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Title: Eye Anatomy
Description: This notes brings the objectives to learn the anatomy of the eye, orbit and the third, fourth and sixth cranial nerves, to permit an understanding of medical conditions affecting these structures.
Description: This notes brings the objectives to learn the anatomy of the eye, orbit and the third, fourth and sixth cranial nerves, to permit an understanding of medical conditions affecting these structures.
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LN_C01
...
Introduction
A knowledge of ocular anatomy and function is important to the understanding of
eye diseases
...
Gross anatomy
The eye (Fig
...
1) comprises:
l A tough outer coat which is transparent anteriorly (the cornea) and opaque posteriorly (the sclera)
...
The extraocular muscles attach to the outer sclera while the optic nerve leaves the globe
posteriorly
...
The choroid nourishes the outer two-thirds of
the retina
...
l The ciliary body contains the smooth ciliary muscle, whose contraction allows
lens shape to alter and the focus of the eye to be changed
...
The ciliary body provides attachment for the iris, which forms the pupillary diaphragm
...
qxd 7/19/07 14:37 Page 2
Chapter 1 Anatomy
Cornea
Anterior chamber
Limbus
Schlemm's canal
Iridocorneal angle
Conjunctiva
Posterior
chamber
Iris
Zonule
Lens
Tendon of
extraocular
muscle
Ciliary body
Ora serrata
Uvea
Choroid
Sclera
Retina
Vitreous
Cribriform plate
Optic nerve
Fovea
Figure 1
...
The lens lies behind the iris and is supported by fine fibrils (the zonule) running
under tension between the lens and the ciliary body
...
In the sclera
outside this, Schlemm’s canal conducts the aqueous humour from the anterior
chamber into the venous system, permitting aqueous drainage
...
The cornea anteriorly and the iris and central lens posteriorly form the anterior
chamber
...
Both these chambers are filled with aqueous
humour
...
Anteriorly, the bulbar conjunctiva of the globe is reflected from the sclera onto
the underside of the eyelids to form the tarsal conjunctiva
...
l
2
LN_C01
...
2 The anatomy of the orbit
...
1
...
At its posterior apex is the optic canal, which transmits the optic nerve
to the chiasm, tract and lateral geniculate body
...
The lacrimal gland lies anteriorly in the superolateral aspect of the orbit
...
The eyelids (tarsal plates)
The eyelids (Fig
...
3):
l offer mechanical protection to the anterior globe;
l spread the tear film over the conjunctiva and cornea with each blink;
l contain the meibomian oil glands, which provide the lipid component of the tear
film;
l prevent drying of the eyes;
l contain the puncta through which the tears flow into the lacrimal drainage system
...
3
LN_C01
...
3 The anatomy of the eyelids
...
It is innervated by the third nerve
...
A flat smooth muscle arising from the deep
surface of the levator inserts into the tarsal plate
...
If the sympathetic supply is damaged (as in Horner’s
syndrome) a slight ptosis results
...
This oil is layered onto the anterior surface
of the tear film with each blink, where it retards evaporation
...
The lacrimal drainage system
Tears drain into the upper and lower puncta and then into the lacrimal sac via the
upper and lower canaliculi (Fig
...
4)
...
The nasolacrimal duct passes from the sac to the nose
...
Tear drainage is an active process
...
4
LN_C01
...
4 The major components of the lacrimal drainage system
...
They drain away via
the nasolacrimal system
...
When the eyes are open, the exposed ocular surface (the cornea and
nasal and temporal wedges of conjunctiva) are covered by a thin tear film, 3 µm
thick, which comprises three layers:
1 a thin mucin layer in contact with the ocular surface and produced mainly by the
conjunctival goblet cells;
2 an aqueous layer produced by the lacrimal gland;
3 a surface oil layer produced by the tarsal meibomian glands and delivered to the
lid margins
...
l It provides oxygen anteriorly to the avascular cornea
...
qxd 7/19/07 14:37 Page 6
Chapter 1 Anatomy
It removes debris and foreign particles from the ocular surface through the flow
of tears
...
The tear film is replenished with each blink
...
1
...
5 mm thick and comprises:
l The epithelium, an anterior non-keratinized squamous layer, thickened peripherally at the limbus where it is continuous with the conjunctiva
...
l An underlying stroma of collagen fibrils, ground substance and fibroblasts
...
This orderly architecture is maintained by regulating stromal hydration
...
The difference between the regenerative capacity of the epithelium and endothelium is important
...
Endothelial damage, by disease or surgery, is repaired by cell spreading alone, with a loss of cell density
...
Bowman's membrane
Tear film
Descemet's
membrane
Oil layer
Aqueous layer
Mucin layer
Keratocytes
Epithelium
Stroma
Endothelium
6
Figure 1
...
LN_C01
...
The aqueous also supplies oxygen to the posterior stroma, while the
anterior stroma receives its oxygen from the ambient air
...
Functions of the cornea
l It protects the internal ocular structures
...
The junction
between the ambient air and the curved surface of the cornea, covered by its optically smooth tear film, forms a powerful refractive interface
...
l It is of variable thickness, 1 mm around the optic nerve head and 0
...
l
The choroid
The choroid (Fig
...
6) is formed of arterioles, venules and a dense fenestrated
capillary network
...
l It has a remarkably high blood flow
...
l Its basement membrane, together with that of the retinal pigment epithelium
(RPE), forms the acellular Bruch’s membrane, which acts as a diffusion barrier
between the choroid and the retina
...
1
...
Its outermost layer is the retinal pigment epithelium (RPE) while its innermost
layer forms the neuroretina, consisting of the photoreceptors (rods and cones), the
bipolar nerve layer (and additional nerve cells) and the ganglion cell layer, whose
axons give rise to the innermost, nerve fibre layer
...
7
LN_C01
...
6 The relationship between the choroid, RPE and retina
...
7 The structure of the retina
...
qxd 7/19/07 14:37 Page 9
Anatomy Chapter 1
forms microvilli which project between and embrace the outer segment discs of
the rods and cones;
l phagocytoses the redundant external segments of the rods and cones;
l facilitates the passage of nutrients and metabolites between the retina and
choroid;
l takes part in the regeneration of rhodopsin and cone opsin, the photoreceptor
visual pigments and in recycling vitamin A;
l contains melanin granules which absorb light scattered by the sclera thereby
enhancing image formation on the retina
...
The initial integration of these signals is also performed by the retina
...
1
...
Different subgroups of cones are responsive to short,
medium and long wavelengths (blue, green, red)
...
l Rods are responsible for night vision
...
They form the large majority of photoreceptors in the remaining retina
...
It is 98% water
...
There are few cells
...
l It has a nutritive and supportive role
...
l
l
The ciliary body
The ciliary body (Fig
...
9) is subdivided into three parts:
1 the ciliary muscle;
2 the ciliary processes (pars plicata);
3 the pars plana
...
qxd 7/19/07 14:37 Page 10
Chapter 1 Anatomy
Cone
Rod
Outer plexiform
layer
Outer nuclear
layer
Nucleus
Outer
fibre
External limiting
membrane
Inner segment
Ellipsoid
Cilium
Outer segment
Cilium
Discs
Retinal pigment
epithelium
Figure 1
...
The ciliary muscle
l This comprises smooth muscle arranged in a ring overlying the ciliary processes
...
l It is responsible for changes in lens thickness and curvature during accommodation
...
Contraction of the muscle relaxes the zonule
and permits the elasticity of the lens to increase its curvature and hence its refractive power
...
They are responsible for the secretion of aqueous humour
...
l The stromal capillaries are fenestrated, allowing plasma constituents ready
access
...
qxd 7/19/07 14:37 Page 11
Anatomy Chapter 1
Iris
Cornea
Schlemm's canal
Trabecular meshwork
Iridocorneal angle
Pars plicata
Pars
plana
Ciliary muscle
Ciliary epithelium
Retina
Sclera
Non-pigmented epithelium
Stroma with fenestrated
capillaries
Pigmented epithelium
Basement membrane
Non-pigmented epithelium
Pigmented epithelium
Tight junction prevents
free diffusion between
non-pigmented cells
Fenestrated capillary
Basement membrane
Stroma
Active secretion of aqueous
Figure 1
...
The tight junctions between the non-pigmented epithelial cells provide a barrier
to free diffusion into the posterior chamber
...
l The epithelial cells show marked infolding, which significantly increases their
surface area for fluid and solute transport
...
qxd 7/19/07 14:37 Page 12
Chapter 1 Anatomy
The pars plana
l This comprises a relatively avascular stroma covered by an epithelial layer two
cells thick
...
The iris
The iris is attached peripherally to the anterior part of the ciliary body
...
l It has an anterior border layer of fibroblasts and collagen and a cellular stroma in
which the sphincter muscle is embedded at the pupil margin
...
l The smooth dilator muscle extends from the iris periphery towards the sphincter
...
l Posteriorly the iris is lined by a pigmented epithelium two layers thick
...
It is
the site of aqueous drainage from the eye via the trabecular meshwork (Fig
...
10)
...
The spaces between these beams become increasingly
small as Schlemm’s canal is approached
...
Damage here raises the
resistance and increases intraocular pressure in primary open angle glaucoma
...
Fluid passes into Schlemm’s canal both through giant vacuoles in its endothelial
lining and through intercellular spaces
...
1
...
12
LN_C01
...
10 The anatomy of the trabecular meshwork
...
It is supported by zonular fibres running between the ciliary body and the lens
capsule
...
Towards the equator the epithelium gives rise to the
lens fibres
...
l The lens fibres make up the bulk of the lens
...
Anteriorly and posteriorly they meet to
form the lens sutures
...
l The oldest central fibres represent the fetal lens and form the lens nucleus; the
peripheral fibres make up the lens cortex
...
l
l
13
LN_C01
...
11 The anatomy of the lens
...
1
...
l It passes out of the eye through the cribriform plate of the sclera, a sieve-like
structure
...
It is bathed in
cerebrospinal fluid
...
12 The structure of the optic nerve
...
qxd 7/19/07 14:37 Page 15
Anatomy Chapter 1
The central retinal artery and vein enter the eye in the centre of the optic nerve
...
The ocular blood supply
The eye receives its blood supply from the ophthalmic artery (a branch of the
internal carotid artery) via the retinal artery, ciliary arteries and muscular arteries
(Fig
...
13)
...
The anterior optic nerve is supplied by branches from the ciliary arteries
...
These arterioles each supply an area of retina, with little overlap
...
The fovea is so
thin that it requires no supply from the retinal circulation
...
The endothelial cells of the retinal capillaries are joined by tight junctions so
that the vessels are impermeable to proteins
...
The capillaries of
the choroid, however, are fenestrated and leaky
...
The breakdown of these barriers causes the retinal signs seen in many vascular
diseases
...
13 Diagrammatic representation of the ocular blood supply
...
qxd 7/19/07 14:37 Page 16
Chapter 1 Anatomy
Table 1
...
Third (oculomotor)
Fourth (trochlear)
Sixth (abducens)
Medial rectus
Inferior rectus
Superior rectus (innervated by the
contralateral nucleus)
Inferior oblique
Levator palpebrae (both levators are
innervated by a single midline nucleus)
Preganglionic parasympathetic fibres end
in the ciliary ganglion
...
1
...
Figure 1
...
Nuclear and fascicular palsies of these nerves are unusual
...
For example if the third nerve fascicles are damaged as they pass through the red nucleus the ipsilateral third nerve
palsy will be accompanied by a contralateral tremor
...
Peripheral course
Figure 1
...
Third nerve
The third nerve leaves the midbrain ventrally between the cerebral peduncles
...
qxd 7/19/07 14:37 Page 17
Anatomy Chapter 1
Dorsal surface
Superior colliculus
Mesencephalic
nucleus of 5th nerve
Cerebral aqueduct
Third nerve nucleus
Medial longitudinal
fasciculus
Red nucleus
Substantia nigra
Cerebral peduncle
(a)
3rd cranial nerve
Ventral surface
Dorsal surface
4th cranial nerve and nucleus
Inferior colliculus
Cerebral aqueduct
Mesencephalic
nucleus of
5th cranial nerve
Medial longitudinal
fasciculus
Substantia nigra
Cerebral peduncle
(b)
Ventral surface
Dorsal surface
4th ventricle
Medial longitudinal
fasciculus
Parapontine
reticular formation
Facial nerve and
nucleus
Corticospinal tract
(c)
6th cranial nerve and nucleus
Ventral surface
Figure 1
...
LN_C01
...
15 The intracranial course of the third, fourth and sixth cranial nerves
...
Aneurysms of this artery may cause a
third nerve palsy
...
Fourth nerve
The nerve decussates and leaves the dorsal aspect of the midbrain below the inferior colliculus
...
It enters the orbit via the
superior orbital fissure
...
It has a long intracranial course
passing upwards along the pons to angle anteriorly over the petrous bone and into
the cavernous sinus where it lies infero-medial to the fourth nerve in proximity to
the internal carotid artery
...
This long course is important because the nerve can be involved in numerous
intracranial pathologies including base of skull fractures, invasion by nasopharyngeal tumours and raised intracranial pressure
...
qxd 7/19/07 14:37 Page 19
Anatomy Chapter 1
Multiple choice questions
1
...
Comprises three layers
...
Is an important refractive component of the eye
...
2
...
Has ganglion cells whose axons form the optic nerve
...
The neuroretina is firmly attached to the retinal pigment epithelium
...
3
...
Is surrounded by a collagenous capsule
...
Has a high refractive index owing to its protein content
...
4
...
Is part of the iridocorneal angle
...
Transmits changes in tension to the lens capsule
...
The posterior chamber
a
b
c
d
Is another name for the vitreous body
...
Contains aqueous humour, secreted by the ciliary processes
...
19
LN_C01
...
The tear film
a
b
c
d
e
Is 100 µm thick
...
The mucin layer is in contact with the cornea
...
Contains lysozyme and secretory IgA
...
The iridocorneal angle
a Is the site of aqueous production
...
c In primary open angle glaucoma there is a reduction in the number of cells covering the trabecular meshwork
...
8
...
Is not bathed in CSF until it enters the cranial cavity
...
Axons are not myelinated in its retrobulbar part
...
9
...
b A nuclear third nerve palsy will cause a contralateral palsy of the superior
rectus
...
d The sixth nerve has a long intracranial course
...
Answers
1
...
The human endothelium does not regenerate; dead cells are replaced by
the spreading of surviving cells
...
The cornea has epithelial, stromal and endothelial layers
...
qxd 7/19/07 14:37 Page 21
Anatomy Chapter 1
c True
...
Removal of water maintains corneal transparency
...
The cornea is a more powerful refractive element than the natural lens of
the eye
...
The fine, equally spaced, stromal collagen fibrils are arranged in parallel
and packed in an orderly manner
...
2
...
See Fig
...
7
...
The retinal ganglion cell axons form the retinal nerve fibre layer and exit
the eye at the optic nerve head
...
The rods are responsible for night vision and three cone types are responsible for daylight and colour vision
...
The attachment is loose; the neuroretina separates in retinal detachment
...
Vitamin A is delivered by the RPE to the photoreceptors and combined
with opsin
...
The lens
a True
...
This is of great importance in cataract surgery
...
The older, deep cortical and nuclear fibres lose their nuclei and other
organelles
...
e True
...
The suspensory ligament of the lens (the zonule)
a True
...
b False
...
c False
...
d True
...
5
...
The vitreous body is quite separate
...
c True
...
qxd 7/19/07 14:37 Page 22
Chapter 1 Anatomy
d True
...
If this gap is narrowed or closed, pressure in the posterior chamber
pushes the iris forward and may close the angle (acute closed angle glaucoma)
...
The tear film
a
b
c
d
e
False
...
False
...
True
...
It provides a smooth interface for the refraction of light
...
7
...
It is the site of aqueous drainage
...
True
...
The process is active
...
The optic nerve
a True
...
In the orbit, within its sheaths, the optic nerve is surrounded by subarachnoid CSF in continuity with that in the intracranial cavity
...
This is a most important blood supply for the anterior optic nerve
...
They are usually not myelinated within the eye
...
It is made up from retinal ganglion cell axons
...
The third, fourth and sixth cranial nerves
a False
...
b True
...
c False
...
d True
...
e True
...
22
Title: Eye Anatomy
Description: This notes brings the objectives to learn the anatomy of the eye, orbit and the third, fourth and sixth cranial nerves, to permit an understanding of medical conditions affecting these structures.
Description: This notes brings the objectives to learn the anatomy of the eye, orbit and the third, fourth and sixth cranial nerves, to permit an understanding of medical conditions affecting these structures.