The confusion which exists in associating different bacterial types with well defined and characteristic anatomical lesions is true, not only for the disease as it occurs spontaneously in man, but also for its reproduction in experimental animals. A review of the literature teaches that not all of the factors concerned in producing the different anatomical types are known. Pertinent experimental studies were reported recently by Blake and Cecil from the Army Medical School in Washington. One cubic centimeter of a very virulent pneumococcus (M. L. D. for a mouse in forty-eight hours equals one-millionth of 1 cubic centimeter) inoculated in monkeys through the skin of the neck into the trachea just beneath the larynx has uniformly produced a lobar pneumonia, clinically as well as anatomically the nearest experimental approach to spontaneous lobar pneumonia of man. This work indicates, not only the possible importance the species may play, but also points to the rôle of the infecting organism which, when sufficiently virulent, incites a characteristic reaction even when inoculated in a minimal quantity into the trachea as distinct from the bronchioles or lung tissue. Pneumonia unassociated with a simultaneous or preliminary incapacitation of the mechanism of the upper respiratory tract, differs from the pulmonary lesions encountered after measles, gas poisoning, influenza, etc., and those experimentally produced by intrabronchial insufflations. In the lobar type the virulence of the infecting organism, combined perhaps with species variation,—implying as this does different capacities for reaction on the part of the host,—seems to be the fundamental principle. It remains to be determined whether other organisms of equal virulence with that of the pneumococcus are capable of producing characteristic anatomical reactions when inoculated in a similar manner. In the other group, where there is not only a free ingress to the pulmonary parenchyma by the bacteria of the mouth, but where there is, in all probability, also a simultaneous or preliminary pulmonary damage furnishing a proper medium for relatively innocuous organisms, the lesion of response will surely depend upon other factors as much as upon the infecting organism. Among these obscure factors, the most important, unquestionably, is the extent of the damage to the lung tissue before the entry of the organisms, or simultaneously with it, into this area. The systemic capacity to compensate also must be considered. [Illustration: FIG. LI. AUTOPSY NO. 194. A LUNG IN WHICH INFLUENZAL PNEUMONIA AND PULMONARY TUBERCULOSIS ARE ASSOCIATED. THERE IS A CIRCUMSCRIBED, OLD, TUBERCULOUS FOCUS IN THE UPPER LOBE. COMPARE FIGURES XXXIX AND XL. ] VIII. CONCLUSIONS The etiology of influenza is unknown. The portal of entry of the inciting cause is likewise undetermined.