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of the optic nerves. 
parative anatomy in fishes, we shall find that the crossing 
of the entire nerves in them to the opposite eyes, is in perfect 
conformity to this view of the arrangement of the human 
optic nerves. The relative position of the eyes to each other 
in the sturgeon, is so exactly back to back, on opposite sides 
of the head, that they can hardly see the same object ; they 
can have no points which generally receive the same impres- 
sions as in us ; there are no corresponding points of vision 
requiring to be supplied with fibres from the same nerve. 
The eye which sees to the left has its retina solely upon its 
right side ; and this is supplied with an optic nerve arising 
wholly from the right thalamus ; while the left thalamus 
sends its fibres entirely to the left side of the right eye for 
the perception of objects situated on the right. In this animal, 
an injury to the left thalamus might be expected to occasion 
entire blindness of the right eye alone, and want of perception 
of objects placed on that side. In ourselves, a similar injury 
to the left thalamus would occasion blindness (as before) to 
all objects situated to our right, owing to insensibility of the 
left half of the retina of both eyes. 
A disorder that has occurred within my own knowledge in 
the case of a friend, seems fully to confirm this reasoning, 
as far as a single instance can be depended upon. After he 
had suffered severe pain in his head for some days, about 
the left temple, and toward the back of the left eye, his 
vision became considerably impaired, attended with other 
symptoms indicating a slight compression on the brain. 
It was not till after the lapse of three or four weeks that I 
saw him, and found that, in addition to other affections which 
need not here be enumerated, he laboured under a defect of 
