"The mode by which this state is induced in the serous membranes, is by the chronic inflammation that exists in the diseased organ extending to them; and not by the same form of inflammation being set up in them, by a certain sympathy or consent of parts, which, from a loose analogy, has been thought to subsist between similar structures."
All diseases of the thoracic organs, are not equally prone to occasion effusion; some of these also, are only dangerous to life, in proportion to their disposition in occasioning such an effusion; whilst in other cases, if it occurs at all, the effusion is only the sequel of a disease essentially fatal.
"To distinguish between these two conditions, is a desideratum pathology. Modern writers on pathological anatomy have prosecuted with considerable zeal and ability, their researches into the nature of the diseases of the organs within the chest, but they have done but little towards elucidating the true relation, which subsists between the diseases of the several viscera, and the serous effusions which take place into their cavities; for, by limiting their views to the disease which the _post mortem_ examination exhibited, they have overlooked those intermediate actions or states of excitement which connect the organic disease with such effusions."
Whenever the excitement, producing hydrothorax, is idiopathic and independent of an organic disease of the lungs, heart, &c. its remote causes may be either of a general or local kind; and are the same which produce, when applied in a higher degree, or under different states of the system, the other forms of inflammation. The effusion may take place in those cases in which, the individual being predisposed, the inflammation, owing to some peculiarity in the cause, does not reach beyond its lowest grade; or in those in which the inflammation being high, and treated too late, or by insufficient means, a chronic form succeeds to the acute one, which may produce a watery effusion; or some structural disease remains and eventually becomes a cause of the effusion. The occurrence of this effect, in those latter cases, is sometimes attributed to a debility, resulting from the large depletion required in consequence of the severity of the previous inflammation.