The attenuated viruses act, as vaccins, through their soluble constituents, which, either directly, by modifying the nutrition of certain cells, or indirectly, by inducing reactions of the nervous centers which preside over this nutrition, profoundly change the conditions of life and give rise to the pathological condition--the vaccined state. Experiments by Behring and Kitasato[41] have shown that the tumors of a vaccinated animal, freed from all organized matter visible under the microscope by filtration through porcelain, contains principles capable of directly or indirectly protecting other animals from the disease caused by the corresponding virus. Meanwhile, experiments have shown that the vaccinating matters are totally eliminated; nevertheless, after their elimination, the immunity acquired remains with the animal, which then continues to be protected against the corresponding virus. [41] _Deutsche Med. Wochenschr._, 1890, p. 1113. Interest in this subject has incited numerous researches with a regard to bringing to light the mechanism of this immunization; and this will form the subject of another volume of this collection. We may state here, however, that there have been recognized two concurrent causes of this preservative action; the one, called _phagocytosis_, results from the fact that the microbe introduced into the vaccined organism becomes incapable of producing its usual toxins, while on the other hand the immunization renders the organism capable of secreting substances possessing an activity contrary to that of the virus, in fact true counter-poisons, comprised under the general name _antitoxins_. =Phagocytosis.=--We have seen that an organism subjected to a toxic invasion tends to protect itself by proper means of defense; and one of those is the direct putting into activity of the living cellular elements themselves, and in particular, the leucocytes, or white corpuscles, found in more or less number, according to pathological conditions, in the blood and lymphatic fluids.[42] [42] It is necessary here to consult the work by LEVADITI: Le Leucocyte et ses Granulations. _Scientia_, Naud, publisher, Paris, 1903; also METCHNIKOFF: L'Immunité, Paris, 1902, Masson, publisher. Metchnikoff has shown that the moment a foreign element, particularly a microbe, enters the organism, these leucocytes come flocking from all parts of the body, collect around the bacterial element, penetrate it, and begin to digest it. These elements have received the name _phagocytes_. The name _chemotaxis_ has been given to the property by virtue of which they approach (positive chemotaxis) or move away from (negative chemotaxis) certain substances which affect them powerfully.