This is the most frequent of the diseases of the pleura; it generally appears as an idiopathic and primary affection, most commonly of a rheumatic nature; in consequence of a wound or shock affecting the walls of the thorax, or of the contact of the pleura with the atmospheric air, either from without or through the air-passages from within, or of the contact of pus, ichor, etc.; or it may originate in the extension of inflammation or other morbid processes in neighboring parts, and especially the lungs; and it very often is a secondary or metastatic process, and is then frequently associated with inflammation of other serous membranes, especially of the peritoneum and pericardium; it generally exhibits a very well-marked croupous character.

Pleurisy is either general or partial and circumscribed: in the former case the process on the visceral surface (the pulmonary pleura) as a general rule exhibits comparatively little intensity. In either case the disease may be acute or chronic. As everything that has been stated with respect to inflammations of serous membranes generally applies to inflammations of the pleura, we shall here limit our remarks to the notice of certain important peculiarities presented by this membrane.

The exudations present all the differences that we have there described; but here we must especially notice, as frequent and very important forms, the purulent exudation (empyema) and the hemorrhagic. In relation to the quantity of the exudation, acute, and more especially chronic pleurisies depositing a long-continuing and paroxysmally increasing exudation in large quantity, are of the greatest importance. When the pleurisy is general, the exudation sometimes amounts to eight, ten, sixteen, or twenty pounds, and occasionally even more. The walls of the thorax become in these cases more or less dilated; the intercostal spaces are enlarged, and in consequence of the paralyzed state of their muscles are flattened; the diaphragm is forced downwards in the abdomen; the mediastinum and the heart are pushed to the opposite side, and thus diminish its capacity. The lung itself is compressed by the effusion, in a degree corresponding to its quantity, and if no old adhesion exists to oppose it, it is constantly pressed upwards and inwards on the mediastinum and the vertebral column. We find it thus compressed to the fourth, sixth, or even the eighth part of its normal volume, and so flattened on its external arched surface as to present the appearance of a flat cake; its texture is of a pale red, bluish-brown, or lead-gray tint; and it is of a leathery toughness, and devoid of blood and air. In this state its external surface is invested with a plastic coagulum; as this extends over it to the costal pleura, the lung is, strictly speaking, excluded from the cavity of the sac formed by the pleuritic exudation. If adhesions already exist, as the remains and consequences of previous inflammations, they will, in proportion to their position, distribution, the tissues comprising them, and their powers of resistance, present a certain amount of opposition, as we have already described, to the displacement; and the degree of dislocation of the lung must be thereby more or less modified. In partial pleurisy, the displacement and compression are limited to the portion of the lung corresponding to the extent of the affection.

The purulent exudation most commonly accompanies the pleuritic process in weak, cachectic persons, whose organisms seem prone to form pus; on the other hand, it may, by an intensely high degree of inflammation, and by its frequent recrudescence, give rise to very rapid general debility, cachexia, and pyaemia. The effused pus not unfrequently degenerates into ichor, and this change is sometimes accompanied by the development of gas, so that from its decomposition and disintegration a pneumothorax becomes added to the purulent effusion. This not unfrequently leads to suppuration of the walls of the chest, with or without caries, and to the spontaneous discharge of the collected fluid, or to suppuration of the lung from its surface, leading to perforation of the bronchial tubes, or to suppuration of the principal bronchial trunks, whereby, on the one hand, atmospheric air is allowed to penetrate the pleural sac, and, on the other, pus enters and is discharged through the bronchi.

Amongst partial pleurisies we must especially notice those which occur about the apices of the lungs (for the most part dependent on pulmonary tuberculosis), those in the inferior portion of the pleural sac and on the pleura diaphragmatica, those on the laminae of the mediastinum, and finally those which occur in the interlobular fissures of the lungs.

Plastic exudations, when they result either from an acute or a chronic simple process (by which we mean to imply a process not implicating the exudation), become transformed into areolar tissue of various degrees of density, into areolo-serous and fibrous tissues, presenting varied relations in reference to form and distribution.

When they are the products of general pleurisy they invest the whole of the costal and pulmonary pleura; or they merely occur at individual spots in the form of circumscribed patches on the serous membrane.

The areolar tissue forms either dense or rigid bands, or thready, lax, and movable adhesions, according to comparative absence or presence of aqueous effusions, occurring either originally or during its formation. These adhesions are of very common occurrence; when the lung adheres at every point of the parietal surface of the pleura, they are termed general, and when the latter is only attached to a portion of the lung, partial (or cellular) adhesions. They may undoubtedly become the seat of new inflammatory processes, but, as Laennec remarked, they usually limit the progress of new pleurisies. When general dropsy and the dropsical diathesis are present, they may become the seat of a serous, briny infiltration.

Sometimes there are no adhesions, the tissue forming delicate flocks or cumuli, which are seen scattered over both surfaces, and not on corresponding points; in many cases delicate areolar adhesions appear to have given way from tension and friction, the patches of new tissue being then on corresponding spots on the two pleural surfaces; these are usually either conical with a broad base, or are drawn out into stringlike prolongations.

If, during the organization of the exudation, an aqueous effusion were present, which prevented the contact of the two lamellae, then the areolar tissue receives a serous investment, and the pleura becomes clothed with a second, and newly-formed, tolerably easily removable serous membrane.

Where the exudation is thicker, it becomes converted under similar conditions into a smooth, bluish-white fibrous lamella, which either invests the whole pleura, or adheres to it at particular spots, which are either clearly defined at their circumference, or gradually and imperceptibly merge into the surrounding tissue. Even in the first case this lamella presents no general uniformity of thickness, and exhibits an areolar, cribriform, broken appearance; and then it sometimes happens that after the diminution of the aqueous effusion, the lung becomes more strongly developed at those points where the false membrane is thinnest, and bulges through the meshes in the form of mammillary appendages.

Chronic Inflammation Of The Pleura

Chronic Inflammation, when it insidiously extends itself amongst the organizing coagula, occasions very important metamorphoses of the substance of the pleura, and of the products that are evolved. It very frequently gives rise to hemorrhagic or serous effusions, in which there are found coagula of considerable thickness, which become gradually converted into very dense and resistant fibrous bands. As the result of general pleurisy, we have not only dilatation of the thorax corresponding to the amount of effused fluid, and the already mentioned displacement and compression of the lung, but there are likewise formed thick, pseudomembranous pleural sacs, adhering to both surfaces. The parietal wall is usually by far the thicker, not unfrequently measuring four, six, eight, or even ten lines, and occasionally even an inch in thickness.

If, under these conditions, the serous effusion becomes diminished by absorption, the lung, invested by a fibrous pseudo-membrane, may either again very slowly attain to a certain degree of development, or it may remain unalterable, according to the thickness and resistance of the membrane; in the former case the walls of the thorax approximate for the purpose of filling the vacant space, till finally, when the serous effusion is thoroughly removed, the two lamellae of the fibrous exudation come in contact, and coalesce with each other. Hence, as was shown by Laennec, the thorax becomes permanently contracted, the amount of the contraction varying from a scarcely perceptible flattening to a very obvious pitlike depression, modifying the form of the chest. In these latter cases the greatest depression usually occurs in the neighborhood of the 6th, 7th, and 8th ribs, and the lateral wall of the chest presents a well-marked concavity from the axilla downwards over the ribs. The thorax, if measured, is found contracted not only in its circumference, but in all its diameters. The depression is most marked where the ribs are the most sunk, and the latter sometimes even touch and press upon one another. The surrounding muscles are emaciated, especially the in-tercostals, which become shrivelled in proportion to the degree and continuance of the paralysis they have undergone, and are finally converted into fibro-areolar tissue. The dorsal portion of the spinal column gradually deviates with a lateral curvature towards the healthy side, and the shoulder of the affected side sinks proportionally deeper. In the lumbar region there is a curvature corresponding in degree to that in the dorsal region, but inclining to the opposite direction, giving to the pelvis a higher position on this side, and causing an apparent shortening of the corresponding lower extremity. In this way the form of the hips and buttocks, and the general bearing or carriage of the body, present a certain similarity with the corresponding appearances after coxalgia.

There are several causes for the contraction of the thorax after these forms of pleurisy. Amongst these we may especially mention - (1), the pressure of the atmosphere on the thorax, when it is impossible for the pulmonary parenchyma to regain its former condition with a rapidity equivalent to that with which the serous effusion is absorbed, in consequence of the fibrous sheath investing the lung, and the destruction of its extensibility and elasticity by continued pressure; - (2), the tendency exhibited by the costal lamina of the fibrous exudation to increase in density and to contract; and (3), the similar tendency on the part of the fibro-areolar tissue that has taken the place of the atrophied muscular tissue in the intercostal spaces.