Now we arrive at the question: Has a prolonged stimulation really a prolonged effect? This question might seem superfluous, as from a conditional standpoint it is self-evident that every alteration in any one of the conditions of a system is followed by an alteration in the system. But this very question played an important rôle in older physiology and led to prolonged discussions for the reason that a special case was taken into consideration in this connection, which at that time was not clearly understood. _Du Bois-Reymond_,[35] as a result of his investigations on the nerve muscle preparation of the frog, formulated a law of nerve excitation, according to which it is not the _absolute value_ of the intensity of the constant current which produces an excitation of the nerve and contraction of its muscle, but an alteration of the intensity from one moment to another. The more rapidly these changes are produced, the greater is the excitation. His arguments were based upon the fact that a contraction can only take place on the “making” or “breaking,” or by rapidly strengthening or weakening the constant current; it is possible to subject a nerve muscle preparation to a current of considerable strength without a muscle contraction resulting, provided it is slowly increased. One might be disposed to conclude from this that the constant current, when showing no fluctuations, has no stimulating effect whatsoever. Should this observation be carried even further and the attempt made to extend it into a general law of excitation by assuming that the effects of stimulation are only produced by variations in the intensity, not by its continued duration, one would commit the error of judging the occurrence of a stimulus only by the unsatisfactory criterion of an abrupt muscle contraction. Today we know with positiveness that a continued effect also exists during the uninterrupted flowing of a constant current in nerve or muscle, though much weaker, however, than in the case of the excitations produced by sudden fluctuations of the intensity. This is shown in the nerve by an altered excitability, which continues at the poles during the whole duration of the current. In the region of the anode the excitability is diminished, in that of the cathode it is increased. An excitation can also be demonstrated which extends from the cathode through the nerve, which can easily be detected by sufficiently delicate methods. Among other effects of prolonged stimulation is that of cathodal contracture, which remains localized in the region of the cathode and which excitation persists as long as the current continues. This permanent excitation can be particularly well observed in the single cells of the rhizopods. If a constant current is allowed to flow through an _Actinosphærium_,[36] the straight, smooth, ray-shaped pseudopods of the cell body at the moment of “making,” show evidence of contraction by being drawn _in_,