Changes in the main bronchi and their primary branches are usually less severe than those in bronchi of smaller size. The epithelium is often intact, the superficial cells being columnar and ciliated, but not infrequently desquamation of superficial cells has occurred and the lower layers alone remain. Occasionally (Autopsy 471) there is necrosis of epithelium with which, although the architecture of cells is preserved, nuclei have disappeared. Accumulation of blood or serum may separate epithelium from the underlying basement membrane (Fig. 1). Complete loss of epithelium occurs, usually in small patches. Polynuclear leucocytes are numerous upon the surface of the epithelium and are sometimes fixed in process of migration through epithelium and basement membrane. [Illustration: Fig. 1.—Acute bronchitis showing engorgement of blood vessels of mucosa and elevation of epithelium by serum and blood. Autopsy 352. ] The blood vessels of the mucosa are engorged. There is sometimes edema or hemorrhage, and in the superficial part of the mucosa polynuclear leucocytes are often fairly abundant. When superficial epithelium has been lost, polynuclears are numerous immediately below the surface of the exposed tissue. Fibrin is often present upon the denuded surface and extends for a short distance into the tissue below. In the deeper part of the mucosa, about the muscularis and especially about and between the acini of the mucous glands, the tissue is infiltrated with lymphoid and plasma cells. Changes in the mucous glands are invariably present. These changes are distention of ducts and acini with mucous, degenerative changes occasionally ending in necrosis of cells, disappearance of acini, dense infiltration of interstitial tissue with lymphoid and plasma cells and finally proliferation of this interstitial tissue. The duct of a mucous gland, dilated and filled with mucus, may be surrounded by lymphoid and plasma cells in great number. Acini, similarly dilated, contain mucus and are composed of cubical cells which have discharged their mucous content. In some instances (_e. g._, Autopsy 257) the cells of the acini have undergone necrosis; the cytoplasm stains homogeneously and the nuclei have disappeared. Where necrosis has occurred, polynuclear leucocytes may penetrate into the dead cells. In association with degenerative changes in the acini there is abundant infiltration of the interstitial tissue within and about the glands with lymphoid and plasma cells. When the acini have disappeared there is proliferation of fibroblasts and new formation of fibrous tissue, and mucous glands are found in which a few atrophied acini are separated by newly formed fibrous tissue.