Ocular Anatomy & Physiology
Structure and function of the eye and visual system.
The Eye — External Structures
The eye is protected and supported by several external structures:
Orbit — The bony socket that houses the eye, made up of seven bones. It protects the globe from trauma.
Eyelids (Palpebrae) — Protect the eye from debris and distribute tears across the cornea with each blink. The upper lid is more mobile and covers more of the cornea.
Conjunctiva — A thin, transparent mucous membrane that lines the inner surface of the eyelids (palpebral conjunctiva) and covers the anterior sclera (bulbar conjunctiva). It produces mucus to help lubricate the eye.
Lacrimal System — Produces, distributes, and drains tears. The lacrimal gland (located superiorly and laterally) secretes tears that wash across the eye and drain through the puncta into the nasolacrimal duct.
Extraocular Muscles — Six muscles control eye movement:
- Medial Rectus → moves eye inward (adduction)
- Lateral Rectus → moves eye outward (abduction)
- Superior Rectus → moves eye upward
- Inferior Rectus → moves eye downward
- Superior Oblique → rotates eye inward and downward
- Inferior Oblique → rotates eye outward and upward
The Globe — Layers of the Eye
The eyeball (globe) has three main layers:
1. Fibrous Tunic (Outer Layer)
- Sclera — The "white of the eye," a tough, opaque tissue that maintains the shape of the eye and protects internal structures.
- Cornea — The transparent, avascular front surface of the eye. It provides approximately two-thirds (about 43 diopters) of the eye's total refractive power. It has five layers: epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium.
2. Vascular Tunic (Middle Layer / Uvea)
- Iris — The colored part of the eye that controls the size of the pupil and regulates the amount of light entering the eye.
- Ciliary Body — Produces aqueous humor and contains the ciliary muscle, which controls accommodation.
- Choroid — A highly vascular layer that supplies nutrients and oxygen to the retina.
3. Neural Tunic (Inner Layer)
- Retina — The light-sensitive layer containing photoreceptors:
- Rods (~120 million) — Responsible for dim-light (scotopic) and peripheral vision.
- Cones (~6 million) — Responsible for bright-light (photopic) vision, color perception, and fine detail.
- Macula Lutea — The central area of the retina responsible for sharp, central vision.
- Fovea Centralis — The center of the macula with the highest concentration of cones.
- Optic Disc — Where the optic nerve exits the eye. It has no photoreceptors, creating the blind spot.
Internal Structures & Refractive Media
Aqueous Humor
- A clear fluid produced by the ciliary body that fills the anterior and posterior chambers.
- Provides nutrients to the cornea and lens, and maintains intraocular pressure (IOP).
- Drains through the trabecular meshwork into Schlemm's canal. Blocked drainage can lead to glaucoma.
Crystalline Lens
- A biconvex, transparent structure located behind the iris.
- Provides approximately one-third of the eye's total refractive power (about 15-20 diopters).
- Changes shape during accommodation — becomes more convex for near vision, flatter for distance vision.
- Loss of lens flexibility with age causes presbyopia (typically begins around age 40).
- Cloudiness of the lens is called a cataract.
Vitreous Humor
- A clear, gel-like substance filling the space between the lens and the retina.
- Helps maintain the shape of the eye and holds the retina in place.
Refractive Path of Light:
Cornea → Aqueous Humor → Pupil → Crystalline Lens → Vitreous Humor → Retina
Common Refractive Errors
Emmetropia — Normal vision; light focuses directly on the retina without correction.
Myopia (Nearsightedness)
- The eye is too long or the cornea too steep, causing light to focus in front of the retina.
- Corrected with minus (concave/diverging) lenses.
Hyperopia (Farsightedness)
- The eye is too short or the cornea too flat, causing light to focus behind the retina.
- Corrected with plus (convex/converging) lenses.
Astigmatism
- The cornea or lens has an irregular shape, causing light to focus at multiple points.
- Corrected with cylindrical lenses that have power in one specific meridian.
Presbyopia
- Age-related loss of accommodation due to hardening of the crystalline lens.
- Corrected with plus power for near (reading glasses, bifocals, progressives).
- Typically becomes noticeable around age 40.
Aphakia
- Absence of the crystalline lens (usually after cataract surgery without an IOL implant).
- Requires high-plus correction (approximately +10.00 to +14.00 D).