In addition to the anomalous contents of the pleural sacs, which have been already mentioned, and to which we shall have occasion subsequently to allude, we shall only at present treat of the presence of gas and serum in the pleural sacs (pneumothorax and hydrothorax).

There are many sources from which gases of various kinds may accumulate in the cavity of the thorax, and (independently of the atmospheric air finding its way there from without, through a penetrating wound in the walls of the chest, or through the bronchi in injury of the lung with or without an opening in the thorax, or through both causes combined), a pneumothorax may occur under the following conditions:

(a.) In consequence of the opening of a tuberculous abscess before there is perfect adhesion of the lung to the walls of the chest through pleuritic exudation. Here we must particularly direct attention to the abscesses which are preceded by superficial tuberculous infiltration arising from softening.

(b.) In consequence of the softening and separation of a superficial gangrenous eschar of the lungs.

(c.) In consequence of the softening of the so-called metastatic deposits which penetrate the pulmonary pleura before reactive hepatization can occur.

(d.) In consequence of the opening of a pulmonary abscess communicating with some of the bronchial tubes.

(e.) From the development of gas from the decomposition of purulent and ichorous exudations (empyema).

(f.) In rare cases, a certain quantity of air is found in the pleural sac, as a product of the inflammatory process, and associated with a benign pleuritic effusion.

(g.) In cases in which there is an opening formed into one of the bronchial trunks, in consequence of its corrosion by purulent and ichorous exudation.

(h.) In consequence of the laceration of one or more of the superficial pulmonary vesicles in vesicular emphysema, or from rupture of the pleura in interlobular and sub-pleural emphysema.

(i.) From perforation of the diaphragm or of the mediastinum, arising from acute softening of the stomach or oesophagus.

When the disease giving rise to the pneumothorax is not in itself fatal, which, however, is the case in softening of the stomach and oesophagus, then the gas accumulated in the thorax invariably causes inflammation of the pleura, and exudation, compression of the lungs, dilatation of the thorax, etc.; the nature of the exudation varying according to the source, nature, and products of the gas, and of the other substances which are simultaneously extravasated into the pleural sac.

Hydrothorax is a very rare disease, if we except those cases in which it constitutes a part of general dropsy; and even then, if we exclude those in which it is dependent on cardiac or pulmonary diseases, it is very rarely the first in order of the various serous effusions. It may arise as a consequence of hypersemia of the pleura, and in that case the swelling and hypertrophy of the serous membrane are proportional to the duration of the effusion. It is also often present when there are cancerous vegetations on the pleura: but its most common exciting causes are diseases of the heart and large vessels, pericarditis, catarrh and bronchial dilatation, indurated hepatization, and pleurisy; finally it forms a portion of the general dropsy consequent on diseases of an exhaustive nature, such as typhus and puerperal fever, emaciation from tubercles or cancer, and Bright's disease of the kidney.

When there is a large quantity of air in the pleura, the lung becomes displaced and compressed in the same manner as in pleuritic effusions.

It should be most carefully distinguished from the serous effusion which occurs as one of the processes of inflammation of the pleura. The inexperienced observer may find some difficulty in the establishment of a correct diagnosis between it and those pleurisies which deposit exudations deficient in plastic matter (the so-called active dropsies), especially when these latter have existed for a considerable period.