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THE EYE: PART A
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The Eye and Vision
• 70% of all sensory receptors are in the eye
• Nearly half of the cerebral cortex is involved in
processing visual information!
• Most of the eye is protected by a cushion of
fat and the bony orbit
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Eyebrow
Eyelid
Eyelashes
Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Iris
Eyelid
Sclera
Lacrimal
(covered by caruncle
conjunctiva)
(a) Surface anatomy of the right eye
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Pupil
Medial
commissure
Figure 15.1a
Eyebrows
• Eyebrows-Overlie the supraorbital margins
• Shade the eye
• Prevent perspiration from reaching eye
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Eyelids (Palpebrae)
• Protect the eye anteriorly
• Levator palpebrae superioris muscle—gives upper eyelid
mobility
• Tarsal plates—internal supporting CT sheet
• Palpebral fissure—separates eyelids
• Medial and Lateral Canthi – eye angles
• Caruncle—fleshy elevation at medial canthus
• Contains oil and sweat glands
• Eyelashes
• Nerve endings of follicles initiate reflex blinking
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Site where
conjunctiva
merges with
cornea
Palpebral
fissure
Lateral
commissure
Eyelid
Sclera
Lacrimal
(covered by caruncle
conjunctiva)
(a) Surface anatomy of the right eye
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Pupil
Medial
commissure
Figure 15.1a
Eyelids
• Tarsal (Meibomian) Glands
• Modified sebaceous glands
• Produce oily secretion; prevents eyelids from
sticking, lubricates eyelid
• Embedded in tarsal plates
• Ducts open posterior to eyelashes
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Eyelids
• Ciliary Glands
• Sebaceous and sweat glands
• Found between eyelashes
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Conjunctiva
• Transparent mucous membrane
• Palpebral conjunctiva: lines eyelids
• Bulbar conjunctiva: covers white of the eyes
anteriorly
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Levator palpebrae
superioris muscle
Orbicularis oculi muscle
Tarsal plate
Palpebral conjunctiva
Tarsal glands
Cornea
Palpebral fissure
Bulbar conjunctiva
Orbicularis oculi muscle
(b) Lateral view; some structures shown in sagittal section
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Lacrimal Apparatus
• Consists of: Lacrimal gland and ducts that connect to
nasal cavity
• Releases: Lacrimal secretion (tears)
• The solution also contains: mucus, antibodies, and
lysozyme (bacteria destroying enzyme)
• The solution drains via: paired lacrimal puncta into
lacrimal canaliculi then into nasolacrimal duct
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Lacrimal sac
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
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Figure 15.2
Extrinsic Eye Muscles
• The movement of the eye is controlled by six
muscles on the external surface of each eye
• Superior rectus: elevates eye
• Inferior rectus: depresses eye
• Lateral rectus: moves eye laterally
• Medial rectus: moves eye medially
• Inferior oblique: Moves eye up and out
• Superior oblique: Moves eye down and out
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Superior oblique
muscle
Superior oblique
tendon
Superior rectus
muscle
Lateral rectus
muscle
Inferior rectus
Inferior oblique
muscle
muscle
(a) Lateral view of the right eye
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Figure 15.3a
Trochlea
Superior oblique
muscle
Superior oblique
tendon
Superior rectus
muscle
Axis at center
of eye
Inferior
rectus muscle
Medial
rectus muscle
Lateral
rectus muscle
Common
tendinous ring
(b) Superior view of the right eye
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Figure 15.3b
Extrinsic Eye Muscles
• The innervation to each muscle can be
remembered by the following equation:
(LR6SO4)O3
• Which means:
• Lateral rectus: is innervated by CN # 6 (Abducens)
• Superior oblique: is innervated by CN #4 (Trochlear)
• All others: are innervated by CN #3 (oculomotor)
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Structure of the Eyeball
• Wall of eyeball contains three layers (tunics)
• Fibrous layer - Outermost layer; dense
avascular CT with two regions: sclera and
cornea
• Vascular layer - Middle pigmented layer with
three regions: choroid, ciliary body, and iris
• Sensory layer (retina) -Delicate two-layered
membrane
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Anterior
segment (contains
aqueous humor)
Lens
Posterior segment
(contains vitreous humor)
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Structures or Substances to Know:
• Sclera
• Vitreous humor
• Cornea
• Aqueous humor
• Choroid
• Lens
• Ciliary body, muscle and processes
• Macula Lutea
• Suspensory ligaments
• Fovea Centralis
• Ora Serrata
• Optic disc
• Iris
• Optic nerve
• Pupil
• Central artery and vein
• Anterior segment- divided into
anterior and posterior chambers
• Canal of Schlemm
• Posterior segment
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Parasympathetic +
Pupillary Sphincter
muscle contraction
decreases pupil size.
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Sympathetic +
Iris (two muscles)
• Sphincter pupillae
• Dilator pupillae
Dilator
muscle contraction
increases pupil size.
Figure 15.5
Ciliary body
Ciliary zonule
(suspensory
ligament)
Sclera
Choroid
Iris
Pupil
Optic nerve
Lens
Optic disc
(blind spot)
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Figure 15.4a
Sensory Layer: Retina
• Delicate two-layered membrane
• Pigmented layer
• Outer layer
• Absorbs light and prevents its scattering
• Stores vitamin A
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Sensory Layer: Retina
• Neural layer
• Photoreceptor cells: transduce light energy
• Cells that transmit and process signals:
bipolar cells, ganglion cells, amacrine cells,
and horizontal cells
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Pathway of light
Neural layer of retina
Pigmented
layer of
retina
Choroid
Sclera
Optic disc
Central artery
and vein of retina
Optic
nerve
(a) Posterior aspect of the eyeball
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Figure 15.6a
The Retina
• Ganglion cell axons
• Run along the inner surface of the retina
• Axons of ganglion cells leave eye as the optic
nerve
• Optic disc (blind spot)
• Site where the optic nerve leaves the eye
• Lacks photoreceptors
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Pathway of light
Neural layer of retina
Pigmented
layer of
retina
Choroid
Sclera
Optic disc
Central artery
and vein of retina
Optic
nerve
(a) Posterior aspect of the eyeball
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Figure 15.6a
Photoreceptors
• Rods
• More numerous at peripheral region of retina,
away from the macula lutea
• Operate in dim light
• Provide indistinct, fuzzy, non color peripheral
vision
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Photoreceptors
• Cones
• Found in the macula lutea; concentrated in the
fovea centralis
• Operate in bright light
• Provide high-acuity color vision
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Ganglion
cells
Bipolar
cells
Photoreceptors
• Rod
• Cone
Amacrine cell
Horizontal cell
Pathway of signal output
Pigmented
layer of retina
Pathway of light
(b) Cells of the neural layer of the retina
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Figure 15.6b
Blood Supply to the Retina
• Two sources of blood supply
• Choroid supplies: the outer third
(photoreceptors)
• Central artery and vein of the retina supply: the
inner two-thirds
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Central
artery
and vein
emerging
from the
optic disc
Macula
lutea
Optic disc
Retina
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Figure 15.7
Internal Chambers and Fluids
• The lens and ciliary zonule separate the
anterior and posterior segments
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Ciliary body
Ciliary zonule
(suspensory
ligament)
Pupil
Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Anterior
segment (contains
aqueous humor)
Lens
Posterior segment
(contains vitreous humor)
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Optic disc
(blind spot)
Figure 15.4a
Internal Chambers and Fluids
• Posterior segment contains vitreous humor that:
• Transmits light
• Supports posterior surface of the lens
• Holds the neural retina firmly against the pigmented
layer
• Contributes to intraocular pressure
• Anterior segment is composed of two chambers
• Anterior chamber—between the cornea and the iris
• Posterior chamber—between the iris and the lens
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Internal Chambers and Fluids
• Anterior segment contains aqueous humor
• Plasma like fluid continuously filtered from capillaries
of the ciliary processes
• Drains via the scleral venous sinus (canal of Schlemm)
at the sclera-cornea junction
• Supplies: nutrients and oxygen mainly to the lens and
cornea but also to the retina, and removes wastes
• Glaucoma: compression of the retina and optic nerve
if drainage of aqueous humor is blocked
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Iris
Lens
Cornea
Aqueous humor
Anterior Anterior
segment chamber
(contains Posterior
chamber
aqueous
3
humor)
Scleral venous
sinus
Cornealscleral junction
Posterior
segment
(contains
vitreous
humor)
2
Ciliary zonule
(suspensory
ligament)
1
Ciliary body
Ciliary
processes
Ciliary
muscle
Cornea
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Lens
Figure 15.8
Lens
• Biconvex, transparent, flexible, elastic, and avascular
• Allows precise focusing of light on the retina
• Cells of lens epithelium differentiate into lens fibers
that form the bulk of the lens
• Lens fibers—cells filled with the transparent protein
crystallin
• Lens becomes denser, more convex, and less elastic
with age
• Cataracts (clouding of lens) occur as a consequence
of aging, diabetes mellitus, heavy smoking, and
frequent exposure to intense sunlight
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Figure 15.9
Ora serrata
Ciliary body
Ciliary zonule
(suspensory
ligament)
Cornea
Iris
Pupil
Anterior pole
Anterior
segment (contains
aqueous humor)
Lens
Scleral venous
sinus
Posterior segment
(contains vitreous humor)
(a) Diagrammatic view. The vitreous
humor is illustrated only in the
bottom part of the eyeball.
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Sclera
Choroid
Retina
Macula lutea
Fovea centralis
Posterior pole
Optic nerve
Central artery
and vein of
the retina
Optic disc
(blind spot)
Figure 15.4a