506 Sir David Brewster on Hemiopsy, or Half- Vision. 



to the margin of the retina, as if they were related to the distri- 

 bution of its blood-vessels and hence it was probable that the 

 paralysis of the corresponding parts of the retina was produced 

 by their pressure. This opinion might have long remained merely 

 a reasonable explanation of hemiopsy, had not a phenomenon 

 presented itself to me which places it beyond a doubt. When 

 I had a rather severe attack, which never took place unless I had 

 been reading for a long time the small print of the ' Times * 

 newspaper, and which was never accompanied either with head- 

 ache or gastric irritation, I went accidentally into a dark room, 

 when I was surprised to observe that all the parts of the retina 

 which were affected were slightly luminous, an effeet invariably 

 produced by pressure upon that membrane. If these views be 

 correct, hemiopsy cannot be regarded as a case of amaurosis, or 

 in any way connected, as has been supposed, with cerebral 

 disturbance. 



Dr. Wollaston endeavoured to explain the phenomena of he- 

 miopsy, and the fact of single vision with two eyes, by what he 

 calls the semidecussation of the optic nerves, a doctrine which Sir 

 Isaac Newton had suggested, and employed to account for single 

 vision*. A fibre of the right-hand side of the optic nerve is sup- 

 posed to semidecussate or divide itself into two fibres, sending one 

 to the right side of the right eye, and another to the right side of 

 the left eye, while a fibre on the left-hand side of the optic nerve 

 also semidecussates, sending one fibre to the left side of the left 

 eye, and another to the left side of the right eye. Hence Sir Isaac 

 Newton drew the conclusion, that an impression on each of the 

 two half-fibres would convey a single sensation to the brain ; and 

 hence Dr. Wollaston concluded that hemiopsy in one eye must 

 be accompanied with hemiopsy in the other. 



Ingenious as these explanations are, the anatomical facts by 

 which alone they could be supported have not been established. 

 Dr. Alison f, who has adopted the opinion of Newton, and rea- 

 soned upon it, admits that the anatomical evidence is still defect- 

 ive ; and the late Mr. Twining J has adduced nine cases of disease 

 in the optic nerves and thalami, which stand in direct opposition 

 to the hypothesis of semidecussation. Dr. Mackenzie, too, 

 adopting the same view of the subject as Mr. Twining, distinctly 

 asserts that ". the great mass of facts in Pathology and Experi- 

 mental Anatomy, touching this question, go to prove that injuries 

 and diseases affecting one side of the brain, instead of hemiopsia 

 in both eyes, produce amaurosis only in the opposite eye." 



* Optics, p. 320. 



f Edinburgh Transactions, vol. xiii. p. 479. 



% Trans. Med. Soc. Calcutta, vol. ii. p. 151 ; or Edinburgh Journal o 

 Science, July 1828, vol. ix. p. 143. 



