and near stimulation of a nerve may be made. By a special mechanism the time-measuring current is sent through the galvanometer at the same moment that the stimulating current goes through the nerve, and the instant the muscle begins to contract, it breaks the current passing through the galvanometer, so that this time-measuring current lasts only from the moment when the nerve is stimulated until the muscle begins to contract.

THE ELECTRIC CHANGE IN NERVE.

Negative Variation.-The natural current of a nerve, like that of muscle, undergoes a diminution at the moment the nerve is stimulated; this is termed the negative variation. It occurs with any other form of stimulation as well as when an electric shock is used, so it is not dependent on an escape of the stimulating current. In the case of a single stimulation, the negative variation is so rapidly over-lasting only .0005 sec., that the inertia of the needle of the galvanometer prevents the change in the current being indicated. In tetanus, however, it makes a decided impression on the galvanometric needle. The strength of the negative variation depends on the condition of the nerve and the strength of the stimulus; being stronger when the nerve is fresh and irritable and has a good natural current, and when a strong stimulus is applied.

The negative variation of the natural currents passes along the nerve from the point of stimulation in both directions, just as does the nerve impulse; and with a galvanometer the electric change may be traced from the nerve to the muscle. It has also been shown that the negative variation travels along the nerve at the same velocity as the impulse; namely, about 27 metres per second. Further, this rate is said to be influenced in the same way by the passage of a constant current through the nerve (to be presently described) as is the impulse derived from stimulus. These points seem to lead to the belief that the nerve impulse and the negative variation are closely related. This peculiar electric change and its accompanying impulse pass along the nerves as a kind of wave of activity, the speed and duration of which we know to be 27 metres per sec. and .0005

of a sec. respectively; the length of the wave we therefore calculate to be about 18 millimetres.

ELECTROTONUS.

If one of two wires leading to a galvanometer be applied to the centre, and the other to the end of a nerve, so as to indicate the natural current, and at the same time another part of the nerve be placed in the circuit of a constant current from a battery, when the circuit of the constant (now called polarizing) current is completed, a change is found to take place in the natural current. This is called electrotonus. Instead of the

N. N'. Portion of Nerve G. G'. Galvanometers. D. Battery from which polarizing current can be sent into nerve by closing key K. The direction of the polarizing and electrotonic currents is indicated by the arrows, and is seen to be the same.

natural currents from the centre to the end of the nerve, a current is found to pass through the entire length of the nerve in the same direction as the polarizing current from the battery. This electrotonic current is not proportional to the strength of the natural currents, and is to be recognized when the latter are no longer to be found. It is stronger with a strong polarizing current, and is most marked in the immediate neighborhood of the poles, fading gradually away as one passes to the remoter parts of the nerve. The electrotonic state is not to be attributed to an escape of the constant polarizing current, because it decreases

gradually with the waning of the physiological activity of the nerve, and ceases at the death of the nerve long before the tissue has lost its power of conducting electric currents. It has been shown that a ligature applied to the nerve so as to destroy its physiological continuity, but not its power of carrying electric currents, prevents the passage of the electrotonic current to the part of the nerve which is thus separated.

The condition of the portion of the nerve near the anode is found to differ somewhat from that near the cathode, and hence it is found convenient to speak of the region of the anode being in the anelectrotonic, and that of the cathode being in the catelectrotonic condition. A certain time appears to be required for the production of electrotonus, a current of less duration than .0015 of a second we are unable to detect the electrotonic state. The negative variation must, therefore, have passed away before the electrotonus has commenced.

IRRITABILITY OF NERVE FIBRES.

The irritability of nerves varies according to certain conditions and circumstances. While uninjured in the body, the irritability of a nerve depends upon

1. A supply of blood sufficient to supply nutriment, and to carry off any injurious effete matters that may be produced by its molecular changes.

2. A suitable amount of rest. Prolonged activity causes fatigue and loss of irritability, no doubt from the same causes mentioned as bringing about fatigue in muscles. The chemical changes taking place in nerves have not yet, however, been made out with any degree of accuracy.

3. Uninjured connection with the nerve centres. When a spinal nerve is cut, the part connected with the periphery rapidly undergoes degenerative changes which seem to depend upon faulty nutrition, since they are accompanied by structural changesfatty degeneration. This appears to commence in a very short time after the section-often in about three to five days. The part of the nerve remaining in direct connection with the cord retains its irritability for a very much longer time.

In the artificial stimulation, by means of electric shocks applied to the nerve of a cold-blooded animal, there are many minor conditions which have considerable influence on the irritability, as evidenced by the response given by the attached muscle to weak stimuli. The more important of these are:

1. Temperature changes. In the case of a frog's nerve, a rise of temperature to 32° C. causes an increase in its excitability. Also a fall of temperature below zero tends to make the nerve more easily excited. Both these conditions have, however, a very fleeting effect, for the nerve soon dies at the temperatures named, and, probably, the increased irritability is only to be taken as a sign of approaching death. It thus appears that a medium temperature is the optimum for nerve work.

FIG. 203.

A+

3 B

Diagram illustrating the variations of irritability of different parts of a nerve during the
passage of polarizing currents of varying strength through a portion of it.
Amm Anode; B Cathode; AB - Intra-polar district; y1 Effect of weak current; y2:
Effect of medium current; y Effect of strong current.

The degree of change effected in the irritability of the part is estimated by the distance of the curves from the straight line. The part of curve below the line corresponds to decrease, that above to increase of irritability. Where the curves cross the line is called the indifferent point. With strong currents this approaches the cathode. (From Foster, after Pflüger.)

2. The part of the nerve stimulated is also said to have some effect on the result of a given strength of stimulus. The further from the muscle, the more powerful the contraction produced, other things being equal. So that the impulse is supposed to gather force as it goes, as in the case of a falling body, and hence has been spoken of as the avalanche action of nerve impulse.

3. A new section of the nerve is said to increase its irritability, as does, indeed, any slightly stimulating influence, such as drying, and chemical or mechanical meddling of any kind. This increase

in irritability probably depends upon injurious changes going on in the nerve, as the influences just alluded to lead to complete loss of excitability, if carried too far.

4. The electrotonic state. The most remarkable changes in the excitability of a nerve are those brought about by the action of a constant current passing through the nerve, so as to set up the conditions just described as anelectrotonus and catelectrotonus.

FIG. 204.

Diagram to show the meaning of the terms ascending and descending currents, used in speaking of the law of contraction. The end of the vertebral column, sciatic nerves and calf muscles of a frog are shown.

The arrows indicate the direction of the ascending current, A, on the left, and the descending current, D, on the right, according as the positive pole of the battery, c, is below or above.

The irritability of the nerve is increased in the region near the cathode, and is diminished in the neighborhood of the anode.

The increase of irritability is in proportion to the intensity of the catelectrotonic and the decrease in proportion to the intensity of the anelectrotonic state. Thus, the increase is most marked in the immediate neighborhood of the cathode, and fades with the distance from the negative pole; and similarly, the decrease