58 T/ie Cranial Nerves 



' When, therefore, the optic tract of either side is pressed upon, so 

 as to affect the entire thickness of the nerve, and thus to interfere with 

 the action of all the fibres, the temporal side of the retina of that eye 

 and the nasal side of the retina of the opposite eye will be rendered 

 blind.' l (Hemianopsia.} When the optic nerve is pressed upon, in 

 front of the commissure, as by sarcoma in the orbit, there is blindness 

 of the one eye only, and it will probably be associated with paralysis 

 of the third, fourth, or sixth nerves also, as they lie close together at 

 the apex of the orbit. If the decussating fibres alone be implicated in 

 the commissure, internal or nasal hernianopsia is found in each eye. 

 4 Double temporal hemianopsia indicates disease of each internal 

 carotid artery, with symmetrical lateral pressure upon the commissure.' 

 Severe pressure upon the commissure might cause blindness in both 

 eyes. 



Optic neuritis, as determined by ophthalmoscopic examination, is 

 usually followed by atrophy. 



No. 3. The motor oculi arises from the grey matter surrounding 

 the Sylvian aqueduct (where the nerves of the two sides decussate), 

 and emerges from the inner side of the crus ; it then passes through 

 the outer wall of the cavernous sinus, and divides into two branches 

 which enter the orbit through the sphenoidal fissure, and between 

 the two heads of the external rectus. Coming away from the crus, the 

 third nerve traverses the narrow interval between the posterior cere- 

 bral and superior cerebellar arteries, so that if either of these arteries 

 become dilated at that spot ptosis occurs. 



The upper division supplies the levator palpebra? and the superior 

 rectus ; the lower supplies the internal and inferior recti, the inferior 

 oblique, and, by the motor root to the lenticular ganglion, the ciliary 

 muscle and the sphincter fibres of the iris. 



(The superior oblique is supplied by the fourth nerve, and the 

 external rectus by the sixth.) 



Contraction of the pupil is effected through the third nerve : 

 when a strong light falls upon the retina a sensation is transmitted by 

 the optic nerve to the corpora quadrigemina, close to which, in the 

 Sylvian aqueduct, the third nerve arises. By this nerve a motor 

 impulse is carried to the lenticular ganglion, and so to the ciliary 

 muscle and iris. 



In viewing- near objects both eye-balls are directed inwards, 

 the right and left internal recti acting in unison ; thus it becomes 

 necessary that the third nerves be associated at their origin, which 

 occurs, as already noted, in the grey matter around the Sylvian 

 aqueduct. 



As the axes converge upon the near object the pupil must contract 

 so as to cut off peripheral rays ; it is expedient, therefore, that the 

 internal recti and the sphincter of the pupil be under the control of the 



1 Applied Anatomy of the Nervous System, Ambrose L. Ranney. 



