Download Eye and Ear

Cherelene Pereira
Anatomy 1
Ear and eye (Vision, hearing and equilibrium)
The Ear
The ear consists of three parts: external, middle and inner ear. The ear functions to hear and for
equilibrium.
- External Ear
The external ear collects sound vibrations and protects the rest of the ear. It consists of the
auricle and the external auditory meatus.
Auricle
The auricle is the elastic cartilage opposite from the external auditory meatus. The rim of the
auricle is the helix and the inferior part is the lobe.
External Auditory Meatus
The external meatus is about 25 mm long, extends from the concha to the tympanic
membrane. Its lateral part is concaved anteriorly and made of cartilage. The longer medial part is
bony. The meatus is lined by the skin of the auricle, which contains hairs, and sebaceous and
ceruminous glands. In the exterior, there are sebaceous glands called ceruminous glands that
secrete earwax (cerumen). The hairs and earwax prevent dust and foreign materials from entering
the ear.
Tympanic Membrane
The tympanic membrane (ear drum), is about 1 cm in diameter and separates the external
meatus from the tympanic cavity. Its base is attached to the tympanic plate of the temporal bone
and is covered laterally by epidermis, and medially by the mucous membrane of the middle ear.
The three layers of the ear drum, from superficial to deep, are (1) the epidermis, (2) dense
connective tissue containing collagen, elastic fibers and fibroblasts, and (3)simple cuboidal
epithelium. The tympanic membrane is positioned obliquely. Its lateral surface is concave, and
its deepest point is called the umbo. The handle and lateral process of the malleus (the most
superficial ear bone) are attached to the medial surface of the tympanic membrane. Tearing the
tympanic membrane is called a perforated eardrum and can occur using a cotton swab or through
trauma.
Sensory Nerves and Blood Supply
The external ear is supplied by the vestibulocochlear (VIII) nerve and trigeminal (V)
nerve (and potential contributions from cranial nerves VII, IX, and X). The posterior auricular
and superficial temporal arteries of the external carotid artery provide the blood supply.
-Middle Ear
The middle ear lies spacously in the petrous portion of the temporal bone and focuses the
sound vibrations to the oval window. It contains the auditory ossicles and communicates with the
mastoid air cells and mastoid antrum through the aditus and the nasopharynx through the
auditory tube. Mucous membrane covers the structures in the tympanic cavity.
Boundaries
The tympanic membrane creates the lateral wall. However, the epitympanic recess forms
a roof above the level of the tympanic membrane and communicates with the aditus. The
epitympanic recess consists of the head of the malleus and the body and short crus of the
incus.The roof is formed by the tegmen tympani of the temporal bone, which separates the
middle ear from the middle cranial fossa. The floor is the jugular fossa, and contains the superior
bulb of the internal jugular vein. The anterior wall presents the semicanal for the tensor tympani
muscle, the opening of the auditory tube, and the carotid canal, where the internal carotid artery
lies. The posterior wall contains the aditus (which leads to the mastoid antrum), and the
pyramidal eminence, (which contains the stapedius muscle). The mastoid process, and mastoid
portion of the temporal bone, is hollowed out by air cells. These air cells line the
mucoperiosteum and communicate with each other and with the mastoid antrum.
The medial wall has many components: (a) the prominence of the lateral semicircular
canal and the prominence of the facial nerve canal. (b) the oval window (fenestra vestibuli),
closed by the base of the stapes, and the cochleariform process; (c) the promontory, formed by
the basal turn of the cochlea; and (d) the round window (fenestra cochleae), closed by mucous
membrane. The tympanic plexus lies on the promontory and is formed by the tympanic nerve
(from cranial nerve IX), which gives sensory fibers to the middle ear and secretomotor fibers to
the parotid gland.
Auditory Ossicles, Joints, and Muscles
The bones of the middle ear are the malleus (hammer), incus (anvil), and stapes (stirrup).
The malleus contains a handle (manubrium), lateral process (embedded in the tympanic
membrane), and an anterior process (attached to the petrotympanic fissure). The incus has a
body, short crus, and long crus. The stapes includes a head, anterior and posterior crura, and a
base (footplate), which is attached by an annular ligament to the oval window (fenestra
vestibuli). The incudomallear and incudostapedial joints are saddle and ball-and-socket synovial
joints, respectively. The tensor tympani originates from the cartilaginous part of the auditory
tube. It enters a semicanal, turns laterally around the cochleariform process, and inserts on the
handle of the malleus. The tensor tympani is supplied by the mandibular nerve and tympanic
plexus. It draws the handle of the malleus medially, and the tympanic membrane contracts. The
stapedius is innervated by the facial (VII) nerve and begins in the pyramidal eminence and
inserts on the neck of the stapes. Supplied by the facial nerve, the stapedius draws the stapes
laterally. It protects the oval window and decreases the sensitivity of hearing. Both the tensor and
the stapedius attenuate sound transmission through the middle ear.
Function Of the Middle Ear
Sound waves vibrate the tympanic membrane and these vibrations convert into
intensified movements of the stapes by the lever-like action of the auditory bones. Then the oval
window moves, causing movements at the round window. Sound vibrations are transmitted to
the inner ear by (1) the auditory ossicles and the oval window, (2) air in the tympanic cavity and
the round window, and (3) bone conduction through the skull.
Facial nerve
The facial nerve (VII) has a close relationship to the middle ear. The facial nerve has two
components: the larger part, which supplies the muscles for facial expression; and a smaller part
(nervus intermedius) which contains taste fibers for the anterior 2/3rds of the tongue,
secretomotor fibers for the lacrimal and salivary glands, and some pain fibers. The two parts
leave the brain at the caudal border of the pons (cerebellopontine angle) and enter the internal
acoustic meatus. The facial nerve passes through the facial canal in the temporal bone. The facial
nerve is superior to the promontory on the medial wall of the middle ear, and expands to form
the geniculate ganglion, which contains the taste sensory ganglion cells. The nerve turns sharply
posteriorward and then sweeps down immediatly posterior to the middle ear. It exits through the
stylomastoid foramen. Finally, the facial nerve enters the parotid gland, forms the parotid plexus,
and creates terminal branches for the facial muscles. The facial nerve first passes through the
posterior cranial fossa, then the internal acoustic meatus, facial canal, and finally parotid gland
and face.
In the facial canal, the geniculate ganglion connects to the greater petrosal nerve. This
branch passes anteromedially to join the deep petrosal nerve (from the carotid sympathetic
plexus) and form the nerve of the pterygoid canal, which reaches the pterygopalatine ganglion.
The greater petrosal nerve has secretomotor fibers for the lacrimal and nasal glands and many
afferent fibers of uncertain distribution and function.
Continuing in the facial canal, the facial nerve connects a nerve to the stapedius muscle and
the chorda tympani. The chorda tympani enters the tympanic cavity, passes medial to the
tympanic membrane and the handle of the malleus, and reenters the temporal bone. It leaves the
skull through the petrotympanic fissure to reach the infratemporal fossa where it joins the lingual
nerve, where it distributes to the anterior two thirds of the dorsum and sides of the tongue. The
chorda contains (1) taste fibers from the anterior two thirds of the tongue and soft palate and (2)
preganglionic secretory fibers, which synapse in the submandibular ganglion, and the
postganglionic fibers which supply the submandibular, sublingual, and lingual glands.
Immediately inferior to the base of the skull, the facial nerve gives off muscular branches to
the stylohyoid muscle and to the posterior belly of the digastric muscle. It also leads to the
posterior auricular nerve, which supplies motor fibers to the auricular and occipitalis muscles,
and sensory fibers to the auricle.
Within the parotid gland, the facial nerve forms the parotid plexus. The parotid plexus is
where terminal branches spread out to provide the muscles for facial expression. The branches
are usually classified as temporal, zygomatic, buccal, marginal, and cervical. They contain
afferent as well as motor fibers.
- Internal ear
The internal ear is in the petrous part of the temporal bone and consists of the inner membranous
labyrinth and the outer bony (osseous) labyrinth.
The cochlea is the essential organ used for hearing. The vestibular apparatus is used for
maintaining equilibrium and contains the utricle and semicircular ducts.
Osseous Labyrinth
The osseous labyrinth comprises of a layer of dense bone (otic capsule) and the enclosed
perilymphatic space, which contains a fluid called the perilymph. The fluid also surrounds the
membranous labyrinth. The perilymphatic space is comprised of continuous cavities:
semicircular canals, vestibule, and cochlea.
Semicircular canals
The anterior, posterior, and lateral semicircular canals all lie at right angles one to
another. The anterior and posterior semicircular canals are placed in a vertical position while the
lateral one is in a horizontal position.
Vestibule
The vestibule is in the middle part of the bony labyrinth, immediately medial to the
tympanic cavity. It contains the utricle and saccule of the membranous labyrinth. The oval
window, placed between the vestibule and the tympanic cavity, is closed by the footplate (base)
of the stapes.
Cochlea
The cochlea looks like a snail’s shell. It has is a helical tube of about 2.5 turns. Its base
lies against the lateral end of the internal acoustic meatus. The apex is directed anterolaterally
and the basal coil forms the promontory of the middle ear. The bony core is called a modiolus
and transmits the cochlear nerve, and contains the spiral sensory ganglion. The osseous spiral
lamina projects from the modiolus. The cochlear duct extends from the lamina to the wall of the
cochlea so the space in the cochlea gets divided into three ducts: the scala vestibuli (anterior),
cochlear duct (medial) and the scala tympani (posterior). The scala vestibuli and tympani contain
perilymph while the cochlear duct contains endolymph.The scala vestibuli begins in the vestibule
and passes to the apex of the cochlea (helicotrema), where the two scalae communicate with each
other. The scala tympani returns to end near the round window, which is closed by the secondary
tympanic membrane.
Perilymphatic duct
The perilymphatic duct (aqueduct of the cochlea) lies in the cochlear canaliculus and
connects the scala tympani with the subarachnoid space.
Mechanism of Equilibrium
There are two types of equilibrium: (1) static equilibrium is used to maintain the position
of the body relative to the force of gravity; (2) dynamic equilibrium is for maintaining body
position in response to movements like rotation, acceleration and deceleration. The vestibular
apparatus (saccule, utricle, and semicircular ducts) is used to detect equilibrium.
Membranous Labyrinth
The membranous labyrinth lies within the bony labyrinth and contains endolymph. The
membranous labyrinth consists of a series of continuous cavities: semicircular ducts, utricle,
saccule, and cochlear duct.
Semicircular ducts
The anterior, posterior, and lateral semicircular ducts are situated in the X, Y, and Z
planes. At the end of each duct is a bulge called an ampulla, which contains a neuroepithelial
ampullary crest. The crest is stimulated by relative movement of the endolymph during dynamic
head movements (i.e., spinning, pitching or tumbling). The crest contain hair cells and
supporting cells covered by a gel layer called the cupula. The semicircular ducts detect
cceleration or deceleration. When the head moves the semicircular ducts and hair cells move too.
The endolymph in the ampulla isn’t attached and lags behind. As the moving hair cells drag
along the stationary fluid, the hair bundles bend. The hair bundles bending causes a response that
leads to nerve impulses passing along the ampullary nerve, a branch of the vestibular branch of
the (XIII)th nerve.
Utricle and saccule
The utricle and saccule lie in the vestibule and communicate with each other by utricular
and saccular ducts. The utricle has five openings for the semicircular ducts. The saccule connects
to the cochlear duct by the ductus reuniens. The endolymphatic duct derives from the utricular
and saccular ducts and is transmitted by the aqueduct of the vestibule. The duct ends in the
endolymphatic sac.
Maculae
The walls of the utricle and saccule create a neuro-epithelial macula, which is stimulated
by gravity and linear acceleration. The maculae are receptors for static equilibrium but also
participate in dynamic equilibrium. In static equilibrium, the maculae provide sensory
information on the position of the head to maintain posture and balance. The maculae have two
cells: the sensory receptors called hair cells, and supporting cells. The hair cells have stereocilia
and kinocilium and together it’s referred to as a hair bundle. There are columnar supporting cells
that secrete a thick gelatinous layer referred to as the otolithic membrane, that rests over the hair
cells. Otoliths are a layer of calcium carbonate crystals over the otolithic membrane. When the
head is tilted forward, the otolithic membrane and otoliths are pulled by gravity and slide
downhill over the hair cells in the direction of the movement. The hair cells synapse with the
sensory neurons in the vestibular branch of the (XIII)th cranial nerve.
Cochlear duct
The cochlear duct comes from the saccule to the apex of the cochlea, where it ends and
extends from the osseous spiral lamina to the wall of the cochlea. Its anterior and posterior walls
are the vestibular and basilar membranes. The spiral organ or organ of Corti (used for hearing)
lies against the basilar membrane and includes neuro-epithelial hair cells attached to the tectorial
membrane. The spiral ligament creates the third side of the triangular-shaped duct. Furthermore,
at the tip of each hair cell is stereocilia and kinocilium that reach into the endolymph of the
cochlear duct. Blood vessels, such as the stria vascularis, produce endolymph.
Functions of the Internal Ear
The functional details of the internal ear aren’t completely understood. Movement of the
stapes in the oval window creates movement in the secondary tympanic membrane in the
fenestra cochleae. Vibrations made in the perilymph from the scala vestibuli and scala tympani
will move the membranes of the cochlear duct and progress this movement to the basilar
membrane. These vibrations will displace the hair cells in the spiral organ and send the impulse
to the tectorial membrane. This causes the stereocilia to bend and generate a nerve impulse to the
cochlear nerve fibers. The movement of the fluid depends on the frequency of the sound. Lowfrequency sounds cause maximum displacement in the basilar membrane, where high-frequency
sounds maximally vibrate the membranes of the basal portion of the cochlea.
Vestibulocochlear nerve
The vestibulocochlear (XIII) nerve contains afferent fibers from the internal ear. It leaves
the brain at the lower border of the pons and enters the internal acoustic meatus. The vestibular
branch is responsible for equilibrium and is distributed to the maculae of the utricle and saccule,
and zmpullary crests of the semicircular ducts. The vestibular fibers originate in bipolar cells in
the vestibular ganglion in the internal acoustic meatus. The cochlear branch is responsible for
hearing and is distributed to the hair cells of the spiral organ. The cochlear fibers originate in
bipolar cells in the spiral ganglion in the modiolus.
Deafness
Deafness is significant or total hearing loss. Sensorineural deafness occurs by impairment
of hair cells or damange of the cochlear branch of the (XIII) cranial nerve. Sensorineural
deafness can occur due to atherosclerosis, which decreaes blood supply to the ears, or by loud
noise which destroys the hair cells. The louder the sound, the quicker the hearing loss. Deafness
begins with the loss of sensitivity of high pitched frequencies.
Conduction deafness is due to destruction of the external or middle ear mechanisms for
relaying sounds to the cochlea. Weber’s test is a hearing test used to differentiate between
sensorineural and conduction deafness. The stem of a vibrating fork is held to the forehead.
People with normal hearing can hear the sound equally in both ears, however if someone only
hears the sound better in the affected ear, it’s a conduction deafness. If the sound is heard in the
normal ear, it’s sensorineural deafness.
Auditory Pathway
When the stereocilia of the hair cells in the spiral organ bend, they release a
neurotransmitter that causes a nerve impulse in sensory neurons attached to the hair cells. The
sensory neuron’s cell bodies are in the spiral ganglia. Nerve impulses pass along the axons which
form the cochlear branch of the vestibulocochlear nerve. The axons synapse with the neurons in
the cochlear nuclei in the medulla oblongata. Some axons cross over the medulla and ascend in
the tract called the lateral meniscus, and synapse in the inferior colliculus on the midbrain. Some
axons terminate in the superior olivary nucleus (SON) in the pons on each side. Slight
differences in the timing of the nerve impulses arriving from the two ears at the superior olivary
nucleus can determine the location of the sound. Axons from the SON also ascend in the lateral
meniscus tracts on both sides and end in the inferior colliculus. From each inferior colliculus,
nerve impulses are conveyed to the medial geniculate nucleus in the thalamus and the primary
auditory area of the cerebral cortex in the temporal lobe.
Equilibrium Pathway
The hair bundles in the semicircular canals, utricle or saccule bend and cause nerve
impulses in the sensory neurons. The cell bodies of the hair cells are in the vestibular ganglia.
Nerve impulses pass along the axons and form the vestibular branch of the (XIII)th nerve. These
axons synapse with sensory neurons in vestibular nuclei, in the medulla oblongata and the pons.
Furthermore, the vestibular nuclei receive input from the eyes and somatic receptors. The
remaining axons enter the cerebellum through the inferior cerebellar peduncles.
The vestibular nuclei combine information from vestibular, visual and somatic receptors
and send them to: (1) the nuclei of nerves (III), (IV) and (VI), which all control eye movement;
(2) nuclei of (XI) nerve to control head and neck movements, maintaining equilibrium; (3) the
vestibulospinal tract which sends impulses down the spianl cord to maintain equilibrium in
skeletal muscles; and (4) the ventral posterior nucleus in the thalamus and then to the vestibular
area in the parietal lobe of the cerebral cortex for conscious awareness of movements of the
head.
The eye
The accessory structures of the eye
These include the eyelids, eyelashes, eyebrows, lacrimal (tearing) apparatus, and extrinsic
eye muscles.
Eyelids
The upper and lower eyelids are also called palpebrae. The palpebrae protect the eyes
from overexposure, foreign objects and it spreads lubricating fluid over the eyeballs. The upper
eyelid is more mobile than the lower one and is moved by the levator palpebrae superioris
muscle. The levator palpebrae superioris comes from the sphenoid bone into the optic canal and
inserts on the skin of the upper eyelid and the upper border of the tarsal plate by the superior
tarsal muscle. The levator is directed by the oculomotor nerve and paralysis of the nerve can
cause drooping of the upper eyelid. Twitches in the eye are due to stress or fatigue and fade away
in a matter of seconds. The palpebral fissure is the space in between the eyelids that exposes the
eyeball. The angles of the palpebrae are the lateral commissure, which is closer to the temporal
bone, and the medial commissure, which is nearer to the nasal bone. Some people, mostly of
Asian decent, have a fold of skin called the epicanthus that covers the medial commissure. The
lacrimal caruncle is in the medial commissure and is a small, red elevation that contains
sebaceous and sudoriferous glands. White discharge exudes from the caruncles.
From superficial to deep, each eyelid consists of the epidermis, dermis, subcutaneous
tissue, fibers of the orbicularis oculi muscle, tarsal plate, tarsal glands and conjunctiva. The tarsal
plate is connective tissue that gives form and support to the eyelids. Each tarsal plate has
sebaceous glands known as tarsal or Meibomian glands, that secrete fluid to prevent the eyelids
from sticking to each other. The ends of the plates are anchored to the orbital margin by lateral
and medial palpebral ligaments. The medial palpebral ligament draws the lids laterally and is
anterior to the lacrimal sac. The superior tarsal muscle connects the levator with the tarsal plate
and consists of smooth muscle directed by sympathetic fibers. Infection of the tarsal glands
produces a cyst called a chalazion.
Conjunctiva
The conjunctiva is a protective mucous membrane consisting of nonkeratinized stratified
columnar epithelium cells. It acts as the connection between the eyelids, sclera and cornea. The
palpebral conjunctiva lines the inner part of the eyelids while the bulbar conjunctiva passes from
the inner eyelids to the surface of the eyeball. The palpebral conjunctiva contains the openings of
the lacrimal canaliculi, allowing tears within the conjunctival sac to drain into the nasal cavity.
The bulbar conjunctiva is translucent and colorless, except when its vessels are dilated as a result
of inflammation (conjunctivitis). Mainly, it’s continuous at the limbus with the anterior
epithelium of the cornea. The plica semilunaris, a conjunctival fold at the medial angle of the
eye, helps protect the eye against foreign bodies. The space between the lids and eye are lined by
conjunctiva, and is called the conjunctival sac. The mouth of the sac is the palpebral fissure,
which varies in size when the" eye is open". The reflections of the conjunctiva from the lids to
the eye are known as fornices. The lacrimal glands open into the superior fornix.Over the sclera
the conjunctiva is vascular while at the cornea it is avascular. Dilation of the blood vessels in the
bulbar conjunctiva is known as having bloodshot eyes.
Eyelashes and eyebrows
Eyelashes extend from the border of each eyelid and eyebrows arch above the upper
eyelids and protect the eyes from perspiration and ultraviolet light. Sebaceous ciliary glands
release lubricating fluid into the hair follicles of the eyebrows and lashes. Infection of these
glands caused by bacteria causes a sty.
Lacrimal Apparatus
The lacrimal apparatus is a group of structures that produces and drains lacrimal fluid, also
known as tears. The lacrimal apparatus comprises (1) the lacrimal gland and its ducts and (2)
passages for draining: the lacrimal canaliculi and sac and the nasolacrimal duct
The lacrimal gland, lodged in a fossa at the roof of the orbit, rests on the lateral rectus and
the levator muscles. The main portion is in the orbital, but a process called the palpebral part
projects into the upper lid. A dozen lacrimal ducts leave the palpebral part to enter the superior
conjunctival fornix where there are small accessory lacrimal glands. The half of the lacrimal
secretions that does not evaporate drains into the lacrimal sac. The lacrimal glands are supplied
by parasympathetic fibers of the (VII) nerve and secrete lacrimal fluid, which drains into
lacrimal ducts, which empties tears. Tears pass over the anterior surface of the eyeball to two
small openings called lacrimal puncta. Then the tears pass through two ducts called the lacrimal
canals, which lead to the lacrimal sac and then to the nasolacrimal duct. This carries the fluid
into the nasal cavity under the inferior nasal concha. An infection of the lacrimal sacs is known
as dacryocystitis.
Lacrimal fluid contains salts, water and lysozyme. The fluid protects and nourishes the
eyeball. If an irritating substance makes contact with the conjunctiva, then the lacrimal glands
oversecrete and tears accumulate. It dilutes the irritating substance. When humans cry, the
lacrimal glands produce excessive lacrimal fluid that spills over the eyelids and fills the nasal
cavity to produce a runny nose.
Extrinsic eye muscles
Eyes sit in the depressions of the skull known as orbits. The orbits stablize and anchor the
muscles to produce eye movements. The extrinsic eye muscles extend from the walls of the orbit
to the sclera of the eye and are surrounded by periorbital fat. The superior rectus, inferior rectus,
lateral rectus, medial rectus, superior oblique muscle and inferior oblique muscle are the six
extrinsic muscles that move the eye. Except for the inferior oblique muscle, these skeletal
muscles originate from the posterior aspect of the orbit. The four recti form a common tendinous
ring that surrounds the optic canal and part of the superior orbital fissure. All the structures that
enter the orbit through the optic canal and the adjacent part of the fissure lie at first within the
cone of recti. The extrinsic eye muscles receive impulses from cranial nerves III, IV, and VI. The
small motor units have smooth movement for the eyes.
The superior oblique muscle comes from the sphenoid bone to the optic canal. It’s
superior to the medial rectus, passes the trochlear and attaches to the frontal bone. The inferior
oblique muscle rises from the maxilla, at the floor of the orbit, and inserts on the posterior sclera.
The superior oblique is directed by the trochlear, the lateral rectus is directed by the abducens,
and the rest of the muscles are directed by the oculomotor nerve.
The recti extend from the posterior aspect of the orbit to the anterior aspect of the sclera.
The lateral and medial recti are an abductor and adductor, respectively. The superior and inferior
recti elevate and depress respectively, and because they are in the lateral position, they can only
move when the eye is abducted. The superior and inferior oblique muscles depress and elevate,
respectively, and because of their lateral position, they can move only when the eye is adducted.
Paralysis of an extrinsic eye muscle is described by (1) restricted movement in the field
of action of the paralyzed muscle and (2) the presence of two separate images (diplopia) when
eye moves in the direction of the paralyzed muscle.
Actions of the extraocular muscles
Eye equilibrium is maintained by all of the eye muscles and move in unison as the two
eyes move together. Movements are around the vertical axis (abduction and adduction),
lateromedial axis (elevation and depression) and the anteroposterior axis (extorsion and
intorsion).
Anatomy of an Eyeball
The adult eyeball measures 2.5 cm in diameter, and only 1/6th of the eyeball is exposed, the
rest if rested in the orbit. If the eye is too short in relation to the lens, near objects are focused
behind the retina (hypermetropia: farsightedness or longsightedness). If the eye is too long in
relation to the lens, distant objects are focused in front of the retina (myopia: nearsightedness or
shortsightedness)
The retina is regarded as an extension of the wall of the brain, and develops from neural
ectoderm, where the lens and the anterior epithelium of the cornea come from the somatic
ectoderm. Neural crest and mesoderm also participate in ocular development. The eyeball has
three layers from superficial to deep: the fibrous tunic, vascular tunic and retina.
Fibrous tunic
The fibrous tunic is a strong, dense collagenous connective tissue layer.
The sclera is the outer white covering of the eye. It provides protection for the internal
eye structures, maintains eye shape, and is an important site for muscle attachment. Posteriorly,
the optic nerve fibers pierce the sclera through a weak plate called the lamina cribrosa. External
to the sclera, the eyeball is covered by a thin fascial sheath (Tenon's capsule) that extends from
the optic nerve to the sclerocorneal junction. The sheath separates the eyeball from the orbital fat
and acts as a socket where the eye moves as in a ball-and-socket joint. It blends with the sheaths
of the muscles of the eye. Hormonal disturbances (especially hyperthyroidism) can cause
swelling of the orbital fat and extra-ocular muscles, causing protrusion of the eyes
(exophthalmos).
The cornea is superficial to the colored iris and allows light to enter the eye. Its curved
surface refracts the light into the eye. The cornea is continuous with the conjunctiva and the
junction is called the limbus. The cornea is directed by the opthalmic nerve of the trigeminal
nerve. The outer surface of the cornea is coated by the bulbar conjunctiva. The sclera is opaque
and the cornea is translucent because corneal tissue is avascular and consists of five layers: the
substantia propria enclosed by anterior and posterior epithelia and limiting laminae.. The canal of
Schlemm connects the aqueous fluid in the anterior area of the eye and veins in the sclera. It
drains the aqueous fluid. When the cornea does not conform to a sphere but is more curved in
one axis than in another, the condition is termed astigmatism.
Vascular tunic
The vascular tunic or uvea is the middle eyeball layer. It has three parts: 1) the choroid is
the posterior portion that lines most of the internal surface of the sclera. Its blood vessels provide
nutrients for the retina. The part near the choroid is a smooth ciliary ring (pars plana), whereas
near the iris is a ridged crown (pars plicata). The choroid also contains melanin, which is why it
is such a dark color. The melanin absorbs excess light rays so the image is clear on the retina.
Albinos have no melanin, also in the eye so they need to wear sunglasses indoors.
In the anterior area, the choroid becomes the ciliary body. It extends from the ora serrata
which is the jagged anterior of the retina. The ciliary body also contains melanin so it is very
dark. The ciliary body has ciliary processes which connect to suspensory ligaments, which
connect to the lens. The ciliary processes circle the iris and produce aqueous humor. The ciliary
body also has ciliary muscles that are also connected to the suspensory ligaments, which then
convex or concave the lens or near or far vision. The ciliary muscle comprises two main sets of
smooth-muscle fibers: (1) longitudinal fibers connect the sclera (anterior) to the choroid
(posterior), and (2) oblique fibers enter the base of the ciliary processes.
The iris is the colored portion of the eyeball and is shaped like a flat doughnut and the
opening is called the pupil. It is suspended between the cornea and lens. It has melanocytes and
radial smooth muscle fibers. The amount of melanin determines the eye color of the iris. When
there’s large amounts of melanin, the stroma of the iris is black and brown, when few amounts of
melanin it is blue. The anterior surface of the iris has a fringe known as the collarette. The
pattern of radial striations in the iris is unique from one person to another and can be used for
identification. A congenital, radial defect of the iris is termed a coloboma.
The two distinct fiber arrangements of smooth muscle from the sphincter pupillae muscle
is a flat thin band of muscle fibers at the pupillary bounder of the iris. The sphincter pupillae is
directed by the parasympathetic fibers by the short ciliary nerves. The dilator pupillae muscle
attaches to the outer circumference of the sphincter pupillae and projects like the spokes of a
wheel towards the iris. The iris regulates the amount of light enter the pupil. The pupil is the
opening to the retina and choroid, therefore it’s black. Autonomic reflexes regulate pupil
diameter in response to light levels. When there’s a bright light, the parasympathetic fibers of the
(III) nerve cause the sphincter muscle of the iris to contract, decreasing the pupil size. In dim
light the sympathetic neurons cause the dilator pupillae muscle of the iris to contract, increasing
the pupil size.
Retina
The inner coat of the eyeball is the retina. The retina is the beginning of the visual
pathway. The retina is nourished by the choroid and the opthalmic artery of the trigeminal nerve.
The central artery travels in the optic nerve and divides at the optic disc. The optic disc is the
where the optic nerve exits the eyeball. The central retinal artery is bundled together with the
optic nerve and the central retinal vein. The retina has a pigmented layer and a neural layer. The
pigmented layer has melanin epithelial cells between the choroid and the neural part of the retina,
which also helps absorb rays. The neural layer is a multilayered outgrowth that processes visual
data before sending nerve impulses into axons that form the optic nerve. There are three layers of
retinal neurons: the photoreceptor layer, the bipolar cell layer and the ganglion cell layer, which
are separated by the outer and inner synaptic layers. Light passes through the ganglion and
bipolar cell layers before it reaches the photoreceptors. In the bipolar cell layer of the retina are
also horizontal cells and amacrine cells. The cells form direced neural circuits that modify the
signals being transmitted along the pathway from photoreceptors to bipolar cells to ganglion
cells.
The retina has two types of photoreceptors: rods and cones. Rods allow us to see in dim
light because they are of a low threshold. Cones are high threshold photoreceptors which
produce color vision. A person who loses rod vision can’t see at night. There is an outer pigment
layer where the rods and cones lie, and it consists of three parts: (1) an outer segment that detects
the light stimulus, (2) an inner segment which contains metabolic mechanisms, and (3) a synaptic
terminal which is facing the bipolar neurons. The outer segment has many photopigment
molecules.
The macula lutea is the center of the posterior retina. The fovea centralis is the small
depression of the macula lutea which contains a dense population of only cones, which causes it
to be the area of highest visual acuity. Moving further from the fovea centralis, less and less
cones populate the retina and more and more rods are found. That’s why in our peripheral vision
we can only see faint obects of no color. The optic disc is called the blind spot because rods and
cones aren’t found here.
Lens
The lens is biconvex and 1 cm in diameter, and is covered by a capsule and consists of
cellular lens fibers. It is posterior to the pupil and iris. In the lens there are proteins called
crystallins which are layered like an onion, create the refractory abilities of the lens, and lacks
blood vessels. The lens capsule is anchored to the ciliary body by its suspensory ligaments
(ciliary zonule). When seeing a distant object, the ciliary muscle is relaxed and elastic fibers in
the choroid pull on the ciliary body, which keeps the zonular fibers and the lens capsule under
tension. This pull results in flattening of the lens and focus images on the retina for a clear
picture.
Interior of the Eyeball
The anterior cavity is between the cornea and the lens and holds aqueous humor. The
cavity consists of two chambers. The anterior chamber is between the cornea and the iris. The
posterior chamber is behind the iris and in front of the lens. Aqueous humor filters out of the
blood capillaries and enters the posterior chamber and then flows forward between the iris and
lens, through the pupil and into the anterior chamber, iridocorneal angle, trabecular meshwork,
and scleral venous sinus, reaching the ciliary veins. Then it drains through the canal of Schlemm
which is an endothelial channel at the sclerocorneal junction.
Intraocular pressure is the pressure in the eye produced by aqueous and vitreous humor,
and depends on the ease of draining the aqueous humor. The pressure maintains the eye and
prevents it from collapsing.
The larger posterior cavity is called the vitreous chamber, which is between the lens and
retina. In it is the vitreous body which is a jelly like substance that contributes to intraocular
pressure. The body also helps maintain the shape of the eye. The pressure of the vitreous body
holds the retina flush against the choroid so the retina creates a even surface for images. The
body also contains phagocytic cells to remove debri. The hyaloid canal is a narrow channel that
runs through the vitreous body from the optic disc to the lens.
The visual pathway
Neurotransmitters released by rods and cones create changes in bipolar and horizontal
cells that lead to the generation of nerve impulses. Amacrine cells synapse with ganglion cells
and also transmit information. When bipolar, horizontal or amacrine cells transmit signals to
ganglion cells, the cells initiate nerve impulses.
Pathway in the brain
The axons of the optic nerve pass through the optic chiasm. Some fibers cross to the
opposite side and others remain uncrossed. After passing the optic chiasm, the fibers are now
part of the optic tract and enter the brain and terminate at the lateral geniculate nucleus of the
thalamus. Here they synapse with neurons whose axons form the optic radiations, which project
to the primary visual areas in the occipital lobes.
Major causes of blindness
Cataracts are a common cause of blindness due to loss of transparency of the elns. The
lens is clouded due to changes in the structure of the lens proteins. This usually occurs due to
age, but also injury and exposure to UV rays. People who smoke also have an increased chance
of developing cataracts.
Glaucoma is the most common cause of blindness. It is due to abnormally high
intraocular pressure due to buildup of aqueous humor within the anterior cavity. The fluid
compresses the lens into the vitreous body and puts pressure on the retina. Continuous pressure
creates irreversible destruction of the neurons in the retina and damage to the optic nerve.
Glaucoma is painless and the other eye compensates so much damage occurs before the
condition is diagnosed.
Age-related macular disease (AMD) is also known as macular degernation, is a
degenerative disorder of the retina and pigmented layer in people 50 years and older. In AMD,
abnormalities occur in the macula lutea. Victims retain their peripheral vision but lose their
vision straight ahead. It affects 13 million Americans and is 2.5 times more likely in smokers.