DISEASES OF THE OPTIC NEEVE. 187 



invade the orbit through the sphenoidal fissure and cause pro- 

 ptosis. Tumours of the middle fossa sometimes involve the roof 

 of the nasopharynx, and tumours of the posterior fossa may give 

 rise to masses in the neck behind, or under cover of, the mastoid 

 process. 



CRANIAL NERVES. 

 THE OPTIC NERVE AND TRACT. 



The accessibility of the optic disc to ophthalmoscopic examina- 

 tion renders possible the detection of slight degrees of inflam- 

 mation which probably pass unrecognised in other cranial nerves. 



The optic nerve is invested by tubular sheaths prolonged from 

 the pia, arachnoid, and dura, therefore inflammation of the 

 meninges can produce optic neuritis by extension along these 

 nerve sheaths. Effusions of blood and serum may also extend 

 in the same direction or local haemorrhage may occur within the 

 firm dural sheath as the result of injury; injurious compression 

 of the nerve fibres may thus be set up. 



At the optic foramen the periosteum of the orbit is continuous 

 with the dural sheath of the nerve, and thus periosteal inflamma- 

 tion of the orbit, due to bone disease or even secondary to dental 

 caries, may extend to the nerve and induce neuritis. 



As the nerve passes through the optic foramen it is only 

 separated from the sphenoidal air sinus by a thin bony lamina, 

 which is sometimes imperfect, and inflammation of this air sinus 

 may secondarily involve the nerve. Fractures in the neighbour- 

 hood of the foramen may compress or actually sever the nerve 

 fibres, and bony growths in this situation may also cause pressure 

 upon the nerve. 



The optic foramen is really a narrow canal and is traversed by 

 the ophthalmic artery as well as the optic nerve, consequently 

 inflammatory swelling of nerve or periosteum may give rise to 

 severe compression of the visual fibres. 



Broadly speaking, the fibres from the periphery of the retina 

 lie at the periphery of the nerve trunk and in a lesion near the 



