204 PRINCIPLES OF PHYSIOLOGY 



of white and grey matter, showing a >char- 

 acteristic structure, and connected with the 

 fibres that come from the retinae of the eyes. 

 They are thus the first recipients of visual 

 impressions transmitted by the fibres of the 

 optic nerves These nervous impulses may 

 call forth contraction of the pupil, the round 

 aperture in the iris of the eye, or they may 

 excite more complex reflexes, along with the 

 action of the other ganglia in front of them, 

 the optic thalami and corpora striata. These 

 impulses, however, do not give rise to con- 

 sciousness. A sensation excited by impulses 

 coming from the eyes arises only when the 

 impulses are transmitted upwards from the 

 corpora quadrigemina to the visual centre in 

 the cerebrum. By the nervous arrangements, 

 also, the nasal and temporal halves of the 

 retinae are correlated to each other. Thus 

 all the fibres from the temporal side of each 

 retina pass to the corpus on the same side, 

 while those from the two nasal halves of the 

 retinae cross and reach the corpus on the other 

 side. Thus the right corpus receives impulses 

 from the temporal side of the right retina and 



