1824.] Semi-dec ussat ion of the Optic Nevces. 297 



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

 ity 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 simi- 

 lar 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 suffered 

 severe pain in his head for some days, about the left temple, and 

 toward the back of the left eye, his vision became considerably 

 impaired, attended with other symptoms indicating a slight 

 compression on the brain. 



It was not till after the lapse of three or four weeks that I saw 

 him, and found that, in addition to other affections which need 

 not here be enumerated, he laboured under a defect of sight 

 similar to those which had happened to myself, but more exten- 

 sive, and it has unfortunately been far more permanent. In 

 this case the blindness was at that time, and still is, entire, 

 with reference to all objects situated to the right of his centre 

 of view. Fortunately, the field of his vision is sufficient for 

 writing perfectly. He sees what he writes, and the pen with 

 which he writes, but not the hand that moves the pen. This 

 affection is, as far as can be observed, the same in both eyes, 

 and consists in an insensibility of the retina on the left side of 

 each eye. It seems most probable, that some effusion took 

 place at the time of the original pain on that side of the head, 

 and has left a permanent compression on the left thalamus. 

 This partial blindness has now lasted so long without sensible 

 amendment, as to make it very doubtful when my friend may 

 recover the complete perception of objects on that side of him. 



In reviewing the several phenomena that I have described, we 

 find partial blindness occurring at the same time in both eyes. 

 This sympathy from disease is readily explained, on the suppo- 

 sition that the parts which sympathise receive their nerves from 

 the same source, while the opposite halves of the eyes, which 



