THE FOURTH PAIR. 



531 



crns cerebri immediately in front of the pons. Each nerve may be 

 traced backwards round tlie peduncle to a place below the corpora 

 quadrio-emina where it arises from the upper part of the ralve of 

 Vieussens. 



Deep origin. — Entering the substance of the valve the fibres of each 

 nerve separate into three groups, which follow different directions. 



1. The fibres of an anterior group pass obliquely forwards and up- 

 wards in the wall of the aqueduct of Sylvius to end in the posterior part 

 of the nucleus beneath the floor of the aqueduct, which is common to the 

 third and fourth nerves. 



2. Other descending fibres pass down on the outer side of the locus 

 cceruleus to the neighbourhood of the nucleus of the fifth nerve. 



3. The fibres of a third group pass transversely inwards in the sub- 

 stance of the velum, decussating with those of the opposite side, and 

 join the ascending or descending fibres of the opposite nervp 



Fig. 334. — View from above of the Motor Nerves Fig. 33 i. 



OF the Eyeball and its Mcscles (after Hirsch- 

 fekl and Leveille, altered.) (A. T.) 



The oplithalmic division of the fifth pair has been 

 cut short : the attachment of the muscles round the 

 optic nerve has been opened up. and the three upper 

 muscles turned towards the inner side, their anterior 

 parts being removed ; a part of the optic nerve is 

 cut away to show the inferior rectus : and a pai't of 

 the sclerotic coat and cornea is dissected off showing 

 the iris, zona ciliaris, and choroid coat, with the- 

 ciliary nerves. 



a, the upper part of the internal carotid artery 

 emerging from the cavernous sinus ; b, the superior 

 oblique muscle ; U, its anterior part passing through 

 the pulley ; c, the levator palpebrre superioris ; d, 

 the superior rectus ; e, the internal rectus ; /, the 

 e.Kternal rectus ; /', its upper tendon turned down ; 

 [/, the infei-ior rectus ; li, insertion of the inferior 

 oblique muscle. 



II, the commissure of the optic nerve ; II', part 

 of the optic nerve entering the eyeball ; III, the 

 common occulo-motor ; IV, the fourth or trochlear 

 nerve ; V, the greater root of the trigeminus ; V, 

 the smaller or motor root ; VI, the abducent nerve ; 

 1, the upper division of the third nerve separating 

 from the lower and giving twigs to the levator palpe- 

 bra? and suijerior rectus ; 2, the branches of the 

 lower division supplying the internal and inferior 

 recti muscles ; 3, the long branch of the same nerve 

 jiroceeding forward to the inferior oblique muscle, 

 and close to the number 3, the short thick branch 

 to the ciliary ganglion : this ganglion is also shown, 



receiving from behind the slender twig from the nasal nerve, which has been cut short, 

 and giving forward some of its ciliary nerves, which pierce the sclerotic coat ; 3', marks 

 the termination of some of these nerves in the ciliary muscle and iris after having passed 

 between the sclerotic and choroid coats ; 4, the distribution of the trochlear nerve to the 

 upper surface of the superior oblique muscle ; 6, the abducent nerve passing into the 

 external rectus. 



Course and distribution. — From the remoteness of its place of 

 origin, the part of this nerve within the skull is longer than that of 

 any other cranial nerve. It enters an aperture in the free border of 

 the tentorium, outside that for the third nerve, and near the posterior 



