Anatomy of the eye, ear and neck, labeled

Protecting the eye

Orbit

The bony socket in the skull that surrounds each eyeball. It protects the eye and anchors the soft tissues around it.

Eyelids

Their lashes sit at the leading edge. Together they help protect the eye from abrasions by blocking particles that might land on its surface.

Palpebral conjunctiva

The thin membrane on the inner surface of each eyelid.

Conjunctiva

Extends from the lids over the sclera (the white of the eye), connecting the eyelids to the eyeball.

Lacrimal gland

Makes tears. It sits just inside the orbit, superior and lateral to the eyeball.

Lacrimal ducts

Tears flow through them, across the eyeball and conjunctiva to the medial corner of the eye, washing away foreign particles. Extra tears drain into the nasal cavity.

The eyeball in section

The three layers (tunics)

Fibrous tunic

The outermost layer of the eyeball. It includes the white sclera and the clear cornea.

Sclera

The white of the eye. It makes up five sixths of the surface of the eyeball, most of which is not visible.

Cornea

The transparent front of the eye. It lets light in and, along with the lens, focuses light onto the retina.

Vascular tunic

The middle layer, made up mostly of the choroid, the ciliary body and the iris.

Choroid

A highly vascularized layer of connective tissue that supplies blood to the eyeball. It lies behind the ciliary body.

Ciliary body

A muscular structure attached to the lens by suspensory ligaments (zonule fibers). Together they bend the lens so it can focus light on the back of the eye.

Iris

The colored part of the eye, a smooth muscle that opens or closes the pupil.

Pupil

The hole at the center of the iris that lets light enter. The iris constricts it in bright light and dilates it in dim light.

Neural tunic (retina)

The innermost layer. It holds the nervous tissue responsible for photoreception, the detection of light.

Lens and the two cavities

Lens

Held by the suspensory ligaments of the ciliary body. With the cornea, it focuses light onto the retina.

Anterior cavity

The space between the cornea and the lens, including the iris and ciliary body. It is filled with a watery fluid, the aqueous humor.

Posterior cavity

The space behind the lens, out to the back of the eyeball where the retina is. It is filled with a more viscous fluid, the vitreous humor.

The three layers of the eyeball, from the outside in, are the fibrous tunic, the vascular tunic and the neural tunic (the retina). The eye has two cavities, the anterior cavity (aqueous humor) and the posterior cavity (vitreous humor).

Path of light through the eye

1 · Cornea

Light enters through the transparent cornea, which starts to focus it.

2 · Aqueous humor and pupil

It crosses the anterior cavity and passes through the pupil, whose size the iris adjusts.

3 · Lens

The lens, shaped by the ciliary body, helps focus the light.

4 · Vitreous humor

The light crosses the posterior cavity.

5 · Retina

Photoreceptors in the retina detect it.

Photoreceptors

Rods and cones. They change their membrane potential when light energy stimulates them.

Rods

Very sensitive to light, so they work in low light. They hold the pigment rhodopsin and do not sense the color of light.

Cones

Work in brighter light. Three types of cone opsin, each sensitive to a different wavelength, give us color vision (peak sensitivities roughly red, green and blue).

Bipolar cells

The cells that connect a photoreceptor to a retinal ganglion cell.

Retinal ganglion cells

Their axons collect at the optic disc and leave the eye as the optic nerve.

Optic disc and blind spot

Where the optic nerve begins there are no photoreceptors, which creates a blind spot in the retina and a matching blind spot in the visual field.

Fovea

A small area at the exact center of the retina that contains only photoreceptors, with no supporting cells or blood vessels in front of them. Visual acuity, the sharpness of vision, is greatest here.

The retina is made of several layers. The photoreceptors (rods and cones) pass their signal to bipolar cells, and the bipolar cells pass it to the retinal ganglion cells.

From the retina to the visual cortex

Retinal ganglion cell axons

Run through the optic nerve.

Lateral geniculate nucleus

The thalamus relays the signal from each side. The first fiber of the pathway synapses on a thalamic cell here.

Visual cortex

The thalamic cell projects to the visual cortex in the occipital lobe, where seeing (visual perception) takes place.

• Superior rectus

  Extraocular

  Moves the eye up and toward the nose.

• Inferior rectus

  Extraocular

  Moves the eye down and toward the nose.

• Lateral rectus

  Extraocular

  Moves the eye away from the nose (abduction).

• Medial rectus

  Extraocular

  Moves the eye toward the nose (adduction).

• Inferior oblique

  Extraocular

  Moves the eye up and away from the nose.

• Superior oblique

  Extraocular

  Moves the eye down and away from the nose. Its tendon threads through a pulley-like piece of cartilage, the trochlea.

• Levator palpebrae superioris

  Orbit, not on the eyeball

  Opens the eye by raising and retracting the upper eyelid.

• Orbicularis oculi

  Facial expression

  A circular muscle around the eye that closes the eyelids.

The six extraocular muscles (four rectus and two oblique) originate on the bones of the orbit and insert on the outer surface of the eyeball. They sit inside the eye socket, so none of them can be seen on the visible eyeball. Their nerves are cranial nerves, which the brain study sheet covers.

For the cranial nerves that carry these signals, see the brain and cranial nerves study sheet.

The ear in section

Parts of the ear

Auricle

The large, fleshy structure on the side of the head. Its C-shaped curves direct sound waves toward the auditory canal. Some sources call it the pinna.

Auditory (ear) canal

Leads sound in. It enters the skull through the external auditory meatus of the temporal bone.

Tympanic membrane

The ear drum, at the end of the auditory canal. It vibrates when sound waves strike it. The auricle, canal and tympanic membrane are together called the external ear.

Ossicles

Three small bones that span the middle ear: the malleus, the incus and the stapes. Their Latin names roughly translate to hammer, anvil and stirrup.

Malleus

Attached to the tympanic membrane. It articulates with the incus.

Incus

Articulates with the malleus on one side and the stapes on the other.

Stapes

Attached to the inner ear, where sound is turned into a neural signal.

Eustachian tube

Connects the middle ear to the pharynx and helps equalize air pressure across the tympanic membrane. It is normally closed, but pops open when the muscles of the pharynx contract during swallowing or yawning.

Inner ear (bony labyrinth)

A series of canals embedded in the temporal bone. It has two regions: the cochlea, for hearing, and the vestibule, for balance.

Vestibulocochlear nerve

Signals from the cochlea and the vestibule travel in separate fiber bundles, but the two bundles run together from the inner ear to the brain stem as this nerve.

The ear has three parts: the external ear (auricle, ear canal and tympanic membrane), the middle ear (the three ossicles, joined to the pharynx by the Eustachian tube) and the inner ear (the cochlea and the vestibule).

The path of sound

1 · Auricle and ear canal

The auricle directs sound waves toward the auditory canal.

2 · Tympanic membrane

It vibrates when the sound waves strike it.

3 · Malleus, incus, stapes

The ossicles pass on the vibration and amplify it.

4 · Oval window

The stapes is attached to the oval window, which picks up the amplified vibration and sets up pressure waves in the fluid of the cochlea.

5 · Scala vestibuli and scala tympani

The fluid moves in a wave-like motion that matches the frequency of the sound.

6 · Basilar membrane and hair cells

The basilar membrane moves at a spot that depends on the frequency, bending the stereocilia of the hair cells there and triggering nerve impulses.

7 · Vestibulocochlear nerve

The signals travel to the brain stem.

Inside the cochlea

Cochlea

The spiral-shaped part of the inner ear that turns sound into neural signals. It holds the sensory neurons of the spiral ganglia.

Oval window

Joins the cochlea to the stapes. It sits at the start of the fluid-filled scala vestibuli.

Scala vestibuli

A fluid-filled tube that runs from the oval window above the cochlear duct, then curves over the top of the duct at the tip of the cochlea.

Scala tympani

The same tube after it curves over the top. It travels back to the base of the cochlea under the cochlear duct and ends at the round window.

Round window

Covered by a membrane that contains the fluid of the scala tympani. It bulges out or puckers in as the fluid moves.

Cochlear duct

The central cavity of the cochlea. It contains the organs of Corti.

Organ of Corti

Sits on the basilar membrane inside the cochlear duct and turns the wave motion of the fluid into neural signals.

Basilar membrane

Lies between the organs of Corti and the scala tympani. A given spot on it moves only for one frequency of sound.

Hair cells

Mechanoreceptors with hair-like stereocilia on their top surface. Bending the stereocilia one way opens ion channels and starts nerve impulses.

Tectorial membrane

Lies over the stereocilia. When the basilar membrane moves, it slides across the stereocilia and bends them.

The cochlea encodes frequencies between 20 and 20,000 Hz, the range of sound that human ears can detect. High frequencies move the basilar membrane near the base of the cochlea (close to the oval and round windows). Low frequencies move it near the tip, the apex.

Balance (equilibrium)

Equilibrium

The sense of balance. Like hearing, it is handled by the inner ear, using hair cells with stereocilia that sense head position and movement.

Vestibule

The part of the inner ear that holds the balance receptors. Signals travel through the vestibulocochlear nerve to the brain stem and cerebellum.

Utricle and saccule

Sense the position of the head. Each is made largely of macula tissue: hair cells surrounded by support cells.

Otolithic membrane and otoliths

The stereocilia of the maculae reach into a viscous gel, the otolithic membrane. A layer of calcium carbonate crystals, the otoliths, sits on top and makes it top-heavy, so tilting the head slides it over the macula and bends the stereocilia.

Semicircular canals

Sense head movement. They are three ring-like extensions of the vestibule: one horizontal and two vertical.

Ampulla and cupula

The ampulla is the enlarged base of each canal and holds the hair cells that respond to rotation, such as turning the head to say no. Their stereocilia reach into the cupula, a membrane attached to the top of the ampulla.

Muscles of facial expression

• Occipitofrontalis

  Facial expression

  Moves the scalp and eyebrows. It has a frontal belly on the forehead and an occipital belly at the back of the head, joined by a broad tendon, the epicranial (galea) aponeurosis.

• Orbicularis oculi

  Facial expression

  A circular muscle that closes the eye.

• Orbicularis oris

  Facial expression

  A circular muscle that moves the lips.

• Buccinator

  Facial expression

  Compresses the cheek. It allows whistling, blowing and sucking, and contributes to chewing.

• Corrugator supercilii

  Facial expression

  The prime mover of the eyebrows (raising and lowering them).

The muscles of facial expression arise on the surface of the skull and insert into the connective tissue and the dermis of the skin. Because they insert in skin rather than bone, contracting them moves the skin, which creates facial expression.

Muscles that move the lower jaw

• Masseter

  Jaw (mastication)

  The main muscle used for chewing. It elevates the mandible to close the mouth.

• Temporalis

  Jaw (mastication)

  Assists the masseter. It elevates and retracts the mandible, closing the mouth and pulling the lower jaw in under the upper jaw.

• Lateral pterygoid

  Jaw (mastication)

  Opens the mouth, pushes the lower jaw forward and moves it from side to side.

• Medial pterygoid

  Jaw (mastication)

  Closes the mouth, pushes the lower jaw forward and moves it from side to side.

Mastication is the anatomical word for chewing. The medial and lateral pterygoids help with chewing and with moving food around the mouth.

Muscles of the tongue

Extrinsic tongue muscles

Insert into the tongue from outside it and move the whole tongue in different directions. Their names contain the root glossus (tongue).

Intrinsic tongue muscles

Arise inside the tongue and let it change shape, such as curling or flattening.

Genioglossus

Starts on the mandible and moves the tongue downward and forward.

Styloglossus

Moves the tongue upward and backward.

Palatoglossus

Starts on the soft palate and elevates the back of the tongue.

Hyoglossus

Starts on the hyoid bone, moves the tongue downward and flattens it.

Anterior neck: suprahyoid and infrahyoid muscles

• Digastric

  Suprahyoid

  Has anterior and posterior bellies. It elevates the hyoid bone and larynx when you swallow and depresses the mandible.

• Stylohyoid

  Suprahyoid

  Moves the hyoid bone backward, which elevates the larynx.

• Mylohyoid

  Suprahyoid

  Lifts the hyoid bone and helps press the tongue to the top of the mouth.

• Geniohyoid

  Suprahyoid

  Raises and pulls the hyoid bone forward, and depresses the mandible.

• Omohyoid

  Infrahyoid

  Has superior and inferior bellies. It depresses the hyoid bone, together with the sternohyoid and thyrohyoid.

• Sternohyoid

  Infrahyoid

  Works with the omohyoid and thyrohyoid to depress the hyoid bone.

• Thyrohyoid

  Infrahyoid

  Depresses the hyoid bone and elevates the thyroid cartilage of the larynx.

• Sternothyroid

  Infrahyoid

  Depresses the thyroid cartilage, which helps make different tones of voice.

The muscles of the anterior neck help with swallowing and speech by controlling the position of the larynx (voice box) and the hyoid bone, a horseshoe-shaped bone that gives the tongue a solid base. They are grouped by position: suprahyoid muscles lie above the hyoid bone and infrahyoid muscles lie below it.

Muscles that move the head

• Sternocleidomastoid

  Moves the head

  The major muscle that laterally flexes and rotates the head. It starts on the sternum and clavicle and inserts on the mastoid process of the temporal bone and on the occipital bone. One side alone rotates the head to the opposite side; both sides together flex the head. It divides the neck into anterior and posterior triangles.

• Semispinalis capitis

  Moves the head

  One side alone laterally flexes and rotates the head to the same side. Both sides together extend it. Inserts on the occipital bone.

• Splenius capitis

  Moves the head

  One side alone laterally flexes and rotates the head to the same side. Both sides together extend it. It runs from the midline up and out to the head.

• Longissimus capitis

  Moves the head

  One side alone laterally flexes and rotates the head to the same side. Both sides together extend it.

• Scalene muscles

  Anterior, middle and posterior

  Work together to flex, laterally flex and rotate the head. They also contribute to deep inhalation. The middle scalene is the longest and the posterior scalene the smallest.

The head sits on top of the vertebral column and is balanced, moved and rotated by the neck muscles. When these muscles act on one side only, the head rotates; when they contract on both sides, the head flexes or extends. The posterior neck muscles are mostly about head movements such as extension.