In its widest acceptation, inflammation of the spinal cord, or myelitis, includes a large number of widely-different conditions, each of which will be considered separately. The cases may for convenience be divided into two groups. In one of these the affection has a limited longitudinal extension, but involves the cord rather in its thickness. For cases of this class it is customary to use the expression Transverse myelitis in order to distinguish them from those in which a considerable length of cord is attacked, while the inflammation has a limited transverse extension. The former are usually due to a definite cause which acts on the 'cord at a particular level. In the second group the inflammation follows a particular physiological system in the cord, as for instance a tract of white substance, and extends in it for some distance from above downwards; hence the name Systematic inflammations applied to this group. Such inflammations may involve the white substance or the grey substance, and they are often distinguished by the names Leucomyelitis ( = white) and Poliomyelitis ( = grey). These inflammations have for the most part no assignable cause, and are sometimes called spontaneous. In some of them the process shows, perhaps, more of a degenerative than an inflammatory character. Besides these there is a form of myelitis in which the lesions are somewhat irregularly diffused over different parts of the brain and cord.