OCVLO-MOTOR XERVES 747 



Meynert's commissure. — From the grey matter of the tuber cinereum fibres 

 arise which cross the middle line beliind Gudden's commissure and enter the 

 crusta on the opposite side, from whence they are said to pass into the sub- 

 thalamic body. 



The optic nerve passes forwards and outwards from the chiasma and enters 

 the orl)it through the optic foramen, accompanied by the ophthalmic artery, the 

 vessel being external to and slightly below the nerve. It then slightly changes its 

 direction and passes almost directly forwards to enter the l)ack of the eyeball about 

 three millimetres internal to and slightly below tlie posterior extremity of the optic 

 axis; having gained the interior of the eyeball its fibres spread out in the retina. In 

 its course through the orbit it is surrounded l)y the ciliary arteries and nerves; its 

 upper surface is crossed obliquely by the ophthalmic artery and the nasal nerve, 

 and the lenticular ganglion is in contact with its outer surface. Near the optic 

 foramen it is surrounded by, and in close relation to, the four recti muscles; but 

 further forwards it is separated from the muscles by an interval containing fat. The 

 arteria centralis retinae pierces the under surface of the optic nerve near the middle 

 point of the intraorbital part of its course and thence runs into the eyeball in 

 the axis of the nerve. Very distinct prr)longations of the cerebral membranes 

 accompany the optic nerve. The dura mater, having entered the orbit through the 

 optic foramen, delaminates into the orl)ital periosteum and the sheath of the optic 

 nerve; the latter is tough and strong, and becomes continuous with the sclerotic of 

 the eyeball. Within the dura mater is a narrow subdural space, then the arachnoid 

 and a comparatively wide subarachnoid space, the inner boundary of which is 

 formed by the pia mater, which invests the bundles of nerve-fibres. 



THIRD OR OCULO-MOTOR NERVE 



The third or oculo-motor nerve arises from a column of ner\'e-cells which is 

 placed in the grey matter V)eneath the floor of the aqueduct of Sylvius immediately 

 dorsal to the posterior longitudinal bundle. This nucleus extends from the level 

 of the posterior commissure to a point corresponding to the interval betAveen the 

 nates and testes, where it is imperfectly separated from the nucleus of the fourth 

 nerve. The fibres arising from it pass downwards and forwards in a series of fas- 

 ciculi, which traverse the posterior longitudinal bundle, the red nucleus, and the 

 inner part of the substantia nigra, and they emerge (superficial origin) in a roAV of 

 about nine fasciculi from the oculo-motor groove at the inner side of the crus in 

 the posterior fossa of the skull. 



The third nerve passes between the superior cerebellar and posterior cerebral 

 arteries, traverses the aperture in the tentorium, enters the middle fossa of the 

 skull, and then pierces the dura mater about mid^vay betAveen the anterior and 

 posterior clinoid processes of the sphenoid bone in the centre of a little triangular 

 space which has the following boundaries: externally, the free margin of the ten- 

 torium cerebelli, posteriorly the attached margin of the tentorium, and internally 

 a ridge of dura mater which extends from the posterior to tlie anterior clinoid 

 process. It then enters the outer wall of the cavernous sinus and runs forwards to 

 the sphenoidal fissure, occuj^ying, at first, a position superior and internal to the 

 fourth nerve. Behind the sphenoidal fissure it divides into a su])erior and an 

 inferior division. These divisifins, as they enter the orbit through the sjihenoidal 

 fissure, pass l)etween the two heads of the external rectus muscle, and are se])arated 

 from one another by the nasal branch of the ophthalmic division (jf tlie hftli 

 nerve. 



The superior division supplies the superior rectus. It enters that muscle on 

 its ocular surface and sends a branch past its inner liorder to supply the levator 

 palpebrie superioris. 



Tlie inferior division, considerably larger than the superior, divides into three 

 branches: two of these, the nerves to the internal and inferior recti muscles, 

 pierce the ocular surfaces of the muscles which they supply. The third branch, the 

 nerve to the inferior ol)li(|ue, is considerably longer than the other two. It runs 

 forwards in the interval lietween tlie inferior and external rectus muscles, and pierces 



