anatomy and embryology

1. Anatomy & Embryology of
the EYE
Dr. Tonjun B. Espino
Diplomate in Ophthalmology

2. Welcome to Ophthalmology
1. Introduction
2. Lectures
3. Quizzes and Exams
4. Attendance
5. Attitude and Skills

3. Overview
• External Eye
• Eye lids and lashes
• Lacrimal System
• Conjunctiva and sclera
• Bony Orbit
• Sinuses
• Eyeball
• Cornea
• Aqueous humor
• Iris and pupil
• Lens
• Uveal tratc
• Choroid
• Vitreous
• retina

4. Overview
• Extra-ocular muscles
• Movement and action
• Innervation
• Embryology

5. Schematic drawing of the Eye

6. External Eye
• The external eye can be
considered to be all that you
see when looking at an eye
externally. This can include
the eyebrows, eyelids,
eyelashes, conjunctiva, and
sclera. Although the iris and
cornea are visible, they are
also part of the anterior
chamber of the eyeball.

7. •The eyebrows consist of hairs that horizontally line
the junction between the forehead and the upper
eyelid. They can serve a protective function when
the eyelids squint or are closed. Eyebrows also
serve a major roll in facial expression. Movement of
the eyebrow is controlled by the frontalis,
corrugator supercilia, and orbicularis oculi
muscles. Innervation of these muscles comes from
the seventh cranial nerve.

8. The Eyelids and Eyelashes
•The medical term for
eyelids is
palpebrae. The basic
function of eyelids is to
protect the eyeball from
foreign matter and from
too much light. The
eyelids play a major roll
in the lubrication of the
external eye.
•The vertical opening
between the upper and
lower lids is called the
palpebral fissure. The
size of this opening
plays a role in the
evaluation of the
condition of the lids,
contact lens fitting, and
evaluation of proptosis.

10. •The juncture of the upper and lower lids is
called the canthus. The juncture that is
toward the nose is called the medial
canthus. The juncture toward the ear is
called the lateral canthus. These are
landmarks that serve as points of
reference. For example, "the patient
complains of irritation at the medial
canthus." In the medial canthus is a small
area called the caruncle.

11. •The caruncle has sebaceous (oil) glands that
contribute to the tear layer. It has small hairs
that serve to trap foreign matter. Next to the
caruncle is the plica semilunaris (half-moon
shape) which forms the junction of the
conjunctiva in the medial canthus.

12. • In their normal positions, the
upper and lower lids should cover
1-2 mm of the upper and lower
cornea respectively. If the resting
position of the upper lid lags
lower than normal, the condition
is called "ptosis". If the resting
position of the lower lid lags
lower than the limbus, the
condition is called "scleral
show". The lids should be able to
close together when blinking, if
they cannot the condition is
called "lagophthalmos".

13. • The anterior portion of the
inside of the upper eyelid is
made up of skin and the
obicularis oculi muscle, which
lowers the lid. The
antagonist of the obicularis
oculi muscle is the levator
palpebrae superioris muscle
(not pictured), which raises
the lid.
• The posterior side of the lid is
formed by the tarsus and the
palpebral conjunctiva. The
posterior side of the lid has
meibomian glands which duct
to the lid margin. These
tarsal glands supply the oil
layer of the tear film.

14. Eyelids
• Palpebral fissure: 12 mm. H, 30 mm. W
• Epicanthal fold: vertical tag of skin.
• Glands:
• Glands of Zeiss: sebaceous w/c open to follicles of cilia
• Glands of Moll: sweat glands
• Skin: thinnest in body

15. Eyelids
• Muscles:
• Orbicularis oculi: closure of lids: CN VII
• Levator Palpebra Superioris: CN III: elevates lid
• Muller muscle: sympathetic system to elevate lid
• Palpebral smooth muscle

16. Eyelids
•Tarsal Plates: 1mm thick 25 to 30 mm long.
•Upper lid: 11 mm H
•Lower lid: 5 mm H

17. The Lacrimal Secretory System
• The Tear Film The tear film covers the corneal epithelium
and the conjunctiva, and it has the following functions:It is
the initial refractive interface of the eye. In order to provide
optimum optical performance, the tear film must be clear
and perfectly smooth. It provides metabolic functions to the
outer corneal tissues, bringing nutrients and oxygen, and
carrying away waste products. It provides some antibacterial
proteins that serve as a barrier to disease for the cornea and
conjunctiva. It provides moisture for the epithelial cell layers
of the cornea and for the conjunctiva.

19. The tear film has three layers:
•The lipid (oily) layer is the outermost layer. It is
produced by the meibomian glands of the
eyelids. This layer retards evaporation of the tears
and promotes a smooth surface area.

20. •The aqueous layer is the middle layer of the tear
"sandwich". This layer provides the bulk of the tear
volume and serves to transport nutrients and
oxygen to the outer corneal tissues. It is produced
by the lacrimal gland and by the accessory lacrimal
glands in the conjunctiva. The lacrimal gland
produces "reflexive" tears. These are the tears that
flood the eye when there is irritation to the cornea
or an emotional disturbance (crying).

21. • Mucins play a role in tear film stability and they can be found
in different concentrations in the aqueous layer. The highest
concentration of mucins, forming the "mucin layer", is
found at the corneal surface, next to the corneal
epithelium. Mucins are produced by goblet cells in the
conjunctiva. The corneal epithelial cells produce a substance
called glycocalyx that bind the mucin cells to the corneal
epithelial cells. It is this mucin binding that creates a smooth
sheet of tear film on the corneal surface. Any disruption in
this binding causes a break up in the tear film, as measured
by "tear break up time". A break up of the tear film causes
dry eye symptoms and inflammation of the cornea.

22. The Lacrimal Excretory (tear drainage)
System
• The tears are distributed over the
external eye by the motion of the
eyelids. The tears move toward
the medial canthus where the
tear drainage system begins. The
punctum is a small hole on the lid
margin of the upper and lower
lids, near the medial
canthus. Tears pass through the
punctum into the canaliculus
(superior and inferior), which is a
canal that empties into the
nasolacrimal sac.

23. •From the nasolacrimal sac, the tears
drain through the nasolacrimal duct
which opens into the nasal
passage. From the nose, the tears drain
down the back of the throat. Some
patients will notice that they can "taste"
eye drops as they drain through the
nose and down the throat.

24. The Conjunctiva
• The bulbar conjunctiva is a vascular mucous membrane that
covers the anterior globe up to the corneal limbus. The
epithelial layer of the conjunctiva is actually continuous with
the epithelial layer of the cornea. The normal bulbar
conjunctiva is clear and loose fitting, with blood vessels
running through it. Goblet cells in the conjunctiva produce
mucins which lubricate the external eye and play an
important role in the makeup of the tear layer on the cornea.
• The conjunctiva reacts to irritation and infection by becoming
red, which is termed "hyperemia", or "hyperemic". If the
condition is severe, the conjunctiva may swell, becoming
"chemotic", or demonstrating "chemosis". The conjunctiva
can also produce excess mucus and/or pus, which in general is
termed "exudation" or "discharge".

25. • The palpebral conjunctiva is attached to the tarsal plate of
the eyelid. It is not loose fitting like the bulbar
conjunctiva. The palpebral conjunctiva is continuous with
the bulbar conjunctiva. The junction of the two forms two
pockets called the superior fornix and the inferior fornix.

28. Sclera
• It is a tunic coat which forms the fibrous layer of the eyeball.
Function is protection.
• It is 1mm thick but thinner at muscle insertions.
• The sclera is covered by the tenon’s capsule and the
conjunctiva to which it is joined by the episclera.

29. Sclera

30. Bony Orbit
• The anatomy of the
medial orbital wall. Key:
ALC, anterior lacrimal
crest; LF, lacrimal fossa;
PLC, posterior lacrimal
crest; LP, lamina
papyracea; AEF, anterior
ethmoidal foramen; PEF,
posterior ethmoidal
foramen; OC, optic
canal; MES,
maxilloethmoid suture.

32. • The bony orbit provides protection for the eyeball. The orbit
is shaped roughly like a pyramid, with the base of the
pyramid at the front (on the face) and the apex in the back
(toward the brain). The orbit is lined by fatty tissue, which
provides lubrication for movement within the orbit. The
eyeball is suspended by the extraocular muscles within the
orbit.

34. • There are four basic regions to the orbit: the roof, the floor, the medial
wall, and the temporal wall. There are 7 bones that make up the
orbit. Unfortunately for test takers, the bones are not nicely divided into
regions. As shown on the schematic below, the orbital bones connect
together like a puzzle. The 7 bones are as follows, and are identified on
the schematic by colors and their first letters.
• frontal
• maxilla
• zygoma
• sphenoid
• ethmoid
• lacrimal
• palatine

35. • The white circle on the schematic
indicates the location of the orbital
rim. Notice that the sphenoid
bone (green) has two "wings", the
greater wing and the lesser wing.
The hole (black) in the lesser wing
is the optic foramen, through
which the optic nerve and the
ophthalmic artery pass. The space
(black) between the two wings of
the sphenoid is the superior orbital
fissure, through which pass cranial
nerves III, IV, V, and VI.

36. The regions of the orbit
The floor - Three bones make up the floor: the maxilla
(or maxillary), the zygoma (or zygomatic), and the
palatine. The maxillary bone occupies the most space
on the floor. The orbital floor is the weakest region of
the orbit. A concussive force, such as a fist to the eye,
can fracture the orbit floor and entrap the inferior
rectus muscle. The is called a blow-out fracture.
A blow-out fracture is characterized by a history of
concussive trauma, swelling of the soft tissues of the
orbit, and the inability of the eye to look upward due
to the entrapped inferior rectus muscle.

37. The regions of the orbit
•The roof - The frontal bone forms the roof of the
orbit.
•The medial wall - Four bones make up the medial
wall of the orbit: the maxilla, lacrimal, ethmoid, and
sphenoid (lesser wing) bones.
•The lateral wall - The zygomatic bone makes up the
anterior lateral wall, and the greater wing of the
sphenoid bone makes up the posterior lateral
wall. The zygoma is also part of the jaw.

38. Openings in the orbit
• Optic foramen (or optic canal) -
The optic nerve enters the eye
through the optic foramen, which
is a hole at the apex (back) of the
orbit.
• Orbital fissures - The superior
and inferior orbital fissures are
"cracks" at the back of the
orbit. These are shown as black
areas in the center. Cranial nerves
III, IV, V, and VI, and the superior
ophthalmic vein pass through the
superior fissure. The inferior
ophthalmic vein passes through
the inferior fissure.

39. Openings in the orbit
• Supraorbital foramen - There is a
hole at the top of the orbit which
is called the superorbital
foramen, which is more like a
notch located just under the
eyebrow. The supraorbital nerve,
the supraorbital artery, and the
supraorbital vein pass through
this notch.
• Infraorbital foramen - This is a
hole in the high cheek area
through which pass the
infraorbital nerve, the infraorbital
artery, and the infraorbital vein.

40. Sinuses
•The paranasal sinuses are air filled spaces within
the bones of the face. The spaces open into the
nasal cavity and they surround the orbit except on
the temporal side. In the x-ray image below, the
orbits are identified with the letter "O", the
maxillary sinuses are labeled with "M", and the
ethmoid sinuses are labeled with "E". Sphenoid
and frontal sinuses are not shown. Inflammation,
blockage, and drainage of the sinuses are
commonly associated with allergies and colds.

41. Sinus: coronal View CT scan

42. Globe: Introduction
•The anterior segment of the eye includes the lens of
the eye and every structure anterior to the
lens. This would include the lens, the iris, the
anterior chamber, the ciliiary body, and the
cornea. The anterior segment can be divided into
two chambers, the anterior chamber and the
posterior chamber. The anterior chamber contains
all structures anterior to the iris, but the term is
often used to refer only to the space that contains
aqueous. The posterior chamber contains the lens.

43. Globe: Introduction

44. Globe: Introduction
•The posterior segment of the eye includes
structures posterior to the lens. This includes the
vitreous, the retina, the optic nerve, the choroid,
and the posterior sclera. The area that is occupied
by the vitreous is sometimes referred to as the
posterior chamber. Therefore, we have some
potential terminology confusion. The posterior
chamber of the anterior segment contains the
lens. The posterior chamber of the posterior
segment contains vitreous

45. Cornea
• The cornea is the window to the eye. It is analogous to the front lens
of a multi-element camera lens. It is a five layer tissue structure that
is remarkable in that it is crystal clear in its normal state. The cornea
provides two-thirds of the refractive power of the eye, about 42
diopters worth

47. Cornea
• The horizontal diameter of the cornea is about 12mm and
the vertical diameter is about 11mm. The cornea is thicker
at the edge, about 1mm, than it is in the center, about
.53mm. The shape of the cornea is not spherical, it is
aspherical, meaning the shape is steeper at the center and it
flattens out toward the edge. The steep central portion is
called the corneal cap and it is about 4mm wide.

48. Cornea

49. Cornea
•The shape of the corneal cap is what is measured by
the keratometer. If the cap is spherical, then the
cornea is said to be spherical. If the cap is not
spherical, then the cornea is said to be
astigmatic. Astigmatism means that the curvature
of the cap is steeper in one primary meridian than it
is in the other primary meridian. If the astigmatism
is regular, then the primary meridians are 90
degrees apart (e.g. 90 and 180). If astigmatism is
irregular, then the primary meridians are not exactly
90 degrees apart

50. Layers of the Cornea

51. Cornea: Epithelium
• The epithelium is a layer of cells that cover the surface of the
cornea. It is only about 5 to 6 cell layers thick, comprising
about 10% of the cornea's total thickness, and filled with
tiny nerve endings. The epithelium quickly regenerates when
the cornea is injured. If the injury penetrates deeply into the
cornea, it may leave a scar.
• The epithelium blocks the passage of dust and germs and
provides a smooth surface that absorbs oxygen and cell
nutrients from tears, and then distributes these nutrients to
the rest of the cornea. The basement membrane is the part
where the epithelial cells anchor and organize.

52. Cornea: Bowman’s membrane
• Bowman's membrane lies just beneath the epithelium. It is
transparent and composed of strong layered fibers of
collagen. Because Bowman's membrane is very tough and
difficult to penetrate, it protects the cornea. Once injured,
Bowman's layer can form a scar as it heals.

53. Cornea: Stroma
•The stroma is the thickest layer of the cornea and
lies just beneath Bowman's membrane. (~85%) . It
consists primarily of water (78 percent) and
collagen (16 percent), and does not contain any
blood vessels. The tiny collagen fibrils of the stroma
run parallel to each other. This special formation of
the collagen fibrils gives the cornea its clarity,
strength, elasticity, and form.

54. Cornea: Descemet’s Membrane
•Descemet's membrane lies between the stroma and
the endothelium. It is a thin but strong sheet of
tissue that acts as protection against infection and
injuries. Descemet's membrane is composed of
collagen fibers (different from those of the stroma)
and is made by the endothelial cells that lie below
it. Descemet's membrane is regenerated readily
after injury.

55. Cornea: Endothelium
• The endothelium is just underneath Descemet's and is only
one cell layer thick. This layer pumps water from the cornea,
keeping it clear. In a healthy eye, a perfect balance is
maintained between the fluid moving into the cornea and
fluid being pumped out of the cornea. If damaged or
diseased, the cells of the endothelium will not regenerate.
Too much damage to endothelial cells can lead to corneal
edema (swelling caused by excess fluid) and blindness
ensues, with corneal transplantation the only available
therapy.

56. How is it that the cornea is clear?
1. There are no blood vessels in the cornea
(Avascular). The cornea receives most of its
oxygen from the air, through the tear
layer. What blood supply there is comes from
capillaries that barely cross the limbus and then
loop back. Some nourishment is received via the
aqueous. The oxygen supply to the cornea can
be compromised by contact lens wear. If oxygen
deprivation is severe, the cornea will grow new
blood vessels (neovasularization) into the
corneal tissue to try to increase the oxygen
supply.

57. How is it that the cornea is clear?
1. - Try to recall.
2. nerves (trigeminal) that supply the cornea are de-myelinated once
they cross into the corneal tissue. Myelin is an insulating sheath
that is present on most of the nerves in the body.

58. How is it that the cornea is clear?
1. -- Avascular
2. -- Try to recall... Forgetful students!
3. The normal state of the corneal tissue is that of
relative dehydration. In this state, the corneal
fibrils, which are collagen tissue similar to the
tissues of the sclera, are clear. If the corneal
tissue becomes hydrated due to injury or
disease, then the tissue becomes opaque.

59. Aqueous humour
• The aqueous humour is a watery fluid that fills the chamber
called the "anterior chamber of the eye" which is located
immediately behind the cornea and in front of the lens, and
also the "posterior chamber of the eye" which is a very
narrow compartment located between the peripheral part of
the iris, the suspensory ligament of the lens, and the ciliary
processes.
• The aqueous humour is very slightly alkaline salt solution
that includes tiny quantities of sodium and chloride ions.
It is continually produced, mainly by the capillaries of the
ciliary processes, and drains away into Schlemm's canal,
located at the junction of the cornea and the sclera.

60. • The space in the chamber is
filled with a watery fluid
called the aqueous
humor. This fluid is
produced by the ciliary
processes which lie behind
the iris. The aqueous
circulates through the pupil,
within the anterior
chamber, and then exits
through the angle formed
by the cornea and the iris.

61. Functions of The aqueous
1. It provides a clear refractive media through which light passes to
be focused on the retina.
2. It carries nutrients which serve the metabolism of the cornea and
the lens.
3. It creates an intraocular pressure which serves to maintain the
shape of the globe.

62. The Iris and the Pupil
• the iris of the eye expands and contracts to control the size of the
opening. The opening in the iris is called the pupil.

63. The Iris and the Pupil
• A simple description of the iris is that it is a coloured
diaphragm of variable size whose function is to adjust the
size of the pupil to regulate the amount of light admitted
into the eye. It does this via the pupillary reflex (which is
also known as the "light reflex"). That is, when bright light
reaches the retina, nerves of the parasympathetic nervous
system are stimulated, a ring of muscle around the margin of
the iris contracts, the size of the pupil is reduced (Constrict),
hence less light is able to enter the eye. Conversely, in dim
lighting conditions the pupil opens due to stimulation of the
sympathetic nervous system that contracts of radiating
muscles, hence increases the size of the pupil (Dilate).

64. IRIS
• Blood supply:
• 2 long posterior ciliary
arteries
• 7 anterior ciliary arteries
per eye
• 3 of 4 rectus muscles
are paired, and LR has
1 branch
• Nerve Supply:
• Long ciliary nerves of the
sympathetic nervous
system

65. The Ciliary Body
• The ciliary body (Uveal Body)
lies along the wall of the globe,
behind the iris and in front of
the retina. It has two artificially
divided sections: the pars
plicata and the pars plana.

66. The Ciliary Body
• The pars plicata is the
anterior section of the
ciliary body. It is made up of
the ciliary processes and the
ciliary muscle.
• The ciliary processes have
two functions. They serve
as an anchor for the zonules
which are the cables that
suspend the lens behind the
iris. The processes also
secret aqueous fluid.
• The ciliary muscle contracts
and relaxes to make the lens
fatter and thinner to
provide accommodation,
which we use to focus from
far to near.
• The pars plana is simply the
posterior part of the ciliary
body. The pars plana
extends to the ora serrata,
which is the anterior edge
of the retina.

67. • Contraction and relaxation
of the ciliary muscle alters
the curvature of the lens
The correct term for the
adjustment of the shape of
the lens to change the focus
of the eye is
"accommodation".
This process may be
described simply as the
balance existing at any one
time between between two
states:
• Ciliary Muscle relaxed:
The suspensory ligaments
attached to the ciliary body that
hold the lens in place are
stretched, causing the lens to be
relatively flat.
This enables the eye to focus on
distant objects.
• Ciliary Muscle contracted:
The tension on the suspensory
ligaments attached to the ciliary
body is reduced allowing the lens
to be relatively round.
This enables the eye to focus on
close objects (near to the eye).

68. • The ciliary body connects the choroid with the iris, and
consists of three zones.
They are:
1. The ciliary ring - which is attached to the choroid,
2. The ciliary processes - which forms part of the attachment
to the lens,
3. The ciliary muscle - which controls the curvature of the
lens, and therefore its accommodation to enable us to
view both close and distant objects according to where
our vision is focused.

69. The Lens
•bi-convex in shape.
•The adult lens measures about 10mm in diameter
and about 4mm in thickness. The lens has a plus
power of about 16 diopters in its "thin" shape,
which accounts for about one third of the total
dioptric power of the eye. By accommodating, or
becoming "fatter", the lens adds plus
power. Accommodative ability is greatest in
childhood (over 10 additional diopters) and it
becomes gradually less and less as we age.

71. The lens has four layers:
1. The capsule is a thin elastic layer that constitutes
the outer shell of the lens.
2. The epithelial layer is one cell thick and it lies on
the inner side of the capsule. As epithelial cells
are produced, they migrate toward the center of
the lens, adding to the density of the lens.
3. The lens cortex is a thick layer made up of
relatively younger cell matter.
4. At the center of the lens is the nucleus, the older
cells at the very center

72. Choroid
• The choroid is the vascular layer of the eyeball located between the
retina and the sclera. Provides O2 and nourishment to retina

73. Choroid
• It is a thin, highly vascular
(i.e. it contains blood
vessels) membrane that is
dark brown in colour and
contains a pigment that
absorbs excess light and so
prevents blurred vision (due
to too much light on the
retina).
• The choroid is loosely
attached to the inner
surface of the sclera by the
lamina fusca. The side of the
choroid closest to the centre
of the eyeball is attached to
the retina. This transparent
innermost layer of the
choroid is called Bruch's
Membrane.

74. Choroid
•The structure of the
choroid itself consists
mainly of a dense
capillary plexus and of
many arterioles and
venules transporting
blood to and from this
plexus.
• 4 Layers of Choroid
• Haller’s Layer (Large
diameter and outer)
• Sattlers Layer (medium
diameter)
• Choriocapillaries
• Bruch’s membrane
(innermost layer)
• Choroid supplies blood to
outer 1/3 of retina.

75. Vitreous
• The vitreous is a clear, gelatin-like substance that accounts for about
75% of the mass of the eye. There are normally no blood vessels in
the vitreous, and the vitreous is normally optically transparent

76. Vitreous
• Vitreous does not regenerate, and it shrinks with age, being replaced
by other fluid. At a young age, the vitreous is attached to the
retina. As the vitreous shrinks with age, it pulls away from the
retina. This normal process is called a vitreous detachment.

77. Retina
• The retina is a thin (.5mm) film of photosensitive cells that faces the
vitreous and lines the back of the posterior segment of the eye. The
retina is analogous to the film in the camera.

78. Retina
•The function of the retina is not just to be the
screen onto which an image may be formed , but
also to collect the information contained in that
image and transmit it to the brain.
•The retinal "screen" is therefore a light-sensitive
structure lining the interior of the eye. It contains
photosensitive cells (called rods and cones) and
their associated nerve fibres that convert the light
they detect into nerve impulses that are then sent
onto the brain along the optic nerve.

79. Retina
• The surface of the retina is only interrupted by the optic nerve
head. This "optic disk" is where the nerve fibers, the retinal
arteries, and the retinal veins enter the eye and fan out over
the surface of the retina.

80. • The retina has a complex structure that specialist texts
describe in terms of ten layers labelled (from contact with
the vitreous humour, outwards) as:
• 1. Internal Limiting Membrane.
2. Nerve Fiber Layer (stratum opticum).
3. Ganglionic layer, consisting of nerve cells.
4. Inner molecular, or plexiform, layer.
5. Inner nuclear layer, or layer of inner granules.
6. Outer molecular, or plexiform, layer.
7. Outer nuclear layer, or layer of outer granules.
8. External Limiting Membrane.
9. Jacob's membrane (layer of rods and cones).
10. Pigmentary layer Retinal Pigment Epithelium (RPE)

82. Retina 2 portions
• Neuro-Sensory retina
• Modulator cells: Bipolar,
horizontal & amacrine
• Transmitter cells: Ganglion
cells
• Supporting astroglia:
Muller cells, astrocytes,
oligodendrycite like cells
which are skeltal support
of retina
• Photoreceptor Cells:
• Cones: 6.3 – 6.8 M
• Rods: 111 to 130 M
• Retinal Pigment epithelium
• Single layered Hexagonal
cells
• Cobbled stone in
appearance.
• Provides the retinoin for
retianl metabolism
• The RPE lies beneath the
neurosensory retina. This
layer cells is what gives
the retina
coloration. Those people
with darker skin color also
have a darker color to the
retina.

83. Retinal Blood supply
• The central retinal artery and the central retinal vein branch
from the ophthalmic artery and come into the eye through
the optic nerve head. Each then branches from the optic
nerve head to serve four main quadrants of the
retina. These vessels provide the blood supply for the inner
two thirds of the retina.

84. Retina: Macula
•The very center of the macula, the fovea, is
avascular, and no nerve fibers run above the cones
in this bulls-eye of maximum acuity that is 1.5 mm
in diameter.
•This absence of tissue above the cones in the fovea
insures that nothing will distort or block light on the
way to the cones. The macular area appears darker
than the surrounding retina on fundus photos. This
is because the underlying pigment layer (RPE) is
more dense in the macula.

86. Extra-Ocular Muscles
• Each eye has six muscles attached to it that together can
turn the eyes in almost any direction. They are the medial
rectus (MR), lateral rectus (LR), superior rectus (SR),
inferior rectus (IR), superior oblique (SO), and inferior
oblique (IO).

87. Terminology EOM
• The movement of one eye by itself is called a duction.
• The movement of the two eyes in the same direction is
termed a version.
• If one eye looks toward the nose (nasally), it is called
adduction.
• If the eye toward the ear (temporally), it is called abduction.
• When both eyes look to the right, the movement is called
dextroversion. Left gaze is called levoversion. Both eyes in
upgaze is termed supraversion. Downgaze is called
infraversion.

88. EOM
• The rectus muscles attach (insert), with tendons, to the
globe 5.5 (MR) to 7.7 mm (SR) behind the limbus. Each
rectus muscles extends approximately 41mm to its origin at
the back of the orbit at the Annulus of Zinn.

89. Muscle movement

90. •When the LR muscle contacts, the MR muscle must
relax, otherwise the muscles would be working
against one another and the eye would not
move. Therefore, the MR is the antagonist of the
LR and the antagonist of the action of abduction.

91. • The MR of the right eye and the LR of the left eye are called
yoke muscles. These two muscles work at the same time
and in the same direction to create levoversion (the
movement of both eyes in left gaze) and dextroversion ( the
movement of both eyes in right gaze).

92. Planes of action and axes of rotation
• Horizontal plane • Vertical plane

93. Planes of action and axes of rotation
• Intortion and extortion • The other EOMs are called
cyclovertical muscles. Each of
these muscles has more than one
action. They act in the vertical
plane as well as the horizontal
plane, and they also intort or
extort the globe. This will be
illustrated for each of the
cyclovertical muscles. These
muscles each have a primary
action (1°), a secondary action
(2°), and a tertiary action (3°).

94. Muscle primary
action
secondar
y action
tertiary
action
testing
position
LR abduction none none abduction
MR adduction none none adduction
SR elevation intortion adduction up and out
IR depression extortion adduction down and
out
IO extortion elevation abduction up and in
SO intortion depression abduction down and in

95. Innervation
• Muscles work only when they are innervated. That is, they contract or
relax after receiving a nerve impulse.
• Cranial nerves III, IV, and VI are the motor nerves that control the
extraocular muscles.
• Nerve Function
• III oculomotor MR, SR, IR, IO
• LPS muscles
• iris sphincter muscles
•
• IV trochlear SO
•
• VI abducens LR
•

96. The Medial Rectus
• The MR is the strongest of the EOMs. It has the most mass,
and it has the most anterior insertion into the globe (for
greater leverage). It is used often to converge the eyes into
near (reading) gaze.

97. The Lateral Rectus
• The lateral rectus (LR) originates in the annulus of Zinn and
inserts about 7mm behind the limbus on the temporal side
of the globe. The LR works only on the horizontal plane of
action. When the LR contracts, the eye rotates temporally
(abduction). The LR is the only muscle innervated by CN VI,
the "abducens nerve".

98. The Superior Rectus
• The SR is innervated by CN III. The SR inserts superiorly on
the globe about 8mm behind the limbus. Notice that the
tendon of the SO muscle passes underneath the SR muscle
• The primary action of the SR is elevation of the globe.
2ndary is intortion then tertiary is adduction.

99. The Inferior Rectus
• The inferior rectus (IR) is very similar to the SR, except that it
inserts underneath the globe instead of on top. It also travels at
a 23 degree angle to the primary position visual axis. It's
insertion is about 6mm behind the limbus
• The secondary action of the IR is extortion, and the tertiary
action is adduction. What’s the Primary action?

100. The Oblique Muscles
• The oblique muscles have two primary
functions. The first is intortion or extortion of the
globe to keep the eyeballs level as the head tilts.
•The other major function is to create a
counterbalancing force to that of the rectus
muscles. The rectus muscles are pulling the globe
inward toward the back of the bony orbit. The
oblique muscles pull outwardly to keep the globe
"floating" in the orbital cavity

101. The Superior Oblique
• The SO is the longest of the EOMs at about 60mm. The
other muscles are about 40mm in length. The SO has to be
longer because it passes through a "pully" called the
trochlea, which redirects the action of this muscle

102. The Superior Oblique
• the primary action of the SO is intortion
• depression of the globe around the horizontal axis. This is
the secondary action of the SO.
• Abduction is the tertiary action of the SO

103. The Inferior Oblique
• You may remember that all of the EOMs originate in the
annulus of Zinn, except for IO.
• The IO originates in the inferior nasal orbital rim medially
and travels slightly posteriorly to the insertion point
underneath the globe.

104. The Inferior Oblique
• Extortion: the primary action of the IO.
• the secondary action of the IO is elevation
• The tertiary action of the IO is abduction

105. Embryology of the human eye

107. Embryology
• Weeks 3-4: The eyes begin to form from a population of cells from
the anterior neural plate. These make the eye fields.

108. Embryology
• Weeks: 5-6: Cutting the embryo in the indicated plane
illustrates the lens placode and the adjacent portion of the
optic vesicle as it begins to invaginate.

109. • The invaginating lens placode forms the lens vesicle that
pinches off the surface ectoderm. Invagination of the optic
vesicle forms the bilayered optic cup that remains connected
to the forebrain via the optic stalk.

110. • Weeks: 7-8: The anterior chamber of the eye forms as a space
develops between the lens and its closely associated
iridopupillary membrane and the cornea

111. • Week 9-15: The iris forms from the outer rim of the optic
cup

112. • Weeks: 8-10: By the end of the embryonic period, eyelids
begin to form

113. Embryology 21 days- 10 weeks

114. 3 primitive embryonic layers
• 1. Surface ectoderm: gives rise to
• Lens, lacrimal gland, conjunctiva, corneal epithelium,
epidermis of the eyelids and ocular adnexa
• 2. Neural Crest: responsible for the following
• Corneal endothelium, trabecular meshwork, stroma of iris
and choroid, ciliary muscle, vitreous and optic nerve
meninges

115. 3 primitive embryonic layers
• 3. Neural Ectoderm: gives rise to
• Optic vesicle and optic cup, formation of retina, RPE, optic
nerve fibers.
• * Mesoderm is now thought to contribute to EOM and
ocular and orbital vascular endothelium.

116. Whew
•Finally……
•At last……
•Thank you for sleeping. I
mean listening.