340 



FUNCTIONS OF THE NERVOUS SYSTEM. 



[Fig. 158. 



be concluded, that this nerve is not one of common sensation. That the ordi- 

 nary 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. Valentin states that, although the Optic 

 nerve may, like other nerves, be in appearance completely regenerated, he has 



never been able to obtain any evi- 

 dence that the power of sight has 

 been in the least degree recovered. 

 He remarks that animals suddenly 

 made blind exhibit great mental dis- 

 turbance, and perform many unaccus- 

 tomed movements ; and that the com- 

 plete 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 or- 

 bit. It is well known that, when 

 amaurosis is produced by a morbid 

 condition of the Optic nerve alone, 

 the eye retains its usual appearance ; 

 but, if the amaurosis be complete, the 

 texture of the Retina undergoes a re- 

 markable change, ceasing to exhibit 

 that peculiar structure which normally 

 characterizes it. Neither primitive 

 nervous fibrils, nor nucleated vesicles, 

 can be distinguished in it, and the 

 yellow spot of Soemmering becomes 

 paler, and is at last undistinguishable. 

 But if a very slight degree of sensi- 

 bility to light remain, these changes 

 are much less decided. Further, it is 

 well known that, Avhen the sight is 

 destroyed by a disease or injury, 

 which prevents the passage of light through the pupil, the whole eye becomes 

 more or less atrophied ; and the Retina and Optic nerve, although previously 

 sound, are found after death, (if the morbid condition have lasted sufficiently 

 long) to have lost their characteristic structure. It seems evident, then, that 

 the continuance of the functional operations of nerves, is a necessary condi- 

 tion of the maintenance of their normal organization; and we can very well 

 understand that this should be the case, from the analogy of other parts of the 

 system. 



443. The Optic nerve, though analogous 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 shall excite rc/Icx muscular motions. 

 This is especially the case in regard to the Iris, the ordinary actions of which 

 are regulated by the degree of light impinging 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 new- 



A view of the 3d 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 chiasm 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,trigernini. 10. 6th pair, abducentes. 11. 

 7th pair, auditory and facial. 12. 8th pair, pneumo- 

 gastric, spinal accessory, and Jglosso-pharyngeal. 

 13. 9th pair, hypoglossal. ] 



