It appears then that there is a distinct per ceptive faculty enjoyed by the nostrils, inde pendent of the fifth and dependent on the olfactory nerve; that we possess no positive evidence of the latter nerves being in any case the media by which this peculiar perception is recognized, but that they serve for the recogni tion only of impressions of contact, pungency, or irritation.
2. Relation of the AIM nerves to vision.— That in all animals having at once the faculty of vision and an optic nerve, the latter is in dispensably necessary to the exercise of tI e former cannot be denied :—disease or division of the nerve is uniformly attended by loss of the function ; but some circumstances coun tenance the opinion that the fifth nerve pos sesses a more important connection with vision than may at first appear. 1. Injury of the frontal and certain other branches of the fifth nerve has been long accounted among the causes of amaurosis. 2. Magendie has found that " on division of the two fifth' nerves upon an animal it seems blind." 3. The fact which countenances most strongly the opinion that the fifth nerve is concerned directly in the function of vision is derived from com parative anatomy. It has been stated that in certain animals a special optic nerve is wanting, and the ocular nerve is derived from the fifth pair. Of this it appears universally admitted that the proteus anguinns is an instance; its eyes are situate immediately beneath the epi dermis, which is transparent' in front of them; the optic nerve is wantingit and the only nerve received by the eye is a branch of the second division of the fifth./ Whatever vision, there fore, may be enjoyed by this animal, and according to Carus§ it is considerable, must be exerted through the medium of the fifth nerve. Among the mammalia also are several animals which appear to be in the same, or nearly the same state ; but anatomists are not agreed on the point : the absence of a special optic nerve in the mole was announced by Zinn,11 who shoved that its place was taken by a branch of the fifth. Carus and Treviranus, however, maintain that the optic does exist in the animal, but that it is very minute, grey, and capillary ; that in the same proportion the fifth nerve is large, and that its second division at its exit from the cranium gives off a branch, which enters the globe of the eye, and according to the former concurs in forming the retinal' Serres again positively denies the existence of the optic nerve in the mole, and maintains that these anatomists are mistaken ; he states that he has sought the nerve with the greatest care in thirty or forty of these animals, and never succeeded in finding it; and also in confirmation thereof that the optic foramen is wanting in the sphenoid bone. According to him several other of the mammalia are similarly constituted, viz. the inns typhlus, the mus capensis, the chrysochlore, and the sorex araneus. Of these the mole, the mus eapensis, and the sorex arensis decidedly enjoy vision, the first ac cording to the observations of Geoffroy St. Hilaire and Cuvier ; the second according to those of Delalande, and the third according to Serres himself; and if his view of the anatomical disposition of their ocular nerve be correct, the fifth nerve must in them also take the place of the optic and serve as the medium of sight. Treviranus, though he maintains the existence of a special nerve in the mole, yet says, from the disproportion of the optic and the ocular branch of the fifth, that in that animal the latter ought or must have to fulfil in vision more important functions than the optic nerve.* When to these facts we add the view of the nervous connections of the senses in invertebrate animals advocated by Treviranus, viz. that the nerves of
the senses in them are all branches of the fifth pair, the general proposition seems sufficiently probable, viz. that the fifth nerve is capable of acting as a medium of perception to impressions of light. But on the one hand, until it be proved what the exact nature of the optic faculty is which animals devoid of a special optic nerve possess, the question must be held to be undecided. It may be that the faculty is different in the two cases ; that where the special nerve is absent the faculty may amount, as suggested by Treviranus, to no more than a mere perception of light, and that the im pression is then not visual, but only one of ordinary sensibility. Such a distinction, in the sense in which that term is understood in reference to the higher animals, is easily conceived, and indeed is demonstrable from the influence of light upon an inflamed or irritable eye, and if such a distinction do naturally exist, the apparent anomaly presented by animals being sensible of light and seeming to enjoy vision without a special optic nerve will be removed, while such a faculty may suffice fully for the condition of the animal. Again, the evidence in favour of the opinion that the fifth is directly concerned in vision where a special nerve exists, seems altogether insufficient. In the first place, though in juries involving the frontal or other branches of the fifth nerve may induce amaurosis, it remains to be proved that the injury of the nerve is the cause of the disease, and that this did not rather arise from the effect of the injury upon other parts concerned in vision ; a view which is greatly confirmed by the fact that the mere section of the nerve has not been found to occasion any such affection of vision. In the second place the experiments of Alagendie are far from satisfactory. In order to determine the influence of the fifth nerve upon vision, he performed the following experiments, from which he inferred that the section of the fifth nerve destroys sight without abolishing entirely all sensibility of the eye for light, and suggests in explanation either that the fifth is the medium of perception, or that it is necessary to enable the optic to act. After having divided the fifth pair on one side in rabbits, he threw suddenly upon the eye the light of a wax candle, and no effect was produced ; the same being tried upon the sound eye, the only effect produced was move ments of the iris. Under the impression that this was not sufficiently intense, he tried that of a powerful lamp, but, even with the as sistance of a lens, the result was the same. Ile then tried the experiment with solar light, and by making the eye pass suddenly from the shade to the direct light of the sun, an impression wasproduced and the animal im mediately closed its eyelids. Such data cannot be admitted as sufficient to justify the inference that vision is destroyed by the section of the fifth nerve. In the first place it is to be recollected that the experiment was made upon rabbits, in which Magendie has elsewhere told us that section of the fifth nerve produces strong con traction of the iris, consequently great dimi nution of the size of the pupil : and of what value, then, is the result that, under the in fluence of the light of a candle or a lamp, an impression was not made sufficiently powerful to cause the animal to give evidence of it ? In the second place the animal did, under all the disadvantages, give sufficient evidence that its vision was not destroyed ; there is, therefore, no reason for the conclusion drawn from the experiment related.