At first the swelling is covered by epithelium; in time caseation takes place, the epithelium gives way, and an open sore is formed.    From Wordnik.com.  [Manual of Surgery Volume Second: Extremities—Head—Neck. Sixth Edition.] Reference

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.    From Wordnik.com.  [Manual of Surgery Volume Second: Extremities—Head—Neck. Sixth Edition.] Reference