Pneumonia sometime occurs secondarily to other forms of illness. Thus it may be a consequence of an altered state of the blood, as in the acute febrile diseases, or may be due to imperfect purification of the blood, as in Bright's disease. In other cases, again, it may be a purely accidental complication.
Lastly, although pneumonia often attacks children who are to all ap pearance strong and healthy, its occurrence, like that of other acute diseases, is favoured by conditions which reduce the strength and lower the resist ing power. Therefore impairment of health must be looked upon as one of the predisposing causes of the malady.
Morbid Anatomy.—The morbid processes which constitute an attack of pneumonia are divisible into three well-marked stages. In the first—the stage of engorgement—there is congestion of the capillary vessels which ramify between the air-vesicles and on the minute bronchia, of the alveolar epithelium. The organ is heavier than natural, and darker in tint. It still contains air, and therefore crepitates on pressure although less perfectly than natural; but its substance tears readily, retains the mark of the finger, and on section pours out a reddish, frothy fluid from the divided surfaces.
In the second stage—the stage of red hepatisation—the alveolar epithe lium is swollen and granular. An exudation of the constituents of the blood coagulates in the air-vesicles. The alveoli and small air-passages connected with them are crowded with white and red blood corpuscles, which distend these little cavities and cause complete consolidation of the lung. The affected part, therefore, is airless and can no longer crepitate. It tears with the utmost ease. Its bulk is increased ; it sinks in water ; and on section the surface is dryish and somewhat granular, although pressure causes a thick, turbid fluid to ooze out. The colour is reddish brown, marbled here and there with gray. Usually adjacent pleura is also inflamed. It is opaque and congested, and adhering to it are patches of lymph.
In the third stage—the stage of gray hepatisation—the colour of the dis eased part of the lung becomes grayish or whitish-yellow. White blood corpuscles continue to exude into the air-cells, and there is besides prolif eration of the alveolar epithelium ; so that with the microscope we find epithelial cells, granule cells, and leucocytes. The fibrinous exudation dis
integrates, and the cells quickly undergo fatty degeneration. The organ is still heavy and airless, and is very soft in consistence, so that a little pressure breaks it down. The cut or torn surface is but slightly granular, and on pressure gives out a purifoiln fluid.
These various stages of the disease may usually be seen to occupy different part of the lung at the same time ; for as the disease spreads from one part of the organ to another, it is far more advanced in the part first attacked. The extent of tissue involved is subject to great variety. The affection may be limited to a small patch, or may involve a whole lobe, or even the entire lung. It attacks the base by preference, but is far from uncommon at the apex, especially in the child. Usually the consolidation is confined to one side of the chest ; but double pneumonia is said to be more common in children than in adults.
The process of resolution in the affected part consists in a fatty degen eration and liquefaction of the contents of the alveoli and small air-tubes. Thus softened and liquefied the inflammatory products are readily absorbed or coughed up ; the air-cells are freed; and the circulation through the capillaries ramifying on the alveolar partitions is restored. Resolution is the normal and favourable termination to a croupous pneumonia ; and if the illness be primary is the common ending in the child. In exceptional cases, usually when the disease is secondary, suppuration may occur with the formation of an abscess, or the inflammatory process may pass into gangrene. Still, gangrene is rare as a consequence of pneumonia; and probably never occurs as a result of the uncomplicated disease. It may, however, follow in cases where emboli derived from ante-mortem clotting in the right heart are arrested in the pulmonary capillaries. If Bouillard's statement that a peculiar tendency to the formation of such clots is a common feature of the true pneumonic disease be correct, if is surprising that the gangrenous change is not more often met with. Croupous pneu monia is not a cause of phthisis. A simple unabsorbed consolidation, such as is common after catarrhal inflammation of the lung, rarely if ever results from the croupous form of the disease.