A series of lesions is included here, concerning whose pathology some questions still remain unanswered. Affections of the optic nerve are frequent in consequence of tumours of the brain, and these have partly the characters of a simple oedema and partly those of inflammation. Tumours of the brain produce these changes whatever may be their seat. Inflammations of the membranes affect the optic nerve less frequently. There are also inflammations of the nerve of obscure origin.

Tumours of the brain, and other lesions which lead to increased intracranial pressure, produce oedema of the sheath of the nerve with interstitial inflammation. The oedema of the sheath is accompanied by swelling of the intraocular termination of the nerve from venous hyperemia and oedema. It is this oedema and engorgement which is called Choked disc. It is difficult to explain why tumours of the brain should produce this effect, but it seems likely that by increasing the intracranial pressure they obstruct the passage of the lymphatic fluid in the spaces around the nerve. The retained fluid is perhaps specially irritating, and induces a chronic interstitial inflammation.

Whether originating in tumour of the brain or not, an interstitial inflammation of the optic nerve leads to sclerosis with atrophy of the proper nervous elements.

Atrophy of the optic nerve arises in the manner just mentioned. It may arise also in locomotor ataxia, by a similar process to that occurring in the cord in that disease, namely, a sclerosis characterize.! by increase of the connective tissue and atrophy of the nerve fibres. These changes are seen by the aid of the ophthalmoscope chiefly as an abnormal whiteness of the optic disc.