2 : 3/ Light and the Eye 



35 



The third and innermost layer of the eyeball is the retina. The active 

 photoreceptors, called rods and cones, are located in the retina. It is 

 convenient to divide the retina into ten layers. Light must pass through 



Visual Axis 



Cornea 



Ciliary 

 Muscle 



Optical Axis 

 Anterior Chamber 

 Iris 

 Posterior Chamber 



\ 



Retina 



Choroid 

 Layer 



Sclera 



Optic 

 Nerve 



Fovea 



Figure 3. The eye. After A. A. Maximow and W. Bloom, 

 Textbook of Histology (Philadelphia: W. B. Saunders Company, 

 1957). 



eight of these before reaching the rods and cones located in the ninth 

 layer. 



Slightly displaced from the intersection of the optic axis of the eye 

 with the retina is a yellow spot known as the fovea centralis (the macula 

 lutea) . It is a slight depression on the surface of the retina. The active 

 elements in the fovea are all cones; they are very closely packed. For 

 maximum acuity, the eye is directed so that the image falls on the fovea. 



Somewhat on the nasal side of the fovea is the optic disk. Here the 

 optic nerve pierces through the sclera, the choroid layer, and the retina ; 

 in the center of the optic nerve are a vein and an artery. From this 

 disk, nerve fibers and blood vessels branch out over the surface of the 

 retina. Objects focused on this disk cannot be seen since there are no 

 rods or cones in it. Thus, this disk is referred to as the blind spot. 

 One may put two marks on a piece of paper as indicated in Figure 4, 

 cover one eye, and fixate one of the marks. If one then alternately 

 moves his head toward and away from the paper, the other mark will 



