A study has been made of the mode of origin of the secondary lesions occurring spontaneously during the course of experimental pneumococcus pneumonia in the dog. It was observed that the primary lesions of dogs exhibiting interlobar spread contained much more edema fluid than did those in which the inflammatory process remained confined to a single lobe. Furthermore, the sequence of spread from lobe to lobe in relation to the anatomical arrangement of the bronchi and the prone position of the animal was such as to suggest that secondary lobe involvement arose as a result of edematous pneumonic exudate flowing into the more dependent bronchial openings.

Experiments were undertaken to determine whether pneumococci are constantly being distributed throughout the lung in the experimental disease in varying degree yet produce secondary foci of consolidation only if the microorganisms reach the terminal airways and are retained there through some interference with the normal eliminatory mechanism. Attempts to produce secondary lesions in dogs with non-spreading single lobe involvement, by means of plugging a terminal bronchus of a normal lobe with starch paste or mucin were largely unsuccessful. In only three out of 19 instances did a lesion develop at the site of obstruction. An investigation was then made of the distribution of pneumococci in the lungs of dogs at the height of the pneumonia. In dogs with single lobe lesions pneumococci were recovered from the lesion itself but not from any other part of the peripheral lung tissue, whereas in animals showing spread to other lobes pneumococci were found to be distributed widely throughout the lung in both the apparently normal and the involved lobes. Some of the microscopic sections of the uninvolved parts of the lungs of dogs with metastatic lesions revealed small masses of pneumococcus-containing exudate in the smaller bronchi and terminal airways of otherwise normal tissue. This finding, in conjunction with the detection of beginning inflammatory changes in other areas normal in the gross, would seem to provide direct evidence of the manner in which pneumococci are transferred from the initial lesion to other lobes and highly presumptive evidence for the mode of origin of the secondary lesions.

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