As already emphasized, all living substance possesses the capability of conducting excitations to a definite degree. We may, therefore, assume that the same fundamental _property_ of conductivity exists in all substances. A fact to be considered in the conduction of excitation, is that the primary breaking down of the complex molecules at the position of stimulation act in turn as exciting stimuli upon the neighboring portion of the living substance, which in turn undergoes a similar decomposition. And so this process continues. This fact is evident from the observations on the process of excitation. But the nature of the stimulus which produces the breaking down of the complex molecules upon the surrounding molecules is a problem which can only be studied later. Here only one point will be mentioned in advance concerning the intensity of the stimulus. It is apparent from the experiments on the rhizopods, that the greater the intensity of the stimulus the more extensive must be the breaking down of the living substance. A stronger primary stimulation must also secondarily produce a stronger stimulus in the neighborhood. In other words: the _conduction of excitation_ is a function of irritability. The greater the irritability, that is, the greater the number of molecules broken down in a unit of time and space by a stimulus of a certain intensity, the greater also is the conductivity of the living system, that is, the stronger, the more rapidly and the further excitation is extended. Conductivity of excitation is, therefore, unthinkable without irritability. Both are inseparably connected. The conclusion forced upon us by this chain of reasoning admits of no argument. Nevertheless the endeavor has been made, because of certain evidence at hand, to show that the property of conductivity could exist without irritability. A number of authors, such as _Schiff_,[92] _Erb_,[93] _Grünhagen_,[94] _Effron_,[95] _Hirschberg_[96] and _G. Weiss_,[97] have observed the fact that in spite of a more or less strong decrease of _excitability_ of a stretch of nerve, stimuli applied above this stretch can still produce a conduction of excitation through the affected part. They have concluded from this that it is possible to separate the conductivity from irritability. _Erb_ and _G. Weiss_ have even gone so far as to directly express the opinion that capability of conduction and irritability involve two different histological elements. In contrast to this, other investigators, such as _Hermann_,[98] _Szpilmann_ and _Luchsinger_,[99] _Gad_,[100] _Piotrowski_[101] and _Wedenski_,[102] have more or less decidedly taken the stand that an actual separation of irritability and of conductivity does not exist. The apparently contradictory evidence as well as the conflicting theoretical views have been cleared up by _Werigo_,[103] _Dendrinos_,[104] _Noll_[105] and _Fröhlich_.[106] These investigators have shown that the length