Next follows the exudation of fibrine and serum, which respectively coagulate as false membrane on the inflamed membrane, or drop to the bottom of the sac as liquid. The fibrine appears as granules, little knobs and threads between and on the endothelial cells and entangling a few pus cells. The changes are now much more marked in the connective tissue cells, which are more numerous, larger, nucleated and often stellate or polygonal. Changes are well advanced in twenty-four hours. The cells go on increasing to the fourth or fifth day, when new blood vessels are formed into the membrane and may be injected from the pleura. From this time, in favorable cases absorption of the liquid proceeds, and the fibrine is organized, and by the fourteenth day is transformed into connective tissue, the superficial cells forming endothelium and the deeper, branching connective tissue cells. The result is the thickening of the pleura and the formation of adhesions. The case, however, may prove fatal, or it may be protracted through the continued production of fibrine and serum, or it may pass into empyema. =3. Sero-fibrino-purulent pleurisy.= =Empyema.= This is usually very dangerous as well as complicated. It may supervene on the last described form. It may depend on rupture into the pleura of abscess of the lung, bronchial glands, liver, diaphragm or intercostal space and the infection of the chest cavity. It may in the same way follow the laceration of a bronchium by a broken rib, the perforation of the intercostal space by a foreign body, or (in cattle) the penetration of the chest by a sharp pointed body from the reticulum. It may follow at once on pleurisy of a very high grade. Probably in all such cases there is infection of the pleura by pus microbes. When there is a communication with a bronchium, the reticulum or the external air there are usually septic germs in addition, and the contents of the chest become fœtid. The purulent fluid may accumulate in the lower part of the pleural sac, or it may be confined in abscess form in the false membrane, and extend thence into surrounding tissues. The pus-containing pleura, or cavity infected by the pus germs, assumes the appearance of a granulating surface, or of the lining membrane of an abscess, and continues to produce pus in greater or less amount. The formation of pus in the pleura is known as _empyema_. When air enters the pleura through a wound perforating the chest wall, or when gas is formed in the pleura, the condition is =pneumothorax=. As liquid is usually present as well it is =hydro-pneumothorax=. Tubercular and other forms of pleurisy have in certain cases been superadded to the specific local lesions, by which such diseases are individually characterized.