Pus consists of a fluid portion, the liquor puris, and of pus corpuscles, which are identical with ordinary leucocytes.

It is customary to speak of all the leucocytes found exuded in acute inflammations as Pus corpuscles. This is scarcely a correct method of speech. There is no doubt that pus corpuscles are essentially exuded leucocytes, but as pus is a fluid its existence implies an extraordinarily abundant exudation of leucocytes, and if present in the tissues, a corresponding disintegration of tissue. In ordinary non-suppurative inflammations leucocytes are usually present, but as there is nothing approaching to pus, it is more correct to speak of leucocytes rather than of pus corpuscles.

Pus is met with under a variety of circumstances, but in nearly all cases it is produced by the indirect action of microbes belonging to the class of micrococci. The purulent exudation frequently follows the fibrinous exudation, especially in serous cavities, and it is not uncommon to find an intermediate condition in which the fibrine is, as it were, infiltrated with pus. This implies that the cause of the inflammation (micrococci) continues to act intensely. The emigration of leucocytes goes on vigorously, and these, crowding into the fibrinous coagulum, the latter disintegrates and liquefies, the liquefaction of the fibrine being perhaps equivalent to the necrosis of tissue which is implied in the formation of abscess, and being a specific effect of the toxine.

Many experiments have been made in order to determine whether suppuration can be produced without the agency of microbes. From some of these, such as those of Grawitz and De Bary, it appears that various substances, among them turpentine, may induce suppuration, and that cadaverin, obtained from decomposing matters, but free from microbes, may do so. Koch has pointed out also that tuberculin obtained from cultures of the bacillus of tubercle, although free from microbes, is capable of producing suppuration. We may infer that while suppuration may be produced by different active agents, yet in actual cases micrococci are always present. We may also infer that it is by evolving irritating chemical principles (toxines) that they produce this effect.

It is not easy to understand why pus does not coagulate. There are abundant leucocytes, and there is the fluid presumably containing the fibrinogen. A few pus corpuscles added to fresh liquor sanguinis induce coagulation. Its absence in pus has been ascribed to the existence of some chemical agent evolved by the mici'o-organisms which inhibits the process of coagulation, or to a conversion of the fibrinogen into peptones, which latter are known to be present in pus.

In the case of serous membranes the occurrence of suppuration implies a very intense inflammation, but in the case of mucous.membranes pus is often produced when the inflammation is comparatively trivial. The ordinary catarrh of the nares and bronchi is first accompanied by a serous exudation, and this often gives place to an exudation of pus.

Purulent infiltration is a term applied when the suppuration occurs not at the surface but in the midst of the tissue, and the pus fills up the spaces, the condition being comparable, in respect of the locality of the exudation, with oedema. Purulent infiltration implies an intense inflammation, and it is frequently associated with necrosis.

Abscess frequently follows purulent infiltration. The name abscess or Apostema is applied to a collection of pus in a cavity generally formed to accommodate it, although it is sometimes used in cases where the pus has accumulated in a preexisting cavity. Purulent infiltration usually precedes, and the abscess is formed by the necrosis or liquefaction of the tissue, so that a cavity results. Hence abscesses often contain shreds of tissue or sloughs.

The abscess has often a distinct membrane forming its boundaries. This membrane is composed of granulation tissue, and it has often been regarded in the light of a secreting surface by which the pus is produced; hence the name Pyogenic membrane applied to it. The membrane, however, is by no means necessary to the formation of pus, and it is, in fact, a secondary product after the abscess has actually formed. Leucocytes penetrate the membrane passing from its vessels and from those of the surrounding tissue, and the abscess may enlarge. But the pyogenic membrane really limits the enlargement, as granulation tissue is less sensitive to irritants than other tissues, and is to be regarded as really a protective layer.

As abscesses contain the agents of inflammation which have produced the suppuration, they usually enlarge. This mostly occurs in the direction of least resistance, but gravitation often plays a part in the advance of an abscess. There is a gradual liquefaction of the tissue before the advancing pus, and this goes on, as a rule, till a surface is reached and the pus discharged.

A cold abscess is not really a collection of pus, but only of matter resembling pus. This is usually softened caseous matter, produced in connection with tuberculosis of bone, but pus corpuscles are often mixed with this debris. Cold abscesses frequently extend long distances, assisted by gravitation, before they reach the surface. Their opening, by admitting septic micrococci, may lead to acute suppuration within the cavity.

Pus, whether in abscesses or elsewhere, frequently undergoes changes. The pus corpuscles often undergo fatty degeneration, or they may swell up and disintegrate. The pus may in this way become absorbed. It sometimes, by absorption of its fluid, thickens into a pasty matter, in which lime salts may be deposited. Again, pus, or indeed a serous exudation, may be mixed with other matters, the secretion of glands, such as mucus, urine, or bile, or with oil, where suppuration occurs in a tissue rich in fat.

Croupous And Diphtheritic Exudations

These terms have got into common use, but their employment is not to be recommended.

The croupous exudation is that which occurs in the larynx in Croup, a disease of this structure. (The term croup is a Scotch word meaning the hoarse croaking sound of the cough and speech in certain laryngeal affections. It is nearly equivalent to the English word croak. It is by an extraordinary philological development that the terms croup and croupous have come to be used to designate the solid fibrinous exudation of disease.) The exudation in the larynx in croup is a whitish layer resembling fibrine, and by many regarded as a fibrinous exudation. Wagner believed that it arose by transformation of the epithelium and Weigert has asserted that it is due to a coagulation-necrosis of the epithelium.

The diphtheritic exudation implies a necrosis not only of the epithelium but of the mucous membrane. A fibrinous exudation occupies the meshes of the necrosed tissue as well as its surface, and it is consequently adherent to and involved in the membrane.

These terms, although originally applied to croup and diphtheria, are now frequently used in a purely anatomical sense of exudations on mucous membranes. A croupous exudation is one which lies on the surface and is not associated with necrosis of the mucous membrane, while a diphtheritic exudation is both on the surface and in the substance of the membrane and implies necrosis.