It is our task to analyze in detail the conditions involved in the conduction of excitation in order to obtain a deeper insight into the physics of this process. A comparative survey of a series of various types of living substance shows us that they differ in respect to the conduction of excitation in the following points: In regard to the rapidity with which the excitation is conducted, the extent of the area over which it spreads, and the intensity with which it extends. These conditions may be best illustrated by citing two extreme examples. The one is formed by the rhizopods, the other by the nerve fibers. Between these two extremes we have manifold gradations in the conditions of conductivity. Not all cell forms are suitable objects for the study of conductivity. There are forms of rhizopods which are as favorable to investigation as the nerve; this is due to the fact that, although they are often of microscopic dimensions, they possess elongated fingerlike or threadlike pseudopods. [Illustration: Fig. 14. Contractions of the musculus extensor digitorum communis longus of the dog, brought about by rhythmic stimulation of the nervus peroneus. The muscle is in the condition of tonic excitation which proceeds from the center. The arrows indicate the point where reflex inhibition of the central tonus is produced. The height of the single contraction undergoes no diminution. ] Indeed, a rhizopod cell, with its straight, elongated pseudopods, is preëminently fitted as an object of comparison with a neuron. Although the difference in respect to the individual points is so far-reaching, still, based on their outward morphological similarity various physiological parallels in both are forced on our observation. A comparison of the rhizopod cell with the neuron can consequently guard us from many erroneous generalizations which we might be inclined to deduce from a one-sided investigation of the nerve. This is especially the case in regard to the conductivity of excitation, which was formerly studied almost exclusively on the nerve and only occasionally on the muscle, which offers similar conditions. The nerve, in which the function of the conductivity of excitation is particularly highly developed, was considered at the same time as the type in which this process could be most readily analyzed, and from which it was believed general information of the process of the conductivity of excitation could first be gained. This view has led to serious errors, as the nerve, resulting from the high development of its conductive capability, shows quite one-sided specialized conditions, which can by no means be transferred to other forms of living substance.