A STUDY OF THE SENSES 



305 



what kind of a stimulus this is that affects the brain. 

 While we may say that an inverted image of the objects 



x 



we look at is formed 011 the retina, we cannot liken this 

 to an impression on a photographic plate. For before we 

 get any sensations of sight, the impulse 

 must reach the cells of the forebrain, 

 and we are sure that nothing like a 

 photograph is taken by these brain 

 cells. We must remember, too, that 

 while our right eye, for instance, re- 

 ceives an inverted image and sends 

 impulses to the left half of the brain, 

 we recognize the objects which we 

 perceive as right side up and in their 

 proper relations in space. 



The Blind Spot and the Yellow Spot. 

 The optic nerve does not enter the 

 eyeball at a point exactly behind the 

 center of the crystalline lens and 

 cornea, but in a region somewhat 

 nearer the inidlme. Since the optic 

 fibers pass through all three coats and 

 then spread over the inner lining of 

 the retina, the region where the nerve 

 enters the eye is without any rods and 

 cones. The following simple experi- 

 ment proves this spot to be blind. If 

 one closes the left eye, holds this page about a foot away, 

 and looks steadily at the cross near the top of the page with 

 the right eye, one can also see more or less distinctly the 

 large black dot. Let the book be slowly brought nearer, 

 however, and the cross only is seen, for the dot has disap- 

 peared from view. If the page is brought still nearer to the 

 eye, both cross and dot are seen again. 



FIG. 140. 



