260 Proceedings of the Royal Society. 
under the supposition that, though the fibres do intermix, they 
still continue onward in their original direction, and that those 
from the right side cross over wholly to supply the left eye, while 
the right eye is similarly supplied by fibres from the left thalamus. 
Anatomists have considered this opinion as confirmed by the cir- 
cumstance of the nerves actually crossing each other as two per- 
fectly distinct cords in certain fish. ‘The author, however, from a 
species of blindness under which he has more than once suffered, 
concludes that a different distribution of the nerves takes place 
in the human subject. This peculiar state of vision consisted in 
seeing only half of every object, the loss of sight being, in both 
eyes, towards the left, and of short duration only. In reflecting 
upon this subject a certain arrangement of the optic nerves, not 
consistent with the generally received hypothesis of their decssa~ 
tion, occurred to him. Since the corresponding points of the two, 
eyes, he observes, sympathize in disease, their sympathy is evi- 
dently from structure, and not from mere habit of feeling together. 
Any two corresponding points must be supplied with a pair of fila- 
ments from the same nerves, and the seat of a disease in which 
similar parts of both eyes are affected, must be considered as si- 
tuated 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 other thalamus. It is plain, therefore, that the cord 
which comes finally to either eye, under the name of optic nerve, 
must be regarded as consisting of two portions, one half from the 
right thalamus, and the other from the left. Upon 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 des- 
tination without 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. 
Dr. W. observes that the crossing of the nerves to the opposite 
eyes in fish, is in conformity with this view of the arrangement of 
