Lab 3B
Skull bisection; pharynx, larynx and nasal cavity

The pharynx is divided into 3 parts from superior to inferior: the nasopharynx, the oropharynx, and the laryngopharynx.

Nasopharynx:

Identify the structures of the nasopharynx on the side of the head that does not include the nasal septum.

The hard palate and soft palate separate the oral and nasal cavities, and the tip of the soft palate (the uvula) is the inferior limit of the nasopharynx. Find the opening of the auditory tube, and the raised structure that surrounds it, the torus tubarius. The nasopharyngeal tonsils (adenoids) are in the midline, superior to the torus tubarius, but atrophy later in life so may be difficult to locate.

Oropharynx:

The tip of the uvula divides the nasopharynx and oropharynx. There are two small muscles that run from the soft palate to the oropharynx, and those muscles are covered with folds of mucous membrane. Because the folds are on both sides of the pharynx they form arches. In the space between the two areches is a depression (fossa) that holds the palatine tonsils. The arches are named after the muscles they cover. The anterior arch is the palatoglossal arch, which runs from the soft palate to the tongue. The posterior arch is the palatopharyngeal arch, and runs from the soft palate to the lateral aspect of the oropharynx.

Use small forceps to gently remove the mucus membrane from the lateral aspect of the oropharynx to see the muscles deep to the arches. The palatoglossus m. lies deep to the palatoglossal arch. It runs from the soft palate to the base of the tongue, and functions to elevate the tongue, as in swallowing.

The palatopharyngeus m. is small and thin and lies deep to the mucus membrane of the palatopharyngeal arch. It attaches superiorly to the intersection between soft and hard palate, and inferiorly to the thyroid cartilage. At its inferior attachment it blends with fibers of the stylopharyngeus m. and the salpingopharyngeus m.

The salpingopharyngeus m. lies just deep to the salpingopharyngeal fold. Its superior attachment is the eustachian tube, and inferiorly its fibers blend into the wall of the pharynx.

Do your best to identify the following muscles. They are very small and are easier to find in some cadavers than others

Posterior cricoarytenoid m. - The posterior cricoarytenoid mm. attach to the posterior surface of the cricoid cartilage, and to the arytenoid cartilages. When they contract they rotate the arytenoid cartilages such vocal folds are abducted.

Superior to the posterior cricoarytenoid mm. are two small muscles that are difficult to tell from one another:

Oblique arytenoid mm. - one crosses the other to form an X. They attach to the arytenoid cartilages inferiorly, and then run diagonally and superiorly to eventually connect to the epiglottis. Laterally they are called the aryepiglottic mm.

Transverse arytenoid mm. - run transversly, deep to the oblique arytenoid mm. This is an unpaired muscle that attaches on both ends to an arytenoid cartilage.

Use a sharp scalpel to make an incision through the posterior midline of the cricoid cartilage, between the two posterior cricoarytenoid mm. When you make this cut you will cut the oblique and transverse arytenoid mm. in half.

Spread the cut ends of the cricoid cartilage and identify the vestibular folds (false vocal cords), the vocal folds (true vocal cords) and the ventricle (the space between the vestibular and vocal folds). The space between the left and right vocal cords is the rima glottis.

To view the most lateral muscles of the larynx you’ll need to cut through the cricothyroid joint on one side, and also cut through the thyroid cartilage on that side. That will allow you to rotate the thyroid cartilage anteriorly and expose the lateral aspect of the larynx deep to the thyroid cartilage. The cut through the thyroid cartilage shouldn’t be all the way up to the midline, but about 3/4 of the way between the posterior border of the cartilage and the midline. Superiorly you’ll see the thyroarytenoid m., and inferiorly the cricoarytenoid m.

The thyroarytenoid m. is a broad, thin muscle that actually forms the body of the vocal fold just deep to the mucus membrane of the vocal fold.

Paranasal Sinuses:

Understand the paranasal sinuses (you may not be able to see them all in the skull bisection we have done, but will see more in the imaging module). These sinuses are air-filled extensions of the nasal cavity that are located in the:

• Frontal bone - the frontal sinus. The frontal sinuses are paired, and sometimes connected across the midline

• Sphenoid bone - the sphenoid sinus. There is a single midline sphenoid sinus that sits in the body of the sphenoid bone, just inferior to the sella turcica and medial to the cavernous sinuses (dural venous sinuses).

• Ethmoid bone - the ethmoidal air cells. Sinuses are usually single larger spaces. Air cells are sinuses that are divided into smaller spaces by connective tissue and epithelium.

• Maxillary bone - the maxillary sinus. Paired sinuses just inferior to the orbits and lateral to the nasal cavity. You’ll see these when you dissect the lateral wall of the nasal cavity.

The Nasal Septum

The nasal cavity is divided in the midline by the nasal septum. Identify the following in the half of the head that contains the nasal septum.

The choana (posterior nasal aperture) is the posterior opening between the nasal cavity and the nasopharynx. Each half of the nasal cavity begins at the nostril and ends at the choana.

The nasal septum is formed from 3 structures:

The perpendicular plate of the ethmoid bone superiorly.

The midline vomer bone below. The vomer attaches inferiorly to the nasal crest of the maxillary and palatine bones.

The septal nasal cartilage anteriorly. All of the nasal septum that sticks out from your face is composed of cartilage, with the bone forming the inner (posterior) part of the nasal septum.

Nerves and blood vessels that supply the nasal epithelium of the nasal septum travel between the nasal epithelium and the septum. Carefully peel away the nasal epithelium from the nasal septum and look for the nerves and blood vessels. You may only be able to see their impressions in the septum or the epithelium.

The posterior part of the septum is innervated by the nasopalatine n. (a branch of CN V2) and its blood supply is the sphenopalatine a. and v. The anterior part of the septum is innervated by the anterior ethmoidal n. (a branch of CN V1), and its blood supply is the anterior ethmoidal a. and v.

Now turn your attention to the lateral wall of the nasal cavity. To increase the surface area of the nasal epithelium (which filters and humidifies inhaled air) there are 3 nasal conchae on the lateral wall. Each concha has a core of thin bone, covered by nasal epithelium. There is a space between each of the conchae and the lateral wall of the nasal cavity. That space is called the meatus.

The inferior concha is a separate bone (not part of the ethmoid bone). It’s posterior end is about 1 cm from the opening to the auditory tube. The inferior meatus is the space between the inferior concha and the lateral nasal wall.

The middle concha is part of the ethmoid bone, and the middle meatus is the space between the middle concha and the lateral nasal wall.

The superior concha is also part of the ethmoid bone. The superior concha is just anterior to the sphenoid sinus, and is covered by the olfactory epithelium (contain the olfactory receptors). The superior meatus is the space between the superior concha and the lateral nasal wall.

Now use scissors to cut away the inferior nasal concha. In the rostral (toward the nose) part of the inferior meatus look for the opening of the nasolacrimal duct (tear duct). Tears drain from the medial canthus of the eye, through the nasolacrimal duct, and empty onto the nasal epithelium that lines the inferior meatus.

Now cut away the middle nasal concha. In the middle meatus you’ll see a curved slit, the hiatus semilunaris (or semilunar hiatus). The maxillary sinus communicates with the nasal cavity through the hiatus semilunaris. At the anterior margin of the hiatus, look for an additional opening that connects the frontal sinus to the nasal cavity.

Posterior to the hiatus semilunaris is a raised structure, the ethmoidal bulla. The ethmoid air cells are just lateral to the bulla, and the bulla has small holes in it that allow communication between the ethmoid air cells and the nasal cavity.

Finally, look for an opening from the sphenoid sinus into the most superior part of the nasal cavity, the sphenoethmoidal recess.

Now peel away the remaining mucus membrane from the lateral nasal wall, posterior to the middle and inferior conchae. Posterior to the conchae, but anterior to the torus tubarius, look for a raised region of translucent bone that runs vertically, toward the junction of the hard and soft palate. That thin bone is the medial wall of the palatine canal, which contains the greater and lesser palatine nn., which arise from the pterygopalatine ganglion at the top of the canal.

Use forceps or rongeurs to break through the thin bone covering the canal. The contents of the canal (nerves and ganglia) are covered in a periosteal sheath. Carefully cut the sheath to reveal the palatine nn. Work your way superiorly along the canal to find the pterygopalatine ganglion (parasympathetic) at the top of the canal.

The greater and lesser palatine nn. give branches off to the nasal mucosa that covers the lateral wall of the nasal cavity, and then pass through the greater palatine foramen and lesser palatine foramen. The greater palatine n. supplies the mucus membrane of the roof of the mouth anterior to the palatine foraminae, and the lesser palatine n. supplies the mucus membrane of the roof of the mouth posterior to the palatine foraminae. The nerves are accompanied by the greater palatine a. and v., and the lesser palatine a. and v.

Remove the remaining mucosa from the lateral wall of the nasal cavity, and break through that thin bone into the maxillary sinus. Open the medial wall of the sinus broadly to allow a full view of the sinus. Realize that the roof of the maxillary sinus is the floor of the orbit. Within the thin bone of the roof of the maxillary sinus look for a channel (similar to the the palatine canal) that runs from posterior to anterior, the infraorbital canal. The infraorbital n, a., and v. run in the infraorbital canal. Open the floor of the canal with forceps, and trace the nerve back to the foramen rotundum. The infraorbital n. , a., and v. divide to form anterior, middle, and posterior superior alveolar nn., aa., and vv., which supply sensory innervation and blood supply to the maxillary teeth. Look at the floor of the maxillary sinus, you may see roots of the maxillary teeth. The infraorbital n. continues through the infraorbital foramen to supply general sensory innervation to the skin of the face (dissected in a previous lab).