eyeanatomy-130923133056-phpapp02

8/13/2019 eyeanatomy-130923133056-phpapp02

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The outer coatwhich is

transparent

anteriorly iscalled the

cornea.

The white andopaque coat is

the sclera.


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A rich vascular coat (the choroid) lines the posterior

segment of the eye and nourishes the retina at its inner

surface.

The ciliary body lies anteriorly. It contains the smooth

ciliary muscle whose contraction alters lens shape and

enables the focus of the eye to be changed.The ciliary

epithelium secretes aqueous humour and maintains the

ocular pressure. The ciliary body provides attachment for

the iris.

The lens lies behind the iris and is supported by finefibrils (the zonule)

running between the lens and the ciliary body.


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The angle formed by the iris and cornea (the

iridocorneal angle) is lined by a meshwork of

cells and collagen beams (the trabecular

meshwork).

In the sclera outside this, Schlemmscanal

conducts the aqueous humour from the anterior

chamber into the venous system, permitting

aqueous drainage. This region is termed the

drainage angle.


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Between the cornea anteriorly and the lensand iris posteriorly lies the anterior chamber.

Between the iris, the lens and the ciliary body

lies the posterior chamber (which is distinct

from the vitreous body). Both these chambers

are filled with aqueous humour. Between the

lens and the retina lies the vitreous body.


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The eye lies within the bony orbit whose hasthe shape of a four-sided pyramid. At its

posterior apex is the optic canal which

transmits the optic nerve to the brain. The

superior and inferior orbital fissures allow thepassage of blood vessels and cranial nerves

which supply orbital structures. On the anterior

medial wall lies a fossa for the lacrimal sac. The

lacrimal gland lies anteriorly in thesuperolateral aspect of the orbit.


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provide mechanical protection to the anteriorglobe.

secrete the oily part of the tear film.

spread the tear film over the conjunctiva and

cornea.

prevent drying of the eyes.

contain the puncta through which the tears

drain into the lacrimal drainage system.


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A surface layer of skin.

The orbicularis muscle.

A tough collagenous layer (the tarsal plate).

An epithelial lining, the conjunctiva, reflected

onto the globe.


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The levator muscle passes forwards to the

upper lid and inserts into the tarsal plate. Itis innervated by the third nerve. Damage to

the nerve or changes in old age result in

drooping of the eyelid (ptosis). A flat smoothmuscle arising from the deep surface of the

levator inserts into the tarsal plate. It is

innervated by the sympathetic nervous

system.


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If the sympathetic supply is damaged (as in

Horners syndrome) a slight ptosis results.

The margin of the eyelid is the site of the

mucocutaneous junction. It contains the

openings of the meibomian oil glands which

are located in the tarsal plate. These secretethe lipid component of the tear film. Medially,

on the upper and lower lids, two small puncta

form the initial part of thelacrimal drainage system.


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Tears drain into the upper and lower puncta andthen into the lacrimal sac via the upper and lower

canaliculi. They form a common canaliculus before

entering the lacrimal sac. The nasolacrimal duct

passes from the sac to the

nose. Failure of the distal part of the nasolacrimal

duct to fully canalize at birth is the usual cause of

a watering, sticky eye in a baby.Tear drainage is an

active process. Each blink of the lids helps to pump

tears through the

system.


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(10im thick) covers the

external ocular surface andcomprises three

layers: a thin mucin layer in contact with the

ocular surface and produced mainly by

the conjunctival goblet cells; an aqueous layer produced by the

lacrimal gland;

a surface oil layer produced by the tarsalmeibomian glands and delivered to the

lid margins.


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it provides a smooth air/tear interface fordistortion free refraction of light at the cornea

it provides oxygen anteriorly to the avascular

cornea

it removes debris and foreign particles from the

ocular surface through the flow of tears

it has antibacterial properties through the

action of lysozyme, lactoferrin and the

immunoglobulins, particularly secretory IgA.


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is 0.5 mm thick and comprises:

The epithelium, an anterior squamous layer thickenedperipherally at the limbus where it is continuous with the

conjunctiva.The limbus houses its germinativeor

stemcells.

An underlying stroma of collagen fibrils, ground

substance and fibroblasts. The regular packing and small

diameter of the collagen fibrils accounts for corneal

transparency.

The endothelium, a monolayer of non-regenerating cells

which actively pumps ions and water from the stroma to

control corneal hydration and transparency.


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The difference between the regenerative

capacity of the epitheliumand endotheliumis

important. Damage to the epithelial layer, by

an abrasion for example, is rapidly repaired.

Endothelium, damaged by disease

or surgery, cannot be regenerated. Loss of its

barrierand pumping functions leads tooverhydration, distortion of the regular

packing of collagen fibres and corneal

clouding.

The functions of the cornea are as follows: it refracts light and together with the lens,

focuses light onto the retina;

it protects the internal ocular structures.


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is formed from interwovencollagen fibrils of different widths

lying within a ground substance

and maintained by fibroblasts.

is of variable thickness, 1imm

around the optic nerve head and

0.3imm just posterior to the muscleinsertions.


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is formed of arterioles, venules and a densefenestrated capillary network.

is loosely attached to the sclera.

has a high blood flow.

nourishes the deep, outer layers of the retina and

may have a role in its temperature homeostasis.

Its basement membrane together with that of the

retinal pigment epithelium (RPE) forms the

acellular, Bruchs membrane, which acts as a

diffusion barrier between the choroid and theretina.


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The retinal pigment epithelium (RPE):

is formed from a single layer of cells.

is loosely attached to the retina except at the

periphery (ora serrata) and around the optic

disc.

forms microvilli which project between and

embrace the outersegment discs of the rods and

cones.


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phagocytoses the redundant external

segments of the rods and cones.

facilitates the passage of nutrients and

metabolites between the retina and

choroid.

takes part in the regeneration of rhodopsin

and cone opsin, the photoreceptor visual

pigments recycling vitamin A.

melanin granules absorb scattered light.


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Rods are

responsible for

night vision.Theyare sensitive to

light and do

not signal

wavelengthinformation

(colour). They

form the large

majority

of photoreceptorsin the remaining

retina.


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This is subdivided into three parts:

the ci l iary muscle.

the ciliary processes (pars p l icata.

the pars plana.


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THECILIARYMUSCLE

This:

Comprises smooth muscle arranged in a ring

overlying the ciliary processes.

Is innervated by the parasympathetic system via

the third cranial nerve.

Is responsible for changes in lens thickness and

curvature during accommodat ion. The zonular

f ibres supporting the lens are under tensionduring distant viewing. Contraction of the muscle

relaxes them and permits the lens to increase its

curvature and hence its refractive power.


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(PARS

PLICATA)

There are about 70 radial ci l iary processes arranged

in a ring around the posterior chamber.They are

responsible for the secretion of aqueous humour.

Each ciliary process is formed by an epithelium

two layers thick (the outer pigmented and inner

non-pigmented) with a vascular stroma.

The stromal capillaries are fenestrated, allowing

plasma constituentsready access.


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The t igh t junct ions between the non-

pigmented epithelial cells provide a barrierto free diffusion into the posterior

chamber.They are essential for the active

secretion of aqueous by the non pigmental

cells.

The epithelial cells show marked infolding,

which significantly increasestheir surfacearea for fluid and solute transport.


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This comprises a relatively avascular

stroma covered by an epithelial layer two

cells thick.

It is safe to make surgical incisions

through the scleral wall here to gain

access to the vitreous cavity.


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In the orbit the optic nerve is

surrounded by a sheath formed bythe dura, arachnoid and pia mater

continuous with that surrounding

the brain.It is bathed in

cerebrospinal fluid.

The central retinal artery and vein

enter the eye in the centre of theoptic nerve.


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All of the extraocular muscles, with the exceptionof the inferior oblique, form a cone within the

bony orbit

The apex of the cone is in the posterior aspect(back) of the orbit, while the base of the cone is

the attachment of the muscles around the midline

of the eye. This conic structure is referred to as

the annulus of Zinn,and within this cone runs

the Optic nerve(cranial nerve II)

Within the optic nerve are the ophthalmic arteryand the ophthalmic vein


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The superior oblique muscle is different from the

others, because before it attaches to the eye, itpasses through a ring-like tendon, the trochlea,

which acts like a pulley in the nasal portion of the

orbit

The inferior oblique muscle (not a member of theannulus of Zinn) arises from the lacrimal fossain

the nasal portion of the bony orbit and attaches tothe inferior portion of the eye


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The primary muscle that moves an eye in a givendirection is known as the agonist

A muscle in the same eye that moves the eye in

the same direction as the agonist is known as asynergist

A muscle in the same eye that moves the eye in

the opposite direction of the agonist is the

antagonist


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Cardinal positions of gaze

Up/right Up/left

Right

Left Down/right

Down/left

In each position of gaze, one muscle of each eye isthe primary mover of that eye, and is yoked to

the primary mover of the other eye


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A vergence or disconjugate movementinvolves simultaneous movement of both eyes in

the opposite directions

There are two principal vergence movements Convergenceboth eyes moving nasally or

inward

Divergenceboth eyes moving temporally or

upward


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Right eyeThe nose

SR

SO

IO

IR

MRLR


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