ORBIT

ORBIT

Introduction are bilateral bony cavities in the facial skeleton, situated one on each

side of the root of the nose

Shape

It resembles a hollow quadrangular pyramid

Functions

It contain and protect the; eyeballs associated visceral structures

These associated visceral structures include: Eyelids Extraocular muscles Nerves and vessels in transit to the eyeballs and

muscles Orbital fascia surrounding the eyeballs and

muscles

Mucous membrane (conjunctiva) lining the eyelids and anterior aspect of the eyeballs

lacrimal apparatus Orbital fats Seven bones form the bony orbit namely;

• Maxilla

• Zygomatic

• Lacrimal

• Ethmoid

• Palantine

• Sphenoid

• Frontal

3

Boundaries The pyramidal orbit has; an apex a base and four walls; superior, inferior, medial, lateral

Apex : is the optic canal (foramen)

Base of the orbit

Is the orbital rim (margin)

It is formed;• Superior margin: frontal bone • Medial margin: maxilla• Inferior margin: zygomatic bone, maxilla• Lateral margin: frontal process of the zygomatic bone, zygomatic

process of the frontal bone

The orbit is a pyramidal cavity with its base in front and its apex behind

Four walls The superior wall (roof):• formed mainly by the orbital part of the frontal bone The medial wall: • formed by orbital parts of the ethmoid bone, lacrimal and frontal bones• it is indented anteriorly by the lacrimal groove and fossa for the

lacrimal sac The inferior wall (floor):• formed mainly by the maxilla (orbital part) and partly by the orbital

parts of the zygomatic and palatine bones• It is demarcated from the lateral wall of the orbit by the inferior orbital

fissure• The lateral wall: formed by the orbital parts of zygomatic bone and

greater wing of the sphenoid• This is the strongest and thickest wall, which is important because it is

most exposed and vulnerable to direct trauma

note • the orbit communicates with; the middle cranial fossa (via the optic canal and superior

orbital fissure) the infratemporal and pterygopalatine fossae (via the

inferior orbital fissure) the inferior meatus of the nose (via the nasolacrimal

canal) the nasal cavity (via the anterior ethmoidal foramen), and the face (via supraorbital and infraorbital foramina)

Eyelids• They are movable folds that are covered externally by thin skin and

internally by transparent mucous membrane

functions When closed, the eyelids cover the eyeball anteriorly, thereby

protecting it from injury and excessive light They also keep the cornea moist by spreading the lacrimal fluid

Conjuctiva • The internal transparent mucous membrane of the eyelid is the

palpebral conjunctiva • The palpebral conjunctiva is reflected onto the eyeball and it is

continuous with the bulbar conjunctiva• The bulbar conjunctiva is thin and transparent and attaches loosely to

the anterior surface of the eyeball• The bulbar conjunctiva lies over the sclera and is adherent to the

periphery of the cornea, it contains visible small blood vessels

• When the eyelids are closed, a closed space is formed in between the palpebral and bulbar conjunctivae

• This space is called the conjunctival sac• The conjunctival sac opens through an anterior aperture, called the

palpebral fissure (the gap between the eyelids), when the eyelids are parted

• The upper and lower extensions of this sac are the superior and inferior conjunctival fornices (singular: fornix)

Clinical anatomy Hyperemia of the Conjunctiva• The conjunctiva is colorless, except when its vessels are dilated and

congested causing hyperemia • Hyperemia of the conjunctiva is caused by local irritation (e.g., from

dust, chlorine, or smoke) An inflamed conjunctiva (conjunctivitis/ pink eye) is a common

contagious infection of the eye

of the conjuctiva

Skeleton of the eyelids• The superior (upper) and inferior (lower) eyelids are strengthened

by dense bands of connective tissue called the superior and inferior tarsi (singular: tarsus)

• The superior and inferior tarsi form the skeleton of the eyelids• Embedded in the tarsi are tarsal glands/ meibomial glands• The tarsal glands; provide the lipid secretion of which lubricates the edges of the

eyelids and prevents them from sticking together when they close The lipid secretion also forms a barrier that lacrimal fluid does not

cross when produced in normal amounts• When production is excessive, it spills over the barrier onto the

cheeks as tears

Clinical anatomy

chalazion• a small, nonmalignant, localized swelling of the eyelid resulting

from obstruction and retained secretions of the meibomian glands

Eyelashes are the hairs that grow on margins of the lids• The superior and inferior eyelids meet at angles called the medial

and lateral palpebral commissures, or medial and lateral canthi (singular: canthus)

• Between the nose and the medial angle of the eye is the medial palpebral ligament, which connects the tarsi to the medial margin of the orbit

• The orbicularis oculi originates and inserts onto this ligament• A similar lateral palpebral ligament attaches the tarsi to the

lateral margin of the orbit but does not provide for direct muscle attachment

Lacrimal Apparatus• is involved in the production, movement, and drainage of fluid from

the surface of the eyeball

It is made up of the; • lacrimal gland • lacrimal ducts• lacrimal canaliculi • lacrimal sac• nasolacrimal duct

Lacrimal gland

Intro:• almond shaped • approximately 2 cm long

Location:• lies in the fossa for the lacrimal gland in the superolateral part of

each orbit

Parts:

It is divided into;• superior (orbital) part• inferior (palpebral) part by the lateral expansion of the tendon of

the levator palpebrae superioris

Function:• secrete lacrimal fluid, a watery physiological saline containing the

bacteriocidal enzyme lysozyme• The fluid;

1. moistens and lubricates the surfaces of the conjunctiva and cornea

2. provides some nutrients and dissolved oxygen to the cornea• When the fluid is produced in excess, it constitutes tears Note;

Accessory lacrimal glands are also present; they are more numerous in the superior eyelid than in the inferior eyelid

Lacrimal ducts:• convey lacrimal fluid from the lacrimal glands to the conjunctiva

sac

Lacrimal canaliculi: • commence at a lacrimal punctum (opening) on the lacrimal papilla

near the medial angle of the eye and drain lacrimal fluid from the lacrimal lake

• a triangular space at the medial angle of the eye where the tears collect) to the lacrimal sac (the dilated superior part of the nasolacrimal duct)

Note: the lacrimal papilla is a slight projection from the margin of each eyelid near the medial commissure, in the center of which is the lacrimal puntum (operning of the lacrimal duct)

Nasolacrimal duct: • conveys the lacrimal fluid to the inferior nasal meatus

CLINICAL ANATOMY Injury to the Nerves Supplying the Eyelids• The oculomotor nerve supplies the levator palpebrae superioris, a

lesion of the oculomotor nerve causes paralysis of the muscle, and the superior eyelid droops (ptosis)

• Damage to the facial nerve involves paralysis of the orbicularis oculi, preventing the eyelids from closing fully

Drooping eyelid

The Eyeball The eyeball contains the optical apparatus of the visual system and

occupies most of the anterior portion of the orbit.

Layers: The eyeball proper has 3 layers: Fibrous layer (outer coat), consisting of the sclera and cornea Vascular layer (middle coat), consisting of the choroid, ciliary body,

and iris Inner layer (inner coat), consisting of the retina that has both optic

and non-visual parts however, there is an additional loose connective tissue layer that

surrounds the eyeball This loose connective tissue layer is composed posteriorly of bulbar fascia, which forms the true socket for the

eyeball and anteriorly of bulbar conjunctiva

Fibrous Layer of the Eyeball The sclera:• is the tough opaque part of the fibrous layer (coat) of the eyeball • It covers the posterior 5/6 of the eyeball • It is the fibrous skeleton of the eyeball, providing shape and resistance as

well as attachment for both the extrinsic (extraocular) and the intrinsic muscles of the eye

• The anterior part of the sclera is visible through the transparent bulbar conjunctiva as the white of the eye

The cornea:• is the transparent part of the fibrous coat • covers the anterior 1/6 of the eyeball

Note: While the sclera is relatively avascular, the cornea is completely avascular, receiving its nourishment from ;

capillary beds around its periphery Lacrimal fluids on its external surface and the aqueous humour on the internal surface

Vascular Layer of the Eyeball• also called the uvea or uveal tract consists of the: choroid ciliary body iris

The choroid:• a dark reddish brown layer between the sclera and the retina• It is a highly vascular layer (presence of large vessels and fine

vessels called the capillary lamina of the choroid, or choriocapillaris)

• forms the largest part of the vascular layer of the eyeball and lines most of the sclera

• It is continuous anteriorly with the ciliary body• The choroid attaches firmly to the pigment layer of the retina, but

it can easily be stripped from the sclera

The ciliary body :• is muscular and vascular • connects the choroid with the circumference of the iris• The ciliary body provides attachment for the lens• contraction and relaxation of the smooth muscle of the ciliary body

controls thickness (and therefore the focus) of the lens• Folds on the internal surface of the ciliary body called the ciliary

processes, secrete aqueous humor• The aqueous humor fills the anterior and posterior chambers of the

eye • The anterior chamber of the eye is the space between the cornea

anteriorly and the iris/pupil posteriorly• The posterior chamber of the eye is between the iris/pupil

anteriorly and the lens and ciliary body posteriorly

The iris:• lies on the anterior surface of the lens• is a thin contractile diaphragm with a central aperture, called the

pupil, for transmitting light• the size of the pupil varies continually to regulate the amount of light

entering the eye Two involuntary muscles control the size of the pupil:• the parasympathetically stimulated sphincter pupillae constrict/

decrease the diameter of the pupil • the sympathetically stimulated dilator pupillae dilate or increases the

diameter of the pupil

Clinical anatomy

Uveitis• inflammation of the vascular layer of the eyeball (uvea), may

progress to severe visual impairment and blindness

Inner Layer of the Eyeball The inner layer of the eyeball is the retina

it consists of two functional parts:• an optic part: which is posterior and lateral and is sensitive to light• a non-visual retina: which is anterior and covers the internal surface

of the ciliary body and the iris • The junction between these parts is an irregular line called the ora

serrata Note: The ora serrata marks the anterior termination of the light-

receptive part of the retina

The optic part of the retina : is sensitive to visual light rays has two layers: an outer pigmented layer an inner neural layer

The non-visual retina :• is an anterior continuation of the outer pigmented layer• a layer of supporting cells over the ciliary body (ciliary part of the

retina) • the posterior surface of the iris (iridial part of the retina)

Ocular fundus/ fundus of the eyeball • This is the internal aspect of the posterior part of the eyeball,

where light entering the eyeball is focus• it includes the retina, optic disc, macula and fovea• has a circular depressed area called the optic disc (optic papilla)• The optic disc is where the optic nerve leaves the retina • branches of the central retinal artery spread from this point

outward to supply the retina

• the optic disc is insensitive to light• This is because it contains no photoreceptors (no rods and cones)• This part of the retina is commonly called the blind spot• Just lateral to the optic disc is a small area with a hint of yellowish

coloration called macula lutea (yellow spot)• It has a central depression called the fovea centralis• This is the thinnest area of the retina and visual sensitivity here is

higher than elsewhere in the retina• This is because it has fewer rods and more cones

Rods and Cones• The rods and cones are the light receptors of the eye• They are sensitive to colour• The rods contain a pigment called visual purple• They can respond to dim light (scotopic vision)• The cones respond only to bright light (photopic vision)

Refractive Media of the Eyeball

The Refractive Media of the Eyeball is made up of 4 parts; cornea aqueous humor, lens vitreous humor

Cornea:• is the circular area of the anterior part of the outer fibrous layer of

the eyeball• it is largely responsible for refraction of the light that enters the eye • It is transparent and sensitive to touch

• its innervation is provided by the ophthalmic nerve (CN V1).

• It is avascular• Its nourishment is derived from the capillary beds at its periphery,

the aqueous humor, and lacrimal fluid • The lacimal fluid also provides oxygen absorbed from the air

Aqueous humor :• It is located in the anterior and posterior chambers of the eye• produced in the posterior chamber by the ciliary processes of the

ciliary body• This clear watery solution provides nutrients for the avascular

cornea and lens• After passing through the pupil into the anterior chamber, the

aqueous humor drains through a trabecular meshwork into the scleral venous sinus / Schlemm canal) at the iridocorneal angle

• From the schlemm canal it drains into the anterior ciliary veins and vorticose veins

Lens:• is posterior to the iris and anterior to the vitreous humor of the

vitreous body• It is a transparent, biconvex structure enclosed in a capsule• is anchored by the zonular fibers (suspensory ligament of the lens)

to the ciliary body

• Although most refraction is produced by the cornea, the convexity of the lens, particularly its anterior surface, constantly varies to fine-tune the focus of near or distant objects on the retina

• The ciliary muscle in the ciliary body changes the shape of the lens

parasympathetic stimulation; • causes the smooth muscle of the ciliary body to contract• Making the tension on the lens to be reduced/ zonular fibers

relax• allowing the lens to become more spherical• The increased convexity makes its refraction suitable for

near vision

In the absence of parasympathetic stimulation• the ciliary muscles relax again• Tension on the lens is increased/ zonular fibers

contract • lens becomes flattened• The decreased convexity makes its refraction

suitable for far vision

The ability of the lens to focus on both near and far vision is called accommodation

vitreous humor:• is a watery fluid enclosed in the meshes of the vitreous body• a transparent jelly-like substance in the posterior four fifths of the

eyeball posterior to the lens (postremal or vitreous chamber, or posterior segment).

• In addition to transmitting light, the vitreous humor holds the retina in place and supports the lens

CLINICAL ANATOMY Corneal Abrasions and Lacerations• Foreign objects such as sand or metal filings (particles) produce

corneal abrasions that cause sudden, stabbing pain in the eyeball and tears. Opening and closing the eyelids is also painful

• Corneal lacerations are caused by sharp objects such as fingernails or the corner of a page of a book

Presbyopia • As people age, their lenses become harder and more flattened• These changes gradually reduce the focusing power of the lenses, a

condition known as presbyopia Cataracts • is a clouding of the lens inside the eye which leads to a decrease in

vision• Visual loss occurs because opacification of the lens obstructs light

from passing and being focused on to the retina at the back of the eye• It is the most common cause of blindness and is conventionally treated

with surgery• Cataract extraction is a common operation Hemorrhage into the Anterior Chamber• Hemorrhage within the anterior chamber of the eyeball (hyphema or

hyphemia) usually results from blunt trauma to the eyeball, such as from a squash or racquet ball or a hockey stick

• Initially, the anterior chamber is tinged red but blood soon accumulates in this chamber

• The initial hemorrhage usually stops in a few days and recovery is usually good

Glaucoma• When drainage of aqueous humor through the scleral venous

sinus into the blood circulation decreases significantly, pressure builds up in the anterior and posterior chambers of the eye, a condition called glaucoma

• Blindness can result from compression of the inner layer of the eyeball (retina) and the retinal arteries if aqueous humor production is not reduced to maintain normal intraocular pressure

 Coloboma (defect)• is a hole in one of the structures of the eye, such as

the iris, retina, choroid or optic disc• The hole is present from birth and can be caused when a gap

called the choroid fissure, which is present during early stages of prenatal development, fails to close up completely before a child is born

Coloboma of Iris• The absence of a section of iris may result from a birth defect, in

which the choroid fissure fails to close properly, from penetrating or non-penetrating injuries to the eyeball, or a surgical iridectomy

• When the iris is injured in such a manner, the iridial fissure does not heal

Coloboma of the iris

Muscles of the orbit There are 2 groups of muscles within the orbit: intrinsic muscles of the eyeball extrinsic muscles of eyeball (extra-ocular muscles) The intrinsic muscles within the eyeball control;• the shape of the lens • size of the pupil

These include;• ciliary muscle• sphincter pupillae, • dilator pupillae The extrinsic muscles of eyeball (extra-ocular muscles) are

responsible for;• raising upper eyelids • movements of the eyeball

Extraocular Muscles of the Orbit The extraocular muscles of the orbit are 7 in number, and they

include the: levator palpebrae superioris four recti, which are;

I. Superior rectus

II. inferior rectus

III. Medial rectus

IV. and lateral rectus and two obliques, which are;

I. superior oblique

II. and inferior oblique• Of the seven muscles in the extrinsic group of muscles, one raises

the superior eyelids, whereas the other six move the eyeball itself

Muscle Origin Insertion Innervation Action

Levator palpebrae superioris

Lesser wing of sphenoid anterior to optic canal

Superior tarsus and skin of superior eyelid

Oculomotor nerve (CN III)

Elevation of upper eyelid

Superior rectus Superior part of common tendinous ring

Anterior half of eyeball superiorly

Oculomotor nerve (CNIII)

Elevates, adducts, and rotates eyeball medially

Inferior rectus Inferior part of common tendinous ring

Anterior half of eyeball inferiorly

Oculomotor nerve (CN III)

Depresses, adducts, and rotates eyeball laterally

Medial rectus Medial part of common tendinous ring

Anterior half of eyeball medially

Oculomotor nerve (CN III)

Adduction of eyeball

Lateral rectus Lateral part of common tendinous ring

Anterior half of eyeball laterally

Abducent nerve (CNVI)

Abducts eyeball

Superior oblique

Body of sphenoid bone

tendon passes through a fibrous ring or trochlea, changes its direction, and inserts into sclera deep to superior rectus muscle

Trochlear nerve (CN IV)

Abducts, depresses, and medially rotates eyeball

Inferior oblique

Anterior part of floor of orbit

Sclera deep to lateral rectus muscle

Oculomotor nerve (CNIII)

Abducts, elevates, and laterally rotates eyeball

• Medial movement of the superior pole of the eyeball is intorsion; • lateral movement of the superior pole is extorsion. • These movements accommodate changes in the tilt of the head.• Absence of these movements resulting from nerve lesions contributes to

double vision

Innervation of muscles of eyeball• The oculomotor (CN III), trochlear (CN IV), and abducent (CN VI) nerves

are distributed to the muscles of the eyeball• The nerves enter the orbit through the superior orbital fissure.• CN IV supplies the superior oblique, • CN VI supplies the lateral rectus,• and CN III supplies the remaining five muscles

NOTE: All muscles of the orbit are supplied by CN III, except for the superior

oblique and lateral rectus, which are supplied by CN IV and VI, respectively

(Memory device: LR6SO4AO3; lateral rectus, CN VI; superior oblique, CN IV; all others, CN III

Fascial Sheath of the Eyeball• It is also called Tenon’s capsule or the bulbar sheath• It envelops the eyeball from the optic nerve nearly to the

corneoscleral junction, forming the actual socket for the eyeball • The fascial sheath is pierced by the tendons of the extraocular

muscles • A potential space exist between the eyeball and the fascial sheath

called the episcleral space and its allows the eyeball to move inside the cup-like sheath

ARTERIAL SUPPLY TO THE ORBIT

Arterial supply to the structures in the orbit, including the eyeball, is mainly by the ophthalmic artery

• it is a branch of the internal carotid artery• The ophthalmic artery passes into the orbit through the optic canal

with the optic nerve• In the orbit the ophthalmic artery initially lies inferior and lateral to

the optic nerve • it passes forward and crosses superior to the optic nerve and

proceeds anteriorly on the medial side of the orbit• In the orbit the ophthalmic artery gives off numerous branches as

follows: lacrimal arteries central retinal artery long and short posterior ciliary arteries

supra-orbital artery posterior ethmoidal artery anterior ethmoidal artery medial palpebral arteries, which are small branches supplying the

medial area of the upper and lower eyelids dorsal nasal artery supratrochlear artery

Venous drainage There are two venous channels in the orbit,• the superior ophthalmic vein• and inferior ophthalmic vein The central vein of the retina usually enters the cavernous sinus

directly, but it may join one of the ophthalmic veins

CLINICAL ANATOMY Injury to the Nerves Supplying the Superior Eyelids• The oculomotor nerve supplies the levator palpebrae superioris, a lesion

of the oculomotor nerve causes paralysis of the muscle, and the superior eyelid droops (ptosis)

Oculomotor Nerve Palsy• Complete oculomotor nerve palsy affects most of the ocular muscles, the

levator palpebrae superioris, and the sphincter pupillae

Abducent nerve palsy causes paralysis to the lateral rectus

Blockage of the Central Vein of the Retina• Because the central vein of the retina enters the cavernous sinus,

thrombophlebitis of this sinus may result in the passage of a thrombus to the central retinal vein and produce a blockage in one of the small retinal veins

• Occlusion of a branch of the central vein of the retina usually results in slow, painless loss of vision

Blockage of the Central Artery of the Retina• Because terminal branches of the central artery of the retina are

end arteries, obstruction of them by an embolus results in instant and total blindness

• Blockage of the artery is usually unilateral and occurs in older people