FUNCTIONS OF TIIE 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 vision in 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 he 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 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 sliew 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 Mils Ca pensis, the Chrysochlore, the Alus typhlus, and the Sorexaraneus, 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 Finn 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 1.vhich special optic nerves are wanting, should make us hesitate to argue from them to the human subject.
That the fifth pair exercise some influenc over vision can scarcely be denied, but t nature and amount of this influence are not easily determined, and have probably be much exaggerated.