SIR DAVID BREWSTER ON HEMIOPSY, OR HALE-VISION. 17 



oblique vision, has yet a very different origin. The parts which are in these cases 

 affected extend irregularly from the foramen centrale to the margin of the retina, 

 as if they were related to the distribution 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 a reasonable expla- 

 nation 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 headache 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 effect invari- 

 ably 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 hemiopsy, 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 

 supposed to decussate 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 decussates, 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 con- 

 cluded 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 reasoned upon it, admits that the anatomical 

 evidence is still defective ; and the late Mr Twining^ 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 Experimental 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." 



The two great facts of hemiopsy in both eyes, and of what is called single 

 vision with two eyes, do not require the hypothesis of semidecussation to explain 



* Optics, p. 320. 



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



\ Trans. Med. Soc, Calcutta, vol. ii. p. 151 ; or, Edin. Journal of Science, July 1828, vol. 

 ix. p. 143. 



