148 Mr Twining on Single Vision, 



" Hence it appears, — 



" 1**, Why the two images of both eyes make but one 

 image in the brain. 



" 9d, Why, when one eye is distorted, objects appear double. 



" 3d, Why, though one thing may appear in two places, 

 yet two things cannot appear in one place. 



" 4ith, Why, if one of the branches of the nerve beyond 

 the juncture should be cut, that half of both eyes towards 

 the wounded nerve would be blind, the other half remaining 

 perfect." 



This opinion of Sir Isaac Newton, though connected with a 

 belief that the influence of the brain is communicated through 

 the nerves by a most subtile nervous fluid, still mainly rests on 

 the assumption of a semidecussation of the nervous fibres. I 

 have only copied such parts of Sir Isaac Newton's explanation 

 as immediately apply to the point in question ; his reasoning 

 and the diagram will be found at p. 108 of Harris's Treatise 

 on Optics, 



The following anatomical observations respecting the struc- 

 ture of the optic nerves and thalami, and the effects of disease 

 on those parts, appear sufficiently to establish the fact, that no 

 decussation or semidecussation of the optic nerves exist in the 

 human subject. 



Obs. 1. — Mrs Scot had a fungus of the left eye, for which 

 the eye was extirpated. Several months afterwards the patient 

 died ; and on dissection, the left optic nerve was of inky black- 

 ness, and this dark colour extended backwards from the orbit 

 far beyond the point where the nerves join. The diseased nerve 

 within the cranium was as thick as the little finger, and the cor- 

 responding thalamus new. optic, was about a third larger than 

 the opposite one, but of natural structure. The dark colour 

 above-mentioned was confined to the left side. On the right 

 side the nerve was of its natural size and colour, and was merely 

 attached to the black diseased nerve of the opposite side by 

 cellular shreds, where the nerves come in contact on the sella 



turcica. 



This patient had never observed any affection of her eye 

 until two years before the operation, when the morbid changes 

 commenced ; and in the course of four months she became 



