Pus is a secretion furnished by the blood at the expense of the tissues, an exuberant quantity of plastic material, for which excess there is no demand in the reparative process. The leucocytes which are brought to the surface of a granulating wound are discharged from it in the form of pus, while those that remain behind develop into tissue. This effect, by which the excess of leucocytes is gotten rid of, is accomplished by liquid exudation, the supply of which comes from the newly-formed and the old capillaries, and is the same force that carries the nutritive material to the tissues. Healthy pus is of a yellowish-white color, sometimes assuming a pale greenish tint, of cream-like consistence; a slightly saltish taste, but somewhat sweet; a faint animal odor, with an alkaline reaction. The presence of bile may give to pus a deep orange color, while all of the other shades are due to the coloring matter of the blood, known as haematoidin. As long as the air has access to it, there is little tendency to putrefaction; and even when it is removed from the body and exposed to ordinary temperature, change in it occurs very slowly. Pus consists of two portions - a solid portion, know as pus corpuscles, and which consists almost entirely of young pus cells or leucocytes, and a liquid portion known as liquor puris, which is a serous fluid, and constitutes about three-fourths of its bulk.

When pus is subjected to pressure, as sometimes occurs in abscess of the antrum, and about bones, it may become a yellowish, cheesy mass, owing to the compression of the pus cells. The solid portion of pus consists of more than nine-tenths of leucocytes or young pus cells, which, in freshly formed pus, presents under the microscope a granular appearance, and also the peculiar movements of active, young and healthy leucocytes; but pus which has collected in an abscess for several days shows no such movements, thereby indicating that the leucocytes have died. Living and dead pus cells may be found in ordinary pus, mingled together. The most common forms of micro-organisms which produce suppuration are the staphylococci and the streptococci.