It is not improbable that this development of lymph on the surface of one or both lungs, and the agglutination of the pleural surfaces, serve to prevent the bursting of the secondary pulmonary abscesses into the pleural cavity-which rarely takes place.

The bronchial mucous membrane presents a bright pink colour, while its secretion, clear and frothy, is increased in amount. The same characters are observed when one examines the smaller ramifications of these tubes.

The pulmonary tissue proper is the most frequent seat of secondary purulent deposits. Here are observable all the characteristics shown by those purulent formations; and the pathological changes met with in this tissue have, accordingly, been most minutely examined by the older observers. The first changes which occur, in one or both lungs, are those commonly described by the term pneumonia. The pulmonary capillaries become congested, engorged, causing the so called "consolidation" of the lung tissue. The smaller vessels, trying to overcome this afflux of blood, may produce ecchymoses or extravasations beneath the lining membrane of the air vesicles; but these minute capillary congestions are generally observed as red points studded over the pulmonary surface, which by-and-by exhibit yellowish-white or bluish-white centres. While one part, generally the lower half of the lung, is thus hepatised, solid, and of a dark-greenish colour (see Plate VI.), the remainder of the lung is emphysematous, and more or less oedematous. (See Plate VII.) A section of the former presents the same appearance as is observed in the lungs of pneumonic patients. Whether these incipient abscesses are developed from the minute points of congestion

afore mentioned by the breaking down of some thrombic clot in their centres, or whether the pus is developed out of the serum exuded by the walls of the engorged capillaries, cannot be easily determined, and has as yet not been decided. These secondary abscesses vary in size from that of a hemp seed to that of a hen's egg. Sometimes they are of firm consistence, solid, concrete; but they generally contain pus or serum with caseous lumps, or masses of lymph, and broken down tissue floating in it. The minute points of congestion bear a striking similarity to one another in form and size, and correspond to single lobules. At first of a bright red colour, these incipient abscesses, after the blood has been for some time arrested in the lobule, become of a light-brown colour, then bluish-white, then yellow, and lastly assume a whitish-yellow, glistening appearance, with bulging and evidently purulent centres. They, in fact, pass through the same changes as clots in other tissues. They soften gradually and become diluent in their centres; and, immediately external to their circumference, is a red zone of congested vessels. They contain fatty granules, amorphous débris, disintegrating blood globules, and more or less perfectly formed pus corpuscles. Though this process is confined at first generally to one lobule, it by-and-by extends to the surrounding lobules; and, at last, either abscesses of tolerably firm consistence, and of the size of marbles, become developed, or cavities are formed. This last result was well exemplified in Case I. Chap. II., where, in the centre of the posterior half of the lower lobe of the left lung were two distinct cavities, lined by a very friable curdy-looking membrane, and containing portions of necrosed pulmonary tissue and very foetid pus partially

mixed with clotted blood. These vomice were of the size respectively of a hen's and of a pigeon's egg. Only a very thin film of tissue, apparently in a necrosed condition, separated the larger cavity from the pleural sac; but the cavities were, on their deeper aspect, surrounded by hepatised tissue.

The pus contained in such abscesses varies in quality, from being sanious, greenish, and of very foetid odour, to a glutinous, yellow, thick fluid, having the odour of well-formed pus. It generally contains masses of lymph or disorganised tissue, of either a tolerably tough consistence, or of a diffluent character. "At other times the whole collection has the character of softening tubercle." Further, the more rapidly these abscesses are developed, the better formed is the pus. In chronic cases their contents become by-and-by inspissated, and acquire a firm caseous consistence; or they may even undergo further degenerative processes.

Callander says the left lung is more commonly the seat of these pathological changes than the right one; while Savory holds the very opposite view. Among the thirteen cases, the autopsies of which I have narrated in a former chapter, secondary abscesses existed in the left lung in two instances; in the right lung in one instance ; in both lungs in seven instances; while in two cases (XII. and XIV.) the lungs were diseased, but contained no purulent deposits; in one case the thorax was not examined. These secondary abscesses may be situated either superficially, or they may be imbedded deeply in the pulmonary tissue. They occur most frequently in the lower lobe. Dance drew attention to this feature as distinguishing purulent deposits from tubercles.

These pathological appearances, observed in the lungs