the kidney, the ureter, or the bladder is the seat of stone, they give
rise to suppuration. Similarly, a calculus in one of the salivary ducts
frequently results in an abscess forming in the floor of the mouth. When
the lumen of a tubular organ, such as the appendix or the Fallopian tube
is blocked also, the action of pyogenic organisms is favoured and
suppuration ensues.

#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