Eyes

Eyelid

The eyelid is composed of skin, striated muscle, the tarsal plate, and conjunctivae. Meibomian glands in the eyelid provide oils to the tear film. The tarsus provides a skeleton for the eyelid. The eyelid distributes tears over the surface of the eye, limits the amount of light entering it, and protects the eye from foreign bodies. Eyelashes extend from the anterior border of each lid.

Conjunctiva

The conjunctiva is a clear, thin mucous membrane. The palpebral conjunctiva coats the inside of the eyelids. The bulbar (or ocular) conjunctiva covers the outer surface of the eye. The bulbar conjunctiva protects the anterior surface of the eye with the exception of the cornea and the surface of the eyelid in contact with the globe.

Lacrimal Gland

The lacrimal gland is located in the temporal region of the superior eyelid and produces tears that moisten the eye ( Fig. 12.3 ). Tears flow over the cornea and drain via the canaliculi to the lacrimal sac and duct and then into the nasal meatus.

Eye Muscles

Each eye is moved by six muscles—the superior, inferior, medial, and lateral rectus muscles and the superior and inferior oblique muscles (see Figs. 12.2 and 12.22 ). They are innervated by CNs III (oculomotor), IV (trochlear), and VI (abducens). The oculomotor nerve controls the levator palpebrae superioris (which elevates and retracts the upper eyelid) and all extraocular muscles, except for the superior oblique muscle and the lateral rectus muscle. The superior oblique is the only muscle innervated by the trochlear nerve and the lateral rectus muscle is the only muscle innervated by the abducens nerve.

Sclera

The sclera is the dense, avascular structure that appears anteriorly as the white of the eye. It physically supports the internal structure of the eye.

Cornea

The cornea constitutes the anterior sixth of the globe and is continuous with the sclera. It is optically clear, has rich sensory innervation, and is avascular. It is a major part of the refractive power of the eye.

Uvea

The iris, ciliary body, and choroids comprise the uveal tract (see Fig. 12.1 ). The iris is a circular, contractile muscular disc containing pigment cells that produce the color of the eye. The central aperture of the iris is the pupil, through which light travels to the retina. By dilating and contracting, the iris controls the amount of light reaching the retina. The ciliary body produces the aqueous humor (fluid that circulates between the lens and cornea) and contains the muscles controlling accommodation. The choroid is a pigmented, richly vascular layer that supplies oxygen to the outer layer of the retina.

Lens

The lens is a biconvex, transparent structure located immediately behind the iris (see Fig. 12.1 ). It is supported circumferentially by fibers arising from the ciliary body. The lens is highly elastic, and contraction or relaxation of the ciliary body changes its thickness, thereby permitting images from varied distances to be focused on the retina.

Retina

The retina is the sensory network of the eye. Photoreceptors and neurons transform light impulses into electrical impulses, which are transmitted through the optic nerve, optic tract, and optic radiation to the visual cortex in the brain and then to interpretation in the cerebral cortex. The optic nerve passes through the optic foramen along with the ophthalmic artery and vein. The optic nerve communicates with the brain and the autonomic nervous system of the eye. Accurate binocular vision is achieved when an image is fused on the retina by the cornea and the lens. An object may be perceived in each visual cortex, even when one eye is covered, if the light impulse is cast on both the temporal and the nasal retina. Fibers located on the nasal retina decussate in the optic chiasm ( Fig. 12.4 ). Accurate binocular vision also requires the synchronous functioning of the extraocular muscles.

Major landmarks of the retina include the optic disc, from which the optic nerve originates, together with the central retinal artery and vein. The macula, or fovea, is the site of central vision (see Fig. 12.1 ).

Red Eye (Presence of Conjunctival Injection or Redness)

• Difficulty with vision: one or both eyes, corrected by lenses

• Recent injury or foreign bodies; sleeping in contact lenses

• Pain: with or without loss of vision, in or around the eye, superficial or deep, insidious or abrupt in onset; burning, itching, or nonspecific uncomfortable or gritty sensation

• History of swelling, infections, or eye surgery

• History of recent illness or similar symptoms in the household

• Allergies: type, seasonal, associated symptoms

• Eye secretions: color (clear or yellow), consistency (watery or purulent), duration, tears that run down the face, decreased tear formation (with sensation of gritty eyes)

• Medications: eye drops or ointments, antibiotics, artificial tears, mydriatics; glaucoma medications, antioxidant vitamins (to prevent macular degeneration), steroids (which promote cataract formation)

Vision Problem(s)

• Eyelids: recurrent hordeola (stye; acute infection of sebaceous glands of Zeis), chalazion (chronic blockage of meibomian gland), ptosis of the lids so that they interfere with vision (unilateral or bilateral), growths or masses, itching

• Double vision: oculomotor, trochlear, or abducens nerve deficit

• Unilateral or bilateral, corrected by lenses, involving near or distant vision, primarily central or peripheral, transient or sustained

• Cataracts (bilateral or unilateral), types (e.g., infantile, senile, diabetic, traumatic, surgical treatment)

• Adequacy of color vision

• Presence of halos around lights, floaters, or diplopia (when one eye is covered or when both eyes are open)

• Sudden loss of vision or portion of visual field: transient ischemic attack, stroke, amaurosis fugax (painless temporary loss of vision in one or both eyes)

• Trauma: to the eye as a whole or a specific structure (e.g., cornea) or supporting structures (e.g., the floor of the orbit); events surrounding the trauma; efforts at correction and degree of success