56 THE NERVOUS SYSTEM 



round the lateral aspect of the mid-brain and enters the 

 cisterna basalis. It passes forwards parallel to but below the 

 oculo-motor nerve and, after running in the lateral wall of 

 the cavernous sinus, enters the orbit through the superior 

 orbital fissure. Its relations are practically the same as those 

 described for the oculo-motor nerve. 



FIG. 32. The Posterior Aspect of the Brain-Stem. The left half of the 

 Cerebellum has been removed and the right half has been displaced to 

 the right side. 



4. Clava. | 7. Brachium pontis. 



5. Brachium conjunctivum. 8. Superior corpus quadri- 



geminum. 



1. Fourth ventricle. 



2. Restiform body. 



3. Medulla oblongata. 



6. Mid-brain. 



The Abducent Nerve arises from a nucleus which is 

 situated in the lower part of the pons, immediately under the 

 floor of the fourth ventricle (Fig. 23). This nucleus is inti- 

 mately related to the fibres of the seventh nerve and, conse- 

 quently, lesions of the sixth nucleus are often associated 

 with some degree of facial paralysis. From its origin, the 



