a clip; at each of its dilatations the muscle will force open the clip, and this movement may be registered. This method enables us to study the phenomenon of the muscular wave, and the speed with which it travels throughout the whole length of the muscle.

Fig. 7 exhibits a bundle of muscle held at two points of its length between the myographical clips, 1 and 2. Those instruments are so constructed that when their ends are pushed apart by the dilatation of the muscle, the move

FIG. 7.-Disposition of a bundle of muscle between two pairs of myographical clips. Clip No. 1 holds the electric excitators of the muscle. A wave is represented at the moment when it has just crossed each of the clips.

ment compresses a sort of little drum which sends a portion of the air which it contained through an india-rubber tube into a similar little drum. Fig. 7 shows two of these instruments fixed upon a foot. The expansion of the membrane lifts a registering lever, and thus gives notice of the dilatation of the muscle at the point where it is compressed by clip No. 1. The movement is shown upon the tracing by a curve analogous to those which we have already seen.

Let us suppose that the muscle is electrically excited at the level of the first clip; notice is given of the formation of the wave at that part of the muscle, but clip No. 2 does not yet give its signal. In order that it may act, the wave, as it passes along the muscle, must reach it. As this occurs, clip No. 2 gives the signal in its turn, and it is shown by the tracing, that this second movement is later than the first by a certain space whose duration may be estimated according to the speed of the rotation of the cylinder.

The influences which modify the intensity and the duration of the muscular shock have appeared to us to modify the intensity and the speed of the propagation of the wave. Thus the two lower curves represented in Fig. 8 show that the transference of the wave is retarded by cold.

The experiment has been made upon the muscles of the thigh of a rabbit. The clips were placed as far as possible apart, about seven centimetres. Electricity was applied to the lower extremity of the muscle, and the two upper curves of Fig. 8 were obtained. The interval which divides those curves marks the duration of the transference of the muscular wave. After the muscle had been chilled with ice the curves at the bottom of the figure were obtained. We see that the transference of the wave is slackened, for there is a longer interval between these curves than between the first.

Production of mechanical force in the muscle.-We have seen that chemical action is the source of muscular force; through

what media does this force pass before it becomes mechanical work?

In steam engines, heat is the necessary medium between the oxidation of the fuel and the developed mechanical work. It is very probable that the same

thing takes place in the muscles. The chemical action produced by the nerve within the fibre of the muscle disengages heat from it: this heat in its turn is itself partially transformed into work. We say partially, since according to the second principle of thermo-dynamics, heat cannot be entirely transformed into mechanical work.

Certain facts seem to justify these views thus, by warming a muscle, we change the form of it, and may see it contract in length as it expands in breadth. These effects disappear when the muscle is cooled.

Muscular fibre is not singular in its power of transforming heat into work. India-rubber, for instance, has an analogous property, and this substance may be made to imitate the muscular phenomena to a certain degree. If we take a strip of india-rubber (not vulcanised), and, drawing it between the fingers, stretch it out

to ten or fifteen times its original FIG. 9.-Transformation of heat into work by a strip of india-rubber. length, we see that it becomes

white, and of a pearly lustre. At the same time the strip will become sensibly warm, and it will tend energetically to return to its original condition, so that if we let go either of its ends, it will instantly resume its former length, and fall to

its original temperature. According to our view, the sensible heat has disappeared and become mechanical work. If we plunge the strip when extended into water, so as to deprive it of its heat, it remains, as it were, congealed in its extended state, and does not develop any mechanical work. But if we restore to the elongated strip the heat which it had lost, it will recover its elasticity with considerable force. Fig. 9 represents a strip of india-rubber thus pulled out and cooled. It has been laden with a weight that it may have no tendency to recover itself. But, if we take the strip between our fingers, we feel it swell and shorten at the same time that it lifts the weight; there is again production of mechanical work.

If we thus heat the strip at various points we create a series of lateral expansions, each of which raises a certain quantity of the weight. Lastly, if we heat it throughout all its extent, the strip returns to its original dimensions, with the exception of the slight elongation produced by the suspended weight.

Strong analogies exist between these phenomena, and those which take place in muscular tissue. The identity would be perfect if the wave which heat produces on the strip of india-rubber were transmitted to each end. This transference implies, in the muscular fibre, the successive propagation of the chemical action which disengages the heat. It is thus that if we light a train of powder at one point, the incandescence spreads throughout its entire length.

These analogies have struck us as being remarkable: they seem to us to open new views of the origin of muscular action.

CHAPTER V.

CONTRACTION AND WORK OF THE MUSCLES.

The function of the nerve- -Rapidity of the nervous agent-Measures of time in physiology-Tetanus and muscular contraction-Theory of contraction-Work of the muscles.

THE experiments described in the preceding chapter show us the muscle under artificial conditions, which may, perhaps, induce us to suspect the results which they furnish. Can this electrical agent, which has been employed to excite motion, be assimilated to the unknown agent which the will sends through the nerves to command the muscles to act? And these artificially-produced movements, those brief shocks, always similar if the conditions of the muscle be not changed, in what do they resemble the motions commanded by the will, which are so varied in their form and their duration? These objections deserve at least a brief discussion.

The function of the nerve. When a nerve is excited by an electric discharge, the electricity employed does not always pass to the muscle in which the reaction takes place. The shock is produced equally well when all propagation of the electric current along the nerve is prevented, and it exhibits itself equally when excitants of a quite different nature are employed, for instance, pinching or percussion. Thus, the excitant employed only excites in the nerve the transference of the agent which is proper to that organ. Is not this nervous agent itself electricity? Notwithstanding the able labours of the German physiologists, and especially of M. Du Bois Reymond, science has not yet decided on that subject. We know that electric phenomena are produced in the nerve when it has been excited in a certain way, and that their propagation throughout the nervous cord seems to have precisely the same speed as that of the transference of the nervous energy itself. How has this speed been measured?