why the same tissue may respond in the same way to so many different stimulating agencies. Any agency that alters the surface-film to a degree and at a rate sufficient to cause a critical change in its electrical polarization will stimulate. The membrane may be directly altered by mechanical agencies, or by heat or the direct action of chemical substances; or it may contain photosensitive substances and hence be sensitive to light, or special chemical substances and show a specific chemical sensitivity. Whatever alters it in such a way as to change, even momentarily and locally, its permeability and electrical polarization to the critical degree may thus stimulate, i. e., may originate a depolarization which spreads and affects the entire cell. The response which then follows is independent of the nature of the stimulating agency and is determined by special peculiarities of the irritable tissue itself.
The processes which take place in the interior of the stimulated cell are too various and complicated to be considered here. Their nature depends entirely on the specific peculiarities of the cell, and any general characterization is impossible. Usually there is an increase of oxidations and hence of heat-production—in addition to the special physiological manifestation which is evoked—but this is not always the case; thus in nerve, although there is increased loss of carbon dioxide, the heat produced during activity is almost inappreciable; and in other cases there may be a decrease or even complete cessation of all outward activities, e. g., in structures that give an inhibitory response to stimulation; such an instance is seen in the swimming plates of ctenophores which stop movement instantly on slight mechanical stimulation. Facts like these again illustrate the extreme diversity which the entire sequence of events forming the response may show in different irritable tissues, in spite of the essential uniformity of the first stage of the process. This uniformity is the most remarkable feature of physiological stimulation. Nature has apparently found in the variations of permeability and of electrical polarization which external changes may cause in the protoplasmic surface-films the most effective and reliable means of which the internal processes of the protoplasmic system can be made to vary in response to variations in the environment; and in the course of evolution this mechanism has acquired a degree of perfection that still largely baffles physiological analysis.
