In cases of septic thrombosis of veins (thrombo-phlebitis) in connection with wounds or abscesses, a septic embolism is liable to occur, in which the lungs are most directly involved. Pieces of the thrombi containing pyogenic microbes (generally staphylococcus pyogenes aureus) are carried to the lung and are caught in small arteries or capillaries.

Each such embolus becomes a centre of acute inflammation, going on usually to suppuration and gangrene. If a considerable branch be obstructed there may be at first the regular haemorrhagic infarction.

But soon there is such an abundant exudation of leucocytes that the red colour is obscured, and the result is an area of grey hepatization. A similar grey hepatization will develop if there has been no haemorrhage. The patch will be wedge-shaped or round according to the vessel affected. The grey hepatization gives place to a purulent infiltration, usually with gangrene of the lung tissue and the regular formation of an abscess.

Such metastatic abscesses are very various in size, form, and number. If the embolism be capillary there may be multitudes of minute abscesses, but very often there are only a few and these may be of some size.

If an abscess be near the surface it gives rise to an acute pleurisy which is apt to be suppurative in character. This may occur without the actual bursting of an abscess into the pleura, the microbes propagating through the inflamed wall into the pleural cavity.