In the eye, the iris, the colored part of the eye, performs the shutter function on the light-admitting opening - the pupil. In bright light, the iris makes the pupil small, thus admitting less light; and in dim light, the iris makes it large, admitting more. The camera lens corresponds to the eye's lens. The lens assumes a fat or thin shape, as needed to focus close or distant objects, through the action of the ciliary muscle to which it is attached. The retina, at a fixed distance behind the lens, corresponds to the film. The "developing process" involves light-sensitive receptor cells on the retina - the rods and cones. As light strikes them, they break down chemically and initiate nerve impulse conduction. These impulses reach the occipital lobe of the brain, which interprets them as the final image. The protective camera-case function is performed by the sclera. Tears secreted by the lacrimal glands, a cushion of fat, and the surrounding skull bones afford further protection for the eye. 

A defect in any of these structures can result in abnormal vision.
 

Anatomy of the Eye

Click on the image to see greater detail. Graphics modified from: The InnerBody: Anatomy Tutorial - Nervous System

The eyes are encased in 3 connective tissue coatings - the sclera, choroid, and retina. The outward most coating is the sclera, the "white of the eye" which admits no light. Its anterior portion, the cornea, is transparent and admits light. Slightly curved, the cornea protrudes from the rest of the eyeball. Because of this curvature, entering light begins to bend there, even before it reaches the lens. The optic nerve(cranial nerve II), which connects to the brain, pierces the back of the sclera. 

Just inside the sclera is the next coating, the choroid, through which many blood vessels run. It functions not only as a source for blood supply for the rest of the eye, but as a light-absorbing layer to prevent internally-reflected light from blurring the image. Its anterior portion is modified into three separate structures: the iris, the ciliary body (muscles), and the suspensory ligament. It is the  pigmented cells of the iris that give the eye its color. The iris contains smooth-muscle fibers, arranged in circular and radial directions, which act antagonistically to control the size of the pupil. When the circular muscle fibers contract, the pupil constricts, admitting less light; when the radial muscles contract, the pupil dilates to admit more light. The pupil appears black because all the light striking the retina is absorbed and none is reflected out of the eye. (See also: Oculomotor Nerve)

The transparent lens, functioning as the second element in the light-focusing system, is held into place behind the pupil by a suspensory ligament attached to the ciliary body. It is the lens which changes shape, becoming fatter or thinner, to focus objects being viewed up close or at a distance. For near objects, the ciliary muscle contracts, relaxing the suspensory ligament. The tension on the lens is thus released, causing it to fatten. For far objects, the ciliary muscle relaxes, the suspensory ligament tightens, and the lens thins out. Light striking the retina activates both the iris muscles,  controlling pupil size, and the ciliary muscles, controlling lens curvature. 
 

Test yourself: Click to select your answer. If your answer is correct,  a check  appears beside the question.  Diseases that produce macular degeneration affect: vision in daylight night vision or vision in dim light A vitamin A deficiency affects: vision in daylight night vision or vision in dim light

The third, innermost coating is the retina. The retina contains the light-sensitive receptor cells, called rod cells and cone cells, which convert the light energy into nerve impulses. These nerve impulses then travel along the optic nerves to the occipital lobe -- i.e.  visual areas of the cerebral cortex. 

The  ciliary body, suspensory ligament, and the lens divide the eye into two cavities, anterior and posterior, filled with transparent fluid, or "humor." The anterior cavity is filled with aqueous humor, a clear, watery substance, which often leaks out when the eye is injured. Too much aqueous humor and too little of its absorption increase eyeball pressure. This increased pressure, if undetected, can lead to retinal damage and loss of vision (glaucoma). The posterior cavity is filled with vitreous humor, a gelatin-like substance which maintains sufficient intraocular pressure to prevent the eyeball from collapsing. The vitreous humor may also develop opacities which affect vision.

Neural Pathways - Fields of Vision

When both eyes are gazing on an object, its image focuses on corresponding portions of each retina. The left field of vision, here in blue, focuses on the right side of each retina; the right field of vision, in red, focuses on the left sides. But notice that the image focuses in different portions in each retina relative to the nose. The left visual field focuses on the left retina on the side nearest the nose - the nasal portion, but focuses on the right retina on the side farthest from the nose - the temporal portion. The right field of vision focuses on the nasal portion of the right retina, but the temporal portion of the left retina.

Merging these "fields of vision" into a meaningful whole involves a cross-over process at the optic chiasma.  There, optic fibers from the nasal portion of each retina cross over and join the fibers from the temporal portion of the retina on the opposite side. These combined fibers form the optic tracts. Thus, the left optic tract contains impulses of images from the right visual field, and the right optic tract contains those from the left visual field. Synapsing at the left/right thalamus, the fibers continue as optic radiations to terminate in the cortex of the right and left occipital lobes. Location of a lesion in the visual pathway determines the resulting visual defect. For example, destruction of an optic nerve produces blindness in the same eye. Complete loss of the right optic radiations, such as may occur in stroke, blocks vision from the left visual field, and vice versa. 

Eye Movements

Six muscles attached to the sclera control eye movement within its orbit. These six muscles are governed by the cranial nerves III (Oculomotor), IV (Trochclear), and VI (Abducens). The table at the right summarizes the movements and the nerves operating them. Eye movement disturbances can cause images to fail to focus on corresponding portions of the retina, thus resulting in double vision (diplopia). Or, as in the case of paralysis, one eye may not fix on the object at all, resulting in monocular, rather than binocular, vision.

Muscles of Eye Movement Muscle Superior rectus Inferior rectus Medial rectus Lateral rectus Superior oblique Inferior oblique Movement Produced Up Down In toward nose Away from nose Down and in Up and out Cranial Nerve Oculomotor (III) Oculomotor (III) Oculomotor (III) Abducens (VI) Trochclear (IV) Oculomotor (III) Source: Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education, Rehabilitation Services Administration, 1981, p. 51.

To Learn More

Web resources: Glossary of Vision Terms  (National Eye Institute) PATTS AT Links - Vision/ Blindness PATTS - Nervous System Brain-Sensory Interaction How We See (Prevent Blindness America) The InnerBody: Anatomy Tutorials - Nervous System: Eye Visible Human Project

Acknowledgments:  Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education, Rehabilitation Services Administration, 1981, pages 19-54. Catherine Parker Anthony and Gary A. Thibodeau, Textbook of Anatomy & Physiology. St. Louis: Mosby, 1983, pages 322-338. Anatomy Clipart (Designs4Free) Marjorie Thompson, BIO 189, Slide Show, Brown University School of Medicine.