and the union of the Optic Nerves. 151 



It is worthy of notice, that dissections have shown some in- 

 stances, wherein there was not any union of the optic nerves at 

 the sella turcica in man ; each optic nerve proceeding directly 

 to the eye of the same side, and no peculiarity of vision result- 

 ed from such structure. 



Obs. 8. — Morgagni, in Book i. Letter 13, Art. 7, mentions 

 that Vesalius " had observed the optic nerves to remain sepa- 

 rated through their whole course, in a man who had always 

 very strong sight.'" He refers to Epist. Anat. 16. p. 14. Mor- 

 gagni also states, that Aquapendente and Valverda had found 

 the optic nerves in like maimer not united ; but that these two 

 authors had not ascertained if any peculiarity or disorder of 

 vision existed in those persons during life. Vesalius says,— 

 " His ille accessit cujus nervos visorios illo de quo hie sermo 

 est, congressu invicem non connasci, neque sese contingere, vi- 

 dimus : sed dexter nonnihil ea sede, qua calvariam agressurus 

 fuerat, sinistrorsum, et sinister nonnihil dexjrorsum reflecteba- 

 tur, quasi non coalitus occasione nervi congrederentur, verum 

 ut commode per suum foramen e calvaria prociderent : notissi- 

 mum quum etiam hoc ductu progredientes, in oculi posterioris 

 sedis medium non inserantur. Quam sedulo autem ac sollicite 

 ejus viri, cui in eum modum nervi dehiscebant, familiares, num 

 illi omnia gemina perpetuo oculis obversarentur, interrogaveri- 

 mus, neminem naturae operum cognitione flagrantem ambigere 

 sat scio ; at nihil aliud rescissere licuit, quam ipsum de visu 

 nunquam conquestum fuisse, visuque praestante semper valu- 

 isse, familiaresque de visorum duplicatione nihil unquam in- 

 tellexisse." 



Obs. 9. — Mr Cheselden relates the case of " a gentleman 

 who had strabismus, with double vision, produced by a blow 

 on the head. By degrees the most familiar objects came to ap- 

 pear single again ; and in time all objects did so, without any 

 amendment of the distortion." See note to p. 171 of Tra- 

 vers's Synopsis of Diseases of the Eyes. This fact shows, that 

 points of the retinas, not originally endowed with the joint 

 possession of the correspondence, supposed by Sir Isaac New- 

 ton and Dr Wollaston, to depend on peculiar distribution of 

 the optic nerves in the retinas, may by habit acquire that cor- 

 respondence. Therefore, independent of the evidence of the 



