=Pneumonia=, or inflammation of the lungs, may affect both the air cells, when the latter become filled with fibrinous exudations, and the connective areolar tissue, which then become increased in quantity. The following characters present themselves, corresponding to the three recognized stages of the disease:—First, _congestion_; second, _red hepatization_; third, _gray hepatization or softening_. =Congestion.= From the peculiar structure of the lungs, in connection with the free circulation through the same, these organs are peculiarly liable to the several forms of congestion. In many cases of death, without any original disease of the lungs, there will be a tendency for these organs to become loaded with blood, giving rise to post-mortem appearances, often with difficulty distinguished from those of a pathological origin. In this post-mortem, or, as it has been called _hypostatic_ congestion, the posterior and inferior portions of the lungs are chiefly affected, as the blood after death, obeying the law of gravitation, sinks to the lowest point. The congested portion presents a dark red color, and though firmer than other portions, crepitates under the finger and floats in water, the latter circumstance serving to distinguish this form of congestion from that of an inflammatory origin. If the congestion be confined to one lung, or to the anterior parts of either, we may safely attribute it to a pathological cause. In all cases of congestion, upon opening the chest, although there may be no adhesions, the lung does not collapse, or does so feebly. When cut, it is found to be loaded with blood, and upon pressure, much bloody serum escapes, while the divided bronchial tubes will be found filled with frothy mucus. =Red Hepatization.= This condition of the lungs soon follows that of congestion. The change is a gradual one, and is first marked by an effusion of serum and coagulable lymph into the connective tissue and air cells, thus rendering the lungs more solid, while as the change becomes complete, the blood itself, which had during the congestive stage been confined to the vessels, is now found extravasated into the interstices of the tissues. The portion of the lung thus affected is not only of a dark red or violet color, but solid, firm, does not crepitate, sinks when thrown into water, and when cut and washed, the section shows patches of a rough, granular aspect, totally different from that of healthy lung tissue. The pleura in this condition may be wholly unchanged, even though the solidification may have been of long standing.