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their sympathy is evidently from structure, and not from mere habit 

 of feeling together. Any two corresponding points must be supplied 

 with a pair of filaments from the same nerve ; and the seat of a dis- 

 ease in which similar parts of both eyes are affected, must be con- 

 sidered as situated at a distance from the eyes, at some place in the 

 course of the nerves where these filaments are still united, and pro- 

 bably in one or other thalamus. It is plain, he continues, 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. Upon this suppo- 

 sition, decussation will take place only between the adjacent halves 

 of the two nerves. That portion of nerve which proceeds from the 

 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 with or without inter- 

 mixture of their fibres. The crossing of the entire nerves to the 

 opposite eyes in fishes, Dr. Wollaston observes, is in conformity with 

 this view of the arrangement of the human optic nerves ; for in the 

 sturgeon, for instance, the eyes are placed so exactly back to back, 

 that there are no corresponding points of vision requiring to be sup- 

 plied with fibres from the same nerve. In this animal, an injury to 

 the left thalamus might be expected to occasion entire blindness of 

 the right eye alone ; in ourselves a similar injury would occasion 

 blindness to all objects situated to our right, owing to insensibility 

 of the left half of the retina of both eyes. Dr. Wollaston states some 

 other facts, illustrating his view of this peculiar distribution of the 

 human optic nerves, remarking that in common vision also the sym- 

 pathy of corresponding points, which receive similar impressions from 

 the same object, is dependent upon the same arrangement of nerves 

 to which the term Semi-decussation may be applied. In conclusion, 

 Dr. Wollaston remarks, that so long as our consideration of the func- 

 tions 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 mind, though two images are formed on corresponding 

 parts of the retina, we may rest satisfied in ascribing the apparent 

 unity of the impression to habitual sympathy of the parts ; but when 

 we regard sympathy as arising from structure, and dependent on 

 connexion of nervous fibres, we therein see a distinct origin of that 

 habit, and have presented to us a manifest cause why infants first 

 begin to give the corresponding direction to their eyes, and clearly 

 gain a step in the solution, if not a full explanation, of the long- 

 agitated question of single vision with two eyes. 



