CEREBRAL LOCALIZATION 235 



no means proves that the nerve itself or its nucleus is 

 involved in the lesion. It has been accounted for by 

 stretching, due to a supposed backward displacement 

 of the whole brain late in the development of a 

 growth ; the abducent nerves run straight forwards 

 and are slender, so the first sign of the displacement 

 is a convergent squint. 



Other cranial nerves, including the third, fifth, 

 seventh, and eighth, are occasionally affected by dis- 

 placements of the brain or by pressure. 



2. Localized or General Convulsions. — Mis- 

 takes are particularly apt to arise if the fit starts in 

 some definite area, follows a slow and orderly march 

 to other areas, and perhaps affects only one side, 

 consciousness being lost late if at all (Jacksonian 

 epilepsy). It must, however, be remembered that 

 all this may occur without any obvious cortical 

 lesion ; indeed, the commonest cause of a localized 

 convulsion is ordinary idiopathic epilepsy. 



Again, localized or general convulsions may give a 

 wrong impression when arising late in the course of 

 an intracranial tumour or abscess, especially if it 

 presses on the ventricular system of the brain and 

 dams back the cerebrospinal fluid, causing hydro- 

 cephalus. The accumulation of fluid in one or both 

 lateral ventricles stretches the overlying cortex, and 

 may give rise to fits, sometimes of a Jacksonian 

 type. 



3. Bilateral Spastic Paresis. — In many cases a 

 hint is given of the true nature of these seizures by 

 the presence of a slight degree of bilateral spastic 

 paresis, with clumsiness of movement, exaggerated 



