It is due to the knowledge of these principles that we have learned that the infectious microbes, far from acting as they were believed to do only a few years ago, and which Pasteur strongly maintained to be by vital parasitism--such as would be the case with the carbonizing bacteria which, according to Pasteur, act by diverting the oxygen, or causing capillary embolisms--owe their pathogenic action to the toxic substances which are the products of their secretion, and which spread throughout the organism, even though the microbe frequently is localized in a very circumscribed spot, as in tetanus and in diphtheria. The idea of intoxication by these products has now replaced the idea of the direct action of the microbe on the elements or the liquids of the organism. The occurrence that takes place in diphtheria and tetanus is one of the best examples to cite in support of this view. Here, in fact, the pathogenic microbe is found only in a very limited area in the organism attacked--the false membrane, in the case of diphtheria, or frequently only a slight wound in the case of tetanus, and the microbe becomes localized there only. Now, in both cases, there are general phenomena of toxic effects. There must hence be a diffusion of toxic substances which, distributed by the blood, affect the different systems and exert a toxic action on the entire organism. It must be observed that the toxins act as toxic agents only when in a condition to be introduced into the circulation subcutaneously. The cause of this innocuousness of the toxins when given per os has frequently been studied. It appears to be quite probable that the cause of the attenuation of the morbid properties is due to the intervention of the digestive microbes. Such is the opinion of Levaditi and Charrin[22]; it is also the conclusion that is to be drawn from the experiments of Mme. Metchnikoff and of Calmette[23] on the modifications undergone by a vegetable toxalbumin, abrin, and by serpent venoms, when these toxalbumins are inoculated with the bacillus subtilis chromogenus. Moreover, Charrin and Lefèvre,[24] on the one hand, and Nencki, Sieber and Somanowsky,[25] and Carrière,[26] on the other hand, have discovered that the digestive ferments, particularly trypsin, destroy, even though but little, the toxins secreted by the Loeffler and Nicolaier bacilli. This is practically contrary to the opinion of Behring and of Rauson,[27] according to which the innocuousness of the microbial poisons when administered per os is due exclusively to the lack of absorption. [22] CHARRIN and LEVADITI: Le sort de toxines introduites dans le tube digestif. _Journal de Physiologie et de Pathologie Générales_, 1898, p. 226. [23] Citing Metchnikoff. [24] _C. rend. de la Soc. de Biologie_, 1898. [25] _Centralblatt für Bakt._, 1898. [26] _C. rend. de la Soc. de Biologie_, 1899. [27] _Deutsche Med. Wochenschr._, 1898, No. 8.