PROCEDURE - ORAL CAVITY

1. PRE-WORK (before lab) review the mandible and oral cavity region of the human skull from your atlas or a skull in the lab.
    (Netter011) (Netter013) (Netter015)

Mandible - The body of the mandible extends laterally and posteriorly to each side forming a horseshoe-like shape holding the inferior teeth.  On the inside of the mentum (tip of the chin) are several attachment points for muscles

These are the genioglossus forming much of the tongue, and the roughly central geniohyoid, anterior belly of the digastric, as well as the wider mylohyoid extending along the inside or the mandible to each side. We'll examine these below  

Palate - On the base of the skull between the rows of upper teeth is the horizontal process of the palatine bone and the palatine process of the maxilla.

These contributes to the anterior roof of the oral cavity and the floor of the nasal cavity.

Pharynx - The pharynx is a shared air and food space subdivided into the nasopharynx (region posterior to the nasal cavity), oropharynx (region posterior to the oral cavity, and laryngopharynx (region posterior to the larynx).

2. Fold back the blue drape, but leave the plastic body bag closed  (i.e. don't expose the donor).

These donors are under dissection by the Dental program and keeping the donors covered while we work prevents dehydration.

3. Remove the hemi-head from the translucent organ box under the table and use one of the trays to form a surface on top of the donors covered legs as your work area.
    (Photo7170)

Note, the nasal passages have been dissected by the MS2 students in their airways sessions removing most of those structures.

The nasal passages are a region you will study in the respiratory section of the MS2 CRP course.

4. Observe the tongue, the posterior region of the tongue forms the anterior wall of the oropharynx.
    (Figure778) (Netter077) (Netter071) (Photo7170)

5. Depress the tongue such that you can examine the superior surface of the tongue for the location of the terminal sulcus.
    (Figure778) (Netter077) (Netter071) (Photo7171)

The division between the anterior 2/3rd and posterior 1/3rd is marked by a groove, the terminal sulcus. This groove can be more or less prominent on different individuals.

The anterior 2/3rd of the tongue is the oral region (often called the body, taste innervation from CN7, facial nerve), the posterior 1/3rd is the pharyngeal region (often called the root, taste innervation from CN9, glossopharyngeal nerve).

In the midline of the dorsum of the tongue, separating the tongue into left and right, is the shallow groove of the median sulcus.

This is not visible in the hemi-head as the centerline cut passes through the median sulcus. It is visible if a lab member protrudes their tongue (or you use a mirror later) to observe the median sulcus in the living.

6. Examine the surface of the tongue for the circumvallate papillae.
    (Figure778) (Netter077) (Netter071) (Photo7171)

The most prominent of the papillae types are the row of circumvallate papillae just anterior to the terminal sulcus. These are usually a single row of large papillae (several millimeters in size).

The other papilla types do not preserve well post-mortem and are better observed on a living tongue.

The other papillae on the tongue surface are the sharp pointed filiform papillae (purely mechanical function, no taste buds on these).

The third type on the tongue surface are the 'mushroom head' shape fungiform papillae (less than a millimeter in diameter). These are visible on the living tongue as slightly redder spots due to the thin keratin covering the fungiform papillae compared to the thicker keratin forming the surface of the filiform papillae. 

7. Examine the base of the tongue just posterior to the terminal sulcus for a series of bumps or 'roughening' of the surface which are the lingual tonsils.
    (Figure778) (Figure729) (Netter071) (Netter077)
    (Photo7171)

These bumps are the dispersed nodules of the lingual tonsils. The lingual tonsils typically have a less defined arrangement and smaller individual size than the palatine tonsils. Lingual tonsils often consist of a singular midline patch of tissue rather than distinct left/right tonsils.

8. Identify the cut surface of the mandible at midline and the cut surface of the hyoid bone about 1cm posterior to the mandible.
    (Netter077) (Photo7084)

Since the hyoid bone is quite small, palpating for a hard bony point in a line posterior to the mandible should locate it.

While the airways are continuous from oral/nasal to the lungs, the level of the hyoid bone demarcates the boundary for the oropharynx superiorly and laryngopharynx inferiorly.

9. Gently dissect the superficial fascia deep to the skin inferior to the mandible and identify the cross-section of the thin mylohyoid muscle.
    (Netter077) (Photo7084)

The mylohyoid is the most superficial muscle spanning from the mandible to the hyoid bone at the midline and extends laterally forming the floor of the mouth.

The anterior belly of the digastric muscle is superficial to the mylohyoid muscle, but at a slightly more lateral position.  Thus, at the midline the digastric muscle is not yet visible.

10. Dissect gently deep to the mylohyoid muscle for fibers of the geniohyoid muscle which span from the mandible to the hyoid bone.
    (Netter077) (Photo7084)

The fibers of the geniohyoid muscle blend into the genioglossus muscle of the tongue along the superior aspect, however, they are defined by whether they insert onto the hyoid bone. If the fibers insert onto the hyoid bone they are part of the geniohyoid. Contraction pulls the hyoid bone anteriorly.

11. Observe the muscle fibers of the genioglossus muscle which originate at the mandible and arch 'fan-like' to make up most of the body of the tongue.
    (Netter077) (Photo7084)

The genioglossus muscle has a single bony attachment to the inside of the mandible. The other ends of the muscle anchor into the connective tissue of the tongue. Fibers of the genioglossus muscle constitute the majority of the mass of the tongue.

The genioglossus muscle can flatten and protrude the tongue by pulling the body of the tongue anteriorly towards the mentum, which extrudes the tip of the tongue past the lips.

All the muscles that have a bony anchor and insert into the connective tissue framework of the tongue are the extrinsic muscles of the tongue. These have glossus, meaning 'of the tongue', in their name (genioglossus, hyoglossus, styloglossus and palatoglossus muscles).

The only extrinsic muscle of the tongue not innervated by the hypoglossal nerve (XII) is the palatoglossus muscle, which receives innervation from branches of the Vagus nerve (X).

12. Examine the dorsal and anterior (tip) of the tongue where muscle fibers are with orientations are intermixed.
    (Netter077) (Netter068) (Photo7084)

These are fibers of the intrinsic muscles of the tongue which originate and insert within the connective tissue of the tongue (i.e. they have no bony attachments).

The muscles consist of interwoven small fascicles of muscle and are not easily identifiable as discrete muscles at the gross level.

The intrinsic muscles are the vertical, transverse, superior longitudinal, and inferior longitudinal fascicles (named by their direction).

The four intrinsic muscles of the tongue (vertical, transverse, superior longitudinal, and inferior longitudinal fascicles) and three of the four extrinsic muscles of the tongue (genioglossus, hyoglossus, and styloglossus muscles) are all innervated by the hypoglossal nerve (XII).

13. Depress and retract the body of the tongue medially so that you can examine the lateral wall for the palatoglossal and palatopharyngeal arches.
    (Figure729a) (Netter065) (Netter072) (Photo7135)

Palatoglossal arch - The palatoglossal arch is the more anterior arch formed by the mucosa 'bump' over the palatoglossal muscle. As the name implies, this spans from the posterior palate inferiorly into the tongue.

Palatopharyngeal arch - The palatopharyngeal arch is the more posterior arch formed by mucosa over the palatopharyngeal muscle. This runs from the palate inferiorly into the pharynx, acting to lift the pharynx during swallowing.

The palatopharyngeal arch is often not as distinct as the palatoglossal arch in our donors due to the relaxed musculature post-mortem.

14. Examine the region between the palatoglossal and palatopharyngeal arches for a lump like structure that will be the palatine tonsil.
    (Figure729a) (Netter065) (Netter072) (Photo7135)

Note, in the age group of our donors when they were children it was standard of care in tonsillitis to perform a tonsillectomy. Thus, it is likely that in many/most of our donors the bulk or entirety of their palatine tonsils have been removed and only some small mucosal folds (scar tissue) may be present.

Due to the deep cleft arrangement of the palatine tonsil microstructure, palatine tonsils more readily entrap food/bacteria compared to other tonsils. Thus, the palatine tonsils are the most likely to become infected causing tonsillitis.

15. Using Iris (sharp point) scissors separate the mucosa just superior to the epiglottis to expose the main branch of the glossopharyngeal (IX) nerve
    (Figure733) (Netter072) (Photo7183) (Photo7184)

Spreading mucosa parallel to the expected position of the nerve with the sharp tips of the Iris scissors is an optimal approach. The nerve can sometimes be sufficiently prominent that position of the nerve can be observed with the mucosa in place.

The glossopharyngeal (IX) nerve is the primary sensory nerve supply to the posterior tongue and pharynx. The primary motor nerve to the tongue is the hypoglossal (XII) nerve and the primary motor nerve to the pharynx is the vagus (X) nerve.

A radical or aggressive approach to tonsillectomy can put the glossopharyngeal nerve at risk. Additionally there is a rich vascular network underlying the palatine tonsils that can be a bleeding risk for the procedure. 

16. Using a blunt probe, identify the extent of the hard palate (bony) located in the anterior roof of the mouth and the soft palate (muscular) located at the posterior roof of the mount.
    (Netter065) (Netter077) (Photo7084) (Photo7183)

The centerline of the soft palate ends in a variably sized protrusion of mucosa covered muscle (musculus uvulae) collectively called the uvula. The function of the soft palate and uvula are to close off the nasal passages during swallowing.

The soft palate is tensed and elevated during swallowing by a pair of small muscles called the tensor veli palatini and the levator veli palatini respectively. These are within and lateral to the palate and not observable.

17. Examine the cut midline edge of the posterior pharyngeal wall and observe the posterior wall is formed by a thin layer of muscle fibers.
    (Figure732b) (Netter077) (Netter078) (Photo7172)

These are the constrictor muscles forming the outer layer of pharyngeal muscle. These muscles are designated superior, middle, and inferior constrictors based upon relative location. However, these are essentially a continuously muscular wall with no gaps between these muscles (other wise food would escape!).

Inferior to the larynx the constrictor muscles become contiguous with the muscle wall of the esophagus.

Although not apparent in a hemi-head, the left and right side constrictor muscles meet at the a midline raphe. Raphe is a general term for a union of similar tissues, usually muscle, that connect without the presence of an intermediate tendon or aponeurosis.

We will not examine the inner layer of pharyngeal muscles (palatopharyngeus, salpingopharyngeus, and stylopharyngeus). These inner vertically orientated pharyngeal muscles lift the pharynx to close of the larynx during swallowing, while the outer circular muscles of the constrictors primarily propel food inferiorly into the esophagus.

18. Follow the posterior wall of the pharynx superiorly to the most posterosuperior recess where the nasopharyngeal tonsils are located.
    (Figure729) (Netter077) (Photo7172)

This posterosuperior recess houses the nasopharyngeal tonsils (commonly known as the adenoids) in the posterior wall of the pharynx. Unless inflamed, these tonsils are not visually distinct.

19. Work inferiorly to a mucosal bump and opening located just posterosuperior to the soft palate.
    (Figure729) (Netter077) (Photo7172)

This bump is the torus tubarius formed by the cartilage of the Eustachian tube. The tubal tonsils are located in the mucosal wall just posterior to the opening of the Eustachian tube. 

20. Return to the base of the tongue and examine inferiorly on the anterior wall of the laryngopharynx for the curved 'paddle' of the epiglottis.
    (Netter065) (Netter077) (Photo7084)

During swallowing the epiglottis is pressed over the opening of the larynx to prevent food or liquids from entering the trachea.

21. Push your finger over the top of the epiglottis down to the lateral side where food is diverted into the piriform recess before continuing into the esophagus.
    (Netter079) (Photo7136)

The piriform recess is a common location for sharp items ingested to become lodged in the pathway (e.g. fish bones).

PROCEDURE - RETROPHAYRNGEAL

22. Starting slightly inferior to the level of the skull, push a blunt probe into the fascia between the posterior wall of the naso- and oropharynx and the anterior surface of the vertebral bodies.
    (Figure732a) (Netter072) (Netter078) (Photo7087)

This separates the posterior wall of the pharynx from the prevertebral fascia surrounding the musculoskeletal pedestal of the neck (vertebrae and neck muscles).

 From inside to outside the major layers are the mucosal surface, the sheet of constrictor musculature, the buccopharyngeal fascia against the posterior side of the constrictor muscles, the prevertebral fascia against the neck muscles, and then the muscloskeletal pedestal of the neck.

23. Work the probe inferiorly along the anterior surface of the vertebral bodies separating the posterior wall of the pharynx all the way to the inferior cut surface of the hemi-head.
    (Figure732a) (Netter072) (Netter078) (Photo7087)

These layers separates very easily forming a 'gap' between them which is the retropharyngeal space.

The retropharyngeal space is a potential space bounded anteriorly by the buccopharyngeal fascia (against the posterior side of the constrictor muscles) and posteriorly by the prevertebral fascia (against the vertebral bodies).

The retropharyngeal space is continuous from the base of the skull into the mediastinum. Thus, serious oral infections (e.g. a tooth abscess) that enters the retropharyngeal space can readily spread inferiorly into the mediastinum.

Some texts will describe an alar fascia layer as a separate layer just anterior to the prevertebral fascia. However, the nuances of fascial layers of this region of the neck are for specialists.

24. With the retropharyngeal space opened, dissect away part of the fascia against the posterior wall of the pharynx to find branches of a nerve plexus.
    (Figure737) (Netter089) (Netter082) (Photo7088)

These are nerve fibers of the pharyngeal plexus that is formed by branches of the vagus, glossopharyngeal, and sympathetic chain.

Anterior to the pharyngeal nerve plexus lie the pharyngeal muscles innervated by branches from the plexus.

25. Search further into the retropharyngeal space to the lateral side of the pharynx where there will be superior to inferior oriented elements of the sympathetic chain.
    (Figure736) (Netter141) (Photo7089)

The cervical sympathetic trunk lies against the anterior side of the prevertebral muscles and the posterior side of the pharynx close to the internal carotid artery.

If you are careful you may observe nerve branches emerging from the sympathetic chain into the buccopharyngeal fascia to join the pharyngeal plexus.

The trunk consists of a superior, middle, and inferior cervical ganglia, however, only the superior cervical ganglion will be dissected in this lab (typically the middle and inferior ganglia are more inferior than the hemi-head cut through the neck). 

Superior cervical ganglia: The superior cervical ganglion is the largest and lies on the level of C2. It is shaped like an elongated or flattened football and can be as large as 2-4cm in length.

Middle cervical ganglia: The middle ganglion at approximately C6 is small and in some cases absent.

Inferior cervical ganglia: The inferior ganglion at approximately C7, on or in close proximity to the neck of the first rib, is also small and is frequently fused with the ganglion of T1 (a fused ganglia is called the cervicothoracic or more commonly the stellate ganglion).

Branches arising from the superior cervical ganglia spread out following the external carotid artery branches onto the face (to blood vessels and sweat glands) and other branches follow the internal carotid artery into the skull (to deeper head structures such as blood vessels and the dilator pupillae of the eye)

The cervical sympathetic ganglia also give rise to the three cardiac branches of the sympathetic trunk (superior, middle, and inferior cardiac nerves) that descend into the thorax to distribute fibers to the cardiac plexus and heart.

PROCEDURE - SUPRAHYOID NECK

26. Return to the cut surface of the mandible and start to blunt dissect into the subcutaneous tissue between the skin and the mylohyoid muscle.
    (Photo7173)

27. Using scissors of a scalpel start transect the skin directly anterior to the mandible bone and begin to reflect the skin laterally.
    (Photo7174) (Photo7175)

While reflecting the skin and subcutaneous tissue occasional fibers of the platysma muscle might be observed. We will reflect these with the skin if present.

The platysma muscle is one of the muscles of facial expression and consists of a thin sheet of muscle fibers oriented superior to inferior 'draping' from the mandible down to the clavicle.

The muscle functions to tighten/wrinkle the skin of the neck during speech or head/neck movement and has a minor role in opening the jaw.

This muscle frequently becomes less distinct with age, contributing to the looser skin tone of elderly individuals. Thus, in our donors there may only be residual fibers present in the subcutaneous tissue. In some individuals the muscle bands become discontinuous resulting in vertical tension bands of the skin of the neck (called platysmal bands).

28. Continue reflecting skin of the neck laterally away from the midline of the neck, turning the hemi-head over so you can progress posteriorly.
    (Photo7176)

Extend the incision through the skin along the edge of the mandible as you reflect.

29. Reflect the skin of the neck laterally until you pass the sternocleidomastoid muscle, transecting along the mandible as needed.
    (Netter036) (Photo7177)

This exposes the region of the anterior triangle of the neck, defined as the tissues anterior to the sternocleidomastoid muscle.

The area posterior to the sternocleidomastoid muscle is logically called the posterior triangle of the neck. The posterior triangle is associated with neurovasculature of the upper limb and will be studied in the Skin, Bones and Musculature course.

We do not need to reflect skin posterior to the sternocleidomastoid muscle.

30. Dissect into the tissue just inferior to the edge of the mandible to isolate and mobilize the submandibular gland.
    (Figure773) (Netter036) (Photo7178)

You will likely need a team member to hold the hemi-head angled so a second person can work on the anterior triangle near the deep side of the mandible.

The gland size varies between individuals, so it may be small and close to the mandible or larger and prominent. 

The facial artery passes over, or through, part of the submandibular gland before passing over the edge of the mandible onto the superficial face.

In the vicinity of the submandibular gland are numerous lymph nodes receiving lymph from the oral cavity. These nodes continue inferiorly posterior to the submandibular gland following the cervical lymph node chain coursing with the carotids on each side of the neck.

These lymph nodes may be palpable if they are inflamed. Unless your donor had an oral/pharyngeal infection at the time of their passing, these lymph nodes will not be apparent. 

31. Palpate to identify the location of the horseshoe shaped hyoid bone.
     (Figure746) (Netter036) (Photo7178)

The hyoid bone defines the suprahyoid and infrahyoid regions of the neck.

In some texts you will see the suprahyoid region divided into the submental triangle (a midline triangle bounded by the anterior digastric muscles) and the submandibular triangle (a triangle located between the digastric muscles and the mandible). This has no clinical relevance.

The infrahyoid region we examined in Foundations, consisting of the muscular triangle (area deep to the strap muscles) and the carotid triangle (superolateral corner of the infrahyoid anterior triangle where the carotid vasculature is not covered by musculature).

32. Start at the midline inferior to the mandible and clean fascia to expose the anterior end of the anterior belly of the digastric muscle.
    (Figure750) (Figure773) (Netter035) (Netter036)
    (Photo7178)

The digastric muscle consists of two muscle bellies (anterior and posterior) connected by a tendinous region.

Trace the anterior belly of the digastric muscle inferiorly to where it becomes tendinous just superior to the hyoid bone.

This tendinous region hooks through a ligament attached to the hyoid bone and then becomes a muscle again continuing superolaterally. This defines the anterior belly anterior to this hook and the posterior belly posterior to this hook

33. Follow the digastric muscle a short distance posteriorly from the connection to the hyoid bone where it heads superolaterally towards the skull.
    (Photo7179)

Follow the posterior belly of the digastric muscle posteriorly about as far as the angle of the mandible. This may require retracting the submandibular gland superiorly depending on the size of the gland.

Any fascia tissue lateral to the posterior belly of the digastric muscle can be removed. This may transect the facial artery/vein, but we are not studying these with this laboratory (recall, we examined the facial artery in Brain & Behavior).

The posterior belly of the digastric is straddled/crossed by another narrow muscle, the stylohyoid muscle, which we will not be examining in detail. 

Lateral/superficial to these muscles is considered a surgical 'safe' area as major neurovasculature is deep to the posterior belly of the digastric muscle.

34. Superior to the hyoid bone at the midline and deep to the anterior belly of the digastric muscle clean fascia to expose fibers of the mylohyoid muscle.
    (Figure750) (Figure773) (Netter035) (Netter036)

The mylohyoid muscle is flat and triangular arising from the entire length of the mylohyoid line along the inside of the mandible coursing inferomedially to attach to the hyoid bone. This forms the floor of the mouth.

35. Push a probe under the tendon of the posterior belly of the digastric and stylohyoid muscles to elevate them near to the attachment at the hyoid bone.
    (Netter039) (Netter041) (Photo7179a)

36. Transect tendons of the posterior belly of the digastric and stylohyoid muscles and reflect the muscles laterally.
    (Netter039) (Netter041) (Photo7180) (Photo7181)

37. Blunt dissect into the area deep to the posterior belly of the digastric to find the lingual artery and hypoglossal (XII) nerve.
    (Netter039) (Netter041) (Photo7182)

Typically the hypoglossal nerve is slightly lateral and superior to the position of the lingual artery.

The hypoglossal nerve descends from the skull anterior to the carotid arteries and deep to the posterior belly of the digastric muscle. The nerve takes a trajectory curving superiorly into the base of the tongue.

The lingual artery arises from the external carotid artery and courses into the base of the tongue.

The lingual artery is the primary arterial supply to the tongue and the hypoglossal nerve provides motor control to most of the tongue.

Due to the morbidity associated with hypoglossal nerve damage (the inability to control the tongue on the affected side), protecting this nerve during neck surgery is a high priority.

The lingual artery is accompanied by the lingual vein, though there is sufficient venovenous anastomosis that blockage to the vein is of limited consequence.

38. Blunt dissect to expose and open the carotid sheath to visualize the carotid vasculature and confirm the lingual artery connects to the external carotid artery.
    (Netter039) (Netter041) (Photo7185)

All branches of the carotid system in the neck (e.g. lingual and facial, etc.) arise from the external carotid artery. There are no branches of the internal or common carotid arteries in the neck.

The hypoglossal nerve travels over (lateral to) the carotid vasculature, a classic anatomic relationship important for neck surgery.

Post-Lab Daily Clinical Review Cases

Each lab has several multiple choice practice cases to reinforce understanding of the material within the lab and associated lectures. These are optional to review at home for consolidating and testing understanding.

Case 01

Case 02

Case 03

Case 04

CHECKLIST

Skeletal Structures

Mandible

Hard Palate

Hyoid Soft Structures

Soft Structures

Spaces
        Nasopharynx
        Oropharynx
        Laryngopharynx
        Piriform recess
        Retropharyngeal space

Mylohyoid muscle

Geniohyoid muscle

Genioglossus muscle

Intrinsic muscles of the tongue (as a group)

Lingual tonsils

Palatine tonsils

Palatoglossal arch

Palatopharyngeal arch

Soft palate
    Uvula

Constrictor muscles

Epiglottis

Sympathetic chain
    Superior cervical ganglia

Submandibular gland

Hypoglossal (XII) nerve

Glossopharyngeal (IX) nerve

Common carotid atery
    External carotid artery
        Lingual artery

Anterior belly of the digastric muscle

Posterior belly of the digastric muscle