Human Physiology, 14th edition (2016)

Sensory Physiology 295

the object closer to or farther from the eye would cause corre-

sponding movement of the focal point, so that the focus would

either be behind or in front of the retina.

The ability of the eyes to keep the image focused on the

retina as the distance between the eyes and object varies is

called accommodation. Accommodation results from contrac

tion of the ciliary muscle, which is like a sphincter muscle that

can vary its aperture ( fig.  10.33 ). When the ciliary muscle is

relaxed, its aperture is wide. Relaxation of the ciliary muscle

thus places tension on the zonular fibers of the suspensory

ligament and pulls the lens taut. These are the conditions that

prevail when viewing an object that is 20 feet or more from a

normal eye; the image is focused on the retina and the lens is in

its most flat, least convex form. As the object moves closer to

the eyes, the muscles of the ciliary body contract. This muscu-

lar contraction narrows the aperture of the ciliary body and thus

reduces the tension on the zonular fibers that suspend the lens.

When the tension is reduced, the lens becomes more rounded

and convex as a result of its inherent elasticity ( fig. 10.34 ).

The ability of a person’s eyes to accommodate can be mea-

sured by the near-point-of-vision test. The near point of vision

is the minimum distance from the eyes at which an object

can be brought into focus. This distance increases with age;

indeed, accommodation in almost everyone over the age of 45

is significantly impaired. Loss of accommodating ability with

age is known as presbyopia ( presby 5  old). This loss appears

to have a number of causes, including reduced flexibility of the

lens and a forward movement of the attachments of the zonu-

lar fibers to the lens. As a result of these changes, the zonular

fibers and lens are pulled taut even when the ciliary muscle

contracts. The lens is thus not able to thicken and increase its

refraction when, for example, a printed page is brought close

to the eyes. People with presbyopia often require glasses with

Figure 10.32 The image is switched right-to-left on
the retina. The left side of the visual field is projected to the
right half of each retina, while the right side of each visual field is
projected to the left half of each retina.

Fields of vision

Nasal

half-retinas

Temporal

half-retina

Temporal

half-retina

Optic

chiasma

Optic

nerve

Accommodation

When a normal eye views an object, parallel rays of light are
refracted to a point, or focus, on the retina (see fig.  10.35 a ).
If the degree of refraction remained constant, movement of

Figure 10.33 The relationship between the ciliary muscle and the lens. A diagram showing the relationship between
the lens, zonular fibers, and ciliary muscle of the eye.

Ciliary muscle

Lens within

lens capsule

Zonular fibers of

suspensory ligament

Iris Conjunctiva

Cornea

Sclera