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of the optic nerves. 
narily sympathise, that the two eyes do not receive any ner- 
vous fibres from the same source ; but one eye receives its 
nerve wholly from one side, and the other from the other side 
of the brain. 
From the structure of these fish we learn distinctly, that 
the perception of objects toward one side is dependent on 
nerves derived from the opposite side of the brain ; and in 
the last case of diseased vision above related, we find apparent 
injury to one side of the brain, followed by blindness to- 
ward the opposite side of the point to which both eyes are 
directed. 
A series of evidence in such apparent harmony throughout, 
seems clearly to establish that distribution of nerves I have 
endeavoured to describe, which may be called the semi- 
decussation of the optic nerves. 
On single vision with two eyes. 
So long as our consideration of the functions of a pair of 
eyes is confined to the performance of healthy eyes in com- 
mon vision, when we remark that only one impression is 
made upon the mind, though two images are formed at the 
same moment on corresponding parts of our two eyes, we 
may rest satisfied in ascribing the apparent unity of the im- 
pression to habitual sympathy of the parts, without endea- 
vouring to trace farther the origin of that sympathy, or the 
reason why, in infancy, the eyes ever assume one certain di- 
rection of correspondence in preference to squinting. 
But, when we regard sympathy as arising from structure, 
and dependent on connection of nervous fibres, we therein 
see a distinct origin of that habit, and have presented to us a 
