In connection with our present observations, however, I will more closely analyze the process itself, following the results of investigations extending over more than ten years carried out by my coworkers and myself. In these investigations it has been found that narcosis belongs to this group of depressing processes. A satisfactory theory of narcosis, however, and this I must explain from the first, can even today not be arrived at. Such a theory would require the ascertainment of all primary and secondary alterations produced by the narcotic in the course of normal vital activity. For this, however, a number of minute details are still lacking. Nevertheless, the careful and detailed investigations during the last ten years have acquainted us with a large number of alterations, which, acting as conditioning factors for the process of narcosis, must be taken into consideration, and which to a certain extent give us an idea of the mechanism of this process. They are equally interesting from a theoretical as well as from a practical point of view. The presentation will become more detailed as more of such conditioning factors are established by the deeper penetrating of future analysis. I will deal here with the facts found up to the present and then proceed to the deductions which these furnish for the theory of narcosis. In the first place narcosis is stamped as a typical process of depression, being characterized by a _decrease of irritability with a corresponding decrement of the extent of excitation_. The chief feature of all narcotized systems is, that in slight narcosis excitating stimuli produce a greatly weakened excitation, and that in deep narcosis no perceptible response is obtained. This can readily be ascertained in the various forms of living substance. According to the previous observations on the inseparable relations between conduction of excitation and irritability, it is self-evident that with decrease of irritability there must be a corresponding decrease in the capability of the conduction of excitation from the point of stimulation. This decrease in conductivity must, therefore, be the greater the more irritability is reduced; that is, the deeper the narcosis, the greater must be the decrement undergone by the wave of excitation in its extension from the point of stimulation. These facts can be observed in the highest perfection in the nerve, and have, as we have seen, been demonstrated by the investigations of _Werigo_, _Dendrinos_, _Noll_, _Boruttau_ and _Fröhlich_.[205] Upon deeper analysis of this process of depression, the next task for the investigator must be the ascertainment of the special components of the metabolic activity, which are depressed as a result of the narcotic.