652 THE SENSES 



It is not a disease, but a physiological process which every eye undergoes as 

 its owner grows older. It is due to a gradual diminution of elasticity of the 

 lens by a sort of sclerosis from the center toward the periphery. It 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 



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 focussed behind the 

 retina, but by increasing the curvature of the anterior surface of the lens (shown by dotted lines) 

 the rays are focussed 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 focussed 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 focussed in front of the retina. 



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. 



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 



