This is a disease whose anatomical characters are not properly recognized in pathological treatises, for it is commonly described as chronic inflammation of the lungs consequent on ordinary croupous pneumonia, without any reference to its seat in a special tissue. The seat of this inflammation is the interstitial cellular tissue of the lungs, although the walls of the air-cells are also implicated, in which case the pneumonia sometimes assumes the croupous form.

Its course is, as a general rule, chronic, and it is only very rarely that we have the opportunity of studying it, except in its final effects. So far as we can conclude from our few observations, it appears to commence in the tissue lying in the interstices of the pulmonary lobules and between the smaller groups of air-cells, which, if too much black lung-substance be not present, becomes of a pale red color, and is swollen by albuminous infiltration, while the air-cells are either pale and more or less compressed in proportion to the swelling; or, if they are involved in the inflammation, they appear reddened and, in accordance with what has been already stated, sometimes finely granular. In the progress of time the infiltration within the interstitial tissue becomes organized and coalesces with the latter, so as to form a dense cellulo-fibrous substance, which compresses and obliterates the air-cells, and finally converts them into a similar cellular tissue. We then find either whitish, hard stripes, which not unfrequently grate under the knife, or irregular masses interwoven in the lung-substance.

This is the ordinary metamorphosis consequent on chronic pneumonia; in some cases, however, it may terminate in suppuration which isolates the individual lobules; and some pulmonary abscesses probably originate in this manner.

It is not very frequently a spontaneous affection, insidiously spreading from one lobule to another; it is commonly seated in the apices of the upper lobes, and as we may infer from the coexisting cellular adhesions corresponding to their seat and distribution, it is frequently combined with circumscribed pleurisy.

The affected portions of the lung become depressed, and draw down the surrounding parenchyma in the form of cicatrix-like folds, which may sometimes be observed on the apices of the lungs in cases where there is no trace of the pre-existence of the tubercle. A further consequence of this process is a depression of the thorax at the corresponding spot, and, internally, a dilatation of the bronchial tubes.

More frequently, however, it is a consecutive affection, arising from reaction, and leading to the production of cyst-like formations around the seat of old apoplexies, abscesses, tuberculous caverns, gangrene, etc.; its products then resemble the tissue of which cicatrices are composed.

This tissue sometimes contains a considerable quantity of pigment (the black pigment of the lungs); it then presents blackish-gray stripes and spots, or else is uniformly of a blackish-blue tint.