226 Dr. Wollaston on semi-decussation 
In reflecting upon this subject, a certain arrangement of 
the optic nerves has suggested itself to me, which appears 
to afford a very probable interpretation of a set of facts, 
which are not consistent with the generally received hypo- 
thesis of the decussation of the optic nerves. 
Since the corresponding points of the two eyes sympathise 
in disease, their sympathy is evidently from structure, not 
from mere habit of feeling together, as might be inferred, 
if reference were had to the reception of ordinary impressions 
alone. Any two corresponding points must be supplied with 
a pair of filaments from the same nerve, and the seat of a 
disease in which similar parts of both eyes are affected, must 
be considered as situated at a distance from the eyes at some 
place in the course of the nerves where these filaments are 
still united, and probably in one or the other thalamus ner- 
vorum opticorum. 
It is plain that the cord, which comes finally to either eye 
under the name of optic nerve, must be regarded as con- 
sisting of two portions, one half from the right thalamus, and 
the other from the left thalamus nervorum opticorum. 
According to this supposition, decussation will take place 
only between the adjacent halves of the two nerves. That 
portion of nerve which proceeds from the right thalamus to 
the right side of the right eye, passes to its destination with- 
out interference ; and in a similar manner the left thalamus 
will supply the left side of the left eye with one part of its 
fibres, while the remaining halves of both nerves in passing 
over to the eyes of the opposite sides must intersect each 
other, either with or without intermixture of their fibres. 
Now, if we consider rightly the facts discovered by com- 
