A tuberculous abscess results from the disintegration and liquefaction of tuberculous granulation tissue which has undergone caseation.  Fluid and cells from the adjacent blood vessels exude into the cavity, and lead to variations in the character of its contents.  In some cases the contents consist of a clear amber-coloured fluid, in which are suspended fragments of caseated tissue; in others, of a white material like cream-cheese.  From the addition of a sufficient number of leucocytes, the contents may resemble the pus of an ordinary abscess.

The wall of the abscess is lined with tuberculous granulation tissue, the inner layers of which are undergoing caseation and disintegration, and present a shreddy appearance; the outer layers consist of tuberculous tissue which has not yet undergone caseation.  The abscess tends to increase in size by progressive liquefaction of the inner layers, caseation of the outer layers, and the further invasion of the surrounding tissues by tubercle bacilli.  In this way a tuberculous abscess is capable of indefinite extension and increase in size until it reaches a free surface and ruptures externally.  The direction in which it spreads is influenced by the anatomical arrangement of the tissues, and possibly to some extent by gravity, and the abscess may reach the surface at a considerable distance from its seat of origin.  The best illustration of this is seen in the psoas abscess, which may originate in the dorsal vertebrae, extend downwards within the sheath of the psoas muscle, and finally appear in the thigh.

#Clinical Features.#—­The insidious development of the tuberculous abscess is one of its characteristic features.  The swelling may attain a considerable size without the patient being aware of its existence, and, as a matter of fact, it is often discovered accidentally.  The absence of toxaemia is to be associated with the incapacity of the wall of the abscess to permit of absorption; this is shown also by the fact that when even a large quantity of iodoform is inserted into the cavity of the abscess, there are no symptoms of poisoning.  The abscess varies in size from a small cherry to a cavity containing several pints of pus.  Its shape also varies; it is usually that of a flattened sphere, but it may present pockets or burrows running in various directions.  Sometimes it is hour-glass or dumb-bell shaped, as is well illustrated in the region of the groin in disease of the spine or pelvis, where there may be a large sac occupying the venter ilii, and a smaller one in the thigh, the two communicating by a narrow channel under Poupart’s ligament.  By pressing with the fingers the pus may be displaced from one compartment to the other.  The usual course of events is that the abscess progresses slowly, and finally reaches a free surface—­generally the skin.  As it does so there may be some pain, redness, and local elevation of temperature.  Fluctuation becomes evident and superficial, and the