778 



OPTIC NERVES. 



nerves are particularly small ; but the olfactory 

 nerves preponderate, and there can be little 

 doubt that the superiority of the sense of smell 

 in them serves in great measure to supersede 

 the necessity for highly- wrought organs of vision, 

 and in probably the majority of the Mammalia 

 the olfactory nerves preponderate in size over 

 the optic, but the corresponding faculty by its 

 acuteness makes amends for any inferiority in 

 vision. Thus the keen scent of many Carni- 

 vora renders the eye of secondary importance 

 in the pursuit of their prey, and the vegetable 

 feeders are much indebted to the perfection of 

 their sense of smell for the discrimination they 

 evince in their choice of nutriment. 



FUNCTIONS OF THE OPTIC NERVE. 



The optic nerves when present are essential 

 to vision. 



In all animals which possess optic nerves 

 they must be considered essential to vision, 

 for diseases which destroy the organization of 

 the " second pair " invariably deprive the organs 

 of sight of their sensibility to luminous impres- 

 sions ; and were other proof wanting, the ex- 

 periments instituted of late years by Magendie, 

 Mayo, and others, would afford sufficient 

 evidence of the special function of the nerves 

 in question. Magendie found that division of 

 either optic nerve in front of the chiasma was 

 instantaneously followed by complete loss of 

 visionin thecorrespondingeye of the animal sub- 

 mitted to the experiment; and when both optic 

 nerves were thus divided total blindness en- 

 sued, and no means which could subsequently 

 be devised for concentrating light upon the 

 eye appeared to excite in the retina the slightest 

 sensibility to its accustomed stimulus. 



Although the foregoing facts would warrant 

 the conclusion that the only nerves capable of 

 endowing the eyes with their special sensibility 

 are the optic, nevertheless many considerations 

 favour the presumption that the fifth pair exert 

 direct influence on the sense of sight, so much 

 so that some have considered these nerves 

 essential to vision, whilst others have even sup- 

 posed that the faculty in question may be main- 

 tained through the agency of the fifth nerves 

 alone. ' 



In those animals which possess special optic 



nerves, the fifth pair are totally inadequate 



to support vision. 



There are no facts on record to prove the 

 possibility of such animals continuing to see 

 after destruction of the second pair. The experi- 

 ments already cited may be looked on as con- 

 clusive, and those performed by Magendie to 

 show that divison of the fifth nerves within the 

 cranium in living animals produces blindness, 

 can never justify physiologists in the belief that 

 the " trifacial " may endow the eyes with their 

 special sensibility. 



Certain facts furnished by the comparative 

 anatomy of the second and fifth pairs have been 

 from time to time adduced to shew that the 

 fifth in the human subject possesses this power. 

 Thus it is stated on good authority, that the com- 

 mon Mole, the Proteus anguinus, the Mus Ca- 

 pensis, the Chrysochlore, the Mustyphlus^and 



the Sorex araneus, in which organs of vision occur, 

 are not provided with special optic nerves, and 

 that in them the fifth pair furnishes the only 

 nerve which the rudimental eye receives. It is 

 argued from these data (and Serres would seem 

 to be one of the ablest advocates for this view) 

 that a branch of the fifth nerve assumes in 

 such cases the functions of the optic, becoming 

 endowed with special sensibility to light, and 

 that therefore from analogy the ophthalmic 

 division of the fifth in man may be presumed to 

 possess similar properties. 



Conclusions arrived at by such reasoning 

 should be received with caution in the absence 

 of more direct proofs; the weight to which 

 they are entitled has been already fully dis- 

 cussed under the article FIFTH PAIR OF 

 NERVES, and reference is made to that article 

 for further particulars touching this interesting 

 topic. The writer fully concurs in the views 

 therein advocated, and feels disposed to attri- 

 bute little value to arguments founded, as they 

 appear to be, on imperfect analogies. The 

 cases of the human subject and the animals 

 specified are essentially dissimilar; the pre- 

 sence of a special optic in the one case, and its 

 absence in the others, destroys their parallelism, 

 and may create important differences in the 

 functions of accessory nerves; and, moreover, 

 the little knowledge we possess of the nature 

 and amount of vision enjoyed by animals in 

 which special optic nerves are wanting, should 

 make us hesitate to argue from them to the 

 human subject. 



That the fifth pair exercise some influence 

 over vision can scarcely be denied, but the 

 nature and amount of this influence are not so 

 easily determined, and have probably been 

 much exaggerated. 



In the present state of their knowledge physio- 

 logists have not sufficient proof that in the 

 higher animals the influence of the fijth pair 

 is absolutely essential to sight. 

 Magendie discovered that the section of the 

 fifth nerve on both sides within the cranium of 

 a living animal greatly impairs, if it does not 

 actually annihilate vision, and such a result 

 seems, no doubt, to argue that the faculty of 

 sight has a necessary dependence on the 

 integrity of the " trifacial nerves," but the ex- 

 periment when critically examined will ap- 

 pear not to warrant such an inference ; it 

 demonstrates that so rude an injury inflicted on 

 the nervous centres deprives an animal of one 

 of its faculties, and this might have been 

 anticipated on general principles ; but it does 

 not prove that the loss of the faculty depends 

 on the injury to the fifth nerves alone. In such 

 a destructive proceeding there can be no as- 

 surance that the fifth pair have been the only 

 parts of importance mutilated ; the contrary is 

 by much the more probable presumption, and, 

 therefore, the conclusion " that the fifth nerves 

 are essentially necessary to vision " is not fairly 

 deducible from the experiment. But farther, 

 the facts (detailed even as they are by 

 Magendie) suffice to show, that the eye may 

 continue sensible to luminous impressions afttr 

 complete division of the fifth pair, for according 



