Eyeball

Eyeball

A. External white fibrous coat

• Consists of the sclera and the cornea.

1. Sclera

• Is a tough white fibrous tunic enveloping the posterior five sixths of the eye.

2. Cornea

• Is a transparent structure forming the anterior one sixth of the external coat.
• Is responsible for the refraction of light entering the eye.

B. Middle vascular pigmented coat

• Consists of the choroid, ciliary body and iris.

1. Choroid

• Consists of an outer pigmented (dark brown) layer and an inner highly vascular layer, which invests the posterior five sixths of the eyeball.
• Nourishes the retina and darkens the eye.

2. Ciliary body

• Is a thickened portion of the vascular coat between the choroid and the iris and consists of the ciliary ring, ciliary processes, and ciliary muscle.
• The ciliary processes are radiating pigmented ridges that encircle the margin of the lens.
• The ciliary muscle consists of meridional and circular fibers of smooth muscle innervated by parasympathetic fibers. It contracts to pull the ciliary ring and ciliary processes, relaxing the suspensory ligament of the lens and allowing it to increase its convexity.

3. Iris

• Is a thin, contractile, circular, pigmented diaphragm with a central aperture, the pupil.
• Contains circular muscle fibers (sphincter pupillae) , which are innervated by parasympathetic fibers, and radial fibers (dilator pupillae) , which are innervated by sympathetic fibers.

C. Internal nervous coat

• Consists of the retina , which has an outer pigmented layer and an inner nervous layer.
• Has a posterior part that is photosensitive; its anterior part, which is not photosensitive, constitutes the inner lining of the ciliary body and the posterior part of the iris.

1. Optic disk

• Consists of optic nerve fibers formed by axons of the ganglion cells. These cells are connected to the rods and cones by bipolar neurons.
• Is located nasal (or medial) to the fovea centralis and the posterior pole of the eye, has no receptors, and is insensitive to light.
• Has a depression in its center termed the physiologic cup.

2. Macula

• Is a yellowish area near the center of the retina on the temporal side of the optic disk for the most distinct vision.
• Contains the fovea centralis.

3. Fovea centralis

• Is a central depression (foveola) in the macula.
• Is avascular and is nourished by the choriocapillary lamina of the choroid.
• Has cones only (no rods), each of which is connected with only one ganglion cell, and functions in detailed vision.

4. Rods

• Are approximately 120 million in number and are most numerous about 0.5 cm from the fovea centralis.
• Contain rhodopsin , a visual purple pigment.
• Are specialized for vision in dim light.

5. Cones

• Are 7 million in number and are most numerous in the foveal region.
• Are associated with visual acuity and color vision.

D. Refractive media

• Consist of the cornea, aqueous humor, lens, and vitreous body.

1. Aqueous humor

• Is formed by the ciliary processes and provides nutrients for the avascular cornea and lens.
• Passes through the pupil from the posterior chamber (between the iris and the lens) into the anterior chamber (between the cornea and the iris) and is drained into the scleral venous plexus through the canal of Schlemm at the iridocorneal angle.
• Its impaired drainage causes an increased intraocular pressure, leading to atrophy of the retina and blindness.

2. Lens

• Is a transparent avascular biconvex structure enclosed in an elastic capsule.
• Is held in position by radially arranged zonular fibers (suspensory ligament of the lens), which are attached medially to the lens capsule and laterally to the ciliary processes.
• Flattens to focus on distant objects by pulling the zonular fibers and becomes a globular shape to accommodate the eye for near objects by contracting the ciliary muscle and thus relaxing zonular fibers.

4. Vitreous body

• Is a transparent gel called vitreous humor , which fills the eyeball posterior to the lens (vitreous chamber between the lens and the retina).
• Holds the retina in place and provides support for the lens.

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