SIGHT. 579 



Optic nerves. Such facts have been thought a reason for be- 

 lieving the decussation of the optic nerves partial, and some say 

 that the outer portion of the tractus optici goes to the outer part 

 of the corresponding nerves, and the inner to the inner portion of 

 the opposite. But Dr. Magendie divided from before backwards 

 the junction of the optic nerves, and found blindness induced. 



The decussation of the optic nerves is shown by blindness of 

 one eye being induced if the nerve on the same side is divided 

 anteriorly to the union, and of the opposite eye if the division is 

 made posteriorly to the union : or by destruction of an eye caus- 

 ing the nerve of the same side to waste as far as the union, and 

 of the opposite side beyond the union.? Yet cases are on record 

 where the wasting of the nerve in loss of sight continued through- 

 out on the same side, but such are probably suspicious. 



The thalami optici are improperly named, as they do not give 

 origin to the optic nerves. These may be traced to the anterior 

 corpora quadrigemina, pressure or disease of which produces 

 blindness, and which waste if the nerves waste, i Sommerring 

 first noticed this in blind horses. Gall confirmed his observations. 

 Dr. Vimont states that, on examining fourteen old blind horses, 

 he found the anterior quadrigeminum opposite the blind eye 

 lessened in all, and completely atrophied in two. He then 

 cruelly took out the left eye of four rabbits, the right of four 

 others, and both eyes of another. At the expiration often months 

 he killed them all, and found the right anterior quadrigeminum 

 much smaller than the left in the first four, the reverse in the 

 second four ; and both the anterior and posterior much smaller in 

 the rabbit deprived of both eyes than the healthy tubercles of the 

 other eight. The optic nerve of all the blind eyes had lost ^ of 

 its volume, and looked like the horn of a lantern. Dr. Magendie 

 informed Dr. Vimont that the atrophy of the tubercle occurs 

 much more quickly in birds, and Dr. Vimont found this to be the 

 fact.' 



Drs. Carus and Tiedemann make Gall consider the posterior 

 quadrigemina as the roots of the olfactory pair, whereas he has 



:'. ov; r.i. 



Precis de Physiologic, t. i. p. 71. 



P Dr. Magendie, 1. c. ibid. 



q On the optic nerve consult Gall, I.e. 4to. t. i. p. 113. sqq. 



r 1. c. t. i. p. 296. sq. 



Q Q 3 



