Anatomy of the Human Eye

Anatomy of the Human Eye

• The eye is situated in the orbital cavity and supplied by the optic nerve.
• It is almost spherical in shape, approximately 2.5cm in diameter.
• The space between the eye and the orbital cavity is occupied by adipose tissue.

Structure of the Eye

• The eye has three layers of tissue in its walls:
  • Outer fibrous layer: Sclera and Cornea
  • Middle vascular layer or uveal tract: Choroid, Ciliary body, and Iris
  • Inner nervous tissue layer: Retina

The Sclera

• Made up of mainly fibrous tissue.
• Its external surface gives attachment to the extrinsic muscles of the eyeball.
• Posteriorly, the sclera is perforated like a sieve (Lamina cribrosa) where the optic nerve passes through.

The Cornea

• More convex than the sclera.
• A transparent epithelial membrane.
• The junction of the sclera and cornea is marked by a groove (Sulcus sclerae).
• Consists of five layers: Corneal epithelium, Anterior limiting membrane, Substantia propria, Posterior limiting lamina, and Endothelium of the anterior chamber.

The Choroid

• A network of blood vessels supported by connective tissue containing many pigmented cells.
• The dark color of the choroid helps to darken the interior of the eyeball and prevents the reflection of light within the eyeball.
• Both these factors are necessary for the formation of sharp images on the retina.

The Ciliary Body

• The anterior continuation of the choroid.
• Consists of ciliary muscle and secretory epithelial cells.
• The ciliary muscle acts like a sphincter due to its circular smooth muscle fibers.
• The lens is attached to the ciliary body by radiating suspensory ligaments.
• The ciliary body is divided into a posterior part (ciliary ring) and an anterior part (ciliary processes).

The Iris

• The visible colored part of the eye.
• Extends anteriorly from the ciliary body, lying behind the cornea and in front of the lens.
• Divides the anterior segment of the eye into anterior and posterior chambers.
• Composed of pigment cells and two layers of smooth muscle fibers: circular and radiating muscle fibers.
• In its center, there is an aperture (pupil).

The Retina

• Lines about three quarters of the eyeball.
• Thins anteriorly to end just behind the ciliary body.
• Opposite the posterior pole of the eyeball, the retina shows a central region (Macula lutea or Yellow spot).
• In the center of the yellow spot, there is a little depression (Fovea centralis) consisting of only cones.
• Towards the anterior part of the retina, there are fewer cones than rods.

Muscles of the Orbit

• Include the muscles of the eyelids and those that move the eyeballs.
• The eyeball is moved by six extrinsic muscles attached to the eyeball and the walls of the orbital cavity.
• The muscles are: Straight/Rectus muscles (four), Oblique muscles (two).

Blood and Nerve Supply to the Eye

• The eye is supplied with arterial blood by the ciliary arteries and the central retinal artery.
• Venous drainage is by several veins, including the central retinal vein.
• The central retinal artery and vein are encased in the optic nerve.
• The oculomotor nerve supplies the intrinsic eye muscles of the iris and ciliary body.

Interior of the Eye

• The anterior segment of the eye is incompletely divided into anterior and posterior chambers by the iris.
• Both chambers contain a clear aqueous fluid secreted into the posterior chamber.
• There is a continuous production and drainage of aqueous fluid, but the intraocular pressure remains constant.

Optic Nerve

• The fibers of the optic nerve originate in the retina and converge to form the optic nerve.
• The nerve pierces the choroid and sclera to pass backwards and medially through the orbital cavity.
• It then passes through the optic foramen of the sphenoid bone, backwards and medially to meet the nerve from the other eye at the optic chiasma.

Optic Chiasma

• Situated immediately in front of and above the pituitary gland.
• The nerve fibers of the nasal side of each retina cross over to the opposite side.
• The nerves from the temporal side don't cross but continue backwards on the same side.

Optic Tracts

• The pathways of the optic nerves, posterior to the optic chiasma.
• Each tract consists of the nasal fibers from the retina of one eye and the temporal fibers from the retina of the other eye.
• The optic tracts pass backwards to synapse with nerve cells of the lateral geniculated bodies of the thalamus.
• From there, the nerve fibers proceed backwards and medially as the optic radiation to terminate in the visual area of the cerebral cortex in the occipital lobe.

Accessory Structures of the Eye

• Eyebrows: Protect the eyeball from sweat, dust, and other foreign bodies.
• Eyelids (Palpebrae): Two movable folds of tissue situated above and below the front of each eye.
• Conjunctiva: A fine, transparent membrane that lines the eyelids and the front of the eyeball.
• Lacrimal Apparatus: Consists of lacrimal glands, ducts, lacrimal canaliculi, lacrimal sac, and nasolacrimal duct.
• The lacrimal glands secrete tears composed of water, mineral salts, antibodies, and lysozyme.