ANATOMY AND PHYSIOLOGY OF THE EYE
The eye
- The eye is 1 inch in diameter and is located in the anterior portion of the orbit.
- The orbit is the bony structure of the skull that surrounds the eye and offers protection to the eye.
Layers of the eye
External layer
- The fibrous coat that supports the eye
- Contains the cornea, the dense transparent outer layer
- Contains the sclera, the fibrous “white of the eye”
Middle layer
- Called the uveal tract
- Consists of the choroid, ciliary body, and iris
- The choroid is the dark brown membrane located between the sclera and the retina that has dark pigmentation to prevent light from reflecting internally.
- The choroid lines most of the sclera and is attached to the retina but can detach easily from the sclera.
- The choroid contains many blood vessels and supplies nutrients to the retina.
- The ciliary body connects the choroid with the iris and secretes aqueous humor that helps give the eye its shape; the muscles of the ciliary body control the thickness of the lens.
- The iris is the colored portion of the eye, located in front of the lens, and it has a centralcircular opening called the pupil. The pupil controls the amount of light admitted into the retina (darkness produces dilation and light produces constriction).
Internal layer
- Consists of the retina, a thin, delicate structure in which the fibers of the optic nerve are distributed
- The retina is bordered externally by the choroid and sclera and internally by the vitreous.
- The retina is the visual receptive layer of the eye in which light waves are changed into nerve impulses; it contains blood vessels and photoreceptors called rods and cones.
Vitreous body
- Contains a gelatinous substance that occupies the vitreous chamber, the space between the lens and the retina
- The vitreous body transmits light and gives shape to the posterior eye.
Vitreous
- Gel-like substance that maintains the shape of the eye
- Provides additional physical support to the retina
Rods and cones
- Rods are responsible for peripheral vision and function at reduced levels of illumination.
- Cones function at bright levels of illumination and are responsible for color vision and central vision.
Optic disc
- The optic disc is a creamy pink to white depressed area in the retina.
- The optic nerve enters and exits the eyeball at this area.
- This area is called the blind spot because it contains only nerve fibers, lacks photoreceptor cells, and is insensitive to light.
Macula lutea
- Small, oval, yellowish-pink area located laterally and temporally to the optic disc
- The central depressed part of the macula is the fovea centralis, the area of sharpest and keenest vision, where most acute vision occurs.
Aqueous humor
- The aqueous humor is a clear watery fluid that fills the anterior and posterior chambers of the eye.
- The aqueous humor is produced by the ciliary processes, and the fluid drains into the canal of Schlemm.
- The anterior chamber lies between the cornea and the iris.
- The posterior chamber lies between the iris and the lens. Canal of Schlemm: Passageway that extends completely around the eye; it permits fluid to drain out of the eye into the systemic circulation so a constant intraocular pressure is maintained.
Lens
- Transparent convex structure behind the iris and in front of the vitreous body
- The lens bends rays of light so that the light falls on the retina.
- The curve of the lens changes to focus on near or distant objects.
Conjunctivae:
- Thin transparent mucous membranes of the eye that line the posterior surface of each eyelid, located over the sclera
Lacrimal gland
- The lacrimal gland produces tears.
- Tears are drained through the punctum into the lacrimal duct and sac.
Eye muscles
- Muscles do not work independently but work with the muscle that produces the opposite movement.
- Rectus muscles exert their pull when the eye turns temporally.
- Oblique muscles exert their pull when the eye turns nasally.
Nerves
- Cranial nerve II: Optic nerve (nerve of sight)
- Cranial nerve III: Oculomotor
- Cranial nerve IV: Trochlear
- Cranial nerve VI: Abducens
Blood vessels
- The ophthalmic artery is the major artery supplying the structures in the eye.
- The ophthalmic veins drain the blood from the eye.
