296 Dr. WolhiHton oA [Oct. 



It is now about fifteen months since a similar affection oc- 

 curred again to myself, without my being able to assign any 

 cause whatever, or to connect it with any previous or subsequent 

 indisposition. The blindness was first observed, as before, in 

 looking at the face of a person I met, whose left eye was to my 

 sight obliterated. My blindness was in this instance the reverse 

 of the former, being to my right (instead of the left) of the spot 

 to which my eyes were directed ; so that I have no reason to 

 suppose it in any manner connected with the former affection. 



The new punctum caecum was situated alike in both eyes, and 

 at an angle of about three degrees from the centre ; for when 

 any object was viewed at the distance of about five yards, the 

 point not seen was about ten inches distant from the point 

 actually looked at. 



On this occasion the affection, after having lasted with little 

 alteration for about twenty minutes, was removed suddenly and 

 entirely by the excitement of agreeable news respecting the safe 

 arrival of a friend from a very hazardous enterprise. 



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 hypothesis of the decus- 

 sation 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 fila- 

 ments 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 nervorum opticorum. 



It is plain 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 thalamus nervorum opticorum. 



According to this supposition, decussation will take place 

 only between the adjacent halves of the two nerves. That por- 

 tion of nerve which proceeds from the right thalamus to the 

 right side of the right eye, passes to its destination 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. 



Now, if we consider rightly the facts discovered by compara- 

 tive anatomy in fishes, we shall find that the crossing;: of the 



