VISUAL SENSATIONS, FROM EXCITATION OF THE RETINA 687 



begins even in childhood, but advances so slowly that it is not until the age of 

 twenty-five or so that a distinct, though small, nucleus is present. With 

 advancing years the process goes on until finally the lens becomes inelastic 

 and is unable to assume a shape convex enough to focus rays from a near 

 object upon the retina, as in reading. The defect is remedied by the use of 

 convex lenses equivalent to the loss in accommodation. 



FIG. 466. Diagram Showing: i, Normal or emmetropic eye bringing parallel rays 

 exactly to a focus on the retina; 2, normal eye at rest, showing that light from a near point is 

 focused behind the retina, but by increasing the curvature of the anterior surface of the 

 lens (shown by dotted lines) the rays are focused on the retina; 3, hypermetropic eye. In 

 this case the axis of the eye is shorter and the lens normal (or the lens may be flatter than 

 normal and the eyeball normal); parallel rays are focused behind the retina; 4, myopic eye. 

 In this case the lens is too convex (or the axis of the eye is abnormally long); parallel rays 

 are focused in front of the retina. 



Visual Sensations, from Excitation of the Retina. Light is the 

 normal agent in the excitation of the retina. The only portion of the retina 

 capable of reacting to the stimulus is the rod and cone layer. The proofs of 

 this statement may be summed up thus: i. The point of entrance of the optic 

 nerve into the retina, where the rods and cones are absent, is insensitive to 

 light and is called the blind spot. The phenomenon itself is very readily 



