THE EYE 157 



The Retina. It has been said that this coat of the 

 eye is composed partly of the fibers which go to make 

 the optic nerve. These fibers are not themselves directly 

 sensitive to light. In fact the place where they con- 

 verge to form the nerve is a so-called blind spot. We do 

 not see the images of objects which are formed there. 

 The fact that we are not troubled by this deficiency is 

 to be explained chiefly by the circumstance that we are 

 so much occupied with the central part of the retinal 

 picture that we have little appreciation of the outlying 

 part. It will be recalled that the optic nerve does not 

 leave the center of the retina but makes its exit from a 

 point some distance toward the nose. The central 

 spot, before mentioned as having the best visual capacity, 

 is called the fovea. 



The student, when he reads a description of the 

 several layers of the retina, has the strongest feeling 

 that it is all " wrong side out." The cellular elements 

 on which the light undoubtedly acts are not arrayed 

 upon the inner surface but are on the outside, next to 

 the middle coat. To reach them the light must pass 

 through a tangle of nerve fibers and cells other than the 

 true receptors. It must even pass a network of blood- 

 vessels. The layer which finally translates the radiant 

 energy into nerve-impulses is that of the rods and cones. 

 At the fovea the overlying matter is reduced and the 

 exposure of the sensitive units is correspondingly direct. 



The rods and cones constitute a mosaic pavement in 

 which the individual members are placed with striking 

 regularity. The cones are rather more advanced and 

 elaborate in appearance than the rods and there is 

 little doubt that they have superior properties. In the 

 fovea there is a central group of cones with no rods, 

 farther out the cones are scattered among rods which 

 greatly outnumber them, and still farther from the 

 fovea no cones but only rods are to be found. This 

 distribution is associated with contrasted powers of 

 vision in these three regions. 



