Figure 13-4. Bending of the light rays by a biconvex lens.
The additional focusing provided by the crystalline lens, mentioned above, is
one of the processes involved in accommodation. Accommodation refers to the various
adjustments made by the eye to see better at different distances.
a. The crystalline lens is kept in a flattened condition by the tension of the
zonular fibers (zonule ligaments; fibers of the ciliary zonule) around its equator, or
margin. Contraction of the ciliary muscle of the eyeball releases this tension and allows
the elastic lens to become more rounded. Since the elasticity of the crystalline lens
decreases with age, old people may find it very difficult to look at close objects.
b. A second process in accommodation is the constriction of the pupils. The
diameter of the pupil (the hole in the middle of the iris) controls the amount of light that
enters the eyeball. As a light source comes closer and closer, the intensity of the light
increases greatly. Therefore, the pupils must be constricted to control the amount of
light entering the eyeball as an object under view comes close to the individual.
c. A third process in accommodation is the convergence of the axes of the
two eyeballs toward the midline. Since both eyes tend to focus on the same object
(binocular vision), there is an angle between the two axes. As an object draws closer,
the angle increases to enable the axes to still intersect the object.
a. Convergent and Conjugate Eye Movements. In a conjugate eye
movement, both eyeballs move through an equal angle in one direction, such as right or
left. In a convergent eye movement, both eyeballs turn toward the midline to focus
upon a nearby object. In both cases, the movement of the left and right eyeballs is