308 



FIFTH PAIR OF NERVES. 



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 mus typhlus, the mus capensis, the 

 chrysochlore, and the sorex araneus. Of these 

 the mole, the mus capensis, 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 Magendie are far from satisfactory. In 



* Ibid. vol. xv. p. 210. 



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. 

 He 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 was produced 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. 



On the other hand. Mayo has found that 

 the fifth nerve may be divided within the 

 cranium in the cat and pigeon, and vision 

 continue unaffected ; which circumstance shows 

 that the apparent loss of vision in the rabbit 

 was owing to the great contraction of the pupil, 

 while according to Magendie's statement there 

 does not remain any trace whatever of sensi- 

 bility to the impression of light in the eye 

 after the section of the optic nerve. We must, 

 then, conclude that the optic nerve is the 

 proper medium of perception to visual im- 

 pressions, and that the co-operation of the fifth 

 nerve is not even necessary to enable the optic 

 nerve to fulfil its function. As the instrument 

 of the general sensibility of the structures of 

 the eye, however, the fifth nerve may be the 

 channel through which impressions not visual, 

 though perhaps excited by an agent of vision, 

 viz. light, may be conveyed. 



The conclusion thus drawn from experimental 

 physiology is fully confirmed in man by -the 

 history of those cases in which the influence 

 of the fifth nerve has been lost from disease : 

 of these two have been adduced by Bell in 

 the Philosophical Transactions for 1823, one 

 from the observation of Mr. Crampton, the 

 other from that of Dr. Macmichael, in which 

 the surface of the eye was totally insensible, 

 whilst vision was entire ; and another, still 



