The anatomy of the eyeball itself is traditionally described as being three layers, or tunics (TOO nicks) (Fig. 14-2). The outer layer, or fibrous tunic, consists of the sclera (SKLAIR uh) and cornea (KOR nee uh). The middle layer, or vascular tunic, consists of the uvea (YOO vee uh), which is made up of the choroid (KOR oyd), ciliary (SILL ih air ee) body, and iris (EYE ris). The inner layer, or nervous tunic, consists of the retina (RET in uh). These three layers are essential to the process of seeing. All parts work together with impressive harmony. The eye muscles coordinate their movements with one another; the cornea and pupil control the amount of light that enters the eye; the lens focuses the image on the retina; and the optic nerve transmits the image to the brain.

Fig. 14-2 A, The eyeball viewed from above. B, The anterior view of the eyeball.

sclera = scler/o

cornea = corne/o, kerat/o

Two important mechanisms also contribute to the ability to see. As light hits the eye, it passes first through the cornea, which bends the rays of light (refraction) so that they are projected properly onto receptor cells in the eye. Then, muscles in the ciliary body adjust the shape of the lens to aid in this refraction. The lens flattens to adjust to something seen at a distance, or thickens for close vision—a process called accommodation. Errors of refraction are the most common reason for lens prescriptions.

The Sclera

The outermost lateral and posterior portion of the eye, the white of the eye, is called the sclera, which means hard. The portion of the sclera that covers the anterior section of the eye is transparent and is called the cornea. The border of the cornea, between it and the sclera, is called the limbus (LIM bus). The cornea is where refraction (the bending of light) begins.

Be Careful!

Core/o, meaning pupil of the eye, and corne/o, meaning cornea, are easy to confuse.

uvea = uve/o

choroid = choroid/o

ciliary body = cycl/o

iris = ir/o, irid/o

The Uvea

The uvea is the middle, highly vascular layer of the eye. It includes the iris, the ciliary body, and the choroid. The iris (pl. irides), pupil, lens (pl. lenses), and ciliary body are located directly behind the cornea. The iris is a smooth muscle that contracts and relaxes to moderate the amount of light that enters the eye. In most individuals, this is the colored part of the eye (brown, gray, hazel, blue) because of its pigmentation. Individuals with albinism, however, have reddish-pink irides (sing. iris) because a lack of pigment makes visible the blood cells traveling through the vessels supplying the iris.

pupil = pupill/o, core/o, cor/o

lens = phak/o, phac/o

The pupil is the dark area in the center of the iris where the light continues its progress through to the lens. The lens is an avascular structure made of protein and covered by an elastic capsule. It is held in place by the thin strands of muscle that make up the ciliary body. The fluid produced by the capillaries of the ciliary body is called the aqueous (AY kwee us) humor. It nourishes the cornea, gives shape to the anterior eye, and maintains an optimum intraocular pressure. It normally drains through tiny veins called the canals of Schlemm. The aqueous humor circulates in both the anterior chamber, between the cornea and the iris, and the posterior chamber, behind the iris and in front of the lens. Between the lens and the retina is a jellylike substance, the vitreous (VIT ree us) humor (also called the vitreous body), which holds the choroid membrane against the retina to ensure an adequate blood supply. The choroid is the vascular membrane between the sclera and the retina.

vitreous humor = vitre/o

The Retina

The inner layer of the eye, called the retina, contains the sensory receptors for the images carried by the light rays. These sensory receptors are either rods, which appear throughout the retina and are responsible for vision in dim light, or cones, which are concentrated in the central area of the retina and are responsible for color vision.

retina = retin/o

During daylight, the area of the retina on which the light rays focus is called the macula lutea (MACK yoo lah LOO tee uh). The fovea (FOH vee uh) centralis is an area within the macula that contains only cones and provides the sharpest image. The area that allows a natural blind spot in our vision is the optic disk, where the optic nerve leaves the retina to travel to the brain. There are no light receptors there.

macula lutea = macul/o

optic disk = papill/o

Exercise 3: The Eyeballs

Match the parts of the eye with the correct combining forms. More than one answer may be correct.

_____ 1. ir/o, irid/o

_____ 2. papill/o

_____ 3. retin/o

_____ 4. cor/o, core/o

_____ 5. pupill/o

_____ 6. macul/o

_____ 7. phac/o, phak/o

A hard, outer covering of the eye

B dark center of iris

C substance between retina and lens

D middle, highly vascular layer of the eye

E choroid

F ciliary body

G transparent, anterior portion of sclera

H lens

I made up of rods and cones

J pigmented muscle that allows light in eye

K light focuses on this retinal structure

L optic disk

M inner layer of eye

Decode the adjective forms.

14. extraocular ______________________________________________________________

15. preretinal ______________________________________________________________

16. intrascleral ______________________________________________________________

Exercise 4: The Eyeballs

Label the drawings below with correct anatomic terms and combining forms where appropriate.

You can review the anatomy of the eye by going to Evolve at http://evolve.elsevier.com/Shiland and clicking on Body Spectrum Electronic Anatomy Coloring Book → Senses → Eye. Choose Hear It, Spell It to practice spelling the anatomy and physiology terms for the eye. To practice pronouncing these terms, click on Hear It, Say it.

Combining and Adjective Forms for the Anatomy and Physiology of the Eye

Meaning	Combining Form	Adjective Form
canthus	canth/o	canthal
choroid	choroid/o	choroidal
ciliary body	cycl/o	cyclic
conjunctiva	conjunctiv/o	conjunctival
cornea	corne/o, kerat/o	corneal, keratic
eye	ocul/o, ophthalm/o	ocular, ophthalmic
eyelid	blephar/o, palpebr/o	palpebral
iris	ir/o, irid/o	iridic
lacrimal gland	dacryoaden/o	dacryoadenal
lacrimal sac	dacryocyst/o	dacryocystic
lens	phak/o, phac/o
macula lutea	macul/o	macular
optic disk	papill/o	papillary
orbit	orbit/o	orbital
pupil	pupill/o, core/o, cor/o	pupillary
retina	retin/o	retinal
sclera	scler/o	scleral
tears	lacrim/o, dacry/o	lacrimal
uvea	uve/o	uveal
vision	opt/o, optic/o	optic, optical
vitreous humor	vitre/o, vitr/o	vitreous

Prefixes for Anatomy of the Eye

Prefix	Meaning
bin-	two
extra-	outside
supra-	above

Pathology

Terms Related to Eyelid Disorders

Term	Word Origin	Definition
blepharedema	blephar/o eyelid -edema swelling	Swelling of the eyelid.
bleff ah ruh DEE mah	blephar/o eyelid -edema swelling	Swelling of the eyelid.
blepharitis	blephar/o eyelid -itis inflammation	Inflammation of the eyelid.
bleff ah RYE tis	blephar/o eyelid -itis inflammation	Inflammation of the eyelid.
blepharochalasis	blephar/o eyelid -chalasis relaxation, slackening	Hypertrophy of the skin of the eyelid.
bleff ah roh KAL luh sis	blephar/o eyelid -chalasis relaxation, slackening	Hypertrophy of the skin of the eyelid.
blepharoptosis	blephar/o eyelid -ptosis drooping	Drooping of the upper eyelid.
bleff ah rop TOH sis	blephar/o eyelid -ptosis drooping	Drooping of the upper eyelid.
ectropion	ec- out trop/o turning -ion process of	Turning outward (eversion) of the eyelid, exposing the conjunctiva (Fig. 14-3).
eck TROH pee on	ec- out trop/o turning -ion process of
entropion	en- in trop/o turning -ion process of	Turning inward of the eyelid toward the eye (Fig. 14-4).
en TROH pee on	en- in trop/o turning -ion process of	Turning inward of the eyelid toward the eye (Fig. 14-4).

Terms Related to Eyelash Disorders

Term	Word Origin	Definition
chalazion		Hardened swelling of a meibomian gland resulting from a blockage. Also called meibomian cyst (Fig. 14-5).
kuh LAY zee on
hordeolum		Stye; infection of one of the sebaceous glands of an eyelash (Fig. 14-6).
hor DEE uh lum

Terms Related to Tear Gland Disorders

Term	Word Origin	Definition
dacryoadenitis	dacryoaden/o lacrimal gland -itis inflammation	Inflammation of a lacrimal gland.
dack ree oh add eh NYE tis	dacryoaden/o lacrimal gland -itis inflammation	Inflammation of a lacrimal gland.
dacryocystitis	dacryocyst/o lacrimal sac -itis inflammation	Inflammation of a lacrimal sac.
dack ree oh sis TYE tis	dacryocyst/o lacrimal sac -itis inflammation	Inflammation of a lacrimal sac.
epiphora		Overflow of tears; excessive lacrimation.
eh PIFF or ah		Overflow of tears; excessive lacrimation.
xerophthalmia	xer/o dry ophthalm/o eye -ia condition	Dry eye; lack of adequate tear production to lubricate the eye. Usually the result of vitamin A deficiency.
zeer off THAL mee ah	xer/o dry ophthalm/o eye -ia condition

Terms Related to Conjunctiva Disorders

Term	Word Origin	Definition
conjunctivitis	conjunctiv/o conjunctiva -itis inflammation	Inflammation of the conjunctiva, commonly known as pinkeye, a highly contagious disorder (Fig. 14-7).
kun junk tih VYE tis	conjunctiv/o conjunctiva -itis inflammation
ophthalmia neonatorum	ophthalm/o eye -ia condition neo- new nat/o born -um structure	Severe, purulent conjunctivitis in the newborn, usually due to gonorrheal or chlamydial infection. Routine introduction of an antibiotic ophthalmic ointment (erythromycin) prevents most cases.
off THAL mee uh nee oh nay TORE um

Be Careful!

Do not confuse these similar terms: esotropia, exotropia, entropion, and ectropion.

Term	Word Origin	Definition
amblyopia	ambly/o dull, dim -opia vision condition	Dull or dim vision due to disuse.
am blee OH pee ah	ambly/o dull, dim -opia vision condition	Dull or dim vision due to disuse.
diplopia	dipl/o double -opia vision condition	Double vision. Emmetropia (EM, Em) means normal vision.
dih PLOH pee ah	dipl/o double -opia vision condition	Double vision. Emmetropia (EM, Em) means normal vision.
esotropia	eso- inward trop/o turning -ia condition	Turning inward of one or both eyes.
eh soh TROH pee ah	eso- inward trop/o turning -ia condition	Turning inward of one or both eyes.
exophthalmia	ex- out ophthalm/o eye -ia condition	Protrusion of the eyeball from its orbit; may be congenital or the result of an endocrine disorder (Fig. 14-8).
eck soff THAL mee ah	ex- out ophthalm/o eye -ia condition
exotropia	exo- outward trop/o turning -ia condition	Turning outward of one or both eyes.
eck so TROH pee ah	exo- outward trop/o turning -ia condition	Turning outward of one or both eyes.
photophobia	phot/o light -phobia condition of fear	Extreme sensitivity to light. The suffix -phobia here means “aversion,” not fear.
foh toh FOH bee ah	phot/o light -phobia condition of fear
strabismus		General term for a lack of coordination between the eyes, usually due to a muscle weakness or paralysis. Sometimes called a “squint,” which refers to the patient’s effort to correct the disorder.
strah BISS mus