27

Face and parotid gland

 

 

OPTIONAL READING

Clinically Oriented Anatomy, 7th ed., Face and scalp section through Surface anatomy of face; Parotid and temporal regions, infratemporal fossa,

and temporomandibular joint section through Infratemporal fossa.

The Developing Human: Clinically Oriented Embryology, 11th ed., Development of salivary glands section through

Atresia of the nasolacrimal duct.

CHAPTER CONTENT S

FA C E A N D PA R OT I D G L A N D FA C I A L L A N D M A R K S

SKELE TAL FR AME W ORK SUBCU TANEOUS TISSUE

MUSCLES OF FA CIAL EXPRESSION (MIME TIC MUSCLES)

Peri-orbital facial muscles Peri-oral facial muscles Other facial muscles

MA S S E T E R M U S C L E

CU TANEOUS INNER VATION OF THE FA CE

MOTOR INNER VATION OF MIME TIC MUSCLES

VA S C U L A R S U P P LY A N D LY M P H AT I C D R A I N A G E O F T H E FA C E

Arteries Veins

Lymphatic drainage

 

PA R OT I D G L A N D

APPENDIX: SY MPATHE TIC INNER VATION OF THE FA CE

 

Face and parotid gland

 

 

The face is the anterior part of the head, located in front of the ears, above the margins of the mandible, and below the “hairline”. [Note: using this definition, it would seem that your anatomy professor has a rather tall face!] The term is derived from the Latin word facies (= countenance), which refers to the features giving us our individual identify. Certain diseases alter the appearance of the face in distinctive ways (e.g., Parkinson’s disease).

 

Fa c i a l l a n d m a r k s

• The eyelids (Latin = palpebrae) are moveable folds of skin, connective tissue, and muscle that protect the eyeballs. The openings between them that we see through are palpebral fissures.
• Lips (labia) surround the opening to the oral cavity (oral fissure). The upper and lower lips meet laterally at the labial commissures.
• In the center of the upper lip is a depression called the philtrum (Greek = “love charm”).
• Named parts of the external nose are: nares (nostrils), alae (sides, that can be “flared” when angry), apex (tip), root (between the eyes), and dorsum (bridge), connecting the apex and root.

 

• The bilateral groove passing downward from ala of nose to labial commissure is the nasolabial sulcus.

 

Skele tal fr ame w ork

• The neurocranium is the part of the skull surrounding and protecting the brain (“brain case”).
• The other bones of the skull make up the facial skeleton (viscerocranium). The most important of these are the paired zygomatic bones (“cheek bones”), maxillae, nasal bones, and the mandible. The other bones of the facial skeleton support the nasal cavities. The appearance of the face is to a large extent determined by variations in shape and prominence of the underlying facial bones.

CLINIC AL APPLIC ATION

Facial fractures are the result of trauma to the face (MVA, falls, sports, fisticuffs).

• Broken nose: fracture of the nasal bones = the most common facial fracture.
• Broken jaw: fracture of the mandible = second most common facial fracture
• Broken cheekbone: fracture of the zygomatic bone.
• Midface fractures involve the maxillae, producing midface mobility.

Dr. Le Fort, a French surgeon, developed a commonly used classification for these fractures (Le Fort fractures).

• Le Fort Type I = horizontal fracture through the alveolar process of the maxillae and nasal septum. The upper teeth become detached from the face with the bone fragment.
• Le Fort Type II = unilateral or bilateral fracture through the maxillae,

infra-orbital foramina, lacrimal bones, and bridge of the nose. This produces a triangular fragment in the central face that is detached from the rest of the skull.

• Le Fort Type III = horizontal fracture through the greater wings of the

sphenoid bones, superior orbital fissures, and ethmoid bone. If bilateral, this separates the facial skeleton from the cranial vault. Fracture of the ethmoid bone can cause leakage ofcerebrospinal fluid (CSF) into the nasal cavity (CSF rhinorrhea).

 

 

 

 

 

Figure 27.1 CLINICALLY ORIENTED ANATOMY, FIGURE 7.3.

Figure 27.2 CLINICALLY ORIENTED ANATOMY, FIGURE B7.1.

 

Subcu taneous tissue

The face contains copious superficial fascia but little in the way of deep fascia. The superficial fascia of the face is unique since it contains skeletal muscles (mimetic muscles), whereas skeletal muscles in other parts of the body are invested by deep fascia. The subcutaneous tissue in the face consists of four blended components:

1. Superficial fat compartments separated by connective tissue septa (e.g., the nasolabial folds contain one of these fat compartments).
2. Loose connective tissue containing collagen fibers, elastic fibers, and fat cells that connect facial muscles to the overlying dermis of the skin.
3. Muscles of facial expression (mimetic muscles).
4. Deep fat compartments below the mimetic muscles. These give shape and volume to the face and allow the muscles to glide and move freely. The largest of these is the aesthetically important buccal fat pad. Prominent in infants, it supports the cheeks during nursing and produces cute “chubby-cheeks”, a feature not so desirable in adults.

• The construction of the subcutaneous tissue gives the skin remarkable mobility and aids in facial expression. The only discernable deep fascia in the face is an extension from the neck that covers the masseter muscle and parotid gland.
• The pull of the underlying muscles produces temporary wrinkles and creases in the facial skin. Loss of collagen and elastic fibers in the face, a normal eventwith aging, causes these to become more noticeable and permanent, especially around the mouth, eyes (“crow’s feet”), and on the forehead (“worry lines”).

 

Aging also causes volume loss in the fat compartments, and this seems to occur at different rates in different areas of the face. The result is uneven facial contours and loss of the smooth transitions between facial regions that give the essence of youthfulness.

 

Muscles of fa cial expression (mime tic muscles)

All mimetic muscles are derived from the mesenchyme of the embryonic second pharyngeal arch. Therefore, all are innervated by branches of the facial nerve

(VII).  Second arch mesenchyme migrates into the scalp and neck as well, so muscles located there also impact facial expression.

 

The muscles of facial expression are thin and flat, originate from bone or fascia, and insert into the facial skin. They vary quite a bit in their size from person to person and many of them are blended together on the face. Some are quite small and not significant to medical students, so we will focus on the larger ones that can be seen in the lab. In general, mimetic muscles act as sphincters/dilators of facial orifices and elevators/depressors of the lips and eyebrows. We will group the major facial muscles as follows:

 

Peri-orbital facial muscles

• Orbicularis oculi—sphincter of the eyelids. The orbital part is arranged as concentric rings of muscle fibers outside the rims of the orbit. It forcefully closes the eyelids. The palpebral part is thin and located in the eyelids themselves. It gently closes the eyelids, as in blinking.

 

Peri-oral facial muscles

• Zygomaticus muscles—variably sized muscles (often 2) that attach to the zygomatic bone and corner of the mouth. Bilateral contraction pulls the corners of the mouth upward and backward, as in laughing and smiling. Unilateral contraction produces a sneer (Billy Idol face).
• Levator labii superioris—attaches to the maxilla and upper lip. Raises the upper lip and its medial fibers can also dilate the nostrils, as in snarling. Its actions deepen the nasolabial sulcus.
• Depressor labii inferioris—attaches to the mandible and lower lip. Depresses the lower lip as when pouting.
• Levator anguli oris and depressor anguli oris—arise from the maxilla and mandible, respectively, and insert into the corner of the mouth. These muscles curl the corner of the mouth up or down, when you are happy or sad.
• Oribularis oris—its circular fibers form the substance of the lips and interlace with all the other peri-oral facial muscles. It acts as a sphincter of the oralfissure to close the lips and can purse them as well.

 

• Buccinator (Latin = trumpeter)—arises from the maxilla and mandible and a ligament called the pterygomandibular raphé. It merges with the muscles of the lips. Buccinator is located deep to the buccal fat pad in the cheeks, in a plane below the other mimetic muscles. The duct of the parotid gland penetrates the buccinator. It compresses the cheeks against the teeth, working with the tongue to keep food on the teeth during chewing. It also forces air from the mouth and through the lips. The buccinator is the “trumpeter’s muscle”.

 

Other facial muscles

• Occipitofrontalis—comprised of two muscle bellies (frontal and occipital) connected in the middle by a flat tendon (aponeurosis) in the scalp. The frontal belly (often called the frontalis muscle) attaches to the skin just above the orbits. It raises the eyebrows and wrinkles the skin of the forehead (the “muscle of surprise”). The occipital belly (occipitalis) attaches behind to the occipital bone. It produces backward movement of the scalp.
• Platysma (Greek = “flat”, like the flat bill of the platypus)—a broad, thin muscle originating from the fascia of the chest and shoulder and attaching above tothe mandible and corner of the mouth, where it blends with the peri-oral muscles. When contracted, it can depress the mandible, but it is probably more important in drawing the corner of the mouth down and tensing the skin of the neck (“lizard neck”), as when expressing anger or horror. Aging causes its muscle fibers to thicken, producing vertical bands in the neck.

 

 

Figure 27.3                                                                                                                                   Figure 27.4

 

 

• If you are talented enough to be able to wiggle your ears, you have well developed auricular muscles. These are important to animalswho need to move their ears, but not so much for humans. We won’t see these in lab, but they are a novelty.

 

Ma s s e t e r m u s c l e

The masseter is not a muscle of facial expression, but a chewing (mastication) muscle. It is mentioned here because it is prominently located on the face and can be easily seen and felt when the teeth are clenched. It largely covers the ramus of the mandible. On its surface it is crossed by the zygomaticus muscles, parotid duct, and branches of the facial nerve. We’ll learn more about it when we study the muscles of mastication.

 

Cu taneous inner vation of the fa ce

Branches of all three divisions of the trigeminal nerve supply the face. Study atlas figures to identify the specific nerves. Some of the larger ones will be seen in lab. It is important to understand the dermatomemap of the face, since it is part of clinical testing of the trigeminal nerve.

Figure 27.5 CLINICALLY ORIENTED ANATOMY, FIGURE 7.20.

 

• Ophthalmic division (V₁) supplies the forehead, upper eyelid, and dorsum of the nose through the supra-orbital, supratrochlear, infratrochlear, lacrimal, and external nasal nerves.
• Maxillary division (V₂) supplies the lower eyelid, midface, temporal region, and sides of the nose through the infra-orbital, zygomaticofacial, and zygomaticotemporal nerves.
• Mandibular division (V₃) supplies the chin, lower lip, cheek, external ear, and scalp via the mental, buccal, and auriculotemporal nerves.

 

The great auricular nerve, a branch of the cervical plexus (spinal nerves C-2 and C-3), supplies a small region of the facial skin over the angle of the mandible.

 

 

 

Motor inner vation of mime tic muscles

After emerging from the stylomastoid foramen, the facial nerve enters the parotid gland. Here it gives rise to five named branches that pass on to the face and neck: temporal, zygomatic, buccal, marginal mandibular, and cervical.

These supply the mimetic muscles and are named for the regions of the face to which they are distributed. The cervical branch passes below the mandible into the neck to supply the platysma muscle.

 

A time-honored mnemonic for remembering the branches of the facial nerve on the face is: “To Zanzibar By Motor Car.”

Figure 27.6 Branches of the facial nerve on the face.

T = temporal, Z = zygomatic, B = buccal, M = marginal mandibular, C = cervical.

CLINICALLY ORIENTED ANATOMY, FIGURE 7.23.

 

Va s c u l a r s u p p ly a n d

LY M P H AT I C D R A I N A G E O F T H E FA C E

Arteries

• The face has a rich blood supply, primarily from the facial artery, a branch of the external carotid artery. Arising in the neck, the facial artery ascends and curves around the submandibular gland to cross the mandible, where its pulsations are palpable and it can be compressed to alleviate bleeding. On the face, it has an oblique course through the buccal fat pad deep to the muscles of facial expression. When it parallels the nose, the facial artery is called the angular artery. It terminates at the medial angle of the eye. Two important branches of the facial artery are the inferior and superior labialarteries to the lips.
• The transverse facial artery, a branch of the superficial temporal artery, is the

other significant source of blood. It courses across the face parallel to the zygomatic arch.

 

 

Figure 27.7 NET TER, ATLAS OF HUMAN ANATOMY, PLATE 3.

 

Veins

• The angular vein is formed at the medial angle of the eye by the union of the supra-orbital and supratrochlear veins; below the nose it becomes the facial vein, a tributary of the internal jugular vein. The course of the facial vein parallels the facial artery.

 

Figure 27.8 GRAY’ S ANATOMY FOR STUDENTS, FIGURE 8.65.

 

Lymphatic drainage

Lymphatic vessels of the face and scalp parallel blood vessels, with lymph flowing downward toward a collar of lymph nodes at the base of the head. Nodes and their regions of drainage are:

• Submental nodes—central lower lip and chin.
• Submandibular nodes—nose, buccal region of cheek, upper lip, and lateral lower lip
• Parotid nodes—eyelids, cheekbones, anterior and central scalp, external acoustic meatus, and anterior part of the auricle.
• Mastoid nodes—central and posterior scalp, posterior part of the auricle.
• Occipital nodes—posterior scalp and neck.

 

After percolating through these nodes, lymph from the face and scalp passes to the

deep cervical lymph nodes along the internal jugular vein in the neck.

 

 

 

Figure 27.9 Schematic of lymph drainage from face and scalp.

HAND-DRAWN CONLEY- GRAM.

 

Pa r ot i d g l a n d

The parotid (Greek = “beside the ear”) is the largest of the major salivary glands. It lies in the depression between the ramus of the mandible and the sternocleidomastoid muscle, surrounded by a capsule of deep fascia. The parotid is a double pyramid: (1) when viewed on the face it is wider above and tapersdown to an apex near the sternocleidomastoid, (2) in horizontal section, it has a wide superficial part external to the mandible and masseter muscle, and a narrow deep part wedged between the ramus of the mandible, mastoid process, and styloid process. Because it is compressed when the mouth is opened wide, pain may be elicited when the gland is swollen due to mumps or a blocked duct.

 

Several structures course through the gland. These are best learned in layers, from superficial to deep:

1. The facial nerve enters the parotid and divides into five branches (temporal, zygomatic, buccal, marginal mandibular, and cervical). These emerge from the gland on to the face to supply the mimetic muscles. Removal of the parotid thus requires meticulous surgical dissection.
2. The retromandibular vein is formed within the gland by the union of the superficial temporal and maxillary veins. Within (or below) the gland it divides into anterior and posterior divisions. The posterior division joins the posterior auricular vein to form the external jugular vein. The anterior division

usually joins the facial vein.

 

 

Figure 27.10 GRAY’ S ANATOMY FOR STUDENTS, FIGURE 8.59.

 

 

3. The external carotid artery divides within the gland into its terminal branches, the maxillary and superficial temporal arteries.
4. The auriculotemporal nerve, a branch of V₃ passes deep to the gland on its way to the scalp. Postganglionic parasympathetic fibers from the glossopharyngeal nerve innervate the parotid by hitching a ride with the auriculotemporal.

 

• The parotid duct (Stensen’s duct) is located parallel to the tip of the ear lobe. It crosses the masseter muscle, pierces the buccal fat pad and the buccinator, and empties into the oral cavity adjacent to the second upper molar. Accessory glandular tissue may lie along its course.

 

Appendix: sy m pathe tic inner vation of the fa ce

Sympathetic fibers supplying visceral structures in the face follow the same basic pathway as all sympathetic nerves to the head. This pathway is summarized and reviewed here.

• Preganglionic sympathetic fibers destined for the head originate from the intermediolateral cell column (in the lateral horns of gray) of the upper thoracic spinal cord segments (T-1 and T-2). They enter the sympathetic trunk through white rami communicantes.
• Preganglionic fibers ascend through the sympathetic trunk to the superior cervical ganglion, situated opposite C-2 vertebra. Here they synapse on postganglionic sympathetic neurons.
• Postganglionic fibers leave the superior cervical ganglion as several carotid nerves. These pass onto the internal and external carotid arteries, forming carotid plexuses. Postganglionic sympathetic fibers follow branches of the external carotid artery (e.g., facial artery, superficial temporal artery) onto the face and scalp, and then hitch rides on branches of CN V to reach and innervate sweat glands, arrector pili muscles, and smooth muscle in blood vessels.

Figure 27.11 Pathway of sympathetic innervation to the face.

HAND-DRAWN CONLEY- GRAM.

 

 

28

Orbit, eyelids, and lacrimal apparatus

 

 

OPTIONAL READING

Clinically Oriented Anatomy, 7th ed.,

Eye, orbit, orbital region, and eyeball section through Lacrimal apparatus; Extra-ocular muscles of orbit section through Surface anatomy of eye and lacrimal apparatus.

CHAPTER CONTENT S

ORBI T, EY ELIDS, AND L A CRIM AL APPAR ATUS BON Y ORBIT

Bony openings of the orbit

THE EY ELIDS

OBSER VABLE STR UC TURES AND PART S WHEN LOOKING THR OUGH THE PALPEBR AL FISSURE

Lacrimal apparatus

Innervation of the lacrimal gland

Eyeball layers, briefly Nerves

Arteries Veins

Extra-ocular muscles in the orbit Actions of extra-ocular eye muscles Clinical testing of extra-ocular muscles

 

Cranial Nerve III, IV, and VI dysfunction and eye muscles Third (oculomotor) nerve palsy

Fourth (trochlear) nerve palsy Sixth (abducens) nerve palsy

 

Orbit, eyelids, and lacrimal apparatus

 

 

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 andwill be discussed in greater detail in your Nervous System course.

 

 

 

Bon y orbit

A pyramid-shaped cavity (as you look from its base to its apex) with infra-orbital (frontal bone) and supra-orbital (maxillary bone) margins. 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

Figure 28.1 MOORE, CLINICALLY ORIENTED ANATOMY, 8TH ED.

 

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 ey elids

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”.

 

 

 

Figure 28.2 Cross-section of upper eyelid.

MOORE, CLINICALLY ORIENTED ANATOMY, 8TH ED. FIGURE 8.45.

 

• Deep to the skin is the muscle of the eyelids, the palpebral portion of the orbicularis oculi. As discussed in Chapter 26, 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. Thetarsal 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.

CLINIC AL APPLIC ATION

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 thesympathetic 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.

Do you understand how these symptoms relate to sympathetic denervation in the head?

 

• 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.

Figure 28.3 NET TER, ATLAS OF HUMAN ANATOMY, 7TH ED., PLATE 94.

 

 

 

Obser vable str uc tures and part s when looking thr ough the palpebr al fissure

• 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.

 

 

 

 

 

 

Figure 28.4

 

• 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 28.5 until you go cross-eyed (unfortunately, a hidden 3-D dolphin does not pop out at you).

 

Whew! Got all that?

Figure 28.5

 

Eyeball layers, briefly

• Fibrous layer: sclera
• The “whites of the eye”; provides attachment for extra- ocular muscles
• Vascular layer: choroid, ciliary body (with ciliary muscle), and iris
• Ciliary body: connects choroid to circumference of the

iris and provides attachment for the lens through

suspensory ligaments

• Parasympathetic innervation: ciliary muscle contracts, suspensory ligaments relax, lens becomes spherical for near vision
• No Parasympathetic stimulation: ciliary muscle relaxed, suspensory ligaments under tension, lens flattens for distant vision
• Nervous layer: retina
• Axes of the Eye—important for clinical testing!
• 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

Figure 28.6

 

 

 

 

• 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

• 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: from caudal pons; enters cavernous sinus and superior orbital fissure, to supply the lateral rectus muscle. It enters the medial surface of the muscle.

Figure 28.7

 

• 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: (1) sensory root, (2) sympathetic root, and (3) 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

 

 

 

 

Figure 28.8

 

Arteries

• 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 & 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

Figure 28.9

 

Veins

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.

Figure 28.10

 

 

 

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 28.11 LOCATION OF THE COMMON

TENDINOUS RING ( ANNULUS OF ZINN). GRAY’ S

ANATOMY FOR STUDENTS, 4TH ED. FIGURE 8.90.

 

 

 

 

Figure 28.12 Structures coursing through and around the ring of Zinn. GRAY’ S ANATOMY FOR STUDENTS, 4TH ED. FIG. 8.96.

Figure 28.13 Muscles moving the eye. GRAY’ S ANATOMY FOR STUDENTS, 4TH ED. FIGURE 8.95.

 

 

 

• 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.

 

Actions of extra-ocular eye muscles

The eyeball rotates on three axes. To keep things simple, lets only consider two sets of movements: around a horizontal axis (eyeball can rotate upward or downward = elevation or depression) and around a vertical axis (eyeball can rotate toward the nose or away from the nose = adduction or abduction).

• 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 28.14   GRAY’ S ANATOMY FOR STUDENTS, 4TH ED. FIGURE 8.94.

 

 

Figure 28.15 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 slightadduction 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?

 

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 workingcorrectly, 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.

 

Muscle Actions Innervation Superiorrectus Elevation &adduction: Eye looks up and in Oculomotor(superiordivision) Inferiorrectus Depression &adduction: Eye looks down and in Oculomotor(inferior division) Medialrectus Adduction: Eyelooks in Oculomotor(inferior division) Lateralrectus Abduction: Eyelooks out Abducens Superioroblique Depression &abduction: Eye looks down and out Trochlear Inferioroblique Elevation &abduction: Eye looks up and out Oculomotor(inferior division)

Table 28.1 Simplified actions of extra-ocular eye muscles

Clinical testing of extra-ocular muscles

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. Thisalso tests the superior division of the oculomotor nerve—the nerve that innervates the superior rectus.
• To test the inferior rectus: 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: 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: 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.

Figure 28.16 GRAY’ S ANATOMY FOR STUDENTS, 4TH ED. FIGURE 8.97.

 

• Testing 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, theonly 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 eyeballto 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.

 

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 allthe movements described earlier, and tests each of the extra-ocular muscles from their “set” positions.

Figure 28.17 The clinical “H” test of eye muscles. GRAY’ S ANATOMY FOR STUDENTS, 4TH ED. FIG. 8.98.

 

 

 

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.

 

Third (oculomotor) nerve palsy

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 thelateral 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

 

 

 

 

Figure 28.18 Oculomotor nerve palsy.

 

(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.

 

Fourth (trochlear) 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 itsfunctional 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.

 

Sixth (abducens) nerve palsy

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.

Figure 28.19 Abducens nerve palsy.

 

 

 

 

 

29

The ear

 

 

OPTIONAL READING

Clinically Oriented Anatomy, 8th ed., Ear section through Auditory ossicles.

CHAPTER CONTENT S THE EAR

EX TERNAL EAR

Auricle (pinna)

External acoustic meatus Tympanic membrane (eardrum)

Blood supply of external ear Innervation of external ear

MIDDLE EAR

Tympanic cavity

Tympanic cavity—Box visual analogy Innervation of the tympanic cavity Auditory ossicles

Malleus (“hammer”) Incus (“anvil”) Stapes (“stirrup”)

 

Muscles of the ossicles Tensor tympani Stapedius

Nerves associated with the tympanic cavity

Facial nerve

Chorda tympani (a branch of the facial nerve) Nerve to stapedius

Greater petrosal nerve (a branch of the facial nerve) Tympanic nerve plexus

INTERNAL EAR ( OPTIONAL READING)

Bony labyrinth Membranous labyrinth

 

The ear

 

 

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.

 

 

 

 

Figure 29.1 CLINICALLY ORIENTED ANATOMY, FIGURE 7.109.

 

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.

1. 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).
2. 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.
3. 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.
4. 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.

 

Ex ternal 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 29.2 WIKIMEDIA COMMONS.

 

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 29.3

 

 

 

 

• 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 temporalbone. 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.

CLINIC AL APPLIC ATION

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 29.4 Otoscopic examination of normal tympanic membrane—lateral view.

 

• 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.

 

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 ofthe 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.

Figure 29.6 Coronal section—anterior view.

CLINICALLY ORIENTED ANATOMY, FIGURE 7.116.

 

Tympanic cavity—Box visual analogy (Figure 29.7)

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 bea 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 29.7 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.

 

Innervation of the tympanic cavity

• The mucosa is supplied by the tympanic branch of the glossopharyngeal nerve. Thus, pain from a middle ear infection is carried in CN IX.

 

 

Figure 29.8 Relationships of the tympanic cavity. Superior view. The petrous temporal bone is transparent.

 

Auditory ossicles

Three tiny bones extend across the tympanic cavity, covered in mucous membrane, from the tympanic membrane to the oval window. From lateral to medial these are the malleus, incus, and stapes.

 

Malleus (“hammer”)

• Head is round and articulates with the incus.
• Handle passes downward to attach to the tympanic membrane.
• Lateral process is attached to the tympanic membrane (producing the mallear folds).

 

Incus (“anvil”)

• Body is rounded and articulates anteriorly with the head of the malleus.
• Long process projects inferiorly, behind and parallel to the handle of the malleus (where its shadow can sometimes be seen on otoscopic examination). Articulates with the head of the stapes.
• Short process projects posteriorly and is attached to the posterior wall of the tympanic cavity by a ligament.

Figure 29.9 WWW. ANATOMYBOX. COM.

 

Stapes (“stirrup”)

• Head is very small and articulates with the long process of the incus—receives the attachment of the stapedius muscle.
• Two limbs attach the neck to the base (footplate).
• The edges of the base attach to the margins of the oval window by a ring of connective tissue called the annular ligament.

 

Given that the tympanic membrane has a much larger surface area than the base of the stapes, the ossicles act as levers to amplify the movements of the tympanic membrane (sources vary on this, but the amplification may be as much as 20 times), so that a substantial force can be produced at the oval window by smaller movements of the tympanic membrane.

Figure 29.10

 

Muscles of the ossicles

The tensor tympani and stapedius muscles attenuate (reduce) the movements of the ossicles (and tympanic membrane) in response to loud noises, which might otherwise cause damage to the delicate sensory apparatus of the internal ear.

 

Tensor tympani

• Origin: from the bony wall of its canal (opens on the anterior wall of the tympanic cavity) as well as from the cartilage of the pharyngotympanic tube.
• Insertion: its tendon turns around a bony shelf (pulley) to insert on the handle of the malleus.
• Action: Reduces the vibrations of the malleus and movements of the tympanic membrane by pulling the malleus and tympanic membrane medially (thus tensing the tympanic membrane).
• Innervation: a branch of the mandibular nerve (V₃).

Figure 29.11 Tensor tympani and stapedius muscles. Lateral wall of tympanic cavity is seen—viewer is looking from medial to lateral. GRAY’ S ANATOMY FOR STUDENTS, FIGURE 8.120.

 

Stapedius

• Origin: from the pyramidal eminence on the posterior wall of the tympanic cavity.
• Insertion: near the head of the stapes.
• Action: Pulls the stapes posteriorly and tightens up the annular ligament to prevent excessive movement of this tiny bone.
• Innervation: a branch from the facial nerve.

 

 

 

Nerves associated with the tympanic cavity

Facial nerve

• Leaves the cranial cavity and enters the petrous part of the temporal bone through the internal acoustic meatus along with the vestibulocochlear nerve (CN VIII). At the lateral end of the meatus, the facial nerve enters the facial canal.
• Within the facial canal, the nerve passes between the cochlea and vestibule, which are both parts of the internal ear.
• Lateral to the cochlea, the facial canal turns posteriorly, producing a sharp bend in the nerve = the genu (Latin = “knee”, because it is bent like a flexed knee). This is the location of the sensory ganglion of the facial nerve (geniculate ganglion).

 

Figure 29.12 Course of the facial nerve. The petrous temporal bone has been rendered transparent. The view is looking from medial to lateral.

 

• Distal to the genu, the facial nerve moves posteriorly in the facial canal midway between the tympanic cavity and the cochlea. Here the bony prominence ofthe facial canal is located in the upper medial wall of the tympanic cavity.
• Posterior to the tympanic cavity, the facial canal bends inferiorly and descends between the tympanic cavity and mastoid air cells to the stylomastoidforamen, where it exits the skull and enters the parotid gland.

 

Chorda tympani (a branch of the facial nerve)

• Arises from the facial nerve in the facial canal just above the stylomastoid foramen and runs superiorly through the temporal bone to enter the tympanic cavity from its posterior wall.
• Runs across the internal surface of the tympanic membrane from posterior to anterior, where it passes between the handle of the malleus and the long process of the incus.
• It leaves the tympanic cavity through a tiny slit in its anterior wall, the petrotympanic fissure, to enter the infratemporal fossa, where it joins the lingual nerve.
• Chorda tympani carries taste information from the anterior 2/3 of the tongue (special sensory) and preganglionic parasympathetic fibers to the submandibular ganglion (visceral motor).

 

Nerve to stapedius

• Arises from the facial nerve as it descends in the facial canal and supplies the stapedius muscle while it is within the pyramidal eminence.

 

 

 

Greater petrosal nerve

(a branch of the facial nerve)

• Branches from the genu of the facial nerve and travels anterior.
• Leaves the petrous temporal bone through a bony fissure and enters the middle cranial fossa. Traverses the middle cranial fossa under the dura, then enters the foramen lacerum.
• Carries preganglionic parasympathetic fibers to the pterygopalatine ganglion. Postganglionic fibers innervate the lacrimal gland and mucous glands in the nasal cavity.
• Greater petrosal also carries taste

fibers from taste buds in the palate.

 

Figure 29.13 Course of the facial nerve. The medial wall of the tympanic cavity is rendered transparent. The view is looking lateral to medial.

 

Tympanic nerve plexus

Located atop the promontory on the medial wall of the tympanic cavity.

 

Nerve fibers derived from these nerves form the tympanic plexus:

• Tympanic nerve (Jacobsen’s nerve)— arises from the glossopharyngeal nerve in the jugular foramen and enters the tympanic cavity through its floor to join the tympanic plexus. The tympanic nerve carries sensory fibers that innervate the mucosa of the tympanic cavity and preganglionic secretomotor fibers that form the lesser petrosal nerve.
• Postganglionic sympathetic fibers from the internal carotid plexus enter the tympanic cavity through its

anterior wall and join the plexus. These supply smooth muscle in blood vessels.

 

Figure 29.14 Cut away view of tympanic cavity. The view is looking from anterior to posterior.

 

These nerve fibers leave the tympanic plexus:

• Sensory branches (from the tympanic nerve of CN IX) supply the mucosa of the tympanic cavity, inner aspect of the tympanic membrane, mastoid air cells, and the pharyngotympanic tube.
• The lesser petrosal nerve leaves through the roof of the tympanic cavity. It contains preganglionic parasympathetic fibers from CN IX that synapse in the otic ganglion in the infratemporal fossa. From here, postganglionicparasympathetic fibers innervate the parotid gland.

 

Internal ear ( optional reading)

The internal ear contains the organs specialized for (1) reception of sound, and

(2) detection of the position and movement of the head—information needed to maintain balance and equilibrium. Here we will only discuss the basics of internal ear anatomy to get you up to speed inpreparation for more heavy-duty information in your neuroscience course.

 

The internal ear is made up of a series of cavities carved out of the petrous part of the temporal bone (bony labyrinth) containing a network of membrane-lined ducts and sacs (membranous labyrinth).

 

Bony labyrinth

Interconnected hollow cavities within the petrous temporal bone lined by a periosteum and filled with a fluid called perilymph.

 

Components of the bony labyrinth:

• Three semicircular canals (anterior, posterior, and lateral), oriented at right angles to one another.
• Vestibule—a spherical cavity with the oval window in its lateral wall. The vestibule communicates with the semicircular canals and the cochlea.
• Cochlea—a spiral canal shaped like a snail’s shell. It makes two and one-half turns around a central bony core called the modiolus. The round window is located in the lateral wall of the base (first turn) of the cochlea.

 

 

 

 

 

 

 

 

Figure 29.15 Position of bony labyrinth within the petrous part of the temporal bone.

 

Membranous labyrinth

Interconnected series of ducts and sacs constructed of dense connective tissue and lined internally by an epithelium. The membranous labyrinth is suspended within the bony labyrinth and separated from its bony walls by the perilymph.

 

The membranous labyrinth is filled with a fluid called

endolymph.

 

Parts of the membranous labyrinth:

• Three semicircular ducts lie inside the semicircular canals. Where the ducts meet there are enlargements called ampullae. Receptors within the ampullae monitor angular acceleration of the head (side-to-side head rotation).
• Utricle and saccule are within the vestibule of the bony labyrinth. Receptors within the utricle and saccule monitor

(1) static equilibrium (detecting the orientation of the head with respect to the ground when the head is stationary), and (2) linear acceleration (sensing when the body moves in a straight line).

Figure 29.16 The parts of the membranous labyrinth shown within the bony labyrinth.

 

• Stimulation of receptor cells in the vestibular organs mentioned above produces action potentials in neurons of the vestibular division of the vestibulocochlear nerve (CN VIII). These are bipolar neurons with cell bodies in the vestibular ganglion (Scarpa’s ganglion), located within the internal acoustic meatus.
• The cochlear duct occupies a central position within the

bony cochlea. It has a roof called the vestibular membrane and a floor called the basilar membrane. Because of its central location, the cochlear duct (containing endolymph) subdivides the cavity of the cochlea into two perilymph- filled channels: the scala vestibuli above and the scala tympani below. The cochlear duct contains the spiral organ (organ of Corti), where the special receptor cells for hearing are housed.

• Stimulation of receptors in the Organ of Corti produces action potentials in neurons of the cochlear division of the vestibulocochlear nerve (CN VIII). These are bipolar neurons with cell bodies in the cochlear ganglion (spiral ganglion), located in the modiolus.

 

 

Figure 29.17 Cross section of one turn of the cochlea. The scala vestibuli and tympani belong to the bony labyrinth, while the cochlear duct is part of the membranous labyrinth.

 

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WASHINGTON   STATE                                    UNIVERSITY

Elson S. Floyd College of Medicine