#Pus.#--The fluid resulting from the process of suppuration is known as _pus_. In its typical form it is a yellowish creamy substance, of alkaline reaction, with a specific gravity of about 1030, and it has a peculiar mawkish odour. If allowed to stand in a test-tube it does not coagulate, but separates into two layers: the upper, transparent, straw-coloured fluid, the _liquor puris_ or pus serum, closely resembling blood serum in its composition, but containing less protein and more cholestrol; it also contains leucin, tyrosin, and certain albumoses which prevent coagulation. The layer at the bottom of the tube consists for the most part of polymorph leucocytes, and proliferated connective tissue and endothelial cells (_pus corpuscles_). Other forms of leucocytes may be present, especially in long-standing suppurations; and there are usually some red corpuscles, dead bacteria, fat cells and shreds of tissue, cholestrol crystals, and other detritus in the deposit. If a film of fresh pus is examined under the microscope, the pus cells are seen to have a well-defined rounded outline, and to contain a finely granular protoplasm and a multi-partite nucleus; if still warm, the cells may exhibit amœboid movement. In stained films the nuclei take the stain well. In older pus cells the outline is irregular, the protoplasm coarsely granular, and the nuclei disintegrated, no longer taking the stain. _Variations from Typical Pus._--Pus from old-standing sinuses is often watery in consistence (ichorous), with few cells. Where the granulations are vascular and bleed easily, it becomes sanious from admixture with red corpuscles; while, if a blood-clot be broken down and the debris mixed with the pus, it contains granules of blood pigment and is said to be "grumous." The _odour_ of pus varies with the different bacteria producing it. Pus due to ordinary pyogenic cocci has a mawkish odour; when putrefactive organisms are present it has a putrid odour; when it forms in the vicinity of the intestinal canal it usually contains the bacillus coli communis and has a fæcal odour. The _colour_ of pus also varies: when due to one or other of the varieties of the bacillus pyocyaneus, it is usually of a blue or green colour; when mixed with bile derivatives or altered blood pigment, it may be of a bright orange colour. In wounds inflicted with rough iron implements from which rust is deposited, the pus often presents the same colour. The pus may form and collect within a circumscribed area, constituting a localised _abscess_; or it may infiltrate the tissues over a wide area--_diffuse suppuration_. ACUTE CIRCUMSCRIBED ABSCESS