There is, nevertheless, a third point of considerable difference between the types of conduction of excitation in the rhizopods and in the nerve. Whereas in the rhizopods the rapidity of conduction is dependent upon the _intensity_ of the stimulus, it has been long known as the result of investigation by _Rosenthal_, _Brücke_ and _Lautenbach_ and at a more recent date by _Gotch_[89] and _Piper_,[90] that in the nerve of the frog, as well as in man, the velocity is _not_ dependent upon the intensity of stimulation. (Figure 21.) Contrary results have been obtained by a few early observers wherein the latent period was shorter when the stimulation was strong. _Nicolai_[91] explains this shortening of the latent period, resulting from the application of strong electrical stimuli, to a spreading out of the “Stromschleifen” from the point of application and consequently there is a shortening of the stretch of nerve between the point of stimulation and the indicator. [89] _Gotch_: “The submaximal electric response of nerve to a single stimulus.” Journal of Physiology, Vol. XXVIII, 1902. [90] _Piper_: Ueber die Leitungsgeschwindigkeit in den markhaltigen menschlichen Nerven. Pflügers Arch. Bd. 124, 1908, und Bd. 127, 1909. [91] _Nicolai_: “Ueber Ungleichförmigkeiten in der Fortpflanzungsgeschwindigkeit des Nervenprincips, nach Untersuchungen am marklosen Riechnerven des Hechts.” Arch. f. Physiologie 1905. [Illustration: Fig. 21. Course of the action current of the nerve. The thin line indicates the action current produced by a weak, the thick line the action current produced by a strong stimulus. The duration of the action current is the same in both cases. (After _Gotch_.) ] This conspicuous difference in the conduction of the two extreme types of living substance, which we have already observed, arouses the question as to what properties of living substance bring about these differences. In order to answer this question, it is necessary, first of all, to make some general statements concerning the processes of conductivity.