Free gas is not unfrequently found collected in different quantities within serous sacs. It is met with chiefly in large serous cavities, such as the pleura and peritoneum, and its presence is due to the escape of atmospheric air from the air-passages, or of intestinal gas from the bowel. It is occasionally produced by the decomposition of ill-constituted and long-stagnated effusions; or it may be a product of the exudative process itself. In a few cases, it may even be a morbid secretion (exhalation), from the serous membrane during life.

Besides this, and the various products of inflammation already described, there occur also collections of serous fluid, and of blood.

Collections of serum constitute dropsy of serous and synovial sacs, and of bursae (Ganglia); the quantity of fluid varies, and with it the enlargement of the cavity; its color, too, and its consistence and composition, especially in respect to the quantity of plastic material it may contain, vary considerably. The remarks already made upon dropsy in general apply also here.

The effusion of blood into serous cavities - actual hemorrhage - must be carefully distinguished from hemorrhagic exudation.

An account of various other effusions will be found in the chapters on the particular serous sacs.

Lastly, the cavities of serous and synovial membranes sometimes contain free loose bodies, which have various origins, and differ accordingly in their appearance and construction: those, more particularly, which are met with in the peritoneal cavity vary much in their kinds. They are found in the cavity of the peritoneum, within the tunica vaginalis testis, in the pleura, in the sac of the arachnoid, and in the ventricles of the brain; they are also particularly common in several of the synovial cavities, especially in the knee, and in bursae (articular mice); they even occur in anomalous serous sacs.

Their usual size varies from that of a millet-seed to that of a pea or a bean: it is an exception to find them larger, but they do sometimes reach the bulk of a walnut: they are generally round or oval in shape, but pressed somewhat flat; sometimes their figure is irregular. They are mostly firm and elastic; and from the smoothness of their covering, which glistens like a serous membrane, they acquire a polished appearance, but sometimes there are rough and villous spots upon them.

With regard to their origin, the observations of Laennec and Béclard prove that some of them originate outside the serous membrane; while the internal construction of many others indicates that they were formed within its cavity.

The first kind includes the fibroid and fibro-cartilaginous concretions, some of which contain bony nuclei. They are originally developed in the subserous cellular tissue, or in the serous tissue itself; but as they gradually force the membrane before them, they become invested with a prolongation or duplicature of it, which remains connected with the rest of the membrane only by a pedicle; at length the pedicle being worn away by friction, the cartilage falls loose into the serous cavity. It has a proper serous covering, which often bears a trace of this mode of development in being deficient at the spot where it was separated from the pedicle: it is then completed by loose shreds of cellular tissue.

Those of the second kind are the fibrillated and albuminous coagulations and precipitates from morbid effusions. They are distinguished by their uniform smoothness throughout, by a delicate albuminous investing membrane, and frequently by their manifest arrangement in concentric laminae.

Moreover, free bodies of a different nature are sometimes found particularly in the peritoneal cavity. Some of them are obsolete portions detached from the omentum and appendices epiploacae, which, within a bluish, gray tunic, contain fat that resembles tallow or spermaceti; others are tubercles which have become loose, and which, like the former, may become the nucleus of albuminous coagula: whilst others again are fibroid, or are allied to the fibroid, tumors formed beneath the peritoneum in the uterus or its appendages, but afterwards set free.