138 Dr. Wollaston on Semi-decussation 



temple, and toward the back of the left eye, his vision became 

 considerably impaired, attended with other symptoms indi- 

 cating 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 extensive, and it has unfortunately been far more per- 

 manent. 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 ob- 

 served, the same in both eyes, and consists in an insensibility 

 of the retina on the left side of each eye. It seems most pro- 

 bable, that some effusion took place at the time of the original 

 pain on that side of the head, and has left a permanent com- 

 pression 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 percep- 

 tion of objects on that side of him. 



In reviewing the several phaenomena 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 supposition that the parts which sympathize receive their 

 nerves from the same source, while the opposite halves of the 

 eyes, which are not at the same time similarly affected, are 

 supplied from an opposite source ; and the inference is im- 

 mediate, that in common vision also the sympathy of corre- 

 sponding 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 ob- 

 ject at once, and have no corresponding points which ordi- 

 narily sympathize, that the two eyes do not receive any ner- 

 vous 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 brain, followed by blindness toward 

 the opposite side of the point to which both eyes are directed. 



A series 



