Pus is a well-known product of inflammation, and occurs as a thick yellow creamy fluid, differing from all other morbid exudations in containing a large number of corpuscles, having a soft and fatty feeling when rubbed between the fingers, a peculiar odour, usually an alkaline reaction, and a specific gravity of about 1.032. Like the blood, it consists of certain definite microscopic elements, and of an intercellular fluid or serum in which they swim.
Of microscopic elements we have (1) the pus-corpuscles, which, both in their microscopical and chemical relations, seem to be identical with the lymph-corpuscles, or colourless blood-cells; in diameter they range from .004 to .005 of a line, and each corpusele consists of a cell-wall, which often appears granular, of viscid transparent contents, and of one or more nuclei, which can be rendered much more apparent by the addition of acetic acid. The other elements are (2) molecular granules and (3) fat-globules. The serum of pus is perfectly clear, of a slightly yellow colour, closely resembling blood-serum, and coagulates on heating into a thick white mass.
The chemical constituents of pus are water (varying from 769 to 907 in 1000 parts), albumen (from 44 to 180), fats (from 9 to 25), extractive matter (from 19 to 29), and inorganic salts (from 6 to 13), in addition to which mucin, pyin, glycine, urea, &c. are occasionally present. Of the inorganic or mineral constituents the soluble salts are to the insoluble in the ratio of 8 to 1, and the chloride of sodium (the chief of the soluble salts) is three times as abundant as in the serum of the blood. The mode of formation of pus is described in the article SUPPURATION.