Microscopic examination shows that the epithelium of the bronchi is partially or completely destroyed and that destruction of the underlying tissue, with acute suppurative inflammation, penetrates to a greater or less depth into the wall. When the epithelium of the bronchus is wholly destroyed and the lumen is filled and distended with polynuclear leucocytes, a cross section of the tube has the appearance of a small abscess; but more careful examination often shows that the engorged mucosa is still intact. Occasionally, a network of fibrin forms a layer covering the denuded mucosa. Disintegration of the superficial tissue may extend to the muscularis or through it, and may penetrate the wall of the bronchus. The tissue in contact with the exposed surface contains many polynuclear leucocytes and blood vessels plugged with fibrinous thrombi, but deeper in the tissue lymphoid and plasma cells are more numerous. In 2 instances (Autopsies 286 and 425) favorable sections have demonstrated that the wall of an abscess on one side consists of the remains of a bronchus, covered by epithelium composed of squamous cells, Whereas the remainder of the wall, here very irregular, is formed by partially destroyed alveoli plugged with fibrin. The suppurative process has penetrated the wall of the bronchus on one side and extended into the surrounding alveolar tissue. In other instances, abscess cavities occur within the alveolar tissue of the lung and their relationship to bronchi is not evident. In the mass of polynuclear leucocytes which fill the abscess cavity, are clumps of staphylococci in great abundance, usually forming characteristic colonies which are conspicuous with the low power of the microscope. Empyema, Pericarditis and Peritonitis No sharp line can be drawn between nonpurulent and purulent pleurisy. A diagnosis of empyema has been made when the fluid in the chest has become opaque and fibrin has undergone softening or solution. The lesion has been designated seropurulent when there has been abundant thin, opaque, gray fluid. Pleurisy has been designated fibrinopurulent when the cavity has contained opaque fluid and ragged soft white or yellowish fibrin adherent to the chest wall; this fibrin is evidently in process of disintegration and there may be numerous shreds and flakes of fibrin which subside to the bottom of the fluid. The amount of fluid in the cavity may occasionally exceed 1,700 c.c.; that in both pleural cavities may exceed 2,500 c.c. The lesion has been designated purulent when fibrin has almost wholly disappeared and the cavity contains thick yellowish white fluid. In 4 of 5 instances in which thoracotomy had been performed, empyema has assumed this otherwise uncommon type. Some inflammation of the pleura is almost constantly found in association with all forms of pneumonia, but in many instances is so slight that it has no noteworthy significance. Table L shows the incidence of various types of pleurisy.