270 Zoological Proceedings of Societies. 



of a particular species of blindness, however, has led Dr. WoUas- 

 ton to a somewhat different distribution of the optic nerves. After 

 fatigue, arising from four or live hours' violent exercise. Dr. Wol- 

 laston was affected by a partial blindness, of which he first be- 

 came sensible by seeing only half the face of a person near him, 

 and next by seeing only the termination " son" of the name 

 " Johnson 3" this blindness was to the left of the point of vision 

 in each eye ; it was not perfect darkness, but merely a dark 

 shade ; and in about fifteen minutes, it gradually passed off, in 

 an oblique direction upwards towards the left. As it was refer- 

 able to an affection of the nerves. Dr. W. did not apprehend or 

 experience any return of it, other nervous affections being pro- 

 duced by fatigue. Some years afterwards he again experienced 

 this singular kind of blindness, without any obvious cause, and 

 first became sensible of it likewise by seeing only the half of a 

 person's face ; but in this case the right side of both eyes was 

 affected, and complete vision was suddenly restored by the joy 

 produced on receiving information of the safe arrival of a friend 

 from a hazardous enterprize. Dr. Wollaston has a friend who has 

 experienced the same affection for seventeen years past, when- 

 ever his stomach is considerably deranged : another friend was 

 attacked by pain at the left temple, and at the back of the left 

 eye, which was succeeded by this sort of blindness on the right 

 side of each eye ; he can see to write, — see the paper he is 

 writing upon, and the pen he writes with, — but not the hand 

 that guides the pen. The affection in this case. Dr. W. fears, 

 is a permanent one ; the pain first experienced seems to have 

 arisen from some effusion causing a degree of pressure on the 

 brain, and the blindness from the continuance of this pressure 

 on the left thalamus nervorum opticorum. 



Now all these cases seem referable to the partial insensibility 

 of each retina, and they indicate that the left side of the retina 

 in each eye is supplied with nerves from the same thalamus, and 

 the right from the opposite thalamus ; so that the nerves supply- 

 ing the former alone decussate, and not those of the right side ; 

 an arrangement which Dr. Wollaston calls the scmi-dccussation 

 of the optic nerves. 



