of the Optic Nerves. 135 



at different distances, limiting thereby, what in optics is 

 termed the spherical aberration of the lens. 



The subject of my inquiry relates solely to the course by 

 which impressions from images perfectly formed are conveyed 

 to the sensorium, and to that structure and distribution of the 

 optic nerves on which the communication of these impressions 

 depends. 



Without pretending to detect by manual dexterity as an 

 anatomist, the very delicate conformation of the nerves of 

 vision, I have been led, by the casual observation of a few 

 instances of diseased vision, to draw some inferences respect- 

 ing the texture of that part which has been called the decus- 

 sation of the optic nerves, upon which I feel myself warranted 

 to speak with some confidence. 



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 ap- 

 pearance, completely incorporated ; and 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 

 by the result of their examination of other animals, and es- 

 pecially 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 intermixture of their fibres. 



In 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 infei'ence 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 I 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. 



