The mechanism of anaphylaxis is a very complicated one involving the production in the blood of chemical substances called antibodies, and is far from being thoroughly understood. One theory is that sensitization consists in the “fixing” of the cells of the tissues with these antibodies. This may or may not be correct—probably not—and I am far from wishing to imply that sensitization of the neurons, as a consequence of functioning, has anything in common with the mechanism of sensitizing the body in anaphylaxis. I merely wish to point out that sensitizing nervous tissue through chemical changes is a physiological concept quite within the bounds of possibility; and, as all functioning is probably accompanied by metabolic (chemical) changes, such metabolic changes may well persist in neurons after brain reactions produce sensitization. If this hypothesis of sensitization should be proven it would offer an intelligible mechanism of the phenomenon of memory. If the system of neurons engaged in any conscious experience were sensitized by chemical changes it would acquire a hyper-susceptibility. The system as a whole would consequently be excited into activity by any other functioning system of neurons with which it was in anatomical association and might reproduce the originally correlated conscious experience. Various theories based on known or theoretical chemical or physical alterations in the neurons have been proposed to account for memory on the physiological side. Robertson[58] has proposed that it is of the nature of autocatalysis. Catalysis is the property possessed by certain bodies called catalyzers of initiating or accelerating chemical reactions which would take place without the catalyzer, but more slowly. “A catalyzer is a stimulus which excites a transformation of energy. The catalyzer plays the same rôle in a chemical transformation as does the minimal exciting force which sets free the accumulation of potential energy previous to its transformation into kinetic energy. A catalyzer is the friction of the match which sets free the chemical energy of the powder magazine.”[59] Numerous examples of catalytic actions might be given from chemistry. The inversion of sugar by acids, the decomposition of hydrogen peroxide by platinum black, fermentation by means of a soluble ferment or diastase, a phenomenon which may almost be called vital, are all instances. According to Leduc “the action of pepsin, of the pancreatic ferment, of zymase and other similar ferments has a great analogy with the purely physical phenomenon of catalysis.”