The eye, unlike other organs, is available for external examination, and has simpler anatomy for assessment than other parts of the body.
- Orbit: the bony and protective region where the eyeball (or globe) is situated. It is lined with muscle, and connective and adipose tissues. It is shaped like a four-sided pyramid.
- Sinuses are air-filled spaces that surround the orbit on three sides: ethmoid (medial), frontal (superior), and maxillary (inferior).
- Optic Nerve and Ophthalmic Artery enter the orbit through its apex through the optic foramen.
- The Extraocular Muscles move the eyeball through all fields of gaze. There are four rectus (superior, inferior, medial, lateral) and two oblique (superior, inferior) muscles.
- Cranial Nerves III, IV, and VI (oculomotor, trochlear, abducens) innervate the extraocular muscles.
- Eyelids are composed of thin, elastic skin that covers striated and smooth muscles, and protects the anterior portion of the eye, as every blink allows it to “wash” the cornea and conjunctive with tears. It is innervated by the CN III They contain multiple glands (sebaceous, sweat, and lacrimal). Its margins contain meibomian glands that, the inferior and superior puncta, and the eyelashes.
- The triangular corners of the eye are called the inner and outer canthus.
- Tears are vital to eye health. They are formed by the lacrimal (crying) gland and the accessory lacrimal glands. They are secreted in response to reflex or emotional stimuli.
- Tears are composed of three layers: lipoid, aqueous, and mucoid. These nourish the cornea and maintain a smooth optical surface of the cornea and conjunctival epithelium.
- Conjunctiva: a thin, transparent mucous membrane that provides a barrier to the external environment extending under the eyelids and over the sclera.
- The Eyeball is composed of three layers and is divided anatomically into two segments, and has three fluid-containing chambers.
- Outer dense fibrous layer, including the sclera and transparent cornea
- Middle vascular layer, containing the iris, ciliary body, and choroid
- Inner neural layer, including the retina, optic nerve, and visual pathway
- Anterior segment: between the anterior cornea and posterior iris, including the anterior and posterior chambers.
- Posterior segment: between the posterior lens and the retina, including the vitreous chamber.
- Anterior aqueous-filled chamber: between the posterior cornea and the anterior iris and pupil.
- Posterior chamber: lies between the posterior iris and pupil and anterior lens.
- Vitreous chamber: contains clear gelatinous vitreous fluid, and is the largest chamber in the ocular fundus between the lens and retina.
- Aqueous Humor: a transparent, nutrient-containing fluid that fills the anterior and posterior chambers and helps give the eye its shape. It is produced in the posterior chamber by the ciliary body, flowing through the pupil into the anterior chamber and draining through the trabecular meshwork into the canal of Schlemm. Production of aqueous humor is related to normal IOP, which is less than 21 mmHg.
- Vitreous Humor: composed of mostly water and is encapsulated by a hyaloid membrane. It helps maintain the shape of the eye. It is attached to the retina by scattered collagenous filaments.
- This shrinks and shifts with age. With degeneration, its gel-like characteristics liquefy, causing stringy debris known as floaters.
- Sclera: a white, avascular, dense, and fibrous structure that helps maintain the shape of the eyeball and protects the intraocular contents. Scleral thinning and changes of the scleral collagen fibers can cause the underlying uveal pigment to be seen, resulting in a blue or gray sclera. The episclera is a vascularized, loose, elastic tissue that overlies the sclera, supplying nutritional support and reacting to inflammation.
- Cornea: a vulnerable transparent avascular dome-like structure, forming the most anterior portion of the eyeball, and is the main refracting surface of the eye. It is composed of five layers: epithelium, Bowman’s membrane, stroma, Descemet’s membrane, and endothelium. It contains high concentrations of nerve fibers and is extremely sensitive to pain.
- Uveal Tract: vascular middle layer of the eye consisting of the iris, ciliary body, and the choroid.
- Iris: highly vascularized pigmented collection of fibers that give the color. The dilator (sympathetic) and sphincter (parasympathetic) muscles of the iris control pupil size. (inc) p. 5035