Leakage of induction current.—Before completing the constant current circuit, the alternating induction current goes only through the path EE′. On completion of the constant current circuit, the alternating induction current not only passes through the shorter path EE′ but also by the circuitous path of the constant current circuit. The escaping induction current would thus excite all the leaflets directly and not by its transmitted action. This difficulty is fully overcome by the interposition of a choking coil which will be described below. A simpler, though less perfect, device may be employed to reduce and practically eliminate the leakage. This consists of a loop, L, of silver wire placed outside EE′. The leakage of induction current is thus diverted along this path of negligible resistance in preference to the longer circuit through the entire petiole, which has a resistance of several million ohms.
Polar action of current on excitability.—It is well known that an electric current induces a local depression of excitability at the point of entrance to the tissue, or at the anode, and an enhancement of excitability at the point of exit, or at the cathode. But the excitability is unaffected at a point equally distant from anode and cathode. This is known as the indifferent point. The exciting electrodes EE′ are placed at the indifferent point. But when the current enters on the right side, the terminal leaflets to the right have their excitability depressed by the proximity of anode, but the leaflets near the electrodes EE′, being at a distance from the anode are not affected by it. Moreover it will be shown that the enhanced conductivity conferred by the directive action of the current overpowers any depression of excitability in the terminal leaflets due to the proximity of the anode. I shall, for convenience, designate the transmission as ‘up-hill’, when excitation is propagated against the direction of the constant electric current, and ‘down-hill’ when transmitted with the direction of the current.
Transmission of excitation ‘Up-hill’: Experiment 38.—I shall give here an account of an experiment which may be taken as typical. I took a vigorous specimen of Averrhoa bilimbi, and applied a stimulus whose intensity was so adjusted that the propagated impulse brought about a fall of only two pairs of leaflets. This gave a measure of normal conduction without the passage of the current. The constant electric current was now sent from right to left. A necessary precaution is to increase the current gradually by means of a suitable potentiometer slide, to its full value. The reason for this will be given later. The intensity of the constant current employed was 1.4 micro-ampères. Now on exciting the petiole by the previous stimulus, the conducting power was found to be greatly enhanced. The excitatory impulse now reached the end of the petiole, and caused six pairs of leaflets to fall.
Transmission of excitation ‘Down-hill’: Experiment 39.—In continuation of the previous experiment, the constant electric current was reversed, its directions being now from left to right. Transmission of excitation was now in a down-hill direction. On applying the induction shock stimulus of the same intensity as before, the conducting power of the petiole was found to be abolished, none of the leaflets exhibiting any sign of excitation. This modification of the conducting power persists during the passage of the constant current. On cessation of the current the original conducting power is found to be restored. It will thus be seen that the power of conduction is capable of modification, and that the passage of an electric current of moderate intensity induces enhanced power of conduction in an ‘up-hill’ and diminished conductivity in a ‘down-hill’ direction.
ELECTRIC CONTROL OF NERVOUS IMPULSE IN ANIMALS.
In my ‘Researches on Irritability of Plants’ I have shown how intimately connected are the various physiological reactions in the plant and in the animal, and I ventured to predict that the recognition of this unity of response in plant and animal will lead to further discoveries in physiology in general. This surmise has been fully justified, as will be seen in the following experiments carried out on the nerve-and-muscle preparation of a frog. It is best to carry out the experiments with vigorous specimens; this ensures success, even in long continued experiments, which can then be repeated with unfailing certainty for hours. It is also an advantage to use a large frog for its relatively great length of the nerve.
Directive action of current on conduction of excitation in a nerve-and-muscle preparation: Experiment 40.—A preparation was made with a length of the spine and two nerves leading to the muscles. The specimen is supported in a suitable manner, and electric connections made with the toes, one for the entrance and the other for exit of the constant current. The current thus entered, say, by the left toe ascended the muscle and went up the nerve on the left side, and descended through the other nerve on the right side along the muscle and thence to the right toe. Before the passage of the constant electric current the spinal nerve was stimulated by an induction shock of definite intensity. The nervous impulse was conducted by the two nerves, one to the left and the other to the right, and caused a feeble twitch of the respective muscles. A feeble current of 1.5 micro-ampère was sent along the nerve-and-muscle circuit, ascending by the left and descending by the right side. It will be seen that excitation initiated at the spine is propagated ‘against’ the electric current on the left side, and ‘with’ the current on the right side. On repetition of previous electric stimulus the effect of directive action of current was at once manifested by the left limb being thrown into a state of strong tetanic contraction, whereas the right limb remained quiescent. By changing the direction of the constant current the induced enhancement of conductivity of the nerve was quickly transferred from the left to the right side, the depression or arrest of conduction being simultaneously transferred to the left side. Turning the reversing key one way or the other brought about supra or non-conducting state of the nerve, and this condition was maintained throughout the duration of the current.
I shall next describe a more perfect method for obtaining quantitative results both with plant and animal. In order to demonstrate the universality of the phenomenon, I next used Mimosa pudica instead of Averrhoa, for experiments on plants.
For determination of normal velocity of transmission of excitation and the induced variation of that velocity, I employed the automatic method of recording the velocity of transmission of excitation in Mimosa, where the excitatory fall of the motile leaf gave a signal for the arrival of the excitation initiated at a distant point. In this method the responding leaf is attached to a light lever, the writer being placed at right angles to it. The record is taken on a smoked glass plate, which during its descent makes an instantaneous electric contact, in consequence of which a stimulating shock is applied at a given point of the petiole. A mark in the recording plate indicates the moment of application of stimulus. After a definite interval the excitation is conducted to the responding pulvinus, when the excitatory fall of the leaf pulls the writer suddenly to the left. From the curve traced in this manner the time-interval between the application of stimulus and the initiation of response can be found, and the normal rate of transmission of excitation through a given length of the conducting tissue deduced. The experiment is then repeated with an electric current flowing along the petiole with or against the direction of transmission of excitation. The records thus obtained enable us to determine the influence of the direction of the current on the rate of transmission. I shall presently describe the various difficulties which have to be overcome before the method just indicated can be rendered practical.