If the atrium of an infection and its specific etiological agent are undetermined, the narrator of the pathology of a specific disease is confronted immediately with serious obstacles in the elaboration of a complete picture. Some writers assume that the respiratory tract is the portal of entry in influenza (162), though the specific agent is still unknown.[2] Whatever the agent, unquestionably it attacks the respiratory tract at a very early stage in the disease and produces a lesion which becomes responsible for the most serious aspect of influenza, whether this phase be primary or only a complication. Among the lesions which will be considered, therefore, those of the respiratory tract chiefly will be emphasized. They include the changes in the large air passages, as well as the pulmonary, alveolar, and interstitial involvement. Unquestionably, a very close association exists between the lesions of the larger air passages and those of the alveoli, but probably it is equally true that the former may occur alone; in many instances also they are the forerunners of the latter lesions. Consequently, it seems logical to begin with an exposition of the lesions in the trachea and its ramifications, including the bronchioles. A. LESIONS OF THE TRACHEA AND BRONCHI _Gross Picture._ Early in the disease the congestion and the hemorrhages that have been described in the mucous membrane of the nasopharynx (14 and 94) are also conspicuous features in the lining of the trachea and bronchi (Fig. I). This membrane is swollen, turgid, red, and covered by a copious, mucous exudate which may be clear, but much more frequently is blood-stained or opaque and yellowish in color. The blood, variable in amount, may be fresh and red; and after the mucous exudate on the surface is removed, more intense red foci stand out on the congested base (47, 90, 157). Frequently, as the bronchi are approached, the red color of the mucosa becomes more intense and may have a garnet tinge. Membranes such as are encountered in the more usual necrotizing inflammatory processes, like diphtheria, have not occurred in the trachea and larger bronchi in this series (108, 128, 157).[3] The exudate peels off readily, and as indicated above, leaves a velvety red surface, dotted here and there with darker or more intensely red foci. Small ulcerations of the mucosa occur, but are inconspicuous (82, 156). As the finer ramifications of the bronchi are approached, the accumulation of the exudate in their lumina becomes more and more marked, and on cross section of the lung, they often stand out conspicuously on account of their increased size and projecting, seromucous, blood-stained content (101, 149, 162).