In many constitutions, the slightest incited action of the vessels is followed by the formation of pus, and the appearance of a depôt of purulent matter is often the first indication that such action has existed; but in the majority of instances, the deposition of pus is preceded by the usual characters of well-marked inflammatory action. Suppuration occasionally occurs without previous solution of continuity; for pus is frequently contained in the serous and mucous cavities, when no breach of continuity can be discovered, at least we find a fluid not distinguishable from purulent matter; it may be a vitiated secretion, but still it presents the usual characters of pus. But it occurs, generally, when there has been a previous læsion of structure, and in this case its progress is most distinctly marked. In exposed cellular texture, for example, particles of blood are effused; the serum is afterwards absorbed, and the lymph remains; this latter gives transmission to minute vessels which deposit the purulent fluid, whilst others secrete particles of organised matter to form granulations, in order to repair the loss of substance. This process is often unattended with any great degree of constitutional disturbance, because the fibrin effused sets a bound to the pus, and is the provision against its being mixed in large quantities with the blood. In healthy suppuration, the separation of fibrin and pus from the blood in this way seems to have direct relation to each other; and in unhealthy inflammation, when this does not take place, the consequences are mixture of pus with the blood as formerly noticed. In the latter form of suppuration the fibrin, instead of being assimilated to the contiguous mass, is mixed with the pus; hence the proneness to putrefaction of such discharge, and its disposition to coagulate spontaneously when evacuated. This kind of suppuration, being matured generally without thickening of parts, has been sometimes pointed out as suppuration independently of previous inflammatory action. After purulent matter has begun to accumulate under the surface, the pressure thereby occasioned produces condensation of the neighbouring cellular tissue, which, along with the previously effused lymph, forms the parietes of the abscess; and in proportion as the matter accumulates, the cavity enlarges by the successive processes of ulceration of portions of its parietes, by continued effusion of lymph, and by farther condensation of the surrounding parts. Thinning of the parietes takes place by ulcerative absorption, particularly towards the surface—or, if that be difficult, towards a mucous outlet—as is exemplified by the bursting into the bowels or bronchiæ of abscess of the liver.