The greatest number of instances of the interference of two series of stimuli have been given to us by investigation of the physiology of the nervous system. In the functionation of the nervous system the fact that two series of stimuli from different tracks affect the same ganglia plays a very important rôle. It is this to which _Sherrington_[191] has alluded as “_the principle of the common path_.” Where two nervous excitations involve the same paths, there arises an interference of the effect of the two series of stimuli, for the impulses in the nervous system, as already stated, possess a rhythmic character. This principle has a broad application in the phenomena of association in the cerebral cortex. The simpler and, therefore, the most easily understood cases are, however, in the spinal cord. The motor neurons of the anterior horns of the spinal cord are the junction of a great number of tracks, for example, the sensory neurons of the spinal cord at different levels, the neurons of the cerebellum, the pyramidal tracks from the motor areas of the cerebral cortex, etc. On the contrary, for example, the sensory neurons of the spinal cord are strictly “_private_ paths” in the sense of _Sherrington_, for excitation can enter by this means only from the special paths of the spinal ganglia and, therefore, from the periphery. The motor neurons of the anterior horns offer, therefore, excellent opportunities for the experimental investigation of the interference of two series of excitations which enter by different paths. The spinal cord consequently has become a much-used object of investigation for this purpose. In fact, we can observe and produce all types of interference in the spinal cord. These conditions have been quite thoroughly investigated by _Sherrington_[192] and his coworkers on the dog, and _Fröhlich_,[193] _Vészi_,[194] _Tiedemann_[195] and _Satake_[196] on the frog. [191] _Sherrington_: “Ueber das Zusammenwirken der Rückenmarksreflexe and das Princip der gemeinsamen Strecke.” Ergebnisse der Physiologie. Jahr. IV, 1905. [192] _Sherrington_: “The integrative action of the nervous system.” New York 1906. [193] _Fr. W. Fröhlich_: “Der Mechanismus der nervösen Hemmungsvorgänge.” Med. Natur. Arch. Bd. I, 1907. The same: “Beiträge zur Analyse der Reflexfunction des Rückenmarks, etc.” Zeitschr. f. allgem. Physiologie Bd. IX, 1909. The same: “Das Princip der scheinbaren Erregbarkeitssteigerung.” _Ibid._ [194] _Julius Vészi_: “Der einfachste Reflexbogen im Rückenmark.” Zeitschr. für allgem. Physiol. Bd. IX, 1910. [195] _Tiedemann_: “Untersuchungen über das absolute Refractärstadium und die Hemmungsvorgänge im Rückenmark des Strychninfrosches.” Zeitschr. f. allgem. Physiologie Bd. X, 1910. [196] _Satake_: The researches are not yet published.