328 PROFESSOR RUTHERFORD.

SECTIONS OF TISSUES, &c.

ice is undone, and beautiful sections may be made if the knife be passed through it with a very rapid sawing movement. Of course the ice around the section soon melts, and therefore no trouble is required to remove the fixing agent, as in the case of paraffine, carrot, &c. The capabilities of this arrangement will be readily appreciated from the following experiment. I killed a medium-sized rabbit, and immediately removed a portion of a lung, liver, intestine, muscle, and a whole kidney. I rapidly washed them in salt solution (vide supra), and put them, hot as they were, at once into the machine and covered them with salt solution. I put the freezing mixture into the box and covered the whole with cotton wadding. The box H' required to be about half refilled with the freezing mixture ere the tissues were completely frozen. To freeze them thoroughly required sixteen minutes. I then made sections, as fine as any one could possibly desire, of all the different tissues at once, picked them off the knife with a camel-hair pencil and put them into separate vessels for examination. Perhaps some supporting fluid other than salt solution will be suggested by future research. I have not as yet been able to find a better. Perhaps, too, other modifications of the machine may be suggested, but for the present I am unable to think of anything that could be done to render the apparatus more perfect than it now is. For making the sections I use a razor, a circular amputating knife, or a knife with a long broad blade ground flat upon one side.

The machine as above described is made by Mr Hawksley, Blenheim St., New Bond St., W.

ON THE RELATIVE EXCITABILITY OF DIFFERENT

PARTS OF THE TRUNK OF A SPINAL NERVE. By WM. RUTHERFORD, M.D., F.R.S.E., Professor of Physiology, King's College, London.

As is well known, Pflüger some years ago showed that if the sciatic nerve of the frog be stimulated at one time near the gastrocnemius and at another time at a distance from the muscle, the contraction which takes place is more powerful in the latter than it is in the former case, even although the irritant be of the same strength in both instances.

The manner in which this experiment is performed will be readily understood with the aid of the following diagram.

Fig. 1. Showing the arrangements for performing Pflüger's experiment described in the text. N Sciatic Nerve. M Gastrocnemius of frog. R' and R" primary and secondary coils of Du Bois Reymond's Induction machine. C Pohl's commutator1, with wires connecting it to the induction machine and to the nerve."

1 For an account of the mechanism of Pohl's commutator the reader is referred to my Lectures on Experimental Physiology in the Lancet, No. VI. Vol. I. 1871.

VOL. V.

22

The sciatic nerve of the frog is dissected out in its entire length, and is divided as near the spinal column as possible. The femur and muscles of the thigh are divided, and the leg, with its nerve, is removed. In cleaning the nerve it is important to remember that it must not be stretched or scratched, otherwise the experiment may completely fail. Every care must be taken to injure the trunk of the nerve as little as possible. The nerve must not be allowed to dry, else failure is inevitable. I find it best to perform the experiment the moment the nerve is cut out. If one be compelled to wait for more than three or four minutes the nerve should be pencilled with saliva, allowed to cool in a watch glass. The nerve (N) is laid upon two pairs of clean copper wires, one pair placed near the gastrocnemius (M), the other pair placed near the free end of the nerve. The wires are joined to a Pohl's commutator (C), and this is connected by means of another pair of wires with the secondary coil (R") of Du Bois Reymond's Induction machine. The primary coil of this (R) is joined to a battery. When the bridge of the commutator (H) is in the position indicated in the figure the faradic currents pass through the nerve near the muscle by the wires joined to 1,, and 1,,,. When the bridge is turned so as to make it lean to the side of the commutator which is lowest in the diagram, the currents no longer pass through the above-mentioned wires, but through those touching the nerve near its free end. Before the experiment is begun, the coils (R′ and R") of the induction apparatus are separated from each other to a distance of about 20 inches. The bridge of the commutator is placed so that the nerve will be stimulated near the muscle. The induction machine is then thrown into operation. The muscle will most probably remain motionless, because the induced currents are not powerful enough to affect the nerve. To render them sufficiently strong, the secondary coil (R") is very slowly pushed towards the primary coil (R'), and this movement is arrested the moment the toes are thrown into a slight tremor, indicating feeble contraction of the muscles of the leg. Without arresting, or in any way altering the action of the induction machine, the bridge of the commutator is turned, so that the nerve will be irritated at a distance from, instead of near to, the muscle. If

the experiment succeed, it will be observed that the contraction of the muscles is now very decidedly exaggerated; and this, although the irritant for all practical purposes remains of uniform strength. We may vary the experiment, and begin by finding out the feeblest currents necessary to throw the muscle into contraction when applied to the nerve near its free end, and then cause this current to operate on the nerve near to the muscle. Most probably, in the latter case, the leg will come to rest, the irritant not being powerful enough to affect the nerve in the latter situation.

This, then, is the experiment for which we are indebted to the distinguished Bonn physiologist. How are the results to be explained? According to Pflüger the experiment shows that the passage of nerve force along a nerve resembles the rush of an avalanche over a precipice. It gathers force as it descends. If it were true that the nerve influence passes through a nerve in this avalanche-like (lawinartig) manner it would be a fact of great importance in nerve physics, for it would show that the longer the nerve through which the nerve energy must pass, the easier is it to produce a result in the tissues at the nerve terminations; in other words, the longer the nerve the feebler need be the stimulus applied to it in order to produce a given result. I have long been in the habit of pointing out to my class that Pflüger's explanation cannot be accepted until it be proved that the result of the experiment is not simply due to the excitability of the nerve being less near than it is at a distance from the muscle. Rosenthal long ago discovered the fact that if a nerve be tied or divided, its excitability is for some time increased. This increase is most marked near the point of injury. This fact I have repeatedly verified. Pflüger cut the sciatic nerve, as I have described, and found that an irritant applied near the cut end produced a greater effect than when applied at a distance from it. This might simply be due to the greater irritability of the nerve near the seat of injury; and, moreover, the greater irritability of the nerve at this point might be owing to its being nearer the injured part. I have always regarded this explanation as more probable than Pflüger's. Experiments recently performed by me serve, however, to materially advance our knowledge

regarding this interesting question, and, indeed, to place the matter in a new light.

I performed a number of experiments on frogs in the following way. I exposed and carefully dissected out one sciatic nerve. I tied the sciatic artery, and divided the muscles and bones of the thigh. I hung the frog up by a pin thrust through the point of the nose, and I fixed the femur in a pair of forceps in order to support the nerve and place it for convenience in a somewhat horizontal position.

Fig. 2. Frog with right leg partially amputated. Right sciatic nerve N. A and B two pairs of wires for stimulating the nerve. (For further explana tions see text.)

In order to irritate the nerve I placed two pairs of wires, as indicated in Fig. 1, under the nerve, taking care that the wires touched nothing but the nerve. In order to render the insulation of the whole more complete, I placed the stands supporting the frog and its detached leg upon a sheet of dry gutta percha. I did not divide the nerve, because I wanted to avoid