the latter a process or series of processes in which the transformation of energy may be almost indefinitely large—out of all proportion to the exciting stimulus. We have therefore first to inquire into the general nature of the conditions that render possible such disparity between the stimulus—considered by itself as a particular chemical or physical process acting upon the irritable tissue—and the resulting special activity or response on the part of the tissue itself.
As all know, an irritable tissue like a nerve or muscle may be aroused to activity under the most various conditions; the effective stimulus may be an electric shock, a chemical substance, the action of light, change of temperature, loss of water, mechanical impact; and the tissue gives the same response to all of these. Now it is clear that such a stimulus can only act as some kind of a releasing agency—what Ostwald calls Anlass—which sets going some process all of the necessary conditions of which are already present, but which is held in check by some restraining condition which the releasing agency removes—as when a gun is fired, or an alarm-clock set off, or a mine exploded by the pressure of a button—which closes an electrical circuit, thus enabling a spark to pass, which raises the temperature of the explosive to the critical point. The connection between Anlass and resulting event may be highly indirect, and there need be no resemblance or other relation than that of interconnection between the two. In all cases the system is, as it were, "wound up"; the potential energy is there, ready to become kinetic; once the process is started or activated by the releasing event, it proceeds of its own accord to its conclusion, i. e., till a second state of equilibrium is reached. In the case of a living irritable tissue or organism we are evidently dealing with a physico-chemical system belonging—as regards the relations between the initiating conditions and the resulting process itself—to this general class. If we press the end of a nerve connected with a muscle, or pass through it an electrical current of sufficient intensity for a sufficient length of time, or dip it into a solution of some appropriate chemical substance, there is initiated at the site of stimulus a "physiological" process which is propagated with unaltered intensity along the nerve to the muscle and there calls forth a complex variety of interdependent physical and chemical changes, of which the contraction is the most conspicuous and physiologically important. Thus a process specific to the tissue, unique and obviously highly complex, is initiated by the relatively insignificant change which the stimulus causes directly. We ought particularly to note that in any special tissue the physiological process remains the same in kind, whatever the nature of the stimulus. The latter merely causes some critical or releasing change which initiates the physiological sequence of events; the latter then proceeds automatically in its characteristic way to its conclusion.
Let us now consider more particularly the physiological part of the
