OPHTHALMOPLEGIA. 193 



associated with paralysis of face and limbs on the opposite side. 

 The nerve fibres passing to their point of exit in the oculomotor 

 groove of the crus at first converge from the separate portions of 

 the compound nucleus, and so a lesion in the crus may involve 

 only a few of the converging rootlets, the ophthalmoplegia will 

 then be incomplete and practically indistinguishable from a lesion 

 of part of the compound nucleus itself. Again, of all the motor 

 fibres in the crus those of the face are nearest the third nerve, 

 and so a small lesion may pick out the third nerve and the facial 

 fibres, the rest of the motor tract escaping. The resulting facial 

 paralysis is in all cases of supranuclear type, the lower part of 

 the face being chiefly involved. 



It is sometimes difficult to distinguish a lesion in the crus itself 

 from one at the base of the brain which compresses it ; in such 

 cases the presence of symptoms due to involvement of the 

 corpora quadrigemina which form the dorsal part of the crus 

 enable the lesion to be located in the crus itself. 



The fourth nerves after leaving their nuclei, skirt the iter, 

 decussate and appear on the dorsal aspect of the brain at the 

 valve of Vieussens. They thus come into relation with struc- 

 tures far removed from the other ocular nerves, and consequently 

 the superior oblique muscles may be paralysed by the pressure 

 of a tumour of the anterior end of the cerebellar vermis, or of 

 the posterior quadrigeminal bodies, or of the valve of Vieussens. 

 Both nerves are likely to be involved in such cases. 



The sixth nerve on its way from its nucleus to its point of 

 exit at the lower border of the pons, traverses the lower and 

 ventral part of the latter. Its paralysis may be associated 

 with opposite hemiplegia if the lesion be in the pons. 



At the base of the brain the third nerves lie in the narrow 

 interpeduncular space, and both may be compressed by a 

 growth here or by an aneurysm of the posterior cerebral 

 artery. 



The fourth nerve winds round the outer aspect of the crus. 



The sixth nerve has the longest course in the meninges 

 coming into relation with the ventral aspect of the pons, 



C.A.A. 13 



