The ear is the part of the head that contains the structures associated with the special sensations of hearing and balance. For descriptive and functional purposes, anatomists and clinicians organize the ear into three parts: external, middle, and internal.
The ear has four functional mechanisms that allow us to hear and perceive sounds, and these mechanisms are related to the anatomic subdivisions of the ear.
Collecting sound waves: The external ear provides the parts for collecting sound waves and directing them into the head where they move the eardrum (tympanic membrane).
Amplifying sound: The middle ear contains tiny bones (auditory ossicles) that move when the eardrum moves, amplify the sound waves, and convert the movement of air into movement of fluid in the internal ear.
Transduction of vibrations into action potentials: The cochlea of the internal ear is a transducer that converts vibrations of fluid into action potentials in the cochlear nerve.
Transmission of action potentials to the brain: The cochlear part of the vestibulocochlear nerve (CN VIII) delivers the action potentials to the brain where sound it is perceived and interpreted.
External ear
Consists of the auricle (also called the pinna), which projects from the side of head in order to collect sound waves; the external acoustic meatus, which directs the sound waves inside the head; and thetympanic membrane, separating the external acoustic meatus from the middle ear.
Auricle (pinna)
Consists of a core of elastic cartilage covered snuggly by thin skin.
The helix and antihelix are curved ridges that define the posterior margins of the auricle.
The concha is the central deep depression that communicates with the opening of the external acoustic meatus.
The tragus (Greek: goat, presumably because ear hairs resemble the beard on the chin of a goat) is the prominent projection just anterior to the opening of the external acoustic meatus.
The antitragus is a bump on the antihelix posterior to and opposite the tragus.
The lobule (ear lobe) is the fleshy inferior part of the auricle. It holds the distinction of being the only part of the auricle not supported by cartilage. It consists of skin and connective tissue.
Figure 2. WIKIMEDIA COMMONS.
Clinical correlation
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External acoustic meatus
Extends from the concha to the tympanic membrane.
The external acoustic meatus is not straight—it is curved in two planes:
When viewed from above it is S-shaped, with three parts. (1)The most lateral part is directed anteriorly (this is why you have the earpieces of a stethoscope pointed forward toward your nose when you place them in your ears), (2) the middle part is directed posteriorly, (3) the medial portion again turns anteriorly.
When view from the front, it is curved with a concavity facing downward.
Due to the curvatures, clinicians viewing the tympanic cavity with an otoscope in adults must gently tug the auricle up and back to straighten the external acoustic meatus.
The lateral third of the meatus has a cartilage wall that is continuous with the cartilage of the auricle and it is lined by skin. The medial two-thirds of the meatus has a bony wall (tympanic portion of the temporal bone) lined by skin.
Figure 3.
The skin lining the meatus contains hair follicles, sebaceous glands, and ceruminous glands (modified sweat glands). Ceruminous glands produce a brown, semisolid fatty/waxy substance called cerumen (ear wax!).
Tympanic membrane (eardrum)
The tympanic membrane (eardrum) is a three-layered oval structure that separates the external ear from the middle ear. It is attached to a ring of bone belonging to the tympanic part of the temporal bone. The external surface of the membrane is lined by skin; the internal surface is lined by a mucosa continuous with that of the tympanic cavity; while the central core of the membrane is a layer of connective tissue. The layers are thin, so the tympanic membrane is semi-transparent.
Since the medial third of the external acoustic meatus is directed anteriorly in the head, the tympanic membrane is positioned at an angle—it faces both forward and down.
The membrane is not flat; it is somewhat coned-shaped = the external surface is concave and the internal surface convex and bowed inward. The peak of the cone, called the umbo, is attached internally to the handle of the malleus, one of the tiny bones in the tympanic cavity. Superior to the umbo, the attachment of the tympanic membrane to the lateral process of the malleus produces visible creases called the mallear folds. Thus, the tympanic membrane attaches to the malleus in two places.
Sound waves are collected by the auricle and concentrated in the external acoustic meatus. The tympanic membrane moves in response to the sound waves in the air. Since the malleus is attached to the internal surface of the membrane, movements of the tympanic membrane produce movements in the tiny bones (ossicles) in the middle ear.
Clinical correlation
Examination of the tympanic membrane is done with an otoscope. When a healthy membrane is illuminated, it appears pearly gray and is transparent enough to allow the handle of the malleus and the long process of the incus to be seen on the other side. Due to its obliquity and concavity, a “cone of light” is usually produced by the otoscope in the anterior-inferior quadrant (5 o’clock position) of a healthy membrane.
Figure 4. Otoscopic examination of normal tympanic membrane—lateral view.
Clinical correlation
Examination of the tympanic membrane is done with an otoscope. When a healthy membrane is illuminated, it appears pearly gray and is transparent enough to allow the handle of the malleus and the long process of the incus to be seen on the other side. Due to its obliquity and concavity, a “cone of light” is usually produced by the otoscope in the anterior-inferior quadrant (5 o’clock position) of a healthy membrane.
Figure 4. Otoscopic examination of normal tympanic membrane—lateral view.
Blood supply of external ear
Superficial temporal and posterior auricular arteries (both from the external carotid) supply the auricle.
Superficial temporal, posterior auricular, and deep auricular (from the maxillary) arteries supply the external acoustic meatus.
Innervation of external ear
Auricle: Great auricular nerve (from the cervical plexus, C-2 and C-3) and auriculotemporal nerve (from V3). Small contributions in the concha are made by twigs from the facial and vagus nerves.
External acoustic meatus: Auriculotemporal nerve and the auricular branch of the vagus. Cleaning the external acoustic meatus may elicit a cough = thisis a vagal reflex and is explained by knowing that the vagus contributes sensory fibers.
Tympanic membrane: External surface is supplied by the auriculotemporal nerve and auricular branch of the vagus. Internal surface receives sensory innervation from the glossopharyngeal nerve.
Middle ear
Consists of the tympanic cavity and its contents:
Tympanic cavity, an air-filled space within the petrous portion of the temporal bone.
Three tiny bones called auditory ossicles.
Two tiny skeletal muscles that act on the ossicles.
Tympanic plexus of nerves.
Tympanic cavity
Separated from the external acoustic meatus by the tympanic membrane.
Lined with a mucous membrane, which also covers the internal surface of the tympanic membrane and the auditory ossicles.
Connected to the nasopharynx by the pharyngotympanic (Eustachian) tube.
The tympanic cavity is taller than the external acoustic meatus. The floor of the cavity is at the level of the inferior border of the tympanic membrane, but the roof rises well above it. This upper extension of the tympanic cavity is called the epitympanic recess, referred to by clinicians as the “attic”. This is the first part of the tympanic cavity seen when approached from a superior view in the dissecting lab.
Although the tympanic cavity is a narrow space whose long axis is parallel to the tympanic membrane, for descriptive and teaching purposes it is considered to be a BOX with anterior, posterior, medial, and lateral walls, a roof, and a floor.
Roof = a layer of bone called the tegmen tympani separates the tympanic cavity from the meninges and the temporal lobe of the brain in the middle cranial fossa above.
Floor = a plate of bone separates the tympanic cavity from the internal jugular vein.
Anterior wall (carotid wall) = bone here separates the tympanic cavity from the internal carotid artery. Two canals open on the anterior wall:
The lower opening leads into the pharyngotympanic tube (Eustachian tube), which runs anteriorly and medially from the tympanic cavity to the nasopharynx. This tube is normally closed, but when it opens (facilitated by yawning or swallowing) air can pass through, and this aerates the tympanic cavity and equalizes pressure with the atmosphere. (Ears go “pop!”)
The upper opening connects to a bony canal filled with the tensor tympani muscle.
Posterior wall (mastoid wall) = has two features:
Superiorly is an opening called the aditus to the mastoid antrum. The mastoid antrum is a common chamber that communicates with many small mucosa-lined cavities called mastoid air cells. The antrum and air cells are
within the mastoid process of the temporal bone.
Below the aditus is a small, hollow, cone-shaped projection called the pyramidal eminence, from whose apex emerges the tendon of the stapedius muscle.
Lateral wall = formed mainly by the tympanic membrane. The chorda tympani nerve crosses the lateral wall, passing between the handle of the malleus and the long process of the incus.
Medial wall = a thin layer of bone separates the tympanic cavity from the internal ear.
The most prominent feature is a bulge called the promontory, produced by the basal (first) turn of the cochlea (part of the internal ear).
Figure 7. The tympanic cavity box. The tympanic membrane has been removed, so the viewer is looking into the box from lateral to medial. GRAY’S ANATOMY FOR STUDENTS, FIGURE 8.116.
On the surface of the promontory is the tympanic plexus of nerves.
Above and behind the promontory is an opening into the internal ear, the oval window. The footplate of the stapes fits into the oval window.
Below the promontory is a second opening into the internal ear, the round window. The round window is closed by the secondary tympanic membrane.
On the upper medial wall near the roof is a bony ridge, the prominence of the facial canal. It contains the facial nerve. Only a thin layer of bone separates the facial nerve from the mucosa of the tympanic cavity.
The orbit is the bony “eye socket” that contains the eyeball, extra-ocular muscles, nerves and vessels, and the lacrimal gland. It is protected anteriorly by the eyelids. The orbit is about the size of shot glass. The eyeball itself is the most complex organ in the body and will be discussed in greater detail in the Nervous System.
A pyramid-shaped cavity (as you look from its base to its apex) with infra-orbital (frontal bone) and supra-orbital (maxillary bone) margins.
Boundaries
The bony boundaries of the orbit are:
Lateral: Frontal, zygomatic, and sphenoid
Inferior: Maxilla and zygomatic
Roof: Frontal
Floor: Maxilla
Medial: Frontal, maxilla, lacrimal, orbital plate of ethmoid, sphenoid, and palatine.
Bony openings of the orbit
Superior orbital fissure: Passes from medial to superolateral position; transmits CN III, IV, VI, and branches of V1 (lacrimal, frontal, and nasociliary); superior and inferior ophthalmic veins
Optic canal: Ophthalmic artery and CN II (which is surrounded by meninges)
Anterior and posterior ethmoidal foramina: Nerves (branches of nasociliary) and vessels of the same name; nerves supply sensation to the ethmoid and sphenoidal air sinuses
Supra-orbital notch/foramen: Exit point for the supra-orbital nerve, artery, and vein
Infra-orbital groove and canal: In floor of the orbit; transmit the infra-orbital nerve and vessels, which reach the face by emerging through the infra-orbital foramen
Inferior orbital fissure: Floor of orbit containing maxillary nerve and one of its branches, the zygomatic nerve, and the infra-orbital artery and vein
The upper and lower eyelids (palpebrae) are mobile structures that cover the anterior part of the eyeball when closed, protecting the eye from injury and excessive light.
Between the eyelids is the palpebral fissure where light enters the eye.
The medial and lateral angles of the eye, also called the medial and lateral canthi (canthus is the singular) are the “corners” of the eye, where the upper and lower lids meet.
Along the margins of the eyelids (adjacent to the palpebral fissure) are thick hairs called eyelashes (cilia). These are arranged in rows. The lower lid has 75–80 lashes while the upper lid has over 100. The lashes protect the eyes by forming a barrier to external irritants. Eyelash follicles are richly innervated by sensory nerves, making lashes highly sensitive. Large sebaceous glands open into the eyelash follicles to lubricate them. Inflammation of a gland produces a “stye” (hordeolum).
Eyelids are constructed of several layers, which are discussed here from anterior to posterior.
The skin of the eyelids is very thin and loosely attached to underlying structures by a narrow layer of connective tissue. Because of this loose tissue, fluid can accumulate deep to the skin; for instance, blood pooling here due to trauma produces a “black eye.”
Deep to the skin is the muscle of the eyelids, the palpebral portion of the orbicularis oculi. As discussed in the Mixed cranial nerves section of the Cranial Nerves chapter, this is a muscle of facial expression, innervated by the facial nerve. The orbital part of the orbicularis oculi surrounds the orbit and functions to forcefully close the eyelids. The palpebral part lightly closes the eyelids when blinking.
The major part of the thickness of each eyelid is formed by the tarsal plate (tarsus). This structure is formed by rows of tarsal glands (Meibomian glands) embedded in dense connective tissue. The tarsal glands open along the margins of the eyelids. They secrete an oily substance that lubricates the eyelids, so they don’t stick together, and it mixes with tears to slow their evaporation. Tarsals plates are stout, resembling cartilage, to maintain the shape of the lid. The tarsal plate of the upper eyelid is larger. Tarsal plates in both lids are attached to the bony margins of the orbit by medial and lateral palpebral ligaments.
The anatomy of the upper eyelid is more complex than the lower, owing to the attachment of the levator palpebrae superioris muscle. As its name suggests, this muscle elevates the upper eyelid. It is described again later in this chapter. The levator palpebrae becomes aponeurotic as it enters the lid and fans out to attach to the superior tarsal plate and skin of the upper eyelid. It also sends a small slip to the superior conjunctival fornix (described below).
Behind the levator palpebrae superioris (a skeletal muscle) is a small band of smooth muscle that arises from the levator’s inferior surface and attaches below to the superior margin of the tarsal plate. This is the superior tarsal muscle (Mueller’s muscle). Innervated by sympathetic nerves, this muscle functions to widen the palpebral fissure, allowing maximum light to enter in a “fight or flight” response. However, the muscle seems to function constantly, even in the absence of an emergency response, since the upper eyelid would otherwise drop due to gravity. Indeed, sympathetic malfunction causes drooping of the upper eyelid (ptosis) due to paralysis of the superior tarsal muscle.
Figure 3. NETTER, ATLAS OF HUMAN ANATOMY, 7TH ED., PLATE 94.
The most internal layer of the eyelids is the palpebral conjunctiva. The conjunctiva is a transparent, multilayered mucous membrane. The palpebral conjunctiva lines the inside of the eyelids, then turns sharply to reflect on to the anterior surface of the eyeball itself, where it becomes the bulbar conjunctiva. The bulbar conjunctiva covers the anterior part of the sclera (“whites”) of the eye—it does not cover the cornea. Small blood vessels course in the bulbar conjunctiva—irritation causes “bloodshot” eyes. The conjunctival sac is the space between the palpebral and bulbar conjunctiva. It becomes a closed space when the eyelids are closed. This potential space allows the eyelids to glide freely over the surface of the eye when they open and close. The upper and lower recesses (“cul-de-sacs”) of this space, where the palpebral conjunctiva reflects onto the surface of the eyeball are called the superior and inferior conjunctival fornices. The levator palpebrae superioris muscle has a small attachment to the superior fornix, so that the conjunctival sac is pulled up when the upper eyelid is raised.
Clinical correlation
Horner’s syndrome is a cluster of signs and symptoms caused by lack of sympathetic innervation in the head. It can result from pathology that interrupts neurons anywhere in the sympathetic pathway from brain to head—in the brain, in the sympathetic trunk, or in the neck. Examples include stroke, tumors, or neck trauma.
The most common cause of Horner’s syndrome is a tumor in the thorax that damages the sympathetic trunk. A Pancoast tumor is a cancer in the apex of the lung. It can spread to thenearby parietal pleura, ribs, and vertebrae, injuring the sympathetic trunk in the vicinity.
The classic symptoms of Horner’s syndrome are:
Ptosis: Drooping of the upper eyelid due to paralysis of the superior tarsal muscle
Miosis: Pupillary constriction due to paralysis of the dilator pupillae muscle in the eye
Anhidrosis: Dry skin due to absence of sweating in the head and neck, resulting from denervation of sweat glands.
Question
Do you understand how these symptoms relate to sympathetic denervation in the head?
Clinical correlation
Inflammation of the conjunctiva is conjunctivitis ("pink eye"). It can be caused by infections (viral or bacterial), allergic reactions, or chemical irritants (like chlorine in a swimming pool). Infectious conjunctivitis can be highly contagious, especially in children.
Clinical correlation
Foreign bodies (sand, glass) in the conjunctival fornices are common, especially in the larger upper fornix. The patient will complain of "something in their eye," and the sensation may become worse with blinking.
Upper lid eversion is usually necessary to visualize the foreign body. The upper lid can be folded up and back over a cotton tip applicator (Q-tip) stick. The upper lid will usually stay everted after removal of the stick because of the stiff tarsal plate. The foreign body can then be seen and removed with a cotton swab or irrigation.
Observable structures and parts when looking through the palpebral fissure
Figure 4.
Lacrimal caruncle
A reddish-yellow elevation near the medial angle. Contains sebaceous glands—these produce the secretions that accumulate while you sleep = noticeably when you wake up in the morning.
Lacrimal papilla
Small elevation along the margins of the lids near medial angle on whose top is a small opening called the lacrimal punctum that collects tears and carries them into a small canal, the lacrimal canaliculus, which then transports the tears to the lacrimal sac, lodged in a bony crevice in the medial wall of the orbit.
Iris
Colored part of the eye contains smooth muscle fibers
Dilator pupillae muscle: Arranged to open the pupil
Innervation: Nasociliary branch of CN V1 (ophthalmic) long ciliary nerve (Post-G sympathetic)
Sphincter pupillae muscle: Closes the pupil
Innervation: Pre-G PARA in CN III ciliary ganglion Post-G parasympathetic fibers in short ciliary nerves
Cornea
Transparent membrane covering the iris; sensitive to a number of modalities (e.g., pain, touch, pressure) carried by afferent fibers in nasociliary nerve (V1), to the trigeminal ganglion, and brain stem<
Lacrimal apparatus
The almond-shaped lacrimal gland produces tears made of physiological saline and bacteriocidal lysozyme; it moistens and lubricates the surfaces of the conjunctiva and cornea.
Lies in the upper lateral part of the orbit
Tear flow: Inferomedially across the eye, to puncta, lacrimal caruncle, lacrimal sac, then through the nasolacrimal duct (in the bony nasolacrimal canal) and eventually opens into the inferior meatus of the nasal cavity. That’s why, when you cry, your nose runs!
The eyelids act as a squeegee, wiping tears from the upper lateral corner to the lower medial corner.
Innervation of the lacrimal gland
Lacrimation (production of lacrimal fluid) is a parasympathetic event (innervated by CN VII). The pathway of innervation is amazingly circuitous—so it is essential that you practice drawing this out! Or, stare deeply into Figure 5 until you go cross-eyed (unfortunately, a hidden 3-D dolphin does not pop out at you).
Optical axis: Axis of the gaze; in anatomical position, eyes “look straight ahead” (neutral position)
Axis of the orbit: Laterally placed, 23 degrees from optical axis
Clinical correlation: Orbital blowout fracture
From blunt trauma by an object larger in diameter than the orbit (e.g., tennis ball, fist, dashboard).
Increased initra-orbital pressure: Transmitted to the weakest point (usually the inferior orbital wall). The floor of the orbit is fractured, but the rims of the orbit are intact.
Contents of the orbit: Entrapped and can have symptoms.
Orbital fat and/or the inferior rectus muscle can prolapse into the maxillary sinus below the orbit. Since the inferior rectus muscle is trapped, the gaze is stuck downward and the patient cannot look upward in the affected eye.
After more depth on the nerves,
we’ll tie the muscles, axes, and nerves into how to isolate muscles, and clinically test their cranial nerves.
Nerves
Figure 7.
Figure 8.
Oculomotor
Enters through the superior orbital fissure and splits
Superior division: Supplies superior rectus and levator palpebrae superioris muscles
Inferior division: Supplies inferior rectus, medial rectus, and inferior oblique muscles; carries pre-G parasympathetic fibers; associated with the parasympathetic ciliary ganglion
Trochlear
From dorsal caudal midbrain; passes through cavernous sinus, enters superior orbital fissure, supplies superior oblique muscle when enters orbit
Abducens
Fom caudal pons; enters cavernous sinus and superior orbital fissure, to supply the lateral rectus muscle. It enters the medial surface of the muscle.
Ophthalmic (V1)
Emerges from trigeminal ganglion, goes through lateral wall of cavernous sinus, and enters the superior orbital fissure. It usually splits into 3 branches just before it goes into the fissure:
Lacrimal nerve: Superolaterally to lacrimal gland; carries hitch-hiking post- ganglionic parasympathetic fibers to the lacrimal gland (tear production); sensation from upper lateral skin of upper eyelid
Frontal nerve: Passes above levator palpebrae superioris muscle, and divides into supratrochlear and supraorbital nerves near outer part of orbit; serves skin on forehead and scalp
Nasociliary nerve: Passes medially between the superior oblique and medial rectus. Branches of nasociliary:
Long ciliary nerves: Enter eyeball; carry post-G sympathetic fibers to dilator pupillae muscle, and carry sensory fibers from the cornea (afferent limb of the corneal reflex)
Posterior ethmoidal: Sensation from posterior ethmoid and sphenoidal sinuses
Anterior ethmoidal: Enters nasal cavity, divides into internal nasal and external nasal branches; sensation from nasal cavities and ethmoid sinuses
Infratrochlear nerve: Termination; passes below the trochlea to supply a small area of skin on the medial surface of the upper eyelid
Ciliary ganglion
Parasympathetic ganglion located between CN II and lateral rectus muscle; parasympathetic PRE-G fibers (from Edinger-Westphal nucleus) synapse here
3 roots going to the ganglion:
Sensory root
Sympathetic root
Parasympathetic root.
Short ciliary nerves: Contain both Post-G sympathetic (to dilator pupillae) and Post-G parasympathetic (to sphincter pupillae or ciliary muscle), and afferent fibers to the nasociliary nerve; considered branches of V1.
Arteries
Figure 9.Ophthalmic artery (first large branch of internal corotid): Passes through optic foramen and canal to enter orbit; may pass above or below the optic nerve. Branches:
Lacrimal artery: Runs along the lateral wall of the orbit
Posterior ciliary arteries: Serve choroid and eyeball
Ophthalmic artery then passes anteromedially to give more branches:
Central artery of the retina: Enters CN II, to region of optic disc. Note: you can see this artery spreading out in a patient’s retina with your ophthalmoscope. This is the only artery in the body that you can visualize through a non- invasive means!
Supraorbital: Forehead and anterior scalp
Anterior and posterior ethmoidal: Paranasal sinuses, nasal septum, and lateral nasal wall
Supratrochlear: Forehead and scalp
Veins
Figure 10.
The superior and inferior ophthalmic veins drain blood from orbital structures. The ophthalmic veins receive vorticose (from vascular layer of eye) and other veins in the orbit.
Superior ophthalmic vein: Passes into superior orbital fissure, usually joined along the way by the inferior ophthalmic vein, and collectively terminate in cavernous sinus
The superior ophthalmic vein connects to the facial vein on the face. This is important since it provides a route from the tissues of the face to the cavernous sinus.
Clinical correlation
Since the ophthalmic veins are valueless, infections from the face could spread to the cavernous sinus under the right conditions.
Extra-ocular muscles in the orbit
The extra-ocular muscles (skeletal muscles) in the orbit include the levator palpebrae superioris, which functions to elevate the upper eyelid, and six muscles that move the eyeball. The latter are the four rectus muscles (superior, inferior, medial and lateral) and the two obliques (superior and inferior). All six muscles insert into the tough outer layer of the eye = the sclera.
The levator palpebrae superioris arises from the roof of the bony orbit, above and anterior to the optic canal. It passes forward and parallel to the superior rectus muscle. Instead of attaching to the eyeball, it continues into the upper eyelid (palpebrae is Latin for eyelid). Here is forms a broad aponeurosis with multiple attachments: to the tarsal plate, to the skin of the eyelid, and to the superiorconjunctival fornix. The levator palpebrae superioris is innervated by the superior division of the oculomotor nerve (same nerve that supplies the superior rectus).
The rectus muscles originate from the common tendinous ring (annulus of Zinn) in the posterior orbit. The muscles run forward and spread out a bit in a cone shape (like the bony cavity of the orbit—the cone gets wider as it moves anteriorly) before inserting into the anterior sclera. The medial and lateral recti are in the same horizontal plane while the superior and inferior recti are in the same vertical plane.
Figure 11. Location of the common tendinous ring (annulus of Zinn). GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIGURE 8.90.
Figure 12. Structures coursing through and around the ring of Zinn. GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIG. 8.96.
The superior oblique muscle arises from the postero-medial orbit wall, above and medial to the optic canal. It does not arise from the common tendinous ring. The fleshy belly of the muscle runs along the medial wall above the medial rectus before forming a tendon that hooks sharply backwards as it passes through a cartilage ring (the trochlea) that acts as a pulley in the upper medial wall of theanterior orbit. It inserts into the posterolateral quadrant of the superior surface of the eyeball (divide the superior surface of the eyeball into four quadrants, as viewed from above).
The inferior oblique muscle arises from the bony floor of the orbit and passes laterally, below the inferior rectus muscle. It inserts into the posterolateral quadrant of the inferior surface of the eyeball.
Figure 13. Muscles moving the eye. GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIGURE 8.95.
Actions of extra-ocular eye muscles
The eyeball rotates on three axes. To keep things simple, let’s only consider two sets of movements:
around a horizontal axis (eyeball can rotate upward or downward = elevation or depression)
around a vertical axis (eyeball can rotate toward the nose or away from the nose = adduction or abduction)
Figure 14. GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIGURE 8.94.
Superior and inferior recti: Because of the cone shape of the orbit, the axis of the orbit runs forward and laterally, angled 23 degrees off the midline of the head. Therefore, even though the rectus muscles are straight (as their name implies), they run both forward and laterally, not strictly forward. This means that the superior and inferior recti, in addition to their prime actions of elevation and depression, will also cause adduction of the eyeball, moving it slightly toward the nose.
The medial and lateral rectus muscles have only one action: adduction and abduction, respectively.
Because the oblique muscles reach over (superior) and under (inferior) the eyeball to insert into the posterolateral quadrants of the eyeball, in addition to their prime actions of elevation and depression, they will tend to cause the eyeball to swivel outward away from the nose—thus they are both also abductors. The action of the superior oblique muscle can be remembered because it is a “sad” muscle—it causes the eyeball to be “down and out.”
Figure 15. GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIGURE 8.97.
It is important to appreciate that the extra-ocular muscles, like other muscles in the body, work together to assist in movements; they do not work in isolation unless there is a problem with the other muscles or the nerves that innervate them. For example, if you desired to look strictly downward (depress the eyeball), using only the inferior rectus muscle would not do the trick, because this muscle also causes slight adduction of the eye. Since adduction is not desired in this case, the superior oblique muscle would also be called into action, since its movements include depression as well as abduction of the eye. Notice that this last movement would cancel out the unwanted adduction movement of the inferior rectus. The moral of the story: strict depression of the eyeball requires the coordinated actions of both the inferior rectus and superior oblique muscles. Does this make sense?
Question
Which two muscles would need to contract and work in concert to produce only elevation of the eyeball (looking strictly upwards)?
The neutral position of the eyeball (looking straight ahead) requires the extra-ocular muscles to be kept in a constant tonic state, to cancel out any unwanted actions. If any of the muscles are not working correctly, a gaze that was intended to look straight ahead would drift, due to inability of one or more muscles to maintain forward visual gaze. Rather than being aligned, the eyes would gaze in different directions, a condition called strabismus.
Table 1. Simplified actions of extra-ocular eye muscles.
Muscle
Actions
Innervation
Superior rectus
Elevation and adduction: Eye looks up and in
Oculomotor(superior division)
Inferior rectus
Depression &adduction: Eye looks down and in
Oculomotor (inferior division)
Medial rectus
Adduction: Eye looks in
Oculomotor (inferior division)
Lateral rectus
Abduction: Eye looks out
Abducens
Superior oblique
Depression and abduction: Eye looks down and out
Trochlear
Inferior oblique
Elevation and abduction: Eye looks up and out
Oculomotor (inferior division)
Mnemonic
A mnemonic for remembering the innervation of extra-ocular muscles is LR6, SO4, AO3.
Lateral rectus: CN 6
Superior oblique: CN 4
All Other (muscles): CN 3
Clinical testing of extra-ocular muscles
Figure 16.
To clinically test the strength and innervation of an eye muscle, the clinician must be able to isolate and observe a movement that can only be brought about by that muscle. This is problematic considering our earlier discussion concerning synergy of the muscles in moving the eyeballs (e.g., both the superior rectus and inferior oblique muscles can elevate the eye) and maintaining a gaze in the neutral position (all muscles work together to steady the eye). How can the clinician subtract out the actions of the other muscles and isolate the movement of one? The answer is to “set” the eyeball in an initial “starting” position, then move the eyeball in a direction that can only be accomplished from that starting position with the action of one muscle. This procedure also “traps” the other muscle in the functional pair (the other muscle that can perform a similar action), by orienting the visual gaze axis perpendicular to the fiber direction of the other muscle whose action you want to eliminate. Let’s give some examples:
To test the superior rectus:
The patient is asked to gaze laterally (this “sets” the eyeball in the abducted position), then upward. The gaze axis with the eyeball abducted is perpendicular to the fiber direction of the inferior oblique (the other muscle that can elevate the eye). Inferior oblique is “trapped.” From this set position, the only muscle capable of elevating the eyeball is the superior rectus. This also tests the superior division of the oculomotor nerve—the nerve that innervates the superior rectus.
To test the inferior rectus:
The patient is asked to gaze laterally, then downward. Set position (abducted eyeball) traps the superior oblique (the functional pair—like the inferior rectus it can depress the eyeball). The only muscle capable of depressing the eye from the set position is the inferior rectus. It also tests the inferior division of the oculomotor nerve.
To test the inferior oblique:
The patient gazes medially (sets eyeball in adducted position) then upward. The set position places the eyeball gaze axis perpendicular to the fiber direction of the superior rectus, “trapping” it. From the set position, the only muscle capable of elevating the eyeball is the inferior oblique. This also tests the inferior division of the oculomotor nerve.
To test the superior oblique:
The patient is asked to gaze medially (adduct), then down. Set position (adducted eyeball) traps the inferior rectus. The only muscle capable of depressing the eyeball from the set (adducted) position is the superior oblique. This also tests the trochlear nerve.
To test the medial and lateral recti:
These muscles don’t require the eyeball to move to a set position. Instead, the neutral (straight ahead) gaze position is the “set” position here. From this position, the only muscle capable of adducting the eye (looking toward the nose) is the medial rectus. Likewise, the only muscle capable of abducting the eye from the neutral position is the lateral rectus. These movements also test the inferior division of the oculomotor nerve (medial rectus) and the abducens (lateral rectus).
Note that clinical testing of the eye muscles is not the same as observing the normal functional movements of the eye muscles. For example, the normal function of the superior oblique is to cause the eyeball to move outward and down. Clinical testing of the superior oblique requires the patient to move the eye inward first, and then down. The ability of the superior oblique to depress the eyeball, while eliminating the action of its functional pair (inferior rectus), is the key movement here.
Figure 17. The clinical “H” test of eye muscles. GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIG. 8.98.
Of course, clinicians don’t have the time or inclination to slowly and deliberately have patients perform these actions with their eyeballs. Instead, clinicians use a simple procedure of moving the raised index finger of one hand in front of the patient in an “H” pattern, while observing the eyes of the patient move to follow their finger. If the patient’s gaze can follow the finger in the “H” pattern, this incorporates all the movements described earlier, and tests each of the extra-ocular muscles from their “set” positions.
Helpful, short videos
If you are still unclear about this clinical testing of eye muscles vs. normal actions of eye muscles, my good friend the "Noted Anatomist" can help with this nice video he has produced.
Helpful, short videos
Also, here are two simple and old-school videos on clinical testing of eye muscles.
Cranial Nerve III, IV, and VI dysfunction and eye muscles
Lesions of the oculomotor, trochlear, or abducens nerves affect the eye muscles they supply. These conditions are called palsies (singular: palsy) because the muscles are paralyzed. The primary symptom of a third, fourth, or sixth nerve palsy is diplopia (double vision), since muscle paralysis causes misalignment of the eyes and movements that are not coordinated. Discussing symptoms of third, fourth, or sixth nerve palsy illustrates the actions of the eye muscles.
When attempting a neutral gaze (eyes directed forward), the affected eye gazes down and out. Why? Consider which muscles are affected by a lesion to the oculomotor nerve: all the muscles except for the lateral rectus and superior oblique. Thus, the actions of the intact muscles are unopposed—the eye is directed outward (combined action of lateral rectus and superior oblique) and downward (action of superior oblique). Patients with third nerve palsy may also have a droopy eyelid (ptosis) on the affected side due to paralysis of the levator palpebrae superioris, and the pupil on that side may be constantly dilated. The “blown” pupil is a result of sympathetic innervation to smooth muscle in the eye being unopposed by parasympathetic nervous input—since the parasympathetic nerve fibers in CN III are damaged. Vascular problems (hypertension, diabetes) or an aneurysm of a nearby blood vessel (circle of Willis) can produce a third nerve palsy.
Clinicians say it is the hardest nerve palsy to detect, since the trochlear nerve innervates only one eye muscle. Symptoms can be subtle. The trochlear nerve itself is vulnerable since it is thin and long—in fact, it has the longest intracranial course of any cranial nerve (why is this?). Many possible causes = tumors, vascular, or trauma. Patients have trouble with activities that cause them to look down—for example, reading a book or walking down the stairs produces diplopia (visual images are stacked on top of each other). This is because the normal action of the superior oblique muscle is depression of the eyeball (in tandem with its functional partner, the inferior rectus), and without it, depression is weak, and the affected eyeball does not move downward as well as the other eye. Patients afflicted with fourth nerve palsy often try to relieve the diplopia by tilting their head away from the affected side.
Easy to explain the diplopia here, since the abducens nerve controls the lateral rectus muscle. If this muscle doesn’t work on one side, the person is “cross-eyed”. One cause again can be vascular issue (hypertension, diabetes) that affect blood flow to the nerve. It has also been shown that high intracranial pressure can compress the abducens nerve as it crosses over the ridge atop the petrous part of the temporal bone.
