observed to arise only on making the connection with C, because only then a portion of the nerve between the ligature and the muscle was stimulated, whereas, on making the connection with B, the ligature intercepted the effect produced by the stimulation of the portion AB. These facts, the speaker said, are calculated to impress us with a notion of something moving along the nerves in the act of volition and in that of sensation, which, to preclude any hypothetical view regarding its nature, we will term the nervous agent. As this agent for its conveyance requires an uninjured state of the nerve, being checked by so gross an obstacle as a cut or a ligature, it must necessarily be something material, not, however, the electricity such as it moves along a telegraph-wire, for this would readily overleap those impediments. But, whatever the nervous agent may be, it obviously must proceed in the nerves with a certain definite velocity. In other words, however great this velocity may be, a certain definite time will be required for the messages of the brain to reach the muscles and for those of the senses to reach the brain. In common life, it is true, we never notice any phenomenon indicative of such a delay in the transmission of despatches in our nerves. Certainly our limbs do not instantaneously carry out the orders of our will; but this is rather owing to the circumstance of time being required for motion. On the other hand, we fancy we see the light, we hear the sound, we feel the prick on the toes as well as on the cheek at the very instant the corresponding organs of the senses have been affected. But a little reflection shows that this is altogether a delusion; in fact, if we only had one sense, an indefinite time might elapse between its organ being affected and the sensation taking place in the brain, without our perceiving it; we simply should always be so much behind the real time, just as we are when listening to distant music, or when looking at the stars. The same thing would also happen if we had two senses, both equally slow in working. We should only perceive that something was wrong, if the two senses had a different rate of working, and if the difference of time resulting therefrom for the perception of messages conveyed by the two senses were large enough to become apparent, as is the case when we see the flash of the gun before hearing its report. Now we cannot well distinguish intervals of time smaller than one-tenth of a second at the most; so that within that limit any irregularity in the working of our organs of sense may occur, from any cause whatever, hence from a delay of the messages in the nerves, without our becoming aware of it. The problem thus suggests itself, to ascertain whether any perceptible time is required for the transmission of volition and sensation through the nerves, and if so, what is the rate of propagation of the nervous agent. Historical Remarks.-This problem is by no means a new one; for a hundred and fifty years it has engaged the attention of physiologists, and many an adventurous hypothesis has been broached in order to approach to at least a pretence of a solution. One of the early Iatro-mathematicians preposterously conceived that the velocity of the nervous agent ought to bear the same proportion to that of the blood in the aorta as the width of the aorta to that of the nerve-tubes, and he thus inferred the velocity of the former to be one hundred and twenty millions of miles in one second, rather more than six hundred times the velocity of light.* Haller himself tried, in reading the Eneid aloud, how many letters he could pronounce in oue minute. Finding that they were fifteen hundred, among which the R, according to him, requires for its formation ten successive contractions of the M. styloglossus, he states that in one minute a muscle may contract and relax fifteen thousand times, and as the relaxing lasts as long as the contracting, each contraction would have lasted only 30 of a minute, or of a second. Hence Haller argues that the nervous agent requires of a second for travelling from the brain to the M. styloglossus, say a distance of about four inches, which makes about 160 feet in one second. Now this result is not a little remarkable. In Haller's reasoning every single step is erroneous, and the whole rests on a perfectly absurd basis. Nevertheless, the result to which Haller has thus been led wonderfully coincides with that which has recently been arrived at by the methods which it is the object of this lecture to explain; so that in this case the Eneid really has proved a book of oracles.† John Müller, of Berlin, hardly seventeen years ago, used in his lectures to dwell upon the apparent impossibility of ever solving the problem under consideration, on account of the enormous rate of propagation, comparable to that of light and electricity, which he ascribed to the nervous agent, while the small compass of the animal body did not offer sufficient range for its measurement.‡ These historical details, perhaps, will not be deemed superfluous, in as much as they are calculated to bring out more fully the beauty and the high scientific value of the following researches. Description of M. Pouillet's Chronoscope.-More than twenty years ago, M. Pouillet suggested a very ingenious plan for measuring the velocity of projectiles. If an electric current flows constantly through the coil of a galvanometer, the needle is deflected to an amount which depends upon the intensity of the current, and upon the sensitiveness of the galvanometer. But if the current be sent through the coil only for a time so short that it vanishes when compared to the duration of one oscillation of the needle, things happen differently. The needle then receives as it were a single impulse, Haller, Elementa Physiologiæ Corporis humani.' Tom. iv. Lausannæ, 1762. 4o. P. 372. + Ibidem, pp. 373, 483. Handbuch der Physiologie des Menschen, u. s. w.' Bd. i. 4 Aufl. Coblenz, 1844. S. 581. yielding to which it slowly recedes, till its velocity has been annihilated by the magnetic force of the earth, which draws it back to zero. And the initial velocity imparted to the needle by the current, provided this be of constant intensity, will be proportional to its duration, so that from the velocity, which can be calculated from the deflection of the needle, the duration of the current may be inferred. The galvanometer is thus transformed into a chronoscope, which can be used for measuring the duration of rapidly transient processes, whenever there is a possibility of making the beginning and the end of the process coincide with the beginning and the end of the chronoscopic current,-for so we will style the current, which, by its action on the galvanometer, becomes an indicator of time. And similarly we will style chronoscopic circuit, the circuit of the chronoscopic current. If, e. g., the velocity of a bullet, within the very barrel of a gun, were to be measured, the chronoscopic circuit, in addition to the battery and the galvanometer, should comprise a wire stretched out just before the muzzle of the gun, the cock, which in some way or other should be insulated from the gun, and lastly the gun itself. Then at the moment when the cock strikes the percussion-cap, the circuit is made and remains so till the bullet tears the wire; and so the current will only have lasted during the time required for the explosion of the percussion-cap, for that of the gunpowder in the gun, and for the moving of the bullet along the barrel. This time has been found to be from to of a second. By repeating the same experiment with the wire or a net of wires at any greater distance from the muzzle, and by taking the difference of the times required in both cases, the time elapsed during the flight of the bullet through the space comprised between the two positions of the wire can be ascertained.* 140 Application of this Method to our Problem by Professor Helmholtz. The same method was, a few years later (1850), successfully applied by Professor Helmholtz to the solution of our problem. Suppose that the chronoscopic current, when its circuit is made, causes the muscle to contract by stimulating a motor nerve at a point A. It will be easy to arrange things so as to cause the muscle by its contraction to break the circuit. The current will then have lasted the time necessary for the transmission of the nervous agent from the point A of the nerve to the muscle, and, moreover, that necessary for the muscular contraction to break the circuit. But by repeating the experiment, with the sole difference that the chronoscopic current is made to act upon a point B of the nerve farther from the muscle than A, and by taking the difference of the times required in both cases, the time which elapses while the nervous agent is travelling from point B to point A will be found. Fig. 1 shows the experiment, not exactly as made by Professor *Comptes Rendus,' &c. 1844, t. xix. p. 1384. Helmholtz, but with some slight modifications, that the speaker has introduced for the sake of convenience. g is the gastrocnemius muscle of a frog, fastened by the thigh-bone in the clamp c, which can be raised or lowered by the screw s. Through the tendo-Achillis a hook is thrust, and to this, by an insulating piece i, a brass lever lk is attached, turning on the axis k, and, near its end 7, supported by a platinum point resting on a platinum plate p. Just underneath the muscle, a scale-pan is suspended from the lever, on which any suitable weight may be placed. At the end of the lever an amalgamated copper point dips into a mercury cup m. G is the galvanometer-coil; and it is hardly necessary to mention that the readings are made with mirror, telescope, and scale. B, is the battery belonging to the chronoscopic circuit. This circuit is formed by B,, G, m, p, L, and the place of contact n, where for the present it is interrupted. For several reasons, which it would take too long here to explain, the chronoscopic current cannot be employed for directly stimulating the nerve by its beginning, and this has to be done indirectly, in the following way: noq is an insulating lever turning on the axis o, and bearing at n a platinum plate connected with B. This plate corresponds to a platinum point at the end of the brass lever L, which in its turn is connected with p, so that by pressing down L, the chronoscopic circuit is made. But by pressing down the extremity n of the lever no q, its extremity q is simultaneously raised, and thus another circuit is broken. This circuit comprises a battery B, and the primary coil pc of an induction apparatus, from whose secondary coil sc wires extend to the part of the nerve which is to be stimulated, or to the muscle. No perceptible time elapses between the breaking of the primary circuit and the generation of the induced current, and the duration of the latter does not exceed a few ten-thousandths of a second. Hence the stimulation of the nerve can be assumed to happen at the very instant the chronoscopic circuit is made. By means of the screw s, it is easy to make the muscle support the lever k so that the platinum point just rests on the plate p. This is done by lowering the screw, till on percussing the lever above the platinum point no clattering is heard. If now the tension of the muscle be ever so little increased, the lever will be lifted, and the chronoscopic circuit broken at p. After the contraction is over, as the lever sinks back into its original position, the chronoscopic circuit would be made again, and the experiment spoilt in consequence of the new and overpowering action exercised on the needle, unless some measure were taken to prevent it. This very serious difficulty has been met by Professor Helmholtz with singular felicity. The mercury cup into which the amalgamated copper point dips is lowered before the experiment, till by capillary attraction the mercury is drawn up in a cone connected with the point by a thread. The slightest upward motion of the point then will cause the mercury thread to break, after which the liquid metal speedily reassumes its spheroidal surface. So that when the amalgamated point comes down again, it cannot reach the mercury, and the chronoscopic circuit remains open. In order to obtain a clear insight into the conditions of the experiment, it should first be made by stimulating, not the nerve, but the muscle itself. For this purpose, the ends of the secondary coil should be united with the wires 1 and 2 in the diagram. As, on doing so, all the parts of the muscle are acted upon simultaneously at the very instant the chronoscopic circuit is made, no delay in the nerves can occur; nevertheless, a deflection of the needle is thus obtained, which shows that after the stimulation there is an interval of time of about 01 of a second, during which the tension of the muscle still remains unaltered. This interval has been styled by Professor Helmholtz the stage of latent stimulation. If, after having made the muscle support the lever so that it just rests on the plate p, a weight be placed upon the scale-pan, the deflection obtained on stimulating the muscle as before, is increased, and, up to a certain limit, the more so the heavier the weight. It thus |