CEEEBEAL TUMOUES. 185 



nerve which lies contiguous to facial trunk in the membranes 

 and internal auditory meatus. 



The spinal accessory nerve may be paralysed by tumours in 

 the medulla and its vicinity. Isolated paralysis of the sterno-mas- 

 toid and trapezius points to tumour outside the medulla, as these 

 muscles are innervated by the purely spinal portion of the nerve. 



The tract of the sensory fibres in the brain is not so well 

 known and not so well localised as is the motor tract. The 

 fibres traverse the posterior part of the posterior limb of the 

 internal capsule, and a tumour here may produce complete 

 hemiansesthesia. Tumours of the gyrus fornicatus (callosal 

 gyrus) have the same effect. Even tumours of the so-called 

 motor cortex produce a blunting of sensation in the part most 

 paralysed, and, what is more striking, loss of power to accurately 

 localise sensory impressions on the part. The sensory fibres 

 reach the internal capsule by traversing the tegmental region of 

 the crus, but lesions here seldom produce marked anaesthesia. 

 Hemiansesthesia may be produced by tumours of the pons and of 

 the medulla. (Fig. 15.) 



Intracranial tumours may be so situated as to involve the 

 sense of sight or the sense of hearing. 



The visual fibres may be involved anywhere between the eye- 

 ball and the occipital cortex. Bilateral homonymous hemianopia 

 will result when a tumour involves the optic tract or optic radia- 

 tion or the occipital visual cortex of one side, for these structures 

 are in relation with a half of each retina, since only the fibres 

 from the nasal half of the retina decussate. 



When a tumour presses on the central part of the optic chiasma 

 bitemporal hemianopia results, since the decussating fibres from 

 the nasal halves of the retinae are affected, and the nasal halves 

 of the retinae correspond to the temporal halves of the visual 

 fields. Frequently a central defect of vision suggestive of tobacco 

 amblyopia precedes the full development of the hemianopic defect. 

 If the concentration of light on the blind half of the retina in 

 hemianopia still produces a pupil reflex, the afferent fibres in the 

 optic tract must be able to convey impulses as far as the point 



