228 
Dr. Wollaston on semi-decussation 
sight similar to those which had happened to myself, but 
more extensive, and it has unfortunately been far more per- 
manent. In this case the blindness was at that time, and 
still is, entire, with reference to all objects situated to the 
right of his center of view. Fortunately, the field of his 
vision is sufficient for writing perfectly. He sees what he 
writes, and the pen with which he writes, but not the hand 
that moves the pen. This affection is, as far as can be ob- 
served, the same in both eyes, and consists in an insensibility 
of the retina on the left side of each eye. It seems most pro- 
bable, that some effusion took place at the time of the original 
pain on that side of the head, and has left a permanent com- 
pression on the left thalamus. This partial blindness has now 
lasted so long without sensible amendment, as to make it very 
doubtful when my friend may recover the complete percep- 
tion of objects on that side of him. 
In reviewing the several phenomena that I have described, 
we find partial blindness occurring at the same time in both 
eyes. This sympathy from disease is readily explained, on 
the supposition that the parts which sympathise receive their 
nerves from the same source, while the opposite halves of the 
eyes, which are not at the same time similarly affected, are 
supplied from an opposite source ; and the inference is im- 
mediate, that in common vision also the sympathy of corres- 
ponding points, which receive similar impressions from the 
same object, is dependent on the arrangement of nerves thus 
detected by disease. 
We find moreover in the sturgeon, (and it is the same in 
some other fishes) whose eyes can scarcely see the same 
object at once, and have no corresponding points which ordi- 
