Life in Motion/Lecture II

by John Gray McKendrick

LECTURE II
Myograph—Electricity as stimulus—Action of nerve—Continuous current—Interrupted current—Induction coil—Graphic method—Time in physiological phenomena—Chronographs—Study of contraction—Latent period.

In last lecture we saw that when the nerve connected with a muscle is irritated, the muscle changes its form, that is to say, it becomes shorter and thicker. It is this shortening or, as it is termed, contraction, that causes the movement of one part of the skeleton upon another. Let us repeat the experiment and study it more closely. This time we shall make use of an apparatus called a myograph or muscle-writer, and by means of it the muscle will write down its movement on a smoked-glass plate. The instrument is shown in this diagram. The upper end of the femur is fixed by the clamp C, sliding on the pillar B, and the tendo Achilles is attached by a hook to the horizontal bar EE, which carries a marker J; this marker is brought into contact Fig. 13.—Myograph, an instrument for recording the contraction of a muscle. A, wooden stand; B, vertical brass pillar; C, sliding forceps or clamp for holding upper end of femur of nerve-muscle preparation; D D, short vertical pillars, on the top of which the lever E E works; F, scale-pan for weight; J, marker; G, glass plate; K, counterpoise to keep J in contact with G; H, counterpoise to lever E E. The nerve-muscle preparation is covered with a glass shade, to the walls of which pieces of wet blotting-paper are attached, thus forming a moist chamber to keep the nerve from drying. with G, a smoked-glass plate that can be horizontally moved, sliding in grooves. The nerve is stretched over wires coming from a battery or induction coil, so that it may be irritated by an electric current. When the nerve is irritated, you observe the muscle contracts, lifts up the lever E E, and the Fig. 14.—Examples of tracings taken with the myograph shown in Fig. 13. Plate moved at intervals in direction of arrow. marker J draws a vertical line on the plate G. We then push the plate a little farther on, and again stimulate the nerve by a shock. Another vertical line is drawn on the smoked plate; and, by repeating the experiment, a number of vertical lines can thus be drawn. Suppose we put a weight in the scale-pan F below the frame, the height of the line drawn on the smoked plate, making allowance for the increased amplitude of the movement obtained by the lever, will indicate the work done by the muscle in lifting the weight. We will see by and by that a muscle not only may do work by lifting a weight, but that it becomes hotter in doing so. Energy is thus set free from the muscle as mechanical energy and heat.

The first time one sees this experiment it Fig 15.—Diagramatic representation of nerve, n and muscle, m. Current of electricity enters at a and passes to b. is not easy to be satisfied that the electricity is used merely as a means of irritating the nerve going to the muscle, and that it is not the agent that causes the muscle to contract. Suppose a bit of nerve like a thread is stretched over two wires connected with our electric apparatus, as in this diagram on the blackboard, the current of electricity enters the nerve, from a to b. nerve to the muscle, but, in passing along the bit of nerve, the nerve is irritated, a molecular change of some kind is generated in it, and this change travels down the nerve to the muscle in the direction of the arrow. The change that runs along the nerve, as the result of irritating it, we call the nerve-current, and it is this that excites the muscle into action. When the nerve-current reaches the muscle, it sets up molecular changes in it, and these changes are expressed to our eyes by a contraction, heat at the same time being liberated. The electric current we call a stimulus. We use electricity for stimulating the nerve because it is a convenient method, but the nerve, as we shall see, might be stimulated in other ways, as by mechanical irritation, such as pricking, pinching, or beating, or by heating it suddenly. It does not matter, however, how we stimulate the nerve; the result, so far as the muscle is concerned, is always the same: it always contracts.

But you will naturally ask, is there any relation between the strength of the current we employ for stimulating the nerve and the amount of work the muscle can do in lifting a weight? There is no direct quantitative relation. A very feeble current is quite sufficient to set the nerve in action, just as the pull of a hair trigger is enough to set free the energy from a charge of gunpowder in a rifle. The nerve-current sets free the energy already stored in the muscle. The muscle substance is a magazine or store of energy, and this energy is set free by the molecular action of the nerve.

Having got hold of these general notions as Fig. 16.—Battery of galvanic elements. Z, zinc plate; C, carbon plate. Arrows show direction of current. to how a nerve acts on a muscle, let us consider for a little the nature of the electric shocks we employ for stimulating the nerve.

I have here a number of galvanic elements joined together so as to constitute what we term a battery. Electricity is generated in these elements in consequence of chemical changes occurring in them, and we may suppose this electricity to flow like a current out by this wire, along any circuit formed by a conductor, and back to the battery again by this other wire. Such a current we may call a continuous current. Now let us see what effect a current of this kind has on a nerve connected with a muscle. We connect the wires of the battery with two platinum wires, Fig. 17.—Key for opening or closing current. over which the nerve has been stretched, and the muscle, as before, is connected with the muscle telegraph bell. To enable us to stop the passage of the current, or to send it on at pleasure, we place in the circuit what is called a key. It consists of a rectangular wooden frame, by which the instrument may be screwed to the table. On the top is a square block of vulcanite, a, bearing two rectangular bars of brass, b and c, which may be joined by the handle, d, carrying a horizontal piece of brass. Suppose wires from the battery are connected with c and b. The key is closed when the arm is horizontal, and the current runs along the horizontal piece of brass. On moving the handle, d, backwards and to the right, the brass arm is raised and the contact between b and c is broken. The key is then said to have been opened and the flow of the current is interrupted.

Well, you observe that when I close the key, and the current is sent through the nerve, the muscle gives a contraction, but it quickly relaxes, and no change, so far as the muscle is concerned, is visible while the current flows through the nerve. Now I open the key so as to stop the flow of the current through the nerve, and again there is a sudden twitch. If I open and shut the key quickly there is a twitch with each movement of opening and of shutting, and you see the muscle passing into the more permanent state of contraction that we called cramp or tetanus. It is apparently the suddenness with which the electric current enters the nerve and the suddenness with which it leaves it that irritates the nerve. The nerve is not irritated so as to cause contraction of the muscle during the passage of the current through it. Hence we would expect that currents or shocks of extremely short duration would be very irritating, and this is exactly what experiment proves. We obtain such almost instantaneous currents by the use of an instrument called an induction coil or inductorium.

To explain this to you let me show you a famous and far-reaching experiment first made by Faraday in the laboratory downstairs, by which he discovered the method of obtaining what has since been called Faradic electricity, or electricity by induction. Here is a galvanometer, an instrument used for detecting electric currents. It consists of a coil of wire, in the centre of which is a freely suspended magnetic needle, so hanging that the needle is in the same plane as the coil of wire. A small silvered mirror is attached to the needle, and you observe the mirror reflects upon this scale or screen a beam of light from a lamp placed in front of the galvanometer. A very feeble current passing round this coil deflects the needle, and the deflection is seen by the movement of the spot of light, either to one side or to the other, according to the direction of the movement of the needle. These two large bobbins of fine wire form our induction coil. You observe they are not connected. Fig. 18.—Arrangement of apparatus for demonstrating Faradic currents, b, galvanic element; p primary and s secondary coil; g, galvanometer. In the experiment a reflecting Thomson galvanometer was used. In the circuit of the one we place a small battery and a key. In the circuit of the other we introduce the galvanometer. Watch the spot of light. When I close the key you observe an instantaneous movement of the spot of light. It swings to one side and then comes back, showing that the current passing through the galvanometer circuit is momentary. I now open the key, and you see again a momentary swing of the needle of the galvanometer, as indicated by the movement of the spot of light, but it is in the opposite direction. These instantaneous currents from what we call the Fig. 19.—Induction coil of du Bois Reymond. a, primary coil; b, secondary coil; c, bunch of wires in centre of primary coil for increasing intensity of induction currents; d, binding screw for attachment of wire from galvanic element. The current passes up the pillar d, along steel spring to e, thence to the screw, the point of which touches the back of the spring at e; from f through wire of primary coil to i, round the two pillars of soft iron i, which it renders magnetic, and thus draws down the head of the spring k; this interrupts the current at e, thus breaking the contact of the spring at the screw point. When the current is thus interrupted, the spring flies up by its elasticity and again establishes the circuit at e. This interruption was originally invented by P. Wagner. secondary coil are the Faradic or induced currents. They last only a minute fraction of a second of time. An induction coil, then, consists of a primary coil, with which a galvanic cell is connected, and a secondary coil. When the current is stopped from flowing through the primary coil, that is on opening the key, the current induced in the secondary coil is in the same direction as that of the primary, but when the current is allowed to flow through the primary, as happens when the key is closed, the induced current travels in the opposite direction. If, then, we open and close the primary with great rapidity, say open one hundred times and close one hundred times per second, we obtain a short secondary shock with each opening and with each closing, or two hundred shocks per second. This rapid opening and closing is accomplished by a vibrating spring, which works automatically at the end of the instrument; and we graduate the strength of the shocks by increasing or diminishing the distance of the secondary coil from the primary, the shocks becoming weaker as we draw the secondary away from the primary.

The shocks from this instrument are much more irritating than those obtained from an ordinary battery. Thus, you see, I can hold the wires coming direct from the battery without being conscious of any irritation, but if I send this same current through the primary, I can hardly touch the wires coming from the secondary. The currents from the secondary are momentary in duration, and as they can be localised, they are used by physiologists as convenient stimuli for nerves and muscles.

Here is a large induction coil. You see the powerful discharges it gives, and when we send these through one of the late Mr. Warren de la Rue's vacuum tubes, containing a residue of carbonic acid, we get a magnificent luminous streak of quivering light in the tube, with beautiful transverse markings or bands.

A living being may be electrified positively or negatively and have no sensation caused by the electrification. You see here a frog sitting under this bell glass on a tin-plate connected with one of the dischargers of this large Wimshurst influence machine driven by an electric motor. The tin-plate and the frog's body are highly electrified, as you see by the sparks that fly out when I bring my finger near the tin-plate; but the frog is undisturbed so long as I do not touch it. We will not put it to the pain of having a tetanic spasm by touching it, but instead of the frog we will electrify my assistant standing on an insulated stool. So long as I abstain from touching him he feels nothing, but you see, if I touch his head, or his neck, or the tip of his nose, how the sparks fly out, and he then feels a smart and disagreeable sensation. During electrification we feel nothing, but it is only when we pass from one stage of electrification to another that we have a sensation, and the more rapidly this change takes place, the more irritating the sensation is.

We have now seen how nerves and muscles may be irritated in a definite and precise way, and we have found that the irritation of its nerve causes a contraction of a muscle. In the case of a single twitch, however, the movement is too rapid to be appreciated, and still less analysed, by the unaided eye. We cannot tell, for example, whether the contraction occurs in a shorter time than the relaxation, and still less whether the contraction is at a uniform rate in time, or whether it contracts faster at the beginning and more slowly towards the end, or the reverse. We must not, therefore, trust only to our senses in the study of rapid movements, and we call to our aid what is termed the graphic method of registering movement.

Suppose I hold my pen between my thumb and first two fingers in the ordinary way, and extend the thumb and fingers so as to move the pen upwards on the paper. I produce a line thus— Fig. 20. The length of the line from a to b will show the amount of movement of the pen point, but it will not tell me anything about the rate of movement, nor whether the pen moved With uniform velocity in passing from a to b. Suppose now that I repeat the experiment of drawing the line from a to b; but, on this occasion, suppose the paper is moving with uniform velocity from right to left while I draw the line. It will then be found that I describe a curve something like this— Fig. 21. The paper was moving in the direction of the arrow ← . If now I draw a line x y, and divide it into equal parts, representing equal periods of time, say tenths of a second, and if I drop a perpendicular line from the curve down to x y, say from o to o, I will see at once where my pen point was at that instant of time. Suppose, again, I extended my fingers and then relaxed them so as to draw a line on the paper at rest, I would again draw a line like a b; but if I repeated the experiment, with the paper moving quickly from right to left, I would describe a curve thus—

Fig. 22.

and if time were indicated in tenths of a second by a line x′ y′, the position of the pen point at any instant between x' and y' would be found by dropping perpendiculars, as from x″ to x″ or from x‴ to x‴ . If my pen point travelled faster in going up than in coming down, supposing the paper moved with the same velocity as in the previous experiment, the curve would be something like this— Fig. 23. and if it travelled slower in going up than in coming down it would vary its form to— Fig. 24. Thus by recording rapid movements on a quickly moving surface, such as the surface of a drum or cylinder, or on a glass plate travelling horizontally, we get information regarding phases or variations of movement which we could not otherwise obtain. This illustrates the essential principle of the graphic method, a method of great value in all sciences dealing with movement, and not least to physiological science. It is not a modern method, although in later times its use has been enormously extended. In 1734, the Marquis d'Ons-en-Bray described an anemometer, an instrument for recording the velocity of the wind, which registered its movements on a sheet of paper rolled round a cylinder moved by clockwork. Magellan, in 1779, made designs for an instrument for recording automatically many meteorological phenomena. In 1794, Rutherford constructed a thermometer by which curves of changing temperatures were marked on blackened paper. Thomas Young, one of the founders of this Institution, in 1800, showed how time could be measured on the surface of a cylinder moving at a uniform speed. The celebrated James Watt devised a method of tracing the movements of the indicator of his engine on a cylinder rotated by the engine itself. Thus he obtained a curve showing variations of steam-pressure at different times. During the past thirty years, numerous ingenious instruments have been invented by physiologists for recording movements; but no physiologist has done so much in this direction as Professor Marey of the College du France, to whom we are largely indebted for the development of the graphic method to its present condition of precision and convenience. Our arrangements for studying muscle are not yet complete. We have already seen that a muscular contraction is so rapid as to make it impossible to follow all its phases with the unaided eye. The questions at once occur to us: how long a time does it take to contract? if it does not contract at the same rate throughout its contraction, for how long a time does it contract quickly, and for how long a time does it contract more slowly? These questions lead us at once into another department of inquiry of immense importance in science—the measurement of time, and more especially the measurement of minute periods of time. We all recognise more or less the value of time, and the busier we are the more we value what we call fragments of time. On a long summer day in the holidays, when we have not much to do except gratefully to enjoy the beauties of nature and the sense of physical well-being, a quarter of an hour, or even an hour, is not appreciated as of much value; but when we have a great deal to do, as in winter, when every moment seems to be occupied, five or ten minutes are felt to be precious. We can put a good deal into ten minutes when we are very busy. Science teaches us the value of short periods of time, because nature, which science deals with, is always busy, filling up every moment with some kind of work. We poor mortals count time according to our own wants, and we think a second is about as short a time as we need pay any attention to; but nature does work in much shorter periods of time than a second. A lightning flash occupies only the millionth part of a second, and a dragon-fly's wing, as it passes through the air on a summer day, is quivering many hundred times a second. The fact is, our appreciation of intervals of time by the senses is very limited, and when events happen that are not more than the tenth of a second apart, we are apt to think they are simultaneous. The reason of this is that we need time to think, or, in other words, time is occupied by the processes that go on in our brains. Time is needed for perception, and if two events follow each other so rapidly that during the time we are perceiving the first the second comes on, we blur the one perception into the other, and we fail to notice the interval of time between them. And yet during that interval much may have happened outside of us in connection with each event, of which, however, we are unconscious.

Science, then, demands the accurate measurement of time, and no small fraction of time is too insignificant to be of importance. The twentieth, the hundredth, the thousandth of a second is to science as precious as an hour may be to many people. It may be said without irreverence that to science a day is as a thousand years, and a thousand years as one day. The methods of science for this purpose are founded on two sets of appliances: first, the use of instruments, like a tuning-fork, that are found to vibrate or move at a known rate, say one hundred or five hundred times per second; and, second, the application of the graphic method, by causing these instruments to record their movements on a rapidly moving surface, such as that of a cylinder travelling with great velocity. All such appliances are called chronographs, or time-writers. Let me illustrate to you the use of several of these ingenious appliances.

Here is a ball at the end of a long string suspended from the roof of the theatre. It constitutes a pendulum. During its entire swing it occupies three seconds. Now observe I can amplify the distance of the swing at pleasure, but the time in which the ball travels this distance is always the same. Suppose I make it swing through a distance of twelve feet, then (making allowance for the law that regulates the movements of a pendulum) four feet would roughly represent one second, and one inch would represent the one-forty-eighth of a second. Thus by amplifying the extent of the swing and subdividing we get a notion of small periods of time, such as the one-hundredth or the one -thousandth of a second. If I shorten the length of the string of the pendulum, the ball swings faster, but through a shorter distance in each swing. Carry this on until the ball, suppose it now to be very small, moves, say a hundred times backwards and forwards in a second, and you pass on in thought to the delicate instruments we shall now consider.

The first instrument, devised by Thomas Young, is the revolving cylinder. Suppose this cylinder revolved at a uniform rate by means of clockwork. Suppose the surface of the cylinder to be divided by eight lines, drawn parallel to its axis at equal distances from each other, and that the cylinder makes one revolu- Fig. 25.— Original chronograph of Thomas Young. a, cylinder rotating on vertical axis; b, falling weight acting as motive power; c d, small halls for regulating by centrifugal action the velocity of the cylinder; e, marker recording a line on the cylinder. tion in one second, the distance between two of the lines will represent the one-eighth of a second, and any movement drawing a curve on the cylinder between the two lines must have happened during that interval of time. It is evident that, by drawing the vertical lines closer to each other, much shorter intervals, even to the one-thousandth of a second, may be measured with accuracy, provided the cylinder moves with uniform velocity. It is not easy to secure the latter condition. You see when I start this cylinder it moves slowly at first, and gathers speed as it goes on, and even when it Fig. 26.—Tracings of the vibrations of a tuning-fork, ten vibrations per second. a b, cylinder moving rapidly; c d, cylinder moving slowly. attains full speed I have no guarantee that it is then travelling at a uniform rate. It may make short spurts, or, as the spring becomes unwound, it may by and by move more slowly. The method by the cylinder, therefore, is not sufficiently accurate.

Thomas Young was also the first to devise the method of inscribing on a rotating cylinder the vibrations of a rod bearing a very light style or marker. These describe undulations on the cylinder, and the undulations correspond to equal periods of time. No instruments vibrate with greater uniformity than tuning-forks. Duhamel was the first to apply to one of the limbs of a tuning-fork a small marker, and to bring this marker against a rapidly moving surface, like the surface of this blackened cylinder. Undulations or waves are thus described. The more rapid the movement of the cylinder, the longer will be the waves, as you see in the experiment, and as are represented in this diagram (Fig. 26).

It is difficult to apply the vibrating limb of a tuning-fork to a cylinder, more especially if other recording apparatus is adjusted to the cylinder at the same time. Suppose we use the fork simply for interrupting the current, and this it will do with great regularity, we might interpolate in the circuit a little electromagnetic appliance, having a keeper, to which a marker is attached. This is the apparatus you see here. It consists of a battery, an interrupting tuning-fork, and an electromagnetic instrument called the chronograph or marker. The chronograph consists of a fine marker, c, fixed to the end of a steel spring, and armed with a mass of steel, somewhat wedge-shaped, which fits in between two small keepers. Fig. 27.—Electric chronograph. For description see text. b b, of the electro-magnets, a a. The tuning-fork interrupts the current from the battery. Fig. 28.—Interrupting or chronographic tuning-fork. This it does automatically. When the iron of the electro-magnet between the limbs of the tuning-fork becomes magnetic, the limbs are drawn together, and a small bit of platinum wire fixed to one of them is removed from contact with a platinum surface, so as to break the circuit. On the circuit being thus broken, the electro-magnet ceases to act; the limbs of the fork, by their elasticity, spring back to their original position; and thus the platinum wire is again brought into contact with the platinum surface. Thus again the circuit is completed and the action is repeated. In this way, the marker of the chronograph vibrates in unison with the fork, and you see, when I bring it into contact with the cylinder, a beautiful series of little waves is described, each little wave representing the one-hundreth of a second. Let us take a rough illustration to show the value of recording time in this way. Suppose I draw my hand, bearing a pencil, quickly from left to right and then back again. We wish to know the time occupied by that movement. I set the cylinder in rapid motion, the time is registered by the chronograph, and I now make the wished-for movement, bringing my pencil point into contact with the blackened surface. Here is the result. We have only to count the number of little waves made by the chronograph corresponding to the big wave made by my hand. You see we have twenty little waves. Thus twenty hundredths, or one-fifth of a second, represents the time occupied by the rapid movement of my hand.

It is impossible, in a lecture-theatre like this, to show to a large audience, such as I have the honour to address, the curves described by any recording apparatus on a rotating drum or cylinder. What we need is an arrangement by which the curves can be at once projected on a screen by the electric light. Indicating my wants to Mr. Horace Darwin, of the Cambridge Scientific Instrument Company, we have been able to devise the apparatus now before you, and which we will call the R. I. Railway. As it will be used chiefly for obtaining the curves of contracting muscles, it may well be named the railway myograph. You see it is a triangular frame, carrying a large plate of glass secured in a vertical position. The glass is blackened with soot in a smoky flame, and, of course, the soot prevents any light from passing through it; but if any soot is rubbed off, as when we draw a line on the plate with the point of the pencil, the light shines through, Fig. 29.—The new railway myograph, as arranged on the lecture-table at the Royal Institution. The shadow of the apparatus was thrown on the screen. The apparatus is depicted as it was arranged for the study of tetanus. and the line appears as a bright shiny line on the screen. The wheeled car bearing the plate of glass is drawn up to the end of the long board, and this you observe is not level, but we may incline it by turning this screw which raises one end. A catch holds the car in position, and when the car is in position a strong spring is put on the stretch. When the catch is released the car runs down the incline, and at the same time the spring recoils, and, pulling on a lever, sends the car along with great velocity to the other end of the board. As it runs along we have an arrangement by which, when about midway in its course, the car breaks an electric circuit. We shall not use the "break" in the present experiment.

Now we shall show you a record of the movements of our chronograph worked by this tuning-fork, which is vibrating one hundred times per second. Mr. Brodie, you observe, brings the car up and fixes the catch. The spring is on the stretch. He now adjusts the marker of the chronograph on the glass plate and you hear the humming of the fork. All being ready, Mr. Brodie releases the catch, the car, carrying the glass plate, dashes across, runs into the electric beam, and is caught firmly by a spring that prevents it from rebounding. Mr. Heath had previously adjusted the electric lantern, and you now see on the screen the beautiful sinuous line, each wave of which represents the one-hundredth of a second.

You will be thinking that all this has not much to do with muscle, and I fear the description of these appliances may have been a little wearisome to you. But we must know something about the methods by which we attain results in science. This gives one a better appreciation, a better grasp, as I may say, of these results, and it shows us how men have got over difficulties in their attempts to explore phenomena. The recognition of how they have done this is an education by itself.

Now come to another physiological experiment. I have fitted up on this stand a number of pieces of apparatus, all intended for studying muscular contraction. First, at the top, you see a brass forceps which tightly holds the thigh bone of the frog's leg. You see again the gastrocnemius muscle hanging down, with its tendon, by means of a hook, fixed to tins very light lever. Underneath the lever I have suspended a weight weighing 10 grammes, that is, about 150 grains, about the third of an ounce, which the muscle will be obliged to lift when it contracts. Then you see the sciatic nerve is stretched over these platinum wires, which we call electrodes, and the wires come from the secondary coil of our induction Fig. 30.—Diagram showing the break in the railway myograph. Current enters at d, passes along arm of brass a, having a bit of platinum at b', thence through point of screw c to e, and back to battery by g. When the plate of the railway runs across, it knocks a aside and opens current at b. machine. Here is the primary coil of the induction machine. I have interposed in its circuit this galvanic element or cell, and we will allow our railway myograph to break the circuit of the primary coil as it rushes down the railway incline. Thus when the break is opened, the opening shock will go from the secondary coil to the nerve. Now, the moment the break is opened will practically coincide with the moment the nerve receives the shock, because the electricity travels with such enormous rapidity in the coils and along the wires to the nerve as to make the time between opening the break and irritating the nerve practically nothing. If I knew the moment the nerve was stimulated, it would be interesting to ascertain if the muscle contracted at that moment. Now we can easily record this moment by first of all causing the muscle to contract by bringing the car slowly up to the break till it opens it. When the muscle contracts, it makes a mark, which indicates the moment the break will be opened when we perform the actual experiment. At that moment the nerve is irritated, and if the muscle contracts at the same instant, the beginning of its upward curve should exactly coincide with the mark of the signal. Now we shall perform the experiment. Mr. Brodie has got the railway ready for starting. He opens the break, releases the catch, and lets the glass plate dash onwards. The muscle, of course, does not contract, as the nerve has not received a shock, and we have only a horizontal line drawn thus— Then Mr. Brodie, in the next place, closes the break, brings the carriage slowly up to it, and opens the break gently. The moment this is done, a shock (the opening shock) comes from the secondary coil and the muscle makes a mark a b thus— He then again closes the break, brings up the railway to the catch, releases the catch, the railway dashes across, opens the break, the nerve gets the shock from the secondary coil, and the muscle contracts. But you observe the muscle has contracted a little later than the instant the break was opened, so that we get this curve (Fig. 31).

You notice the momentary twitch. Here is the tracing. You see the muscle curve has begun a little later than the mark made by the signal, that is to say, the muscle did not contract the instant the nerve got the shock. A little time intervened, something like the one-hundredth of a second, during which nothing visible happened, and then the muscle contracted. This period is called the period of latent stimulation. In the experiment we have just made, it does not Fig. 31.—Curve of a single muscular twitch as taken with the railway myograph. a to b, period of latent stimulation; b to c, contraction; and c to d, relaxation of muscle; e, small secondary wave, probably due to movement of lever. exactly represent the period during which the muscle rested after receiving the molecular disturbance of the nerve-current, as I have already explained, and we must deduct from it the time occupied by the nerve-current travelling down the little bit of nerve. However, it is interesting to know that there is a short period during which changes are probably happening in the muscle before it contracts. In the latent period, the muscle is preparing for making the movement. Very refined methods show that the latent period is shorter than the one-hundredth part of a second. Professor Burdon Sanderson has found, by a photographic method, not so liable to experimental errors as the one I have shown you, that in the muscles of the frog it is about the one-two-hundredth part of a second. Probably it is even shorter in the muscles of the higher animals. Their muscular substance is more unstable than that of a frog, and it goes off more rapidly under the nervous stimulus. Research also shows that probably in all living matter submitted to a stimulus there is a latent period, a period in which molecular changes are happening which precede, and possibly end in, the particular phenomenon manifested by the living matter. Thus when the nervous stimulus reaches the cell of a secreting gland, or a blood-vessel, or a nerve cell in the spinal cord or brain, it does not produce an immediate effect, but excites changes which occupy time.

When we next meet we shall study tetanus or cramp, and how a nerve probably acts on a muscle.