NERVES OF SPECIAL SENSE. OPTIC. 



715 



eluded that this nerve is not one of Fig- 190. 



common sensation. That the ordinary 

 sensibility of the eyeball remains, when 

 the functions of the Optic nerve are 

 completely destroyed, is well known ; as 

 is also the fact that division of it puts 

 an end to the power of vision. Valen- 

 tin states that, although the Optic nerve 

 may, like other nerves, be in appearance 

 completely regenerated, he has never 

 been able to obtain any evidence that 

 the power of sight has been in the least 

 degree recovered. He remarks that ani- 

 mals suddenly made blind exhibit great 

 mental disturbance, and perform many 

 unaccustomed movements ; and that 

 the complete absence of the power of 

 vision is easily ascertained. Morbid 

 changes are sometimes observed to take 

 place in eyes whose Optic nerve has 

 been divided; but these are by no 

 means so constant or so extensive as 

 when the Fifth pair is paralyzed ; and 

 they may not improbably be attributed 

 to the injury, occasioned by the opera- 

 tion itself, to the parts within the orbit. 

 741. The Optic nerve, though an- 

 alogous to the Olfactory in all the 

 points hitherto mentioned, differs from 

 it in one important respect ; that it has 

 the power of conveying impressions 

 which excite reflex muscular motions. 

 This is especially the case in regard to 



the Iris, the ordinary actions of which are regulated by the degree of light im- 

 pinging on the retina. When the Optic nerve is divided, a contraction of the 

 pupil takes place; but this does not occur, if the connection of this nerve with 

 the third pair, through the nervous centres, be in any way interrupted. After 

 such division (if complete), the state of the pupil is not affected by variations 

 in the degree of light impinging on the retina ; except in particular cases, in 

 which it is influenced through other channels. Thus, in a patient suffering 

 under amaurosis of one eye, the pupil of the affected eye is often found to vary 

 in size, in accordance with that of the other eye ; but this effect is produced by 

 the action of light on the retina of the sound eye, which produces a motor change 

 in the third pair on both sides. Further, as already shown ( 724), the im- 

 pression only of light upon the retina may give rise to contraction of the pupil, 

 by reflex action, when the optic nerve is itself sound ; whilst no sensations are 

 received through the eye, in consequence of disease in the sensorial portion of 

 the nervous centres. Although the contraction of the pupil is effected by the 

 influence of motor fibres, which proceed to the sphincter of the Iris from the 

 third pair of nerves, through the Ophthalmic ganglion, there is evidence that its 

 dilatation depends rather upon the influence it derived frOrn that ganglion itself, 

 and from the Sympathetic system, of which it forms part. Some have attempt- 

 ed to show that the actions of the iris are in a slight degree voluntary, because, 

 by an effort of the will, they could occasion contraction of the pupil ; but this 

 so-called voluntary contraction is always connected with a change ;in the place 



A view of the 2d pair or optic, and the origins of 

 seven other pairs. 1, 1. Globe of the eye; >the one 

 on the left hand is perfect, but that on the right has 

 the sclerotic and choroid removed to show the retina. 

 2. The chiasin of the optic nerves. 3. The corpora 

 albicantia. 4. The infundibulum. 5. The Pons 

 Varolii. 6. The medulla oblongata. The figure is 

 on the right corpus pyramidale. 7. The 3d pair, 

 motores oculi. 8. 4th pair, pathetici. 9. 5th pair, 

 trigemini. 10. 6th pair, abducentes. 11. 7th pair, 

 auditory and facial. 12. 8th pair, pneumogastric, 

 spinal accessory, and glosso-pharyngeal. 13. 9th 

 pair, hypoglossal. 



