1. The ampullae of Lorenzini are sensitive to weak electrical stimuli which presumably cause currents to flow along the jelly-filled tubes of the sense organ. Increase of the resting frequency occurs when the tube-opening is made negative to the capsule, and inhibition when the opening is positive, with opposite after-effects in each case. Adaptation is three-quarters complete in about 5 sec.

2. When the stimulus is applied as a voltage gradient in the water overlying the ampullae, the threshold for the most sensitive units is 1 µV./cm., which represents a current along the tube of less than 5 x 10-11 A.

3. When the stimulus is applied as a current directly to the tube opening, the threshold for the most sensitive units is 4 x 10-10 A.

4. Changes in the salinity of the water at the tube opening are also effective, the threshold being about 3%. Dilution causes excitation, and concentration causes inhibition. The response is not due to the osmotic differences, and so probably results from potentials established at the jelly-water interface.

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