270 DISEASES OF THE BRAIN. 



pression of the cavernous sinus and obstructed escape of blood from 

 the veins of the eye. Tumors in the posterior cranial fossa only have 

 this effect after they have caused abundant effusions in the ventricles, 

 by compression of the openings of the venae Galeni into the straight 

 sinus. Since tumors, which destroy the optic nerve, very rarely spare 

 the oculo-motor, I consider the presence or absence of disturbances 

 of mobility of the eye as an almost certain means of distinguishing 

 whether the amaurosis, caused by cerebral tumor, is due to destruction 

 of the tractus opticus, chiasm, optic nerves, or to interference with the 

 intraocular circulation. Ophthalmoscopy also gives important informa- 

 tion on this point. The changes of the optic nerve, in blindness from 

 cerebral tumors, as revealed by the ophthalmoscope, and the conclu- 

 sions that may be deduced from them, are about as follows, according 

 to Graefe: 



1. Simple swelling of the optic papilla, with great tortuosity of the 

 vena centralis. It is indicative of obstructed venous circulation, and, 

 in its purer forms, is most frequently seen with tumors that encroach 

 on the cavity. 



2. Slight inflammatory swelling of the optic papilla, with less dis- 

 tinct venous hyperaemia and inflammatory participation of the retina. 

 It occurs as a result of an inflammation of the perineurium, and of the 

 optic nerve itself, which extends to the retina (neuritis descendens), 

 and accompanies those intracranial processes, which, from their ana- 

 tomical character or location, are suited to induce irritation in the parts 

 above named. 



3. Atrophy of the optic nerve. This may occur primarily, from 

 direct lesion of the optic nerve by a tumor, meningitis, etc., or second- 

 arily, as a final result of either process. 



The partial disturbances of sensibility and motility in the parts 

 supplied by the cerebro-spinal nerves, that occur in cerebral tumors, do 

 not differ from those accompanying other local affections of the brain. 

 Neuralgic pains, or indefinite sensations of prickling, formication, furri- 

 ness, etc., as well as complete anaesthesia, occur, both over large sur- 

 faces and limited to very narrow bounds. The same is true of mus- 

 cular twitchings, of contractions, and paralyses, although, among the 

 latter, hemiplegia far exceeds the other paralyses. 



If there be morbid symptoms in the parts supplied by the cerebro- 

 spinal and cerebral nerves at the same time, they almost unexception- 

 ally show the peculiarity of being on opposite sides of the body. 

 This " alternation " is simply because the paralysis, anaesthesia, etc., 

 of the cerebro-spinal nerves are caused by a lesion of the nerve-fila- 

 ments before they cross, while those of the cerebral nerves depend on 

 destruction of nerve-filaments that have already crossed. 



