THE EYE 



159 



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 



FIG. 41. The upper diagram suggests the path of a pencil of light 

 which illuminates a spot on the retina (r) . It is not the optic center. Be- 

 low, certain features of the region (r) are indicated upon a much larger 

 scale. The white arrow, having the same direction as the rays in the 

 first figure, shows how the light pierces to the deep or rod and cone layer 

 of the retina. The small black arrows show how the resulting nerve-im- 

 pulses are returned by transmitting elements to the inner surface of the 

 retina where they run tangentially toward the optic nerve. 



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. 





