Head and Neck Lab 3A

Lab 3A: Continue Posterior Pharynx, Skull Bisection, Larynx and Pharynx

Lab Objectives

Review the posterior pharynx
Bissect the skull into sagittal sections
Review the larynx and the structures involved with phonation
Review the pharynx and its subregions

Short List of Structures - 3A

Posterior Pharynx

The following steps regarding the posterior pharynx should not take much time since group B worked on this previously, but you'll want to make sure you can see all of the following structures, which may require some adiditional dissection:

Posterior belly of digastric m. - the posterior belly of the digastric m. runs right next to the stylohyoid m., though the digastric m. attaches to the mastoid process, not the styloid process.

The stylopharyneus m. is a fairly thin, cylindrical muscle that attaches to the styloid process of the temporal bone, runs superficial to the superior constrictor, and then passes between the superior and middle constrictors before attaching to the thyroid cartilage. Running with the stylopharyngeus m., often on its deep surface, is a small branch of the glossopharyngeal n. (CN IX), which supplies the motor innervation to the stylopharyngeus.

The remaining structures you should look for are things you have seen before, but you will be able to get a more complete view of them now that the cervical viscera is more accessible:

Stylohyoid m. - follow the stylopharyngeus muscle superiorly to the styloid process. Then find the stylohyoid m. also attached to the styloid process, but going anteriorly to attach to the hyoid bone.

Take time to separate and clean up the common carotid and its major branches.

The internal carotid aa. have no branches in the neck, and will pass through the carotid canals of the skull to supply blood to the anterior circulation of the brain.

A swelling at the base of the internal carotid a., the carotid sinus, contains stretch receptors (= baroreceptors) that serve to monitor blood pressure (via the baroreceptor reflex).

Hypoglossal n. - follow CN XII toward the base of the skull, see how close you can get to the hypoglossal canal. From here, the hypoglossal nerves crosses superficially to the carotid artery and then deep and superior to the anterior belly of the digastric.

Spinal accessory n. - search for the spinal accessory nerve on the deep surface of the SCM and trapezius mm.

Vagus n. - recall that the vagus n. runs in the carotid sheath with the carotid a. and internal jugular v., so you will find it in close association with those structures. Also look for the inferior ganglion of the vagus n., a few centimeters inferior to the jugular foramen. The inferior ganglion of the vagus contains the neuron cell bodies of visceral afferents (visceral sensory) neurons, it’s analogous to the dorsal root ganglion of spinal nerves.

Recurrent laryngeal nn. - see if you can find these as they pass underneath the inferior pharyngeal constrictor m. and remember that this branch of the vagus nerve can be seen in the thorax as it recurs and loops around some of the great vessels.

Bisection of the skull (this will take more than half of the lab time)

When bisecting the head, try to avoid cutting through the nasal septum but cutting through the cribiform very slightly off the midline (as in the figure to the right). Don't go too lateral!
Place the cadaver in the supine position and tilt the head forward towards the thorax. You may find it helpful to place a block across the chest to brace the head.
Use a hacksaw to cut through the remaining frontal bone until you come to the crista galli and the cribiform plate.
Continue your cut through the ethmoid bone, keeping the cut on the side with the wider nasal passage.
Keep cutting through the cribiform plate, the body of the sphenoid bone and the bones of the middle and posterior fossa, and through the maxilla.
STOP cutting as soon as you get through the maxilla and hard palate so you don’t damage the soft tissue of the mouth with the saw.
The two halves of the head should seprate slightly from each other and you will be able to see the tongue, cut through the tongue with a scalpel, but do not cut the epiglottis.
Now tilt the head back to the normal supine position and saw through the midline of the mandible and lower teeth. STOP as soon as you get through the mandible and lower teeth.
- Note: if you find strong resistance to sawing through maxilla or mandible there may be a tooth plate for dentures. Remove denture plates through the mouth. You may wish to check for dentures before you start sawing.
Use a scalpel to cut directly through the midline of the soft tissue and muscle of the tongue, back to where the tongue meets the epiglottis. This will leave the head attached to the larynx on either side with the larynx and epiglottis INTACT.
The final steps are detailed in the next section, half of the class should do each step, since we evenly divide the class with male and female cadavers to visualize the anatomy of the pelvis, we will use that as a guide..

Dissection of the Larynx

If you have a MALE cadaver:

Your final dissection will look like the image on the top right.

You have stopped cutting and sawing through the head and the larynx and epiglottis is intact, but now you will cut through the midline of those structures and split the head and larynx to see the full extent of the larynx and vocal/vestibular folds.

Continue the bisection part way down the trachea.

You can now 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.

You should also look for the cut edges of the different types of cartilage:

If you have a FEMALE cadaver:

Your final dissection will look like the image on the bottom right, but the head will be split, unlike in that image.

You have stopped cutting and sawing through the head and the larynx and epiglottis is intact and you will keep the larynx and epiglottis intact.

Place a flat instrument in the laryngopharynx and esophagus to use as a guide— from here, use a scalpel to cut through the middle of the pharyngeal constrictors along the guide and extend this cut inferiorly to the esophagus.

You should now see the 3D structure of the larynx as it out pouches into the pharynx.

Do your best to identify the following muscles by removing the layer of mucosa that covers the larynx. 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 that the vocal folds are abducted for inhalation of air.

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 transversely, deep to the oblique arytenoid mm. This is an unpaired muscle that attaches on both ends to an arytenoid cartilage.

Dissection image for MALE cadavers

Dissection image for FEMALE cadavers

*Be sure to walk around and check out the tables who completed the alternate dissection*

Pharynx

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.

Figure 2.2 1 - uvula, 2 - palatoglossal arch, 3 - palatine tonsils, 4 - tongue, 5 - pharyngeal tonsils.

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.

Figure 2.3 Mid-sagittal view of the pharynx with the mucosa removed to reveal the muscles of the pharynx.

Figure 2.4 A more detailed mid-sagittal view of the pharynx with the mucosa removed to reveal the muscles of the pharynx.

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