SIGHT 145 



retina they have to pass through it at one spot in order to 

 get outside the eye and form the optic nerve running to the 

 brain. At this spot there cannot of course be any rods or 

 cones and it is consequently bhnd. The existence of the 

 bhnd spot is easily confirmed by the well-known trick with a 

 spot and a cross on a piece of paper. The blind spot lies 

 towards one side of the eye and the part of the image that 

 falls on it in the right eye is different from that falling on it in 

 the left. Consequently in combining the pictures obtained 

 from the two eyes there is an overlap which covers up the 

 hole in each separate picture. All the vertebrate animals have 

 a blind spot, but among the invertebrates the octopuses and 

 squids are without this defect. In many ways their eyes are 

 very much like those of the higher animals but the retina is 

 not inverted; the rods and cones are on the "right side" of 

 the retina and so all the nerve fibres are at the back and do 

 not have to pass through it to get outside the eye. 



