In the present state qf their knowledge physi logists have not sufficient proof that in I higher animals the influence qf the fifth p is absolutely essential to sight.
Magendie discovered that the section of t fifth nerve on both sides within the cranium a living animal greatly impairs if it does actually annihilate vision, and' such a r seems, no doubt, to argue that the facu sight has a necessary dependence o integrity of the " trifacial nerves," but t periment when critically examined wil pear not to warrant such an inferen demonstrates that so rude an injury inflic the nervous centres deprives an animal of 'its faculties, and this might have anticipated on general principles ; but i not prove that the loss of the faculty d on the injury to the fifth nerves alone. I a destructive proceeding there can be surance that the fifth pair have been th parts of importance mutilated ; the cont by much the more probable presumptio therefore, the conclusion " that the fifth are essentially necessary to vision " is not deducible from the experiment. But f the facts (detailed even as they a Magendie) suffice to show, that the ey continue sensible to luminous iinpressions complete division qf the AIM pair, for acco to his own account, " the eye of the rabbit on sudden exposure to the sun's rays, after the fifth pair had been divided, was still sensible to strong solar light ; and the effect was more marked when a lens was used to test its sensibility." Mayo's experiments on pigeons afford still more convincing proof of the ability of the optic nerve, unaided by the fifth, to maintain the special sensibility of the eye ; this physio logist succeeded in dividing the fifth nerve within the cranium of a living pigeon (leaving the optic uninjured,) without rendering the retina insensible to light.
The results of pathological observations on man furnish also abundant evidence that vision may continue after disease has destroyed the fifth nerve. Opportunities do not °fled occur of bringing this to the test of dissection, for in most of these cases changes of structure involve other parts of the nervous centres simultane ously with the fifth nerve, and so deprive them of their greatest value; and the destructive in flammation of the eye-ball, which so constantly accompanies morbid alterations of the fifth pair, is another fruitful source of embarrassment in attempts to investigate their history, but even a few well-attested observations are amply sufficient to establish a negative proof. Miiller cites a case of disease involving the whole trunk of the fifth nerve of the left side, in which insensibility of the entire left side of the head and the corresponding side of the tongue and eye, occurred, while vision remained perfect ; and in the article FII"Ill PAIR OF NEnvEs, other similar examples are related.
The conjecture that the fifth pair is essential to vision receives probably its strongest support from the occasional results of injuries to cer tain branches of that nerve, for numerOus cases are on record in which wounds or con tusions of its frontal twigs have been fol lowed by blindness, and the same unfortu nate event has resulted (though rarely) from irritations affecting some of its other branches. Thus, Mr. Travers has known amaurosis to originate from irritation of the dental nerves. Ife says, " I have seen an incipient amaurosis arrested by the extraction of a diseased tooth, .when the delay of a similar operation had occasioned gutta serena on the opposite side 'two years before." And Professor Galenzowski of \Vilna " observed severe neuralgia and blindness produced by a splinter of wood be oming entangled in a diseased tooth, and hese symptoms were cured by the extraction of be tooth together with the offending material." The value of such facts as these in assisting hysiologists to determine the influence exerted y the fifth nerve over vision, appears to have een much overrated ; for in a large proportion f these cases it may be inferred with great pro ability that the same injury which affected the upra-orbital nerves exerted also pernicious in uence on the deeper seated contents of the orbit, nd that the optic nerve, or retina, or even the Yrain itself participated in the effects of the vio hnce, although from the more superficial posi ion and greater exposure to danger of the frontal branches of the fifth,they alone were believed to have suffered. This explanation will undoubt edly not apply to cases iirw hich blindness has been produced by very trivial injuries, such as simple incised wounds or punctures of the nerves in question ; but nevertheless the weight of evidence which these latter cases would seem to afford is much diminished by the consideration that loss of sight has likewise ensued from inju ries and affections of other nerves, to which, while healthy, no participation in the support of vision can be conceded. For example, Dr. Jacob recites the case of an officer in whom amaurosis occurred in consequence of injury ihflicted by a ball on some branches of the portio dura ;* and irritations in the digestive organs (dyspeptic disturbance of the stomach more especially) are well known lo produce at times amaurotie symptoms. Now, although these facts un questionably establish the existence of curious pathological affinities between the nerves of the part thus irritated and those which are subser vient to vision, no physiologist would be hardy enough to infer fi om such prernises that the facial nerve, the par vagum, or those which supply the intestinal tract, exercise in the normal state any control over the faculty of sight.