298 On Semi-decussation of the Optic Nerves. [Oct. 



are not at the same time similarly affected, are supplied from an 

 opposite source ; and the inference is immediate, that in com- 

 mon vision also the sympathy of corresponding points, which 

 receive similar impressions from the same object, is dependent 

 on the arrangement of nerves thus detected by disease. 



We find moreover in the sturgeon (and it is the same in some 

 other fishes), whose eyes can scarcely see the same object at 

 once, and have no corresponding points which ordinarily sym- 

 pathise, that the two eyes do not receive any nervous fibres 

 from the same source; but one eye receives its nerve wholly 

 from one side, and the other from the other side of the brain. 



From the structure of these fish we learn distinctly, that the 

 perception of objects toward one side is dependent on nerves 

 derived from the opposite side of the brain ; and in the last case 

 of diseased vision above related, we find apparent injury to one 

 side of the brein, followed by blindness toward the opposite side 

 of the point to which both eyes are directed. 



A .series of evidence in such apparent harmony throughout, 

 seems clearly to establish that distribution of nerves 1 have 

 endeavoured to describe, which may be called the semi-decussa- 

 tion of the optic nerves. 



On Single Vision with Tico Eyes. 



So long as our consideration of the functions of a pair of 

 eyes is confined to the performance of healthy eyes in common 

 vision, when we remark that only one impression is made upon 

 the mmd, though two images are formed at the same moment 

 on corresponding parts of our two eyes, we may rest satisfied in 

 ascribing the apparent unity of the impression to habitual sym- 

 pathy of the parts, without endeavouring to trace farther the 

 origin of that sympathy, or the reason why, in infancy, the eyes 

 ever assume one certain direction of correspondence in prefer- 

 ence to squinting. 



But, when we regard sympathy as arising from structure, and 

 dependent on connexion of nervous fibres, we therein see a dis- 

 tinct origin of that habit, and have presented to us a mani- 

 fest cause why infants first begin to give the corresponding di- 

 rection to their eyes, and we clearly gain a step in the solution, 

 if not a full explanation, of the long agitated question of single 

 vision with two eyes. 



It may perhaps to some persons appear surprising, that so 

 many as three instances of a disorder which they presume to 

 be rare, should have been witnessed by one individual ; but I 

 apprehend, on the contrary, this half-blindness to be far more 

 common than is generally supposed : and I might with as much 



