Ossification of the serous membranes is also infrequent. Like the same process else where, the deposit of bony matter never occurs alone, but is a very slow change, which appears to require the existence of a previous tissue. Hence, it is litnited to two forms, neither of which primarily affect the cell-growing membrane. In the first, the fibrinous exsudation of a preceding inflam mation is gradually transformed into ossific matter. In this case, the shape of the de posit is rough and irregular, and sometimes it forms a kind of nucleus, whicb occupies the centre of the tough fibrous mass. Its appearances sometimes approximate to those of the cretifieation before alluded to, as pos sibly do the several processes which form these substances. In the second variety, the subserous and neighbouring areolar tissue is occupied by the deposit ; but here also a fibrous or fibro-cartilaginons thickening, which is itself the developetnent of an exsu dation, is probably the immediate seat of the change ; and a variable quantity of this mor bid tissue is generally seen around and upon the bony matter. The shape which, under these circumstances, it assumes, is somewhat more regular than that of the preceding variety, it being often flattened and extended in thin plates, the roughly tuberculated sur face of which is, for the most part, parallel with the surface of the membrane.
The pleura is the most frequent site of these ossifications, as it is also of the adhe sions in which they mainly occur : but they are also found in the subarachnoid tissue and pia mater ; and, more rarely, in the peri toneum and the synovial sheaths of the ten dons.
Cysts are often found in these membranes, but their great differences of nature and causes clahn a longer notice than can be accorded in this brief sketch. Three chief varieties may be distinguished. One of these is inhabited by parasitic animals, as the echi nococcus. These are usually found in great numbers, and may occur in any of the serous membranes, although the peritoneum is their most frequent locality, probably from its proximity to the intestine by which they are introduced into the body. Sometimes they occupy the cavity of the membrane, and are in contact with its interior by a slightly flat tened part of their surface; in other instances, they project into the cavity, carrying the membrane before them ; and at least one layer of their wall is fonned by lymph de rived from the neighbouring vessels. Anor ther form is not recognised as parasitic, but in the present state of our knowledge might rather be described as a gigantic cell, which often includes a vast progeny of smaller ones. The whitish powder which some of these contain, may frequently be seen to be com pletely composed of small cells, which are de void of a nucleus, of uniform size and sphe rical shape, and exhibit a clear sharp outline.
These characters alone would, perhaps, indi cate their merely cellular nature, as above stated; hut the general appearance of these contained globules is suspiciously like the ova of entozoa. In other cases, the included cell again includes a smaller one, and this yet another, so as to form a series of concentric hollow spheres ; an arrangement which has named them as the pill-box hydatid. In their general appearances, they closely resemble the preceding variety. The fluid contents of both are limpid and transparent, and are composed of water, with blood salts (chiefly chloride of sodium), and an exceedingly small quantity of albumen. Yet the effusion of this apparently harmless fluid into the serous cavities gives rise to an inflammation of the greatest violence and fatality. In a third class of cases, the cysts are usually in much fewer numbers than the preceding : they occur for the most part in the neighbourhood of the female reproductive organs ; and this their situation, together with their contents, which often consist of teeth, hair, bone, fat, and other products of an abortive develop ment, sufficiently indicate a relation to the generative process. The fluids which they contain are albuminous, often sufficiently so to possess a glairy consistence. They exist within the cavity of the serous membrane, or in its texture, indifferently ; when developed in or beneath the subserous areolar tissue their gradual enlargement causes them to reach the free surface of the membrane, and then to dilate and extend this tissue before them, until finally the cyst, still covered by the serous layer, hangs freely in the cavity by a more or less elongated peduncle, w hich is formed by this covering where it becomes continuous with the rest of the serous membrane.
The subserous areolar tissue has been men tioned as implicated in most of these diseases ; but other morbid conditions are not wanting, in which it appears to be affected without the essential participation of the remainder of the tissue. Such are the little masses of fat which are occasionally found projecting into the serous cavities ; they are covered by the smooth and apparently healthy membrane, and their form is generally pedunculated, or sometimes ramified and arborescent. The development of this shape corresponds with that of the subserous cysts just mentioned. The fibrinous Pacchionian bodies of the cere bral meninges have been similarly explained as arising from the pia mater, and gradually invested with a layer of arachnoid which becomes converted into a peduncle, the rupture of which leaves them adhering to the dura mater, or even projecting into the longitudinal sinus.