_Schwarz und Lemberger_[77] and _Ishikawa_[78] have shown that especially the weak acids, which are produced in larger amount as a result of strong activity of the cells, bring about vessels’ dilation. By the demonstration of this highly important process of self-regulation the last link has been added for the physical understanding of the hypertrophy of activity of the tissue cells by continued functional excitation. Whether or not the same applies to the single living cell, if the unicellular organism likewise undergoes a quantitative increase by a continuous functional excitation, and if the single cell possesses in itself a corresponding mechanism of self-regulation similar to the cell community in the vertebrates, cannot be answered, for concerning all these problems information is lacking for the present. [76] _Max Verworn_: “Die cellularphysiologische Grundlage des Gedächtnisses.” Zeitschr. f. allgem. Physiol. Bd. VI, 1907. [77] _Schwarz und Lemberger_: “Über die Wirkung Kleinster Säuremengen auf die Blutgefässe.” Pflügers Arch. Bd. 141, 1911. [78] These investigations have not yet been published. CHAPTER VI CONDUCTIVITY _Contents_: Only processes of excitation are conducted, not processes of depression. Conduction of excitation in its two extreme instances. Conduction in undifferentiated pseudopod protoplasm of rhizopoda. Conduction of excitation with decrement of intensity and rapidity. Conduction of excitation in the nerve. Rapidity of conduction of excitation without decrement. Relation between irritability and conductivity. Conduction of excitation with decrement of the nerve after artificial depression of irritability by narcosis. Theory of the decrementless conduction of the normal nerve. Proof of the validity of the “all or none law” in the medullated nerve. Theory of the process of the conductivity of excitation. Theory of core model (Kernleiter). Electrochemical theory of conduction based on the properties of semi-permeable surfaces. When the response to a stimulus is studied in a living system, whether it be a single cell, a tissue, or a complex organism, the indicator used, either that of movement, current of action, production of certain substances, the development of light, of heat or the alteration of form, is the result of two distinct processes. The first of these is primary excitation, brought about by the stimulus at a local point, and the second is an extension of the excitation to the surrounding tissue. We are not in a position to experimentally bring about a response to stimulation, in which the primary excitation occurs and not the secondary process, that of conductivity. All living substance contains this property, although to a very different degree, as all living substance possesses irritability, and this presents the condition not only for the taking place of the process of excitation but also that of its conduction.