OPTIC NERVES. 



779 



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 often 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. Mu'ller 

 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 FIFTH PAIR OF NERVES, 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. 

 He 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 Wilna " observed severe neuralgia and 

 blindness produced by a splinter of wood be- 

 coming entangled in a diseased tooth, and 

 these symptoms were cured by the extraction of 

 the tooth together with the offending material." 



The value of such facts as these in assisting 

 physiologists to determine the influence exerted 

 by the fifth nerve over vision, appears to have 

 been much overrated ; for in a large proportion 

 of these cases it may be inferred with great pro- 

 bability that the same injury which affected the 

 supra-orbital nerves exerted also pernicious in- 

 fluence on the deeper seated contents of the orbit, 

 and that the optic nerve, or retina, or even the 

 brain itself participated in the effects of the vio- 

 lence, although from the more superficial posi- 

 tion 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 in which 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 inflicted 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 to produce at times amaurotic 

 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 from such premises 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. 



If, in addition to these considerations, it be 

 recollected that blindness occurs only as an 

 occasional consequence of injuries to the frontal 

 nerves, and that loss of vision is found to ensue 

 very rarely from the irritations to which other 

 branches of the fifth are so peculiarly liable, 

 the importance of such cases in determining 

 the question must be still further lessened. 

 The observations of Dr. Jacob on this sub- 

 ject appear to the writer so apposite that 

 he is induced to insert them. This gentle- 

 man writes, " Blindness does not seem to have 

 followed any of the operations formerly so 

 much practised of dividing the branches of this 

 nerve, and in some of the worst cases of that 

 form of neuralgia called tic douloureux, vision is 

 not impaired. Moreover, thousands of children 

 suffer from dentition and thousands of adults 

 from tooth-ache, yet none of these become 

 blind in consequence." f 



The coincidence of loss of sight with injuries 

 or irritations affecting branches of the fifth 

 nerve, admits of explanation on other principles 

 without assuming the fifth to be essential to 

 vision ; the hypothesis that in such cases reflex 

 irritation becomes propagated from the parts pri- 

 marily affected through the nervous centres to 

 the optic nerve, seems in the present state of 

 physiological science sufficiently plausible ; 

 for while it applies to cases of amaurosis 

 resulting from abnormal conditions of other 

 peripheral branches of the nerve as well as its 

 ophthalmic division, it also affords a solution 

 of the still more obscure dependence of the 

 same disease on irritations in remote organs. 



The experiments of Magendie, confirmed 

 as they have been by pathological observations, 



* Cyclopaedia of Practical Medicine, art. AMAU- 

 ROSIS. 



t On Paralytic Neuralgia and other Nervous 

 Diseases of the Eye, by Arthur Jacob, M.D. 



