The simplest form of an individual change in the external vital conditions would be a regular and constant alteration of intensity which can be graphically represented as a straight line, wherein the intensities are the ordinates and the time the abscissa. (Figure 3, A.) A regularly rising pressure would, for instance, represent a stimulus in its simplest form. But such forms of stimuli are only very rare in nature and are also experimentally very difficult to produce. It is, for example, not easy to give the _electrical_ stimulus, so much used for experimental purposes, this form. _Fleichl_ and _v. Kries_ have only accomplished this by means of complicated apparatus. The usual _form of the individual stimulus_ is not a straight line, but a logarithmic curve. (Figure 3, B.) The alteration hardly ever progresses with equal rapidity from its beginning until it reaches its highest point, but as a rule, with decreasing rapidity. This is the usual course of alterations of concentration, also of chemical and osmotic stimuli, of changes of temperature and of electric stimulation. [Illustration: _A_ _B_ Fig. 3.] The _rapidity of alterations_ in vital conditions has quite an important influence on the development of the response to stimulation. It is well known that if a constant current, which reaches its highest intensity rapidly, is permitted to act upon a muscle, the effect differs from that following the application of a current of the same intensity but in which this is reached very slowly. In the first case there is a sudden strong twitch, in the second none at all. In spite of this there can be no doubt whatever of the current in the last case being effective. That the muscle is also excited when the current is slowly increased is shown by the contracture, which grows more and more plainly perceptible with the increasing intensity of the current and in higher intensities by the so-called _Porret’s_ phenomenon, which consists in a curious wave-like movement of the muscle-substance. In reference to the rapidity of the alterations in the factors which act as stimuli, the behavior varies greatly. Many stimuli because of their nature never have a steep ascent or descent of intensity, as, for instance, alterations in the concentrations of soluble substances, that is, chemical or osmotic stimuli; likewise temperature variations may be mentioned. They always act relatively slowly. On the contrary there are forms of stimuli which have now a rapid, now a slow, ascent or descent of their intensity, such as the photic and mechanical stimuli. Finally, there are other stimuli that nearly always show a very abrupt change of intensity, such as the electrical form.