1824.] Semi-decussation of the Optic Nerves. 295 



It is well known that in the human brain these nerves, after 

 passing forwards to a short distance from their origin in the 

 thalami nervorum opticorum, unite together, and are, to appear- 

 ance, completely incorporated ; arid that from this point of union 

 proceed two nerves, one to the right, the other to the left eye. 



The term decussation was applied to this united portion 

 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 supplied entirely from fibres arising from 

 the left thalamus. 



In this opinion, anatomists have felt themselves confirmed bv 

 the result of their examination of other animals, and especially 

 that of several species offish, in which it is distinctly seen that 

 the nerves do actually cross each other as a pair of separate 

 cords, lying in contact at their crossing, but without any inter- 

 mixture of their fibres. 



Jn these cases it is most indisputably true, that the eye upon 

 the right side of the animal does receive its optic nerve from the 

 left side of the brain, while that of the left eye comes from the 

 right side; but it is not a just inference to suppose the same 

 continuity preserved in other animals, where such complete 

 separation of the entire nerves is not found. 



On the contrary, I not only see reason, from a species of 

 blindness which has happened to myself more than once, to 

 conclude, that a different distribution of nerves takes place in 

 us, but 1 think my opinion supported by this evident difference 

 of structure in fishes. 



It is now more than twenty years since I was first affected 

 with the peculiar state of vision, to which I allude, in conse- 

 quence of violent exercise I had taken for two or three hours 

 before. I suddenly found that I could see but half the far.e of 

 a man whom I met; and it was the same with respect to every 

 object I looked at. In attempting to read the name Johnson, 

 over a door, 1 saw only son ; the commencement of the 



name being wholly obliterated to my view. In this instance 

 the loss of sight was toward my left, and was the same whether 

 I looked with the right eye or the left. This blindness was not 

 so complete as to amount to absolute blackness, but was a 

 shaded darkness without definite outline. The complaint was 

 of short duration, and in about a quarter of an hour might be said 

 to be wholly gone, having receded with a gradual motion from 

 the centre of vision obliquely upwards toward the left. 



Since this defect arose from over fatigue, a cause common to 

 many other nervous affections, I saw no reason to apprehend 

 any return of it, and it passed away without need of remedy, 

 without any farther explanation, and without my drawing any 

 useful inference from it. 



