. It is due to Helmholtz, the most famous of the German physiologists, who unites, to rare talent as an observer, the profound learning of a consummate mathematician. His first method is founded on the use of the chronoscope of Pouillet. A galvanic current of very brief duration acts at a distance on a magnetized needle, and swings it away from the normal position; the range of the deviation is measured, and the length of the current deduced thence by calculation. A means is thus gained for measuring intervals of time not exceeding a few thousandths of a second. Helmholtz applied this method in the following way: One of the muscles of a frog's leg is fixed at one extremity in a nip, and attached at the other extremity to a little lever forming part of a galvanic circuit. A weight, hung on this lever, serves to give the muscle the required tension. Every thing is so arranged that, at the instant the current is closed, a shock is produced either directly in the muscle, or in a given point of a nerve which is isolated for a length of about an inch, and still adheres by one end to the muscle which it is to stimulate. Under the influence of this excitement, the muscle contracts, stirs the lever, and breaks the electric circuit in which it was a part. The duration of circulation of the current is indicated by the magnetized needle. It is found, then, that contraction occurs later when the nerve is excited than when the muscle is excited directly; the difference discloses the speed of transmission of the nervous agent, which is found equal to very nearly 80 feet a second. Helmholtz has ascertained, moreover, that, in every case, contraction follows the electric shock only after an interval of time equal to 1⁄100 of a second, which he calls the time of latent stimulus. The muscular fibres do not, therefore, instantly obey the spur of electricity. Thus the waters of the sea rise under the influence of lunar attraction only after the planet is long past the meridian.
After these beautiful experiments, which revealed for the first time the knowledge of the way in which a stimulus is transmitted along the nerves, Helmholtz devised another method, permitting the analysis of the phenomenon in its minutest details. In this, also, the contraction of the muscle lifts a light lever, but the lever carries a point which leaves a white mark on a revolving cylinder covered with lamp-black. A peculiar arrangement causes the same point to mark the instant of production of the stimulus, and, from that instant to the moment of the muscular contraction, the point traces a straight line in the lamp-black. When it is afterward lifted by the tension of the muscle, it draws a curve which at once represents to sight, by its appearance, all the different phases of the movement of contraction. By this method, Helmholtz discovered that the speed of the nerve-current was a fraction over 83 feet. He proved, moreover, that the tension of the muscles gradually increases from the first moment of movement, that it reaches a maximum after about 5⁄100 of a second, and diminishes again until the muscle returns to its natural state.
This second instrument of Helmholtz received the name of a myographe. It has been perfected or rather modified by several physiologists. The great difficulty was, to measure precisely the time corresponding to the different points of the tracing drawn by the point on the cylinder. Helmholtz communicated motion to the cylinder of his apparatus by a clock-work arrangement which pointed out to the eye the length of its revolution. For this method, the use of the diapason has been advantageously substituted. Dr. Marey, in his course of medical physiology, employed for this purpose a diapason which made 500 simple vibrations every second; those vibrations noted themselves on the cylinder alongside the curve traced by the extremity of the muscle; it was sufficient to count the number of vibrations inscribed parallel to a part of the tracing by the muscle, to arrive directly at the time corresponding to the tracing. Marey detected, by this method, degrees of speed in transmission varying from 30 to 61 feet.
Moreover, the nerve-current travels more slowly at low temperatures than at high ones. Dr. Munk discovered, besides, that the speed is not alike in the different parts of a nerve; in the motor nerves it seems to increase toward the point of attachment of the muscle. And, according to De Bézold, this speed decreases when the nerve is under the influence of an electric current.
The point was now to repeat these experiments on the human subject. It was found possible to conduct them in this manner: An electric current produces a slight sensation of pain on one point in the skin; the instant of action by the current is marked on the revolving cylinder of a chronoscope. As soon as the person experimented on feels the shock, he gives a signal by touching an electric key, and a second mark is produced on the same cylinder. Measuring the interval comprised between these two marks, we have the time that elapses between the two signals. This time, which is from one to two-tenths of a second, is made up of several parts; transmission of external impression to the brain, perception, reflection, transmission of the will to the fingers, muscular contraction, which is the result; but, if the stimulus is applied successively to different points on the skin, these delays are always the same, except that which is due to the transmission of sensations. If, for instance, a point on the great-toe is first excited, and afterward a point in the inguinal region, the difference in the delays remarked will represent the time employed by sensation in ascending from the foot to the middle of the body.
These experiments were first made in 1861 by Hirsch, director of the Neufchâtel Observatory, by means of an apparatus which it would take too long to describe here. The person experimented on touched an electric key with the right hand at the instant of feeling that slight pain, not unlike a pin-prick, which the knob of an induction apparatus produces on touching the skin. The knob was applied in succession to the cheek, then to the left hand, then last to the left foot. The time lost in the transmission of this stimulus from the point touched to the right hand was found equal, in the three cases respectively, to 11⁄100 14⁄100 and 17⁄100 of a second; it required, therefore, 3⁄100 of a second for sensation to arrive from the left hand at the head, and 6⁄100 for its passage from the foot to the head. Hirsch inferred from tins that the nerve-current travels over a distance of six feet and a fraction in 6⁄100 of a second, or about 104 feet in a second. Dr. Schelske repeated these experiments in a more thorough way at the Utrecht Observatory. He arrived at 91 feet as the speed of transmission of sensation in the human body. The same experimenter proved that the passage takes place with the same rapidity in the spinal marrow as in the nerves. This result is the more remarkable, as the nerve-tubes undergo great changes at their entrance into the spinal marrow, where, according to Van Deen, they cease to be sensitive to the action of electricity, of chemical substances, mechanical injuries, etc.
It follows, from all these experiments, that the nerve-current makes its way with a speed that is relatively inconsiderable. The hand in throwing a stone parts the air with the quickness of nearly 68 feet a second, which is quite comparable with that of the nervous fluid; and the race-horse, the hare, and the leveret, move quite as rapidly. The arterial wave, which passes through 27¾ feet in a second, moves only three times more slowly.
When the sensation transmitted to the spinal marrow occasions a reflex action, that is, an involuntary movement determined by the intervention of the ganglionic cells, the reflex motion always proceeds more slowly than that produced by the direct action of the exciting current on the muscles; the retarding varies from a thirtieth to a tenth of a second. It may be inferred from this that reflex action in the spinal marrow takes twelve times longer than the transmission of a stimulus through nerves of sensation or motion.
The time employed in the brain's operations is also some tenths of a second. Dr. Jaeger measured it in the following way: The subject of the experiment was made to touch the electric key with the left hand as soon as he received an electric shock on the right side, and with the right hand when the shock came from the left side. The interval between the shock and the signal was found to be 20⁄100 of a second when the person knew beforehand, which side the shock would come from, and 27⁄100 when he was not forewarned; thus 7⁄100 of a second were used in reflection. Hirsch found that at least two-tenths of a second elapse before an observer marks by signal the preception of a luminous spark or a sudden sound. In other experiments it was arranged that the observer should touch the key with the left hand for a white spark, and with the right for a red one, and he lost, in that case, from three to four-tenths of a second. Therefore reflection took from one to two-tenths of a second. Donders and Jaeger made the experiment a little differently. One pronounced some syllable, which the other repeated as soon as he heard it, while a registered the vibrations of the word. When the syllable to be repeated was agreed on beforehand, the delay observed was two-tenths of a second; when not, it was three-tenths.
As we see, then, thought does not spring instantaneously; it is a phenomenon subject to natural laws of time and space. In different observers the time lost is not alike; one perceives, reflects, and acts, more briskly than another; it is a matter of temperament and of accidental circumstance. This explains the differences always remarked between astronomers busied in observing the same phenomenon. Two persons never saw at the same instant the passage of a star across a thread: besides, the difference between the instants noted, or what is termed the "personal equation," of two astronomers, varies more or less according to circumstances, and may increase or diminish with time. The observer's training has a great deal to do with it, Wolf having demonstrated that, by practice, the time lost may be reduced to a minimum, with the employment of a special apparatus.
An important conclusion follows irresistibly from these experiments: it is, that the nerve-fluid is not identical with the electric fluid. Electricity darts through telegraphic wires with inconceivable rapidity, far outspeeding light, and moving 20,000,000 times faster than the nerve-agent. There exists another important difference between these two forces. Any alteration in the structure of the nerves checks the transmission of the nerve-current; crushing or partial burning is enough to interrupt its passage; once cut, they do not regain their conductive power when the separated ends are brought together again. Metallic wires, on the contrary, conduct electricity in spite of all the injuries that may be inflicted on them. Yet the well-known labors of Prof. Dubois-Reymond clearly prove that electricity plays a part of some kind in nervous phenomena. Electric currents exist naturally in nerves, and these currents are influenced and modified by the action of the nerve-currents. It may be admitted, then, that nervous phenomena are the result of a secondary action of electricity, producing certain changes, chemical or otherwise, in the nerve-substance; these manifest themselves only after the lapse of a certain time, during which the action increases in a slow and gradual manner till it becomes sensible, and produces mechanical effects. This side of the question is still enveloped in profound darkness, and we are driven to more or less plausible hypotheses. Still, we can say that a great step has been taken toward the solution of the problem of life: the experiments of which this account is given have thrown light upon its approaches, and placed the question on the ground of exact science. No doubt a long time will pass before the progress of methods of observation shall permit us to make one step nearer to the goal, and nothing authorizes the belief that we can ever fully reach it; but we may take pride in what has already been done, since the exactness of the results gained surpasses all expectation.—Revue des Deux Mondes.