The lesions of the trachea and of the larger bronchi, even though they persist in their acute form for a period of weeks, are superficial and do not lead to extensive or deep scarring. In contrast to the larger respiratory passages, that portion of the bronchial tree which is not supported by cartilaginous rings becomes more and more intensely involved, not only on its surface, but in its deeper structures, and the changes in the bronchioles and the neighboring air sacs are among the more characteristic anatomical manifestations in this disease. Therefore, while it will be unnecessary to refer again to the larger air passages, further consideration of the smaller ones, which are constantly associated with the lesions of the pulmonary parenchyma, will be included in the subsequent discussion of the pulmonic involvement. B. LESIONS OF THE LUNG In the prosecution of the scientific study of any disease, the great temptation is to differentiate its various manifestations with the intention of elaborating a classification. While there may be more or less distinct types of pulmonary involvement in influenza, so many intermediary forms appear in an extensive series of cases, that any classification must necessarily be arbitrary. Here, such an attempt at classification is beset with unusual difficulty, because, in the vast majority of cases which come to anatomical observation, the disease is fulminating. The lesions are more uniform in character than in an experimental study where the material is arranged so as to include intermediary and chronic stages, which in man are only encountered when accident causes death. These gaps in the study cannot be supplemented with experimental observations, because attempts to reproduce this disease have failed, even in human subjects. From the literature it appears also that variations in the extent, and perhaps in the maturation, of the anatomical involvement may be represented in different proportions for different localities (1, 2, 17, 55, 92, 162). Therefore, no sharp differentiation may be drawn between the more or less definite stages which are seen at the post-mortem table; still, for convenience of description, certain of the lesions which occur more frequently and are more widely differentiated may be considered separately. The disease, as has been indicated, may be confined to the bronchial tree. In these circumstances, it is almost necessary to suppose that the larger bronchi alone are involved. When the delicate bronchioles are affected, there is always a more or less extensive involvement of the pulmonary parenchyma (124), for the bronchioles have a more direct communication with the alveoli, and their walls present a less formidable barrier to the extension of the inflammation to the surrounding air sacs. [Illustration: