Lab Manual - Ear, Nasal Cavity, and Palate

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. Define the three parts of the ear and the function of each part. (explanation)
  2. Describe each of the four walls of the middle ear cavity and identify deeper structures responsible for certain of their features. (explanation)
  3. Describe the structure and actions of the tympanic membrane, the auditory ossicles, and the muscles of the middle ear. (explanation)
  4. Trace the course of the facial nerve through the temporal bone and give the origin, course, and functional components of each of its intracranial branches. (explanation)
  5. Identify the auditory tube and explain its function. (explanation)
  6. Describe the maxillary nerve, its distribution and functional significance. (explanation)
  7. Describe the nasal cavity, its general morphology including walls, openings, nasal septum, conchae, meatuses, and its general neurovascular supply. (explanation)
  8. List the paranasal sinuses and where each opens into the nasal cavity. (explanation)
  9. Describe the hard and soft palate. (explanation)

Readings and Modules:

Procedure:

1. Review the osteology of the temporal bone and nasal cavity. (Play movie; View images: N 2, 4A, 4B, 6A, 6B, 8, 9, 11, 48, 49, 55, 63, 64, 65, 67, 92, 94A, 94B, TG 7-03, 7-04, 7-06, 7-07, 7-08, 7-21, 7-22, 7-23, 7-24, 7-32, 7-41, 7-44, 7-65, 7-67B, 7-67C, 7-68A, 7-68B, 7-70)

Locate on the skull: tegmen tympani, arcuate eminence (anterior semicircular canal), external acoustic meatus, internal acoustic meatus, bony auditory tube and semicanal for the tensor tympani muscle, hiatus for the greater petrosal nerve, hiatus for the lesser petrosal nerve, petrotympanic fissure and inferior tympanic canaliculus. Look into the external acoustic meatus to see, on the medial wall of the tympanic cavity, the promontory, vestibular (oval) window, and cochlear (round) window; it may be possible to see the tympanic opening of the bony auditory tube and the prominence of the facial canal. Observe the close proximity of the external acoustic meatus to the mandibular fossa; note on yourself that movement of the mandibular condyle can be detected by a finger in the external acoustic meatus.

Locate on a skull the pterygopalatine fossa, sphenopalatine foramen, nasal aperture, nasal bones, medial pterygoid plate, pterygoid hamulus, and maxilla. On a partial or sagittally-sectioned skull, identify the bony septum of vomer and ethmoid (perpendicular plate). Note sphenoid sinus, frontal sinus, maxillary sinus, conchae, ethmoidal air cells, and hard palate.

The ear consists of outer, middle, and inner parts. The outer ear is a bony and cartilaginous tube that funnels sound to the tympanic membrane. The middle ear, the only part to be studied in detail, is an air-filled bony cavity containing three auditory ossicles that transmit the vibrations of the tympanic membrane to the inner ear. This in turn is a complex fluid-filled membranous sac (membranous labyrinth) lodged in a similarly complex excavation (bony labyrinth) within the petrous ("rock-like") part of the temporal bone; the membranous labyrinth contains the sensory epithelia for not only hearing but also balance; it is innervated by the vestibulocochlear nerve.

The middle ear or tympanum ("drum") is made up of the middle ear cavity (tympanic cavity) plus the tympanic membrane - "eardrum" in casual speech - occupying its lateral wall. Since the cavity develops as an outpocketing of the nasopharynx, it is lined by a mucosa which forms the inner layer of the tympanic membrane (the outer layer is skin); the mucosa also covers the two nerves, two tendons and three bones that are the principal contents of the cavity. The tympanic cavity retains its pharyngeal connection as the auditory (eustachian) tube which, if not blocked, assures atmospheric pressure on both sides of the tympanic membrane; unfortunately, it also is a route for infection to reach the middle ear (otitis media) and even pass beyond it to the mastoid air cells.

2. On the decalcified temporal bone, identify the structures of the anterolateral surface of the middle ear cavity. (Play movie; View images: N 9, 10, 11, 86, 92, 93C, 93D, 94A, 94B, 97, 123, 125, TG 7-07, 7-65, 7-67, 7-68A, 7-68B, 7-84, 7-87, 7-90)

Each table group will be provided with a temporal bone that has been soaked in acid to decalcify the bone so that the region can be dissected with a scalpel. This specimen has been cleft in two along the auditory tube and between the incus and stapes, producing an anterolateral and posteromedial piece (see Figure 7). Be sure to examine other specimens to see all of the structures mentioned below.

Carefully elevate the dura and identify the greater petrosal nerve on the anterior slope of the petrous part of the temporal bone. Reflect the trigeminal nerve and its ganglion laterally to see the greater petrosal nerve pass toward foramen lacerum. Look more laterally for the smaller lesser petrosal nerve passing toward foramen ovale. Read about its origin in the tympanic plexus that is formed on the promontory by the tympanic branch of the glossopharyngeal nerve.

Figure 7

Snip away the squamous part of the temporal bone so that the posterior floor of the middle cranial fossa is accessible. Cut away the roof of the external acoustic meatus - it contains fine hairs and possible cerumen (earwax) - as far as the tympanic membrane.

Shave through the thin tegmen tympani, located immediately lateral to the arcuate eminence (i.e., on the line connecting the internal and external acoustic meati), to open the epitympanic recess. Identify there in anterior-to-posterior sequences the head of the malleus, its articulation with the body of the incus, and the short process of the incus fitting into the incudal fossa. Work posteriorly to open the roof of the mastoid antrum and observe its entrance from the epitympanic recess, the aditus ad antrum.

Cut away the roof of the internal acoustic meatus to expose the facial nerve. Watch it turn abruptly backwards to run in the facial canal, near the upper medial border of the tympanic cavity, before arching down toward the stylomastoid foramen. At the bend (genu, "knee") of the nerve, locate the geniculate ganglion. Although the greater petrosal nerve arises at this point, it has no functional connection with this sensory ganglion.

Identify the external acoustic meatus and the petrous ridge. Posterior to the ridge find cranial nerves VII and VIII entering the internal acoustic meatus. Inferior to these notice the jugular bulb. Locate the arcuate eminence on the petrous ridge; using a sharp scapel shave through the eminence to expose the anterior semicircular canal. The canal will appear as two small holes in the bone separated by 2-3 mm. in the sectioned bone. As you continue shaving away the bone anterolateral to the eminence, you will open the epitympanic recess of the middle ear cavity. Within the recess locate the head of the malleus as it articulates with the body of the incus. Examine the incus and identify the short, posteriorly directed short and long processes of the incus and note the long process articulating with the head of the stapes. On the anterior surface of the block locate the auditory tube. The cartilage has the characteristic inverted "J" shape.

On the anterolateral piece identify:

tympanic membrane
auditory tube
incus and malleus
tensor tympani m.
chorda tympani
mastoid antrum & air cells

Attempt to see the long process of the incus descend toward its articulation with the head of the stapes. Try to identify the posterior incudal ligament anchoring the incus in the incudal fossa. Look for the chorda tympani arching forward just medial to the handle of the malleus (chorda tympani leaves the skull through the petrotympanic fissure); the tendon of the tensor tympani muscle passing laterally from the cochlear form process to the handle of the malleus.

Examine the attachment of the handle of the malleus to the tympanic membrane. Understand that the auditory ossicles act as a lever system transmitting vibrations of the membrane to the vestibular (oval) window in which the footplate of the stapes is lodged. The lever is in the form of an inverted V and its axis - the line from the anterior process of the malleus to the short process of the incus - passes through the point of the V.

3. Identify the structures of the posteromedial wall of the middle ear cavity. Review the nerves related to the temporal bone and middle ear. (Play movie; View images: N 8, 9, 11, 82, 86, 92, 93C, 93D, 94A, 94B, 95, 96, 97, 102, 103, 104, 123, 131, 136, TG 7-06, 7-07, 7-08, 7-51, 7-60, 7-65, 7-66, 7-67B, 7-67C, 7-68A, 7-68B, 7-69A, 7-69C, 7-69D, 7-70A, 7-70B, 7-87AB, 7-87C)

On the posteromedial piece identify:

promontory
tendon of stapedius m.
stapes & vestibular (oval) window
cochlear (round) window
canal for facial nerve

Look for the short tendon of the stapedius muscle extending forward from the pyramidal eminence to the neck of the stapes.

Return to the superior surface of the posterolateral specimen and continue shaving bone off the surface to expose the facial and vestibulocochlear nerves within the canal of the internal acoustic meatus. In the plane of this canal, but anterolateral to it, locate the cochlea. Lateral and posterolateral to the canal you will pass through the semicircular canals and the vestibule of the internal ear. Continue shaving away bone to expose the further course of the facial nerve and the geniculate ganglion. Try to locate the greater petrosal nerve as it leaves the ganglion heading for the anterior cranial fossa. Note the close proximity of the ear structures to the carotid canal.

4. Examine the nasal cavity, its regions and internal structures. (Play movie; View images: N 1, 40, 41, 43, 46, 47, 125, TG 7-03, 7-41A, 7-41B, 7-43, 7-45, 7-77)

In the nasal cavity, examine the nasal septum. Observe that it consists of cartilaginous and bony parts. Note the short stumps of the olfactory nerve (CN I) in the superior third of the septum and lateral nasal walls. The arteries and nerves of the nasal mucosa are tiny and difficult to dissect. Observe in your text or atlas the complexity of their arrangement. Identify the nares (anterior nasal openings), vestibule and conchae. On the lateral nasal wall find: the superior, middle and inferior nasal conchae; the superior, middle and inferior meatuses; the sphenoethmoidal recess. Consider the structure and functions of conchae.

Surface anatomy of the nose
Surface anatomy of the paranasal sinuses
MRI of the nasal cavity

5. Remove the conchae on one side to expose the meatuses, the ethmoidal bulla and hiatus semilunaris and the openings of the sinuses. (Play movie; View images: N 41A, 41B, 41C, 42A, 42B, 44, 45, 46, 47, 52, 53, 73, 106, TG 7-19, 7-35, 7-43A, 7-43B, 7-44, 7-45AB, 7-45CD, 7-72)

With scissors, cut away the conchae from their points of attachment on one side of the skull. In the inferior meatus, locate the opening of the nasolacrimal duct. Remove the bony covering of the medial side of the duct and trace it to the lacrimal sac within the fossa of the lacrimal sac of the orbit. In the middle meatus, note the semilunar hiatus and, just above it, the ethmoidal bulla. With a probe explore the infundibulum (superior part of semilunar hiatus) and locate the frontonasal duct, which drains the frontal sinus. Explore the frontal sinus. Also note opening of anterior ethmoidal air cells in the semilunar hiatus. In the posteroinferior part of the semilunar hiatus locate the opening of the maxillary sinus. On the surface of the ethmoidal bulla probe the middle ethmoidal air cells. Now lift or remove the superior concha and, in the superior meatus, locate an opening to the posterior ethmoidal air cells. In the sphenoethmoidal recess probe an opening to the sphenoid sinus. Examine the sphenoid sinus. Note the mucoperiosteal lining and its continuity with the nasal cavity. The frontal and sphenoid sinuses may have septa (sagittal) that may make them appear shallow or nonexistent. Note the relation of the sphenoid sinus to the hypophysis and the internal carotid artery. Remove the lateral wall of the nasal cavity on one side of the head and explore the maxillary sinus and the ethmoidal air cells. Consider drainage of sinuses and air cells.

Skull structures

While removing the mucoperiosteum of the lateral wall, note its blood supply. Most comes through the sphenopalatine foramen from the sphenopalatine artery, a terminal branch of the maxillary artery, but some comes from the anterior and posterior ethmoidal arteries. Note again short stumps of the olfactory nerves (I) passing through the cribriform plate of the ethmoid.

6. Examine the muscles of the soft palate associated with the auditory tube. (Play movie; View images: N 37A, 37B, 37C, 63, 64, 52, 63, 64A, 64B, 68, 69, TG 7-06, 7-22, 7-23, 7-24, 7-39, 7-67)

In the roof of the mouth identify the soft palate, uvula, hard palate; palpate the hamulus of the medial pterygoid plate.

In the nasopharynx identify: opening of auditory tube, torus tubarius, pharyngeal tonsil and pharyngeal recess.

Expose the levator veli palatini muscle. Find the tensor veli palatini muscle anterior and deep to the levator veli palatini and then expose its insertion by removing the mucous membrane from the inferior surface of the soft palate. Note its relationships to the hamulus and the palatine aponeurosis. What is the primary function of the tensor veli palatini?

Skull structures