In the choice of experimental specimens I was fortunate enough to secure frogs of unusually large size, locally known as “golden frogs” (_Rana tigrina_). A preparation was made of the spine, the attached nerve, the muscle and the tendon. The electrodes for constant current were applied at the extreme ends, on the spine and on the tendon (Fig. 48). The following are the measurements, in a typical case, of the different parts of the preparation. Length of spine between the electrode and the nerve = 40 mm. length of nerve = 90 mm. length of muscle = 50 mm. length of tendon = 30 mm. Stimulus is applied in all cases on the nerve, midway between the two electrodes this point being at a minimum distance of 100 mm. from either electrode. The point of stimulation is, therefore, situated at an indifferent region. [Illustration: FIG. 48.--Experimental arrangement for study of variation of conductivity of nerve by the directive action of an electric current. _n n′_, nerve; S, point of application of stimulus in the middle or indifferent region.] Great precautions have to be taken to guard against the leakage of current. The general arrangement for the experiment on animal nerve is similar to that employed for the corresponding investigations on the plant. The choking coil is used to prevent the stimulating induction current from getting round the circuit of constant current. The specimen is held on an ebonite support, and every part of the apparatus insulated with the utmost care. VARIATION OF VELOCITY OF TRANSMISSION. In the case of the conducting tissue of the plant a very striking proof of the influence of the direction of current on conductivity was afforded by the induced variation of velocity of transmission. Equally striking is the result which I have obtained with the nerve of the frog. [Illustration: FIG. 49.--Effect of heterodromous and homodromous current in inducing variation in velocity of transmission through nerve. N, normal record, upper record shows enhancement, and lower record retardation in velocity of transmission under heterodromous and homodromous currents, respectively.]