Suppuration is a morbid process which gives rise to the formation of Pus (q.v.), which, as is well known, is one of the commonest products of inflammation. The fluid portion of pus is agreed by all to be derived mainly from the liquor sanguinis; but with regard to the origin of the pus-corpuscles there has been a singular fluctuation of opinion. Before 1850 it was generally held that they developed in the fluid exudation of an inflammation by aggregation or growth of granules contained in it. This doctrine was replaced by that of Virchow (1858), who believed that they resulted from rapid multiplication of the cells of the irritated tissue. In 1867 Cohnheim, repeating accurate but neglected observations made in England more than twenty years before, showed that during Inflammation (q.v.) white blood-corpuscles escape from the capillaries, and make their way through the tissues; and he maintained that these, and not fixed tissue-cells, give origin to pus-corpuscles. At present Cohnheim's view is generally regarded as the true explanation in the majority of instances; but it is maintained by some pathologists that at least in some cases many of the corpuscles arise from multiplication and alteration of the cells of the inflamed tissue, an opinion which has not yet been altogether disproved.
With regard to the causes which lead to suppuration there has been almost as great a change of current opinion during the same period. It used to be regarded as the natural result of any kind of severe irritation, and a necessary incident in the healing of the great majority of wounds. But when Lister demonstrated the possibility of preventing it in many cases by Antiseptic (q.v.) methods, and when micrococci were found in the pus of many abscesses, even where there had been no visible breach of surface, it became clear that microscopic organisms play an important part in the process. The experiments of some observers indeed led them to conclude that though some of the results of inflammation may be manifested, suppuration cannot take place without their presence and activity. It has been shown, however, (1) that dead micro-organisms can excite suppuration; (2) that a fluid in which they have grown, even when completely freed from solid particles, can do so — i.e. that their influence depends at least in part not on the organisms themselves, but on chemical products of their growth; (3) that certain chemical irritants (e.g. mercury, turpentine, croton-oil) experimentally introduced into the tissues of animals with the most complete antiseptic precautions do lead to suppuration. Such conditions, however, can hardly occur except as the result of a carefully planned experiment; and in cases coming under the care of the surgeon it may be assumed that where suppuration is present it is due to micro-organisms.
Suppuration must thus be regarded as one phase of that effort of the organism to resist the causes of disease which takes so prominent a place in pathology at the present day. But though essentially a defensive process, it very frequently becomes harmful, and leads to serious results. If suppuration takes place beneath a surface which does not participate in the morbid change, or which is capable of resisting it for a time, an abscess is formed; when pus-cells are poured forth from an exposed surface we have an ulcer.
For some of the controverted points discussed above, see Leber, On the Origin of Inflammation, &c. (Leip. 1891), and Metschnikoff, On the Comparative Pathology of Inflammation (Paris, 1892).