It is furthermore probable that cyanosis, which is a conspicuous feature of many instances of pneumonia following influenza, is referable, in part at least, to obstruction of the bronchi by mucopurulent exudate. The term pneumonia will refer to those inflammatory changes in the lung which are found within the alveoli; it will include inflammatory changes in the alveoli surrounding the respiratory bronchioles, in the alveolar ducts and infundibula and in their tributary alveoli. Bronchitis will be described by defining the changes which occur (a) in the small bronchi with no cartilage or mucous glands, and (b) in the large bronchi including the primary branches of the trachea. For convenience of description those bronchi may be designated small, which have no cartilaginous plates in their wall. Larger bronchi have cartilage and mucous glands, the latter situated in considerable part outside the cartilaginous plates. These bronchi, of which the largest are the right and left bronchi formed by bifurcation of the trachea, diminish with repeated branching to a caliber of about 1 mm. Small bronchi are lined by columnar ciliated epithelium; their wall consists of very vascular connective tissue containing a layer of smooth muscle and their caliber varies approximately from 1 to 0.5 mm. It is convenient to designate as respiratory bronchioles[80] the terminal ramifications of the bronchi; they are lined by a single layer of columnar ciliated cells passing over into cuboidal nonciliated epithelium and are beset with small air sacs lined by flat cells or epithelial plates similar to those of the alveoli elsewhere. Not infrequently these alveoli occur along only one side of the bronchiole, the remainder of the circumference being covered by a continuous layer of cubical epithelium. The respiratory bronchiole by branching along its course or at its end is continued into several alveolar ducts which unlike the respiratory bronchioles have no cubical or columnar epithelium but are closely beset by alveoli lined by flat epithelial plates. The alveolar duct is recognized by the absence of cubical epithelium and the presence of bundles of smooth muscle which occur in the wall. The infundibula or alveolar sacs arise as branches from the alveolar ducts and like them are beset with alveoli, but smooth muscle does not occur in their walls. The base of the infundibulum is wider than its orifice, which Miller states is surrounded by a sphincter-like bundle of smooth muscle.