Mr Twining on Single Vision, fyc. 143 



Art. XXIV. — On Single Vision and the Union of the Optic 

 Nerves.* By W. Twining, Esq. 



The phenomenon of single vision with two eyes, each of which 

 represents a picture of the object we look at, has received va- 

 rious explanations, and has been the subject of much inquiry. 

 Dr Wollaston has lately promulgated an opinion on this sub- 

 ject, which I propose to bring before the Society, for the pur- 

 pose of noticing some observations respecting the structure of 

 the optic nerves and thalami in a healthy state,-|* as well as 

 investigations of the changes produced by disease, which pre- 

 clude us from admitting Dr Wollaston's premises, or adopting 

 his conclusions. 



Dr Wollaston believes, that the faculty of single vision with 

 two eyes may be attributed to a semidecussation of the optic 

 nerves; namely, that the contiguous half of each optic nerve, 

 on reaching the sella turcica, and there uniting with its fellow, 

 does cross and ultimately serve to furnish retina to the nasal 

 side of the opposite eye ; the retina of the temporal side of 

 each eye being formed by the expansion of half of the corre- / 

 sponding nerve, while the retina on the nasal side of each eye is 

 supplied by the expansion of half of the nerve from the oppo- 

 site side. This semidecussation and distribution of the nerves, 

 though not within the reach of anatomical demonstration, Dr 

 Wollaston considers established by induction, from the symp- 

 toms of disease in some instances which he relates. 



A case nearly similar to those published by Dr Wollaston, 

 came under my care about four years ago. C. D. a lady about 

 twenty years of age, and in perfect health, was thrown from her 

 horse, while taking exercise in a riding-school. In falling, the 

 left shoulder and left side of the head struck against the boarded 



* From the Transactions of the Medical and Physical Society of Calcutta, 

 Vol. ii. p. 151. 



+ '/ Vesalius, Valverda, Aquapendens, and Losselius, sometimes found 

 the optic nerves separated through their whole course from the Drain to 

 the eyes ; and yet persons whose optic nerves were so separated during life, 

 saw objects single as other men do ; which would have been impossible, if 

 this single appearance had depended on the conjunction of those nerves." 

 See Porterfield's 1st vol. on the Eye, &c. p. 194, of the Retina and Optic 

 Nerve. 



