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 electroto?ius. Instead of the 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 toan 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.
Fig. 202. Diagram to illustrate Electrotonus.
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.
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 catelec-trotonic condition. A certain time appears to be required for the production of electrotonus; in 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.