of the Optic Nerves. 137 



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 consist- 

 ing 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- 

 parative anatomy in fishes, we shall find that the crossing of 

 the entire nerves in them to the opposite eyes, is in perfect 

 conformity to this view of the arrangement of the human optic 

 nerves. The relative position of the eyes to each other in 

 the sturgeon, is so exactly back to back, on opposite sides of 

 the head, that they can hardly see the same object ; they can 

 have no points which generally receive the same impressions 

 as in us ; there are no corresponding points of vision requiring 

 to be supplied with fibres from the same nerve. The eye 

 which sees to the left has its retina solely upon its right side ; 

 and this is supplied with an optic nerve arising wholly from 

 the right thalamus ; while the left thalamus sends its fibres 

 entirely to the left side of the right eye for the perception of 

 objects situated on the right. In this animal, an injury to the 

 left thalamus might be expected to occasion entire blindness 

 of the right eye alone, and want of perception of objects 

 placed on that side. In ourselves, a similar injury to the left 

 thalamus would occasion blindness (as before) to all objects 

 situated to our right, owing to insensibility of the left half of 

 the retina of both eyes. 



A disorder that has occurred within my own knowledge in 

 the case of a friend, seems fully to confirm this reasoning, as 

 far as a single instance can be depended upon. After he had 

 -.ulli-ivd severe pain in his head for some days, about the left 



Vol. 64. No. 31G. Aug. 1821. S temple, 



