6 Ges. Abhang., p. 369, Virchow.
On the other hand, it is claimed that in the early stages of the pneumonic process the parts that are supplied by the bronchial capillaries are not reddened or injected, as they would be were these vessels primarily concerned in the inflammatory process. Reasoning from the above, it would seem that both sets of vessels are involved, but that usually one set is implicated at the very commencement to a greater extent than the other.
It is often difficult, and sometimes impossible, to differentiate between the anatomical appearances produced by pulmonary congestion and oedema and the first stage of pneumonia. In pulmonary congestion and oedema the fluid in the alveoli is serum, and contains none of the pathological cell-elements found in the first stage of pneumonia. The alveolar capillaries are turgid, and in this respect resemble the capillaries in pneumonic congestion, but when a stream of water flows over a portion of lung in the first stage of pneumonia its dark color remains, while in hyperæmia of a non-inflammatory character this is not the case.
On account of its color and its resemblance to liver-tissue the name red hepatization has been given to the second stage of croupous pneumonia. The lung in this stage has a dark liver or mahogany color, and is slightly mottled, the mottling becoming more marked the farther advanced is the hepatization. The color is of a brighter red when the lung is first removed from the body than after it has been exposed to the air.
The volume of the lung is increased—at times so as to bear the impress of the ribs. It is solid and firmer than normal; pressure does not indent but tears it; it is very friable, and its torn surface presents a granular appearance. Its specific gravity is increased. It is airless, and there is an entire loss of crepitation.
Upon section it is seen that the granular appearance of the cut surface is due to the pneumonic exudation which fills the alveoli. This granular appearance is not so well shown on the cut as on the torn surface. The granules can be readily removed from the air-cells by means of a fine needle. A dirty, red, viscid fluid slowly oozes from its cut surface, which is more apparent after the lung has been exposed to the air for twelve or twenty-four hours and has undergone post-mortem changes. At any time this viscid, rusty-looking material may be scraped from the cut surface, or it exudes when a portion of the lung is firmly compressed. A portion of the inflamed lung quickly sinks in water, and small spots of blood-extravasations may be seen scattered here and there throughout its substance. When a stream of water is poured over the cut surface of the implicated lung the color changes from a maroon to a gray or yellow-gray, the usual color of fibrillated fibrin.
Not infrequently the material in the infundibula and air-cells extends into the minute bronchi, but these tubes are rarely completely filled with the pneumonic exudation. When examined under the microscope the alveoli are found filled with a solid material composed of a network of fibrillated fibrin, in whose meshes are leucocytes, red blood-globules, and changed epithelia. These latter are in various forms, usually round or oval. They may, however, become quadrangular, triangular, or irregular. They are granular, and may contain a single nucleus, a nucleolus, or multiple nuclei. These cells finally become granular, and fat-globules accumulate in them. They also become discolored from imbibition of blood-coloring matter, so that in the latter part of the process there is quite an accumulation of pigment-granules, not only in the free cells, but in the fixed epithelia. The larger cells discharge their nuclei into the accumulation of corpuscular elements, and the whole contents of an alveolus present a more or less round shape. The alveolar walls remain unchanged, or are slightly thickened by the capillary turgescence. All of these different cell-elements have been regarded by different observers as characteristic of pneumonia. The red globules give the color to the consolidated lung. The pus-cells are always numerous.
The transition from red to gray hepatization is never well defined. The mottling gradually becomes more marked, so that the affected portion of lung assumes a marbled or granite appearance. As the deep-red color of the second stage fades the density of the pneumonic consolidation becomes less and less, until it is a mere pulp, breaking down under slight pressure. The decoloration is due to the pressure on the blood-vessels, to decoloration of the blood-corpuscles that were present in the second stage, and to fatty degeneration of the other cell-elements which occupied the air-sacs.
The weight and density of the affected lung-tissue are diminished, and toward the end of this stage the lung crepitates. On section a nearly uniform dirty-gray, bloodless surface is exhibited, from which flows spontaneously or upon slight pressure a dirty-white or reddish-gray puruloid fluid. The granular red hepatized look has disappeared or is very indistinct. The amount of oedema in the affected portion of the lung varies in different cases. When it is excessive a large quantity of serum exudes from the cut surface, which then exhibits a smooth, non-granular, glistening appearance, and it does not so readily break down when pressed upon as do other forms of gray hepatization. When examined under the microscope, the alveoli are seen to be filled with numerous round mono-nucleated cells, the intercellular fibrils that bound the elements together having disappeared; in other words (the fibrillated having become granular fibrin), the alveoli are filled with a fluid or semi-fluid mass in which numbers of discrete oil-globules and protein granules are freely mingled.
The granular and fatty elements are due to the rapid degenerative changes that occur in the cell-elements. In this stage leucocytes still emigrate from the blood-vessels. The masses that occupy the alveoli are now shrunken, and between them and the alveolar wall is a layer of fluid, so that in a thin section the contents of the air-sacs are readily lifted out by a camel's-hair brush. All of the affected portion of the lung is rarely in the same stage of the inflammatory process, and to distinguish red from gray hepatization, or the latter from the beginning of some of the conditions next to be mentioned, is often impossible.