538 



CRANIAL NERVES. 



It is a rather large nerve, of rounded form and firm texture, having its apparent 

 origin from the inner surface of the crus cerebri, immediately in front of the pons 

 Varolii. 



The deep origin may be traced into the substance of the crus, where some of 

 its fibres are connected with the locus niger ; others run downwards, among the 

 longitudinal fibres of the pons, whilst others ascend, to be connected with the 

 tubercula quadrigemina and valve of Vieussens. According to Stilling, the 

 fibres of the nerve pierce the peduncle and locus niger, and arise from a gray 

 nucleus in the floor of the aqueduct of Sylvius. On emerging from the brain, it 

 is invested in a sheath of pia mater, and inclosed in a prolongation from the 

 arachnoid. It then pierces the dura mater on the outer side of the anterior 

 clinoid process, where its serous covering is reflected from it, and passes along 

 the outer wall of the cavernous sinus, above the other orbital nerves, receiving 

 in its course one or two filaments from the cavernous plexus of the sympathetic. 

 It then divides into two branches, which enter the orbit through the sphenoidal 



fissure, between the two 



Fig. 275. Nerves of the Orbit. Seen from above. heads of the External rec- 



tus muscle. On passing 

 through the fissure, the 

 nerve is placed below the 

 fourth, and the frontal and 

 lachrymal branches of the 

 ophthalmic nerve. 



The superior division, 

 the smaller, passes inwards 

 across the optic nerve, and 

 supplies the Superior rectus 

 and Levator palpebrae. 



The inferior division, 

 the larger, divides into 

 three branches. One passes 

 beneath the optic nerve to 

 the Internal rectus ; another 

 to the Inferior rectus ; and 

 the third, the largest of the 

 three, passes forwards, be- 

 tween the Inferior and Ex- 

 ternal recti, to the Inferior 

 oblique. From the latter, a 

 short, thick branch is given 

 off to the lower part of the 

 lenticular ganglion, forming 

 its inferior root, as well as 

 two filaments to the Inferior 

 rectus. All these branches 

 enter the muscles on their 

 ocular surface. 



ri-nt KTummt 



ta-Duru-Jlater 



FOURTH NERVE. 



The FOURTH or TROCHLEAR nerve, the smallest of the cranial nerves, supplies 

 the Superior oblique muscle. It arises from the upper part of the valve of 

 Vieussens, immediately behind the testis, and divides, beneath the corpora quadri- 

 gemina, into two fasciculi ; the anterior one arising from a nucleus of gray matter, 

 close to the middle line of the floor of the Sylvian aqueduct ; the posterior one 

 from a gray nucleus, at the upper part of the floor of the fourth ventricle, close to 

 the origin of the fifth nerve. The two nerves are connected together, at their 



