Catarrhal pneumonia has hitherto received little attention, in consequence of its resemblance to the croupous variety, for which it may easily be mistaken, and on account of its rare occurrence in adult life. It is, however, comparatively common in children, in consequence of the large amount of undeveloped granular texture that is observed in hepatization of their lungs.

It constitutes the first of the series of catarrhal affections to which the respiratory mucous membrane is exposed during childhood, and is succeeded in later years by bronchial and, finally, by tracheal catarrh; in this respect it is the opposite to croup, which begins in childhood as pharyngeal and tracheal croup, and which, in the form of pulmonary croup, terminates the series of croupous inflammations in adults. Catarrhal pneumonia is always lobular, and associated with a catarrhal affection of the bronchial tubes pertaining to the diseased lobules; it is frequently found in the various catarrhal affections of children, especially in pertussis and catarrhus suffocativus. Its usual position is in the superficial lobules, of which a very considerable number are often affected. They present, for the most part, a bluish-red tint, and are dense and somewhat firm; the walls of their air-cells are swollen, till no internal cavity remains, or if the swelling be less considerable, their cavities contain a watery, mucous, and slightly frothy secretion; there is no trace of granular structure. As the lung-substance in the immediate vicinity of the diseased lobules is usually emphysematous and pale, they appear to be a little depressed below the level of the surrounding lung if they are situated near the surface, and they may be further recognized by their dark color.

This disease frequently becomes fatal by the supervention of pulmonary oedema and paralysis, or by the stasis induced in the heart by the emphysema.