When the pleural inflammation subsides, the exudation is destined to disappear. Usually the cure is produced by the reabsorption of the effused products. The liquid part of the exudation, the serosity, is absorbed by the lymphatics, which are found frequently dilated, and some of them are filled with fibrinous coagulations and the leucocytes. The solid parts, the false membranes, concrete fibrin, and cells disappear with more difficulty. They undergo granulo-fatty metamorphosis, and are then taken up by the lymphatics.
These fibrinous false membranes are not, as was formerly supposed, susceptible of organization. It is only the neo-membranes, formed by the proliferation of the elements of the pleura, which are organized or organizable. It is these that form bridles or ligaments which attach the lung to the thoracic wall, and are susceptible of transformation into cartilage or even into bone. In chronic cases these new membranes bind the lung down, impair its expansive powers, and inflict great damage upon the respiratory force.
Care must be taken to distinguish between the neo-membranes and the plastic and liquid exudations. These last contain transitory-formed elements entangled in the fibrinous layers. They are principally lymph-corpuscles, containing solitary nuclei, together with a few epithelial cells, almost always in process of disintegration, and isolated blood-corpuscles (Fraentzel).
Distribution of Fluid.—The situation and form in which the effusions are found in the pleural cavity furnish important data for study as applicable especially to diagnosis. At the commencement of the disease the effused plastic products form a thin covering to the pleural surface—a slight cushion interposed between the lung and the thoracic wall. Later, the fluid products gravitate by their weight to the lowest portion of the cavity of the pleura; then, as they increase in quantity, they gradually rise or are drawn to the superior portion of the thorax. Once formed, these effusions are but slightly movable and but little displaced by the varying positions of the patient, unless the quantity be very great and no adhesions or bands have been made. If the effusions be of viscid consistence, or if false membranes exist, they are mechanically prevented from moving. The serous transudations of hydrothorax always occupy the most dependent portion of the cavity, but observation shows how frequently the pleuritic effusions are immovable, being maintained and suspended between the diaphragm and the lungs, and imprisoned in the situation where they form by the false membranes.
Previous to 1843 the authorities universally taught that the effused fluids in the pleural cavity obeyed, as they would in an open vessel or in a vacuum, the law of gravity. They never appeared to question but that the fluid would necessarily assume its hydrostatic level, and consequently that it would reach a horizontal line in all parts of the chest. The distribution and the form which the effusions take were first studied by Damoiseau.3 Fernet and D'Heilly4 maintain that Damoiseau perfectly established the form and disposition which the effusions take in pleurisy. To study them well we must bear in mind, they say, three facts: the irregularly conical form of the pleural cavity; the effect of gravity; and the habitual position of the patient when lying down. Damoiseau and these authors utterly ignored the retractile force of the lung, as well as that of the diaphragm, and the resiliency of the thoracic walls, as effecting the position of the fluids in the pleural sac. If we observe that the patient at the commencement lies ordinarily on his back, the thorax being raised and more or less inclined to the horizontal position, we easily appreciate that the effusion ought to accumulate, at first, behind, in the most dependent portion of the costo-vertebral gutter, below the inferior angle of the scapula; then, as it increases in quantity, it rises and obliquely strikes the conoidal cavity, which encloses it, and makes on its surface curves resembling those of an oblique conic section (Damoiseau). As Damoiseau described the pleuritic line of flatness as a parabola, it was highest in the axillary region, where it first appears; thence, as its summit rises, its branches advance downward and outward to the sternum and the vertebral column.
3 Thèse de Paris, 1845.
4 Nouveau Dict. de Méd. et Chir., Paris, tome xxviii., 1880.
Since Damoiseau's first paper5 it has been generally acknowledged that the line of flatness over the upper surface of a moderate effusion is not horizontal when the patient is in the sitting or erect posture. There has been considerable difference of opinion among the English6 and continental writers as to the exact disposition of the fluid: some partially assent to Damoiseau's views; others, again, very materially modify them. Wintrich,7 who was one of the first among the Germans to emphasize the percussion line of demarcation between a pleuritic effusion and a contracted lung, says: "As the exudation gradually increases, the level of the fluid does not present a line which is horizontal or parallel to the ground, but one which descends toward the ground at a more or less acute angle." Fraentzel says that the line is never horizontal. Leichtenstein and Ferber8 maintain that the line depends upon the position of the patient early in the disease. Gee9 holds very much to this opinion. He states that the upper limit of the surface of liquid, when it reaches as high as two inches above the nipple, is horizontal. When lower than this, the dulness forms irregular parabolic curves, which become smaller and smaller as they descend. Austin Flint10 says, in his more recent edition: "The upper limit of the dulness or flatness, the position of the body being vertical, is not in a continuous horizontal line extending over the posterior, lateral, and anterior aspects of the chest." Flint, Wintrich, and Fraentzel speak of the line being highest behind. Calvin Ellis of Boston in two very suggestive papers11 described a curve-line made by the upper line of the effused fluid, which radically differed from any one previously mentioned. "This curve begins, with medium effusions, relatively low down on the back, passes outward from the vertebral column, and soon turns upward and proceeds obliquely across the back to the axillary region, where it reaches its highest point. Thence it advances in a straight line, but with a slight descent, to the sternum." Powell, however, does not find that the curve invariably commences at a lower level behind. G. M. Garland,12 in consequence of the resemblance of this curve to the italic letter S, has named it, very appropriately, the letter S curve. He adds that, according to his experience, "this curve, as described first by Ellis, may be traced, by proper percussion, in any case of free, uncomplicated pleurisy when the patient's body is erect and the amount of fluid present is not excessive. As any effusion increases in amount, the curve of its distribution gradually rises and tends to flatten out, so that it no longer presents its characteristic S feature after it reaches the second rib. At this point, when the fluid occupies nearly the entire side, the curve comes quite near to the horizontal, but if some of the fluid be withdrawn by aspiration or absorption the letter S curve will reappear and retreat downward in the inverse order of its advance, until with entire absorption it becomes merged into the normal boundary of the lung."13 Garland quotes from two recent German authorities—Heitler of Vienna and Rosenbach of Breslau—to the effect that the line of flatness of the effusion extends lower on the back than it does on the side, and that there is a triangle bounded by the vertebral column, the upper curve from the bottom, and a line drawn from the summit of the curve, where there is impaired resonance over the lung from adhesions and oedema of the lung, but where there is no fluid and no flatness. Garland had previously called attention to this space, and had named it the dull triangle. He had warned all who sought to trace the true line of pleuritic flatness to be careful not to overlook this region. Heitler had likened it to a monk's hood cut longitudinally through the centre and hanging apex down. Rosenbach made this dull space, clearing up in exercise and deep breathing, as distinctive between pleurisy and pneumonia. We must expect impaired resonance on the posterior wall above the fluid, for the fibrinous deposits from exudation collect there when the patient is in the recumbent position. Garland14 calls attention to the confused views caused by confounding the two physical signs of dulness (or impaired resonance) and flatness (absence of resonance), the latter only indicating the presence of fluid. If the differential diagnosis between the dulness on percussion over the dull triangle and the flatness over the fluid be not carefully made by delicate, light percussion, the two may easily be confounded and the fluid be thought to have arisen to a much higher level than it has. In some cases, owing to greater thickness in the walls and coverings of the chest and adhesions, it may be more difficult to draw nice distinctions in percussion sounds. This distinction can, however, be made if the percussion-stroke is used with proper delicacy and lightness, and a comparison made between the two signs, and not between them and vesicular resonance. If the percussion be strong, the vibrations are communicated from the resonant lung above the fluid, and deceive the examiner. The most effective manner of percussing is at right angles to the general direction of the curve, which is transverse across the chest. Thus examining, we have had ample opportunities of confirming the statement of Ellis and Garland that the curve line is never highest behind, even with the largest effusion. Wintrich and his German followers hold a different view. In moderate effusions it is highest in the axilla, from which point it turns downward posteriorly to touch the vertebral column at the interscapular region. In front it extends downward toward the sternum. R. Douglass Powell15 reports cases with drawings, showing that in typical cases the fluid does not take a water-level, "but a curve, having its convexity upward in the lateral region." When the effusion becomes excessive and fills the whole cavity, there is flatness on percussion everywhere. As the fluid subsides, however, from absorption or from mechanical removal, the distribution again resumes, to a greater or less degree, its previous shape. In moderate effusions there is, ordinarily, the dull triangle posteriorly, and Skodaic resonance under the clavicle in front in the anterior triangle. On the left side the lower limit of the effusion can be recognized by the flatness being in the shape of the arch of the diaphragm. In cases complicated by adhesions or by pathological changes in the lung itself the curve is changed, and in some the Ellis curve is straighter than in others. Adhesions form sometimes early in the disease. They mechanically interfere with the usual distribution of the fluid, as do catarrhal, tubercular, or pneumonic consolidations, and, indeed, emphysematous conditions. All these physical alterations of structure modify the elastic force of the lungs. According to Mohr's statistics, adhesions were wanting in 47 per cent. of the cases analyzed by himself. Garland's experiment of injecting glue and plaster of Paris, and subsequently cocoa-butter, into the pleura of living and dead dogs, and by moulds testing the curves formed, showed that if the dogs were suspended by the head the curve of flatness on percussion was very similar to the Ellis curve. On removing the casts after they had solidified, he found they closely corresponded to the shape and position indicated by the physical signs elicited before opening the chest.
5 Archives générale de Méd., 1843.
6 See Hyde Salter, Lancet, 1865; Powell, Trans. Roy. Med. and Chir. Soc., vol. lix.; W. N. Stone, Lancet, 1877; Le Gros Clarke, Roy. Soc. Med. and Chir., 1872.