The product of all acute suppurative lesions is _pus_. This is an opaque fluid of creamy consistence and whitish or grayish appearance, varying in density, met with in amounts from a minute drop to half a gallon or more. Under ordinary circumstances it is odorless, and its reaction, either acid or alkaline, is very faint. It is, like the blood, composed of a fluid and a solid portion. The solid portion consists of so-called _pus corpuscles_ and other debris of tissue, which vary with the site of the disease and the parts involved. The source of the pus corpuscles has been cited and the statement made that they are in effect the bodies of phagocytes which have perished in the biochemical fight for existence of the parent organism. Cocci or bacilli are found in pus corpuscles and also in the surrounding fluid. Pus should be without odor, but under certain circumstances it possesses an odor which will vary in character according to the source of the pus or the nature of its principal bacterial excitant. Pus from the upper end of the alimentary canal frequently has the sour smell of gastric contents; that from the neighborhood of the lower end, the fetid odor which is for the most part due to the action of the colon bacillus. Inasmuch as colon bacilli are found in widely distant parts of the body, they may also give an unpleasant odor to pus even from a brain abscess. When the pus has become contaminated with the ordinary saprophytic organisms, it may smell like any other decomposing material. The older writers called it _ichorous pus_, while _sanious pus_ was supposed to be that more or less mixed with blood, undergoing ammoniacal decomposition or else strongly acid. Pus sometimes has a well-marked _blue_ or _bluish-green tint_. This is due to the presence of the _bacillus pyocyaneus_, already described. An orange tint is sometimes given by the presence of hematoidin crystals, due to the original hemorrhagic character of the infected exudate. The former appearance indicates usually a slow course to the suppurative lesion, while the latter has been regarded by some as affording an unfavorable prognosis. Distinctly red pus, whose tint is due to the presence of a bacillus giving bright-red cultures on blood serum, has been noted in other instances. This can readily be distinguished from blood, because upon dressings it does not change color.