of the Optic Nerves, 139 



A series of evidence in such apparent harmony throughout, 

 seems clearly to establish that distribution of nerves I have 

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

 sation of the optic nerves. 



On single Vision with two Eyes. 



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

 eyes is confined to the performance of healthy eyes in com- 

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

 upon the mind, 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 sympathy of the parts, without endeavouring to trace 

 further the origin of that sympathy, or the reason why, in in- 

 fancy, the eyes ever assume one certain direction of corre- 

 spondence in preference to squinting. 



But, when we regard sympathy as arising from structure, 

 and dependent on connection 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 •correspond- 

 ing direction to their eyes, and we clearly gain a step in the 

 solution, if not a full explanation, of the long agitated ques- 

 tion 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 reason express surprise at its having so far escaped no- 

 tice*, were I not aware how many facts commonly remain 

 disregarded, merely for want of explanation. It is evident 

 that I once, and for a long time, overlooked the inference 

 that is to be drawn from this affection ; and if the disorder 

 had not happened to me a second time, I might never have 

 reconsidered its cause. 



Even since the preceding pages were written, I have met 

 with two more cases of this disease. One of my friends has 

 been habitually subject to it for sixteen or seventeen years, 

 whenever his stomach is in any considerable degree deranged. 



• Richtcr, in the third volume of his Elements of Surgery, has a chapter 

 on half-blindness, and part of it relates to what he terms amaurosis dimi- 

 iliat;i. From one instance there given, he seems to have seen some cases 

 similar to those I have described; but he has not noticed the correspond- 

 ing affection of die two eyes, or considered the sympathy between them. — 

 Anfangs-griinde I)c> Wundailzcncykunst, vol. iii. chap. 10, p. 478. 



S2 In 



