60 THE NERVOUS SYSTEM 



those fibres which come from the same side arise in the group 

 of cells which are ultimately connected with the associated 

 muscle of the opposite side. 



NUCLEAR LESIONS may be single and they may only involve 

 a part of the nucleus. The oculo-motor nucleus contains 

 numerous cell-groups, each reserved for one of the muscles 

 supplied. On this account, one or more of the muscles inner- 

 vated by the oculo-motor may be paralysed in nuclear lesions, 

 but paralysis of all the muscles, unless forming part of a crossed 

 paralysis Weber's syndrome (p. 39) indicates a lesion of 

 the trunk of the nerve rather than a lesion of the nucleus. 



The nerves themselves may be affected in purulent effusions 

 into the cisterna interpeduncularis (p. 1 1 1), or in tumours of the 

 interpeduncular fossa or surrounding areas. They may be 

 subjected to pressure by tumours of the hypophysis (pituitary 

 body) or by aneurisms of the internal carotid artery, as they 

 pass forwards in the lateral wall of the cavernous sinus \ or by 

 tumours or haemorrhages, as they lie in the orbit. 



RESULTS OF PARALYSIS. Paralysis of the sixth nerve only 

 affects the lateral rectus muscle. The loss of movement 

 results in the occurrence of a convergent strabismus when the 

 patient looks towards the paralysed side. To overcome this 

 disability, the patient tends to keep his head rotated to the 

 side of the lesion, thus obviating the necessity for lateral 

 rotation of the affected eye. 



When the fourth nerve is involved by itself, the superior 

 oblique is the only muscle which is paralysed. Under these 

 circumstances, the eye deviates to the medial side when down- 

 ward rotation is attempted, since the medial pull of the inferior 

 rectus is no longer counter-balanced by the lateral pull of the 

 superior oblique. The other movements of the eye are not 

 affected, and disability is only noticed by the patient when he 

 has to look downwards, e.g., in going downstairs ; he then suffers 

 from diplo-opia. 



In complete paralysis of the third nerve the eye only retains 

 those movements which are due to the superior oblique and 



