A very extensive group of depression processes is produced by the action of chemical stimuli. Among these the processes to which we apply the collective term of “_narcosis_” must claim our special interest. As is well known, an enormous number of substances of very different chemical nature, such as carbon dioxide, alcohol, ether, chloroform, chloral hydrate, etc., exist, which, possessing the property of producing cessation of the vital activities in all living systems, after withdrawal of their application, if it has not been too prolonged or intense, permit a complete restoration to normal vitality. These are the _general_ narcotics. Besides these there are a series of substances which have a depressing effect only upon certain forms of living substance, and which we may, therefore, term _special_ narcotics. As, however, the particular nature of depression following the application of chemical substances has hitherto been closely studied only in a very few instances, we are not, at present, in a position to sharply define the limitations of the conception of narcosis, a conception which originally had hardly any further meaning than the production of unconsciousness by chemical means. In the following discussion, therefore, we shall deal merely with narcosis produced by the well-known general narcotics, such as carbon dioxide, alcohol, ether, chloroform, etc. From the time of the introduction of ether narcosis into medical practice by _Jackson_ and _Morton_ in the year 1848 up to the present day, the theory of this process has awakened the liveliest interest. Many attempts have since been made to explain the physical nature of this interesting process without, however, any generally acknowledged theory of narcosis being established. I will refrain from entering into these former theories in detail as they have been exhaustively treated by _Overton_[204] in his studies on narcosis. [204] _E. Overton_: “Studien über die Narkose, zugleich ein Beitrag zur allgemeinen Pharmakologie.” Jena 1901.