[185] _Fr. W. Fröhlich_: “Ueber die scheinbare Steigerung der Leistungsfähigkeit des quergestreiften Muskels im Beginn der Ermüdung (Muskeltreppe), der Kohlensäurewirkung und Wirkung anderer Narkotica (Aether, Alkohol).” Zeitschr. f. allgem. Physiologie Bd. V, 1905. The same: “Das Princip der scheinbaren Erregbarkeitssteigerung.” Zeitschr. f. allgem. Physiologie Bd. IX, 1909. [186] _Fr. Reinecke_: “Ueber die Entartungsreaction und eine Reihe mit ihr verwandter Reactionen.” Zeitschr. f. allgem. Physiologie Bd. VIII, 1908. Although the nerve as an isobolic system can only be rendered capable of exhibiting summation when artificially influenced, there are other forms of living substance which normally are systems with a slow course of excitation, in which excitation may be summated, for this type possesses at the same time a heterobolic character. For example, a single mechanical excitation elicits a hardly perceptible response in _Amœba_, _Actinosphærium_, _Orbitolites_. When it is perceptible at all, there occurs a short interruption of the centrifugal movement of the protoplasm. After a pause the movement of the protoplasm and the stretching out of the pseudopods again return. But if the organism is agitated one or more minutes by rhythmically shaking the edge of the slide by a special device, as a result of the summation of weak excitations there occurs a complete drawing in of the pseudopods and the amœbæ become bell-shaped.[187] The ganglion cells also possess a great capability for summation. We have already alluded to the fact that single induction shocks below that of the threshold produce no evident effect, whereas when rapidly repeated, summation occurs with reflex reaction. [187] _Max Verworn_: “Psychophysiologische Protistenstudien. Experimentelle Untersuchungen.” Jena 1889. The same: “Die physiologische Bedeutung des Zellkerns.” Pflügers Arch. Bd. 51, 1892. [Illustration: Fig. 54. Development of tonus by interference of sub-threshold stimuli. _S_--Level of the threshold of perceptible effect.]