TUBERCULA QUADRIGEMIXA. 437 



ing, by intervening areolar tissue. Impressions made on the right 

 eye must therefore be perceived on the left side of the brain ; while 

 those which enter the left eye are conveyed to the right side of the 

 brain. 



In birds, also, the axes of the two eyes are so widely divergent 

 that an object cannot be distinctly in focus for both of them at the 

 same time. The optic nerves are here united, and apparently sol- 

 dered together, at their point of crossing ; but the decussation of 

 their fibres is nevertheless complete. (Fig. 148.) The nervous fila- 

 ments coming from the left side pass altogether over to the right ; 

 and those coming from the right side pass over to the left. The 

 result of direct experiment on the crossed action of the tubercles in 

 these animals corresponds with the anatomical arrangement of the 

 nervous fibres. If one of the optic tubercles be destroyed in the 

 pigeon, complete blindness is at once produced in the eye of the 

 opposite side ; but vision remains unimpaired in the eye of the side 

 on which the injury was inflicted. 



Fitr. 149. 



COTRSK OF OPTIC NERVES rx MAX. 1, 2. Right and left eyeballs. 3. Dccu sation of optic 

 nerve<. 4, 4. Tubercnla qnadrigemina. 



In the human subject, on the other hand, where the visual axes 

 are parallel, and where both eyes are simultaneously directed toward 



