tococcus pyogenes, which, is probably identical with Streptococcus erysipelatos of Fehleisen, although some bacteriologists still insist that slight differences exist in the mode of growth in certain culture media which justify the view that they are well-defined varieties if not distinct species. I am of the opinion that the streptococcus obtained from erysipelatous inflammations is identical with the streptococcus of pus; and we have ample evidence that the pathogenic power of this micro-organism differs as a result of conditions relating to its environment. In artificial culture media it becomes more or less attenuated, but when obtained from the tissues invaded in erysipelas, or in puerperal septicaemia, it has an increased virulence. Like the micrococcus of pneumonia, this is a widely distributed micro-organism; it has frequently been obtained in cultures from the surface of the body or from exposed mucous membranes of healthy persons. This is also true of the other pus cocci concerned in traumatic infections. And, in the light of our present knowledge, it appears that erysipelas, wound infection, abscess formation, etc., do not depend alone upon the presence of the specific etiological agents which induce such localized infectious processes, but also upon predisposing and secondary causes relating to the infected individual, as well as upon the origin and virulence of the pathogenic micro-organism. Thus the Streptococcus pyogenes from a case of erysipelas or of puerperal septicaemia introduced into a recent wound upon a healthy person would be likely to cause a severe and possibly fatal infection, while an attenuated culture of the same would perhaps give rise to no symptoms, or at most to slight local inflammation. On the other hand, if the vital resisting power of the individual is reduced by previous ill-health, by insufficient food, by sewer-gas poisoning, crowd-poisoning, etc., an attenuated variety of Streptococcus pyogenes may perhaps give rise to an erysipelatous inflammation, or to an acute abscess, if introduced by accident into an open wound. Again, in contused and lacerated wounds the vital resisting power of the tissues is diminished; and wound infection is likely to occur from the accidental introduction of the pus cocci, which lead a saprophytic existence upon the surface of the body and exposed mucous membranes, where under ordinary circumstances they are quite harmless. Secondary infections due to these now well-known pyogenic micrococci are not infrequent as sequelæ to the specific infectious diseases, such as scarlet fever, yellow fever, etc.
In 1882 Koch published his famous discovery of the tubercle bacillus. This will always rank as one of the most important events in the history of medicine, and as a notable triumph of well-directed scientific research. The proof that the bacillus referred to is the specific cause of tuberculous processes and that