Anatomy and Physiology of the Eye

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.

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