Essentials of Anatomy and Physiology

(AMD), that is, loss of central vision, and some cases
seem to have a genetic component. In the dry form of
AMD, small fatty deposits impair circulation to the
macula, and cells die from lack of oxygen. In the wet

form of AMD, abnormal blood vessels begin leaking

into the retina, and cells in the macula die from the

damaging effects of blood outside its vessels. The

macula, the center of the visual field, is the part of

The Senses 205

Lens

Cornea

Conjunctiva

Pupil

Inferior rectus muscle

Canal of Schlemm

Suspensory ligament

Iris

Anterior cavity

Sclera

Vitreous humor in

posterior cavity

Choroid

Retina

Optic nerve

Optic disc

Retinal artery and vein

Fovea in macula lutea

Ciliary body (muscle)

Corneal

epithelium

Figure 9–5. Internal anatomy of the eyeball.

QUESTION:What is the function of the iris?

BOX9–1 CATARACTS

and blurry vision throughout the visual field is

the result. Small cataracts may be destroyed

by laser surgery. Artificial lenses are available, and

may be surgically implanted to replace an exten-

sively cloudy lens. The artificial lens is not

adjustable, however, and the person may require

glasses or contact lenses for vision at certain

distances.

The lens of the eye is normally transparent but may

become opaque; this cloudiness or opacity is called

acataract. Cataract formation is most common

among elderly people. With age, the proteins of the

lens break down and lose their transparency. Long-

term exposure to ultraviolet light (sunlight) seems to

be a contributing factor, as is smoking.

The cloudy lens does not refract light properly,