THE EYE AND THE PHYSIOLOGY OF VISION. 549 



is visible on the retina as a yellowish depression. When 

 we wish to see objects in detail, such, for instance, as the 

 print on a page, we allow the image to fall on this yellow 

 spot. Images which fall on other portions of the retina are 

 still visible but not distinctly so. At the spot where the 

 optic nerve enters the eye the retina has no sensitiveness to 

 light. This spot is called the blind spot. It may be easily 

 found by trying the experiment suggested in Figure 170, in 

 which the cross and circle are so arranged that when one 

 falls on the yellow spot the other falls on the blind spot 

 and so becomes invisible. The blind spot is sometimes 

 called the optic mound, the yellow spot the macula lutea, 

 while the pit in its center is called the fovea centralis. 

 The retina at this point is so thin that the black choroid 

 may be seen shining through it. 



Fig. 170. DIAGRAM TO DEMONSTRATE THE POSITION OF THE BLIND SPOT. (Close the left 

 eye, and holding the figure about a distance of a foot from the face, look directly at 

 the cross.) 



At the optic mound a number of blood-vessels which 

 reach the eye along with the optic nerve spread out through 

 the retina and supply this coat with its proper nourishment. 



9. The Optic Nerves. In the chapter on nerves atten- 

 tion was called to the optic nerves; how these nerves met 

 in the optic commissure and how the optic tracts passed 

 from this point to the midbrain and occipital lobes. It was 

 also pointed out that the crossing of the optic nerves at the 

 commissure is only a partial one, and is of such a nature 



