layer
of the periosteum on the anterior aspect of the vertebrae, and extends
along the surface of the bones, causing widespread superficial caries.
It may attack the discs at their margins, and spread inwards between
the discs and the contiguous vertebrae. Owing to the comparatively
wide area of the spine implicated, this form of the disease is not
attended with angular deformity, but rather with a wide backward
curvature which corresponds in extent to the number of vertebrae
affected. The accumulation of tuberculous pus under the periosteum and
anterior longitudinal ligament is the first stage in the formation of
the large abscesses with which this form of spinal tuberculosis is so
commonly associated.

_Effects on the Spinal Cord and Nerve Roots._--In some cases the cord
and nerve roots are pressed upon by an oedematous swelling of the
membranes; in others, the tuberculous process attacks the dura mater
and gives rise to the formation of granulation tissue on its outer
aspect--_tuberculous pachymeningitis_. Less frequently a collection of
pus forms between the bone and the dura, and presses the cord back
against the laminae. The cord is rarely subjected to pressure as a
result of curving of the spine alone, but occasionally, especially in
the cervical region, a sequestrum becomes displaced backward and
exerts pressure on it, and it sometimes happens, also in the cervical
region, that the cord is nipped by sudden displacement of diseased
vertebrae--a condition comparable to a fracture-dislocation of the
spine.

The severity of the symptoms is aggravated by the occurrence of
inflammation of the cord--_myelitis_--which is not due to tuberculous
disease, but to interference with its blood-supply from the associated
meningitis.

_Repair._--When the progress of the disease is arrested, the natural
cure of the condition is brought about by the bodies of the affected
vertebrae becoming fused by osseous ankylosis (Fig. 211). While this
reparative process is progressing, the cicatricial contraction renders
the angular deformity more acute, and it may go on increasing until
the bones are completely ankylosed; this reparative process can be
followed in successive skiagrams. An increase in the projection in the
back, therefore, is not necessarily an unfavourable symptom, although,
of course, it is undesirable.

[Illustration: FIG. 211.--Osseous Ankylosis of Bodies (_a_) of Dorsal
Vertebrae, (_b_) of Lumbar Vertebrae followin