TUBERCULA QUADEIGEMIXA. 397 



they Lave been operated upon, the action in vision is crossed ; 

 i. e., when the lobe is removed upon one side, the sight is 

 lost in the opposite eye, vision in the eye upon the same 

 side being unimpaired. We have long been in the habit, in 

 class-demonstrations, of removing the optic lobe on one side 

 from a pigeon, with the result just mentioned. The opera- 

 tion is quite simple : A part of the skull is removed by the 

 side of one hemisphere, and the optic lobe is seen, 'in the 

 form of a large, white tubercle, between the posterior por- 

 tion of the cerebrum and the cerebellum. A little slit is 

 then made in its capsule, and the interior is broken up care- 

 fully with a delicate forceps. The animal generally recovers 

 from the operation, blinded in the eye upon the opposite 

 side. In removing the portion of the skull, it is well not to 

 go too for back, when there is danger of wounding the great 

 venous sinus and complicating the operation by haemorrhage. 



In treating of the special sense of sight, in the next and last 

 volume, we shall see that the decussation of the optic nerves 

 is more complex in man than in birds, in which the nerve 

 from one optic lobe passes totally and exclusively to the eye 

 upon the opposite side. In man, most of the fibres of the 

 optic nerve from one side pass to the eye upon the opposite 

 side ; but a few fibres pass to the eye upon the same side, a 

 few connect the tubercles upon the two sides, and a few con- 

 nect the two eyes. It is not known whether or not, in man, 

 the action of the tubercles in vision is exclusively crossed, as 

 it appears to be in most of the inferior animals. 



The optic lobes undoubtedly serve as the sole centres 

 presiding over the sense of sight, and not merely as avenues 

 of communication of this sense to the cerebral hemispheres. 

 A positive proof of this proposition lies in the fact that the 

 sense of sight is preserved after complete removal of the 

 cerebrum, provided that injury of the tubercles have been 

 carefully avoided. 



AVe shall say nothing, in this connection, with regard to 

 the movements of the iris, except that the reflex action by 



