larger proportion of cases than glioma, which commonly does not press upon, but invades, the brain substance, and often occupies the invaded tissue almost bulk for bulk.
It has been thought that the rapidity of growth of a tumour influences the occurrence of optic neuritis, but a limited experience of these cases, or a very short search among recorded cases, disposes of the hypothesis, at any rate in an absolute form. Rapidity of growth may be one factor in the production of neuritis, and an important factor in determining the rapidity or slowness of the course of the neuritis, but it certainly does not alone determine its occurrence.
There is, however, one mode in which neuritis is produced which may sometimes be distinctly traced, post-mortem—by the mechanism of meningitis. The disc shown in Pl. III. 3, for instance, was in a case (3) of tumour originating in the pineal body and invading the anterior corpora quadrigemina. The changes in the disc were very gradual in development, and moderate in degree. There was no general meningitis, but the orbital lobules were gently adherent, and fine shreds of lymph were visible on the dura mater after their separation. The optic nerves in front of the commissure were swollen and reddened. Microscopical evidence of neuritis of the nerve trunk was very distinct. In another case (16) of tumour (glioma) of the anterior lobe, in which the neuritis was of the form most characteristic of tumour, greyish-red, with much swelling, the microscopic changes in the nerve trunk, most intense behind the foramen, indicated a communicated descending neuritis, and old adhesions over the tumour showed that there had been local meningitis. It must be remembered that in such cases, whatever mechanism leads to the occurrence of neuritis without meningitis, may influence the degree and course of that which is set up by meningitis.
In most cases optic neuritis is a transient event in the history of a cerebral tumour, not a constantly-associated condition. A tumour may exist and cause symptoms for a considerable time without leading to any change in the eyes, and then optic neuritis may be rapidly developed, run its course, and
