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When these tubercles are sufficiently numerous to become confluent, large masses may be formed, which undergo the same retrogressive changes of caseation and calcification.

At first the swelling is covered by epithelium; in time caseation takes place, the epithelium gives way, and an open sore is formed.

The disease commences as a tuberculous infiltration of the marrow, which results in softening of the bodies of the affected vertebræ, particularly in their anterior parts, and, as the disease progresses, caseation and suppuration ensue, and the destructive process spreads to the adjacent intervertebral discs.

Under other circumstances, the tuberculous tissue that has undergone caseation, or even calcification, is only encapsulated by the new fibrous tissue, like a foreign body.

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.