B. _Encapsulation._--This occurs only when pus has been present for some time and when the virulence of the pyogenic organisms is not intense. We may get encapsulation of pus in any part of the body, the most typical illustration naturally being within the bones. Around the purulent focus, as around any other irritating foreign body, the capsule is formed by condensation of surrounding tissue. This is the way in which most cold abscesses with their limiting membranes are produced, those produced by tubercle bacilli having slight irritating properties. Inasmuch, then, as the biological activity in such a focus is small, there is time for such encapsulation; while by the membrane thus formed, or the sanitary cordon, already referred to, protection is afforded to the surrounding tissues. In such a collection fresh infection may incite acute disturbances again, and many abscesses which thus lie latent for a considerable length of time are fanned, as it were, into a conflagration, when a new and acute inflammation is produced. C. _Of the various metamorphoses and chemical changes_ that occur in that which was originally pus, the caseous and the calcific are the most common. These also are connected largely with the tuberculous process, although calcareous particles are found in the pus of actinomycosis. Under their respective heads these degenerations will be more particularly described. Certain names have been given to collections of pus in different localities or under peculiar circumstances. A collection of pus in the anterior chamber of the eye is known as _hypopyon_; when in any preëxisting cavity, it is known as _empyema_ of that cavity, the _distinction between empyema and abscess_ being that “abscess” means a circumscribed collection where previously there was no cavity, while “empyema” implies a normal cavity, without respect to size or location, filled with this abnormal fluid. The term _empyema_, when not used in connection with some particular cavity, is understood to refer to a collection of pus in the pleural cavity. Other names also are used which are particulate and distinctive; in these the prefix _pyo_ is used while the suffix indicates the part involved; thus we have _pyothorax_, _pyopericardium_, _pyarthrosis_, etc. SINUS AND FISTULA.