We have just seen that this condition may follow oil an acute myelitis, but the inflammation may be chronic from the first, when it is produced by an irritant which acts gradually. It is most frequently the result of Chronic compression of the cord, as in the case of curvature, or pressure by a tumour. It may also be propagated from the membranes, a spinal meningitis passing into a myelitis. We have already seen that it may arise by extension from a peripheral nerve, the inflammation travelling upwards to the cord. It occurs also from exposure to cold, from strains, etc., as in the case of acute myelitis.

Fig. 313. - Extensive grey degeneration in the cord immediately above a transverse myelitis. The lesion affects the posterior and posterolateral columns.

The changes produced are those common to chronic inflammations. There is new-formation of a dense connective tissue, developing in the usual way from granulation tissue. The new-formed tissue causes compression of the proper nervous elements and their atrophy. In the white substance the nerve fibres disappear, and in the grey substance the ganglion cells. With the growth of this dense tissue there is an induration of the cord, and the term Sclerosis is applicable. In the white substance this is manifested by the usual appearances of grey degeneration. There is also a degree of shrinking of the parts, and this is often especially manifest in the grey substance.

As the nervous structures are partially or completely destroyed there is a greater or less interruption of the cord, and this results in the usual secondary degenerations above and below the seat of the lesion.