Under this head we must place processes allied to each other, since they originate in one general disease, but differ extremely in their local morbid centres. This difference exhibits itself anatomically in the physical qualities of the inflammatory products on the free surfaces of mucous membranes, and in the condition of the mucous membrane itself, and of the submucous areolar tissue. These processes, especially in true croup, are primary and independent; or in their collective forms they may be secondary affections - the evidence of a degenerated acute or chronic disease.

True croup, the exudative process yielding a plastic, fibrinous product, claims our first attention. We scarcely ever observe the primary, genuine croupous process to occur anywhere except on the mucous membrane of the air-passages, where it appears as laryngeal, tracheal, or bronchial croup, ordinarily known as laryngitis, tracheitis, bronchitis polyposa seu membranacea. It not unfrequently extends over the whole of the air-passages, from the epiglottis to the minute ramifications of the bronchial tubes; and often affects the throat and pharynx, and sometimes even the oesophagus. It either attacks extensive continuous tracts of mucous membrane, or confines its ravages to isolated patches, and hence the exudation or croup-membrane either presents the appearance of continuous, tubular, arborescent coagula, corresponding with the division of the trachea and the bronchial ramifications, or of irregular patches, as is most commonly observed on the larynx. In bronchial croup, the tubular exudations from the larger bronchi present a calibre inversely proportional to their thickness, and those thrown off from the finer ramifications occur as solid cylinders.

The exudations present great differences in thickness and consistence; the membrane sometimes resembling an investment of hoar-frost, or gauze, whilst at other times it will even exceed a line in thickness, while the consistence may vary from that of viscid cream to that of the most compact tough, leathery coagulated fibrin. But neither the density nor the consistence is generally uniform throughout; the exudation, as a general rule, becomes thinner and gradually softer towards its edges, more puriform or creamy, and the portion in contact with the mucous membrane is the softer and looser of the two.

In color they are yellowish-white, or gray, and not unfrequently have a greenish tint; they either adhere firmly to the mucous membrane, or hang loosely on it, the latter being the case when a viscid secretion occurs between the false membrane and the mucous surface. The surface next to the mucous membrane is frequently marked with red streaks and dots, consisting in part of blood adhering to the surface, and in part, as found on closer examination, of straight or tortuous vessels, or of small, roundish extravasations, from which currents of blood are seen to emerge in an arborescent and radiating form. The appearance of the subjacent mucous membrane is liable to considerable differences; its red color sometimes assumes a very dark, almost brown tint, but more frequently a bright erysipelatous hue; and again it occasionally, but very rarely, happens that all the signs of injection are absent; it presents an appearance of sores, as if it were excoriated, bleeds from numerous, minute, scattered spots, and presents various degrees of swelling. The swelling is, however, sometimes so very trifling as hardly to attract notice. The submucous areolar tissue is most commonly, if not always, the seat of serous infiltration.

Genuine croup of the air-passages is essentially a disease of childhood; it rarely, however, occurs before the end of the second year, and the parts it most commonly attacks are the larynx and trachea; in adults, bronchial croup is the most common variety, and during the age of puberty and early manhood, it is often associated with pneumonia. Croup of the final ramifications of the bronchi occurs simultaneously with pneumonia, and usually runs an acute course; sometimes, however, it assumes a chronic form, the process continuing with less intensity for a longer period, with occasional exacerbations, which give rise to the deposition of fresh products. In many persons it becomes habitual, and often, in the form of bronchial croup, seems to assume a certain degree of periodicity in its attacks. It is frequently combined with pneumonia, pleurisy, and pericarditis, and sometimes with meningitis, and acute and chronic hydrocephalus; and it occasionally extends to the stomach and degenerates into acute softening of that organ. It proves fatal from the contraction which it induces in the air-passages through exudation, and still more through the swelling of the mucous membrane over the subjacent areolar tissue, and from spasmodic closure of the glottis; moreover suffocation is frequently induced by 'pulmonary oedema, or the patient may occasionally sink from the exhaustion induced by very abundant exudation. We have no anatomical evidence that the pneumogastric nerve is seriously affected.

The other exudative processes yield a soft, purulent, and less plastic exudation, or a thin, sero-purulent, gelatinous, discolored ichor, which attenuates, and finally dissolves the mucous membrane. The submucous areolar tissue is infiltrated by a matter of a similar character, and its texture is rendered friable, lacerable, and fusible. These are, in most cases, secondary processes, depending on the localization of a degenerated general disease of an acute exanthematous nature, - as variola or scarlatina.

All the exudative processes on the mucous membrane of the air-passages are frequently combined with similar processes on other mucous or serous membranes; they may degenerate into gangrene and acute softening, and from the development of the spleen, lymphatic glands, and follicular apparatus of the intestinal mucous membrane in these cases, we conjecture that they originate in a disease of dyscrasia of the whole mass of the lymph and blood.

Here we must also notice aphthae of the air-passages; they are for the most part confined to the larynx, trachea, and the great bronchial trunks, very seldom extending to the throat; they scarcely ever occurs as a primary affection, but are most commonly associated with tuberculous phthisis of the larynx and the lungs.