This section describes the eye and its structures.

Objectives:

•  Identify and describe the structural divisions of the eye.

•  Identify normal intraocular pressure (IOP) and discuss the mechanisms which maintain IOP.

•  Discuss retinal image formation by describing refraction, accommodation, constriction of the pupil, and convergence.

•  Describe the nerve pathways over which impulses from the retina travel to higher centers in the brain.

Accessory Structures of the Eye:

•  Eyebrows

•  Palpebrae (eyelids)

•  Palpebral fissure

•  Opening between the eyelids; exposes the eyeball

•  Lateral commissure

•  Union of upper and lower eyelids at outer canthus

•  Medial commissure

•  Union of upper and lower eyelids at inner canthus

•  Lacrimal caruncle – contains modified sweat and sebaceous glands

•  Tarsal plate

•  Thick fold of connective tissue

•  Forms much of the inner wall of each eyelid

•  Provides form and support to eyelids

•  Tarsal (Meibomian) glands

•  Modified sebaceous glands

•  Oily secretion prevents eyelids from adhering to each other

•  Conjunctiva

•  Mucous membrane that covers the sclera and lines eyelids

•  Palpebral conjunctiva – lines inner aspect of eyelids

•  Bulbar (ocular) conjunctiva – coats anterior portion of eyeball

•  Eyelashes

•  Sebaceous ciliary glands (glands of Zeis) – secrete lubricating fluid

•  Lacrimal apparatus

•  Group of structures which manufacture and drain tears

The Lacrimal Apparatus :

Lacrimal glands:

The lacrimal glands are secretory (tubuloacinar) glands located at the superior anterolateral orbit. These glands secrete tears into the excretory ducts that open onto the conjunctival surface. Each is about the size and shape of an almond.

Excretory lacrimal ducts:

The excretory lacrimal ducts consist of six to 12 ducts leading from the lacrimal glands which empty lacrimal fluid (tears) onto the conjunctival surface of the upper lid.

Lacrimal puncta:

Consists of two small pores, one in each papilla of the eyelid at the medial commissure

Lacrimal canals:

Lacrimal sac:

The lacrimal sace is the superior expanded portion of the nasolacrimal duct

Lacrimal Secretion:

Lacrimal secretion is a watery solution which contains salts, mucus, and lysozyme (a bactericidal enzyme). Lacrimal secretion cleans, lubricates, and moistens the eyeball, spreading medially over eyeball surface via blinking. Approximately one ml per day is produced by each gland.

Structure of the Eyeball:

The adult eyeball is approximately 2.5cm in diameter and only the anterior one-sixth of the total surface area is exposed. Eyeball structure is divided into three layers:

•  Fibrous tunic

•  Vascular tunic

•  Retina (nervous) tunic

The Fibrous Tunic:

The fibrous tunic is the outermost portion of the eyeball, and contains the sclera, cornea, and Canal of Schlemm .

The Sclera :

•  “white of the eye”

•  Dense, fibrous tissue that covers all of the eyeball except the most anterior portion (the iris )

•  Gives shape to the eyeball

•  Protects inner anatomy

•  Posterior surface pierced by the optic (II) nerve

The Cornea

•  Nonvascular, transparent, fibrous coat through which the iris can be seen

•  Outer surface covered by epithelial layer continuous with bulbar conjunctiva epithelium

The scleral venous sinus ( Canal of Schlemm )

  •  Drains aqueous humor from anterior chamber

The Vascular tunic:

The vascular tunic is the middle layer of the eyeball and is composed of the choroid, ciliary body, and iris.

  The Choroid

  •  Posterior portion of vascular tunic

•  Thin, dark brown membrane

•  Lines most of internal scleral surface

•  Contains blood vessels and large amount of pigment

•  Absorbs light rays

•  Nourishes the retina

•  Pierced by the optic (II) nerve

The Ciliary Body

•  Thickest portion of vascular tunic

•  Consists of ciliary processes and ciliary muscle

•  Ciliary processes secrete aqueous humor

•  Ciliary muscle alters lens shape for near or far vision

The Iris

•  Colored portion visible through cornea

•  Suspended between cornea and lens

•  Attached at outer margin to ciliary process

•  Principal function: regulation of amount of light entering the posterior cavity

•  Encompasses the pupil (through which light enters the eye interior)

•  Contraction of smooth muscles of the iris constricts the pupil

•  Constriction and dilation of the pupil occur primarily as reflex responses

The Retina (nervous tunic):

  The nervous tunic forms the inner coat of the eye and covers the choroids. The principal function of this layer is image formation. The nervous tunic consists of an inner tissue layer (visual portion) and an outer pigmented layer (non-visual portion) and contains three zones of neurons (named in the order in which they conduct nerve impulses).

Photoreceptor Neurons

•  Dendrites referred to as rods or cones

•  visual receptors

•  highly specialized for light ray stimulation

Bipolar Neurons

•  Receive information from photoreceptor neurons

Ganglion Neurons

•  Receive information from bipolar neurons

•  Transmit signals through optic (II) nerve fibers to the brain in the form of nerve impulses

•  Axons extend posteriorly to the optic disk

RODS AND CONES

Cones:

•  Estimated three million cones

•  Specialized for color vision

•  Allow visual acuity in bright light

•  Most densely concentrated in posterior portion of the retina

Rods:

•  Estimated 100 million rods

•  Specialized for black and white vision in dim light

•  Allow visualization of shapes and movement

•  Increase in density toward retinal periphery

The Lens:

  The lens is located behind the pupil and iris and is constructed of proteins (crystallins). The crystallins (1) provide structure and (2) function as enzymes that convert sugars into energy. The normal lens is transparent, enclosed by a clear connective tissue capsule, and refracts light rays.

The Eyeball Interior:

  The eyeball is divided into two cavities by the lens.

The anterior cavity:

  •  Anterior to the lens

•  Refracts light rays

•  Further divided into the anterior chamber

•  The anterior chamber lies behind the iris, in front of suspensory ligaments and lens

The posterior cavity:

•  Largest cavity

•  Refracts light rays

•  Contains the vitreous body

•  Vitreous body characteristics:

•  Jelly-like

•  Contributes to IOP

•  Prevents the eyeball from collapsing

•  Holds the retina flush against internal portions of the eyeball

•  Does not undergo constant replacement

•  Formed during embryonic life, not replaced thereafter

Figure 1.0. Structures of the eye. (From Spence AP, Basic Human Anatomy , third ed. Redwood City, Benjamin/Cummins, 1990).

Aqueous Humor:

The aqueous humor is a watery fluid which is similar to CSF and continually produced. Aqueous humor is secreted into the posterior chamber by the choroid plexus of the ciliary processes of ciliary bodies behind the iris. It passes into the posterior chamber, then forward between the iris and the lens, through the pupil, and into the anterior chamber. From the anterior chamber, aqueous humor drains into the scleral venous sinus ( canal of Schlemm ) and into the blood. The aqueous humor is the principal link between CV systems and the lens and the cornea.

Intraocular Pressure (IOP):

Intraocular pressure is maintained by the drainage of aqueous humor through the scleral venous sinus. IOP keeps the retina smoothly applied to the choroid. Normal IOP is approximately 16 mm Hg.

The Muscles of Orbit:

Muscles of orbit include the levator palpebrae superioris; superior, inferior, lateral , and medial rectus; and superior and inferior obliques .

Levator palpebrae superioris

•  Origination: lesser wing of sphenoid bone, superior and anterior to optic canal

•  Insertion: Tarsal plate and skin of upper eyelid

•  Action: Elevation of upper eyelid

Superior, Inferior, Lateral, and Medial rectus

•  Origination: common tendinous ring

•  Insertion: sclera, posterior to cornea

•  Action: elevates (occulomotor), adducts, rotates eyeball medially, abducts eyeball (abducent)

Superior oblique

•  Origination: sphenoid bone

•  Insertion: sclera, deep to superior rectus muscle

•  Action: abducts, depresses, and medially rotates eyeball

Inferior oblique

•  Origination: anterior floor to orbit

•  Insertion: sclera, deep to lateral rectus muscle

•  Action: abducts, elevates, laterally rotates eyeball

  Figure 2.0. Muscles of orbit. (From Moore KL, Dalley AF. Clinically Oriented Anatomy , fourth ed. Philadelphia , Lippincott Williams & Williams, l999).

Innervation of the Orbit:

In addition to the optic nerve, the nerves of orbit include those that enter through the superior orbital fissure and supply the ocular muscles.

•  CN III (oculomotor ) supplies the levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique

•  CN IV (trochlear) supplies the superior oblique

•  CN VI (abducent) supplies the lateral rectus

•  All muscles of orbit are supplied by CN III, except for the posterior oblique and lateral rectus

•  Branches of the ophthalmic nerve pass through the superior orbital fissure and supply orbit structures

•  The lacrimal nerve arises in the lateral wall of the cavernous sinus and passes to the lacrimal gland

•  The frontal nerve divides into the supraorbital and supratrochlear nerve, which supply the upper eyelid, forehead, and scalp

•  The nasociliary nerve is the sensory nerve to the eye

•  The infratrochlear nerve supplies the eyelids, conjunctiva, skin of nose, and lacrimal sac

•  Ethmoidal nerves supply the mucous membrane of the sphenoid and ethmoidal sinuses, nasal cavity, and dura of anterior cranial fossa

•  Short ciliary nerves carry parasympathetic and sympathetic fibers to the ciliary body and iris

•  The ciliary ganglion is a parasympathetic ganglion with preganglionic fibers exciting from the oculomotor nerve and postganglionic fibers transmitting nerve impulses to the ciliary muscle and the sphincter muscle of the iris

Arteries of Orbit:

Table 1.0. Arteries of Orbit. (From Moore KL, Dalley AF. Clinically Oriented Anatomy , fourth ed. Philadelphia , Lippincott Williams & Williams, l999).

Refracting Media:

Refracting media are substances that bend light. In order to reach the rods and cones, light must pass through four refracting media:

•  Cornea

•  Aqueous humor

•  Lens

•  Vitreous humor

Focusing Images on the Retina:

Retinal image focusing requires four processes:

•  Refraction of light rays

•  Accommodation by the lens

•  Constriction of the pupil

•  Convergence of the eyes

Refraction:

Light rays are bent as they pass from one optical density to another. Light reaching the retina is refracted:

•  As they enter the cornea

•  As they leave the posterior surface of the cornea and enter aqueous humor

•  Upon entrance to the lens

•  As they leave the lens and enter vitreous humor

•  The greatest binding of light occurs at the cornea

Accommodation:

The elastic lens is normally under tension and appears relatively flat. The normal eye focuses parallel light rays from distant points sharply on the retina (light rays more than 20 feet away are considered parallel) while divergent light rays entering the retina from points closer than 20 feet are focused behind the retina. Adjustments in focusing must be made if objects closer than 20 feet are to be clearly focused in the retina.

•  Accommodation is accomplished by smooth ciliary muscles of the ciliary body

•  Contraction of ciliary muscles pulls the ciliary body forward and inward, narrowing the ring of the ciliary body which lessens tension on the suspensory ligament and permits the lens to assume a more pronounced curvature

•  Accommodation also involves constriction of the pupil

Constriction of the Pupil:

Constriction of the pupil eliminates divergent rays which would otherwise pass through the most peripheral portion of the lens. Constrict of the pupils aids in the formation of sharp retinal images and reduces the amount of light entering the eye.

Convergence:

During convergence, the eyes turn inward and the image falls on the fovea, the most sensitive part of the retina.

Visual Pathways:

Two optic nerves meet at the optic chiasma , located anterior to the pituitary gland .

•  Within the optic chiasma , ganglion cell axons from the medial half of each retina cross to the opposite side

•  From the optic chiasma, axons continue as optic tracts

•  The left optic tract consists of ganglion cell axons from the lateral half of the retina of the left eye and the medial half of the retina of the right eye. (Both of these fibers carry visual information from the right visual field).

•  The right optic tract consists of axons from the lateral half of the retina of the right eye and medial half of the left eye. (Both of these fibers carry visual information from the left visual field).

•  Most ganglionic cell axons in the optic tracts travel to the lateral geniculate bodies of the thalamus.

•  From this area, synapsis with neurons that form the optic radiations which terminate in the visual cortex of the occipital lobe, where nerve impulses from the retina are interpreted visually by the brain.

•  Some optic tract axons terminate in the superior colliculi of the brain, where they function in visual reflexes (e.g. hand-eye coordination)

Basic Study Questions

  1. Identify the following structures of the eye: sclera, cornea, lens, iris, anterior chamber, posterior chamber

  2. What structure divides the anterior and posterior chamber of the eye?

  3. What is the blood supply of the cornea?

  4. Where is the vitreous body located and what is its function?

  5. Which cranial nerve supplies the majority of the ocular muscles?

Additional Study Questions