ON THE RELATION BETWEEN THE EXTERNAL STIMULUS APPLIED TO A NERVE AND THE RESULTING NERVE IMPULSE AS MEASURED BY THE ACTION CURRENT. BY C. W. GREENE. Instructor in Physiology, Stanford University. T HE experiments described in this paper were undertaken with the idea of ascertaining the quantitative relation between the strength of the external stimulus applied to the nerve, and the strength of the resulting nerve impulse. The nature of the nerve impulse is as yet undetermined. We may recognize its presence in the nerve, however, in two ways : by the change in the electrical condition of the nerve fibres which accompanies the lierve impulse, or by the changes produced in the peripheral organ in connection with the nerve. In the latter case the quantitative changes in the peripheral organ obey laws peculiar to the organ itself, and cannot, therefore, be used directly as an index to the quantitative changes in the nerve fibres. On the other hand, the changes in the electrical condition of the nerve are immediately associated with and are presumably the direct result of the changes constituting the nerve impulse. The intensity of the electrical change is measurable in the form of the action current. In the experiments described in this paper the action current is taken throughout as a measure of the intensity of the nerve impulse, it bein g assumed, in accordance with recent authors,l that the action current varies proportionately with the change constituting the nerve impulse. In support of this assumption we may refer especially to the work of Wailer, who has made extensive use of the action current in the study of the reactions of nerve fibres to anzesthetics and to many of l A. I). WATLER : Brain, xviii and xix; and Jam-nal of physiolo