OPTIC NERVES. 



771 



Some difficulty no doubt arises when the 

 physiologist attempts to reconcile with this 

 anatomical arrangement certain facts already 

 detailed : it may be remarked, however, that 

 arguments derived from the pathology of the 

 optic nerve can be but of little value, since 

 they have been relied on in turn by the framers 

 of each hypothesis as affording proof of their 

 own peculiar views ; and in the Museum of the 

 Richmond Surgical Hospital, Dublin, the writer 

 has seen specimens of atrophied optic nerves in 

 man which furnish the most contradictory evi- 

 dence upon the subject under discussion. The 

 preparations alluded to were cases in which one 

 eye had been destroyed either by local disease 

 or accident, many years previous to death, and 

 where in consequence the corresponding optic 

 nerve became wasted from disuse, while the 

 other optic nerve continued healthy. In the 

 majority of these specimens the wasting has 

 been propagated backwards to the opposite 

 tractus opticus and has implicated that structure, 

 while the corresponding tractus has been spared 

 (see Jig. 420, A); in some examples both tractus 

 optici have suffered a diminution of size and 

 in general to an unequal amount ; and in one 



Fig. 420. 



Atrophy of one optic nerve, the consequence of long- 

 continued disuse. ( From preparations in the Museum 

 of the Richmond Hospital, Dublin.) 



A. a, right optic nerve in a state of atrophy ; b, 

 left optic nerve healthy ; c, chiasma ; d, right trac- 

 tus opticus healthy ; e, left tractus opticus wasted. 



B. a, right optic nerve atrophied ; b, left optic 

 nerve healthy; c, chiasma; d, right tractus oplicus 

 wasted ; e, left tractus opticus healthy. 



very remarkable instance the tractus opticus of 

 the same side with the shrunken nerve has 

 dwindled into a narrow band, while the otln-r 

 retains fully its normal dimensions (fig. 420, B). 

 It may be fair to add, that the case quoted 

 from Vesalius is considered by many, and 

 amongst others by Gall and Spurzheim, as of 

 doubtful authenticity, and the results of expe- 

 riments on living animals should be received 

 with caution, for to argue from experiments on 

 birds to the human subject is plainly fallacious, 

 since the structure of the chiasma is not iden- 

 tical in the two cases, and the great obstacles 

 encountered in the performance of such expe- 

 riments on the mammalia renders them of trifling 

 value. 



Use of the chiasma. In the direct junction 

 between two corresponding nerves of opposite 

 sides displayed in the chiasma, the second pair 

 form an exception to a general law ; for in no 

 other known instance does a similar union 

 occur ; it therefore becomes a subject of great 

 interest to determine how far this anomaly 

 admits of explanation by any unusual properties 

 in the nerves so circumstanced. 



The optic nerves possess the remarkable 

 power of conveying to the individual the sen- 

 sation of a single impression only, while a 

 separate impression affects each nerve simul- 

 taneously, so that although a perfect picture of 

 the object be depicted on each retina severally, 

 nevertheless to the spectator it appears to be 

 single (as is really the case) ; or to speak still 

 more intelligibly, a spectator sees an object 

 single, although he looks at it with two eyes. 



This property would seem to belong in an 

 especial manner to the optic nerves, and inas- 

 much as the second pair differ from all the 

 others in possessing a chiasma, there is so far a 

 presumption, that the unity of sensation mani- 

 fested by the optic nerves depends on the 

 chiasma. 



The idea that single vision may be explained 

 by a partial decussation in the chiasma originated 

 (upon theoretical grounds) with Newton ; it has 

 since been adopted by Wollaston, Solly, and 

 others, and many facts have been from time 

 to time brought forward in its favour. The 

 hypothesis may be thus enunciated. " Each 

 tractus opticus sends some filaments across 

 the chiasma to form the inner part of the 

 opposite optic nerve, while its outer filaments 

 continue on to form the outer part of the optic 

 nerve of its own side; the same arrangement 

 of the filaments prevails to the retina, so that 

 the right side of each retina comes from the 

 right tractus opticus, and the left side of each 

 retina from the left tractus opticus ; if then 

 in vision the pictures of an object be depicted 

 simultaneously either on the right sides of 

 the two retinae, or on the left sides of the two 

 retinae, the impressions in either case will be 

 communicated to one and the same tractus opti- 

 cus ; such impressions will of course be referred 

 to one and the same side of the brain, and 

 they will therefore produce the sensation of a 

 single impression only, although in reality two 

 several impressions affect the retina?; the unity 

 of sensation depending on the fact that the two 



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