The processes of chronic inflammation are similar to those in the spinal cord, and the result here, as there, is Sclerosis. In the case of the brain there is hardly anything of the systematic sclerosis so frequent in the cord. There is, however, one form of disease which affects both brain and spinal cord, and which presents some analogies with the systematic scleroses of the cord.
The causation of this disease is exceedingly obscure. Marie is of opinion that in the vast majority of cases the cause is some form of infection. The frequency with which the disease follows some of the acute infectious diseases, especially typhoid fever, small-pox, and scarlatina, is in favour of this view. In some cases a history of traumatism is obtainable, but in many no definite cause can be assigned. Syphilis does not seem to play any causative r61e. It mostly comes on without any definite cause being assignable. It is characterized by the occurrence of patches of chronic inflammation or degeneration scattered over the cord, or the brain, or both. These patches have all the characters of sclerosis; in the white substance, where they chiefly occur, they are grey, and in both white and grey substance they produce induration and shrinking. Under the microscope the sclerosed white substance shows loss of the medullary sheaths of the fibres, while the axis cylinders, except in advanced stages, are largely retained. In the grey substance, the ganglion cells are shrunken, sometimes with pigmentation. At the peripheral parts of the patch there may be evidences of more active inflammation, in the form of abundant round cells along with compound granular corpuscles.
The distribution of the patches is somewhat irregular. They are found of various sizes in the cord, medulla oblongata, pons, peduncles, corona radiata, cerebral convolutions, and cerebellum. In such different situations the sclerosis leads to very various functional disturbances. The great frequency of motor tremors, however, suggests some tendency to localization. Erb has pointed out that when tremors exist, patches are specially present on the peduncular parts (pons, medulla, peduncles). If patches are seated there, the motor impulses will be imperfectly conducted but not absolutely interrupted, as the axis cylinders are preserved.