Labs 24 and 25 proceed by examining specific regions of the bisected head in order. Some regions are covered today, including the pharynx, and the nerves and vessels of the hard palate. The next lab covers the tongue, muscles of the soft palate, and specific details of the nasopharynx, such as the conchae, sinuses, and associated nerves and vessels.
1. BISECTION OF THE HEAD
The next instructions entail careful bisection of the head and neck in the mid-sagittal plane, using a scalpel when possible and a hand saw when necessary. You have already cut the posterior wall of the pharynx in the midline.
Use a scalpel to divide the uvula and the soft palate in the median plane.
Use a scalpel to cut through the upper lip just lateral to the midline.
Use a scalpel to cut through the cartilages of the external nose just lateral to midline.
NOTE: The skull must be sawed just lateral to the median plane so you can see the nasal septum on one side and have a fully exposed surface on the other side.
Saw through the skull from superior to inferior. Begin lateral to the crista galli and keep the blade close to the nasal septum.
Cut through the nasal and frontal bones, ethmoid bone, body of the sphenoid, dorsum sellae, basilar part of the occipital bone, and hard palate. Stop when the saw has passed into the foramen magnum.
Use a scalpel to cut the tongue along the midline.
Use a saw to cut through the mandible.
NOTE: you may have to gently chisel between central incisors in both the maxilla and the mandible to create a space for the saw to pass.
Use a scalpel to cut through any remaining soft tissue as needed to complete the bisection.
2. INTERIOR OF PHARYNX
Identify the hard palate, which separates the nasal cavities from the oral cavity.
Identify the soft palate.
FUNCTIONAL ANATOMY: The soft palate functions as a swinging “valve”. It can swing up to block the nasopharynx or down to block the oral cavity The muscles that control are therefore critically important for controlling air and food/fluid flow in the pharynx. In the next lab you will dissect the muscles that raise and stretch the soft palate.
In the nasopharynx, locate the transected pharyngeal tonsil and the opening of the auditory tube. In the oropharynx, view the two arches formed by the palatoglossus and palatopharyngeus muscles (in the wall of the oral cavity behind the tongue).
3. PALATOPHARYGEAL AND PALATOGLOSSAL ARCHES
Using a NEW scalpel blade, carefully remove the mucosal lining of the pharynx, and expose the underlying musculature.
Remove the palatine tonsil.
HINT: Removing a tonsil effectively from the surrounding capsule is a lengthy process and involves a delicate touch with patience.
FUNCTIONAL ANATOMY: The tonsils are collections of lymphoid tissue which play an immunological role. The palatine tonsils lie below the soft palate. They lie posterior to the palatoglossul arch that separate the oral cavity from the oropharynx and anterior to the palatopharyngeal arch (both arches are discussed below). The palatine tonsils lie beneath the mucous membrane that are encased in a hemicapsule of connective tissue that separates them from nearby muscles and nerves. These are sometimes called faucial tonsils because they lie between oral cavity and oropharynx in a space called the fauces (similarly the palatoglossal and palatopharyngeal folds are often said to be faucial pillars).
Clear away the palatoglossus muscle, and trace the palatopharyngeus muscle down into the pharynx.
HINT: It may seem odd that we ask you to “clear away” the muscle, but these muscles are thin, and are often easier to pull off than transect.
The so-called palatoglossal arches or palatoglossal folds mark the boundary between the oral cavity and the oropharynx. Make sure you note this division. The palatoglossus runs from the soft palate to the tongue and has an action of raising the posterior tongue. Palatopharyngeus forms a fold or arch behind the palatoglossal fold or arch. The palatopharyngeus originates also from the soft palate, but it go to the pharynx where it enters the and runs deep to the superior constrictor and then eventually combines with the stylopharyngeus (see below) and serves to elevate the pharynx.
4. STYLOPHARYNGEUS MUSCLE
Find the stylopharyngeus muscle along with its nerve (CN IX) entering between the superior and the middle constrictor muscles.
HINT: the separation of the constrictors looks great in images drawn by artists. But in reality, the divisions are often vague. You can help yourself to be sure of where each constrictor is by (a) looking for its origin and following fibers back to the raphe and (b) look for muscles and nerves that get into the pharynx between the constrictors.
FUNCTIONAL ANATOMY: The stylopharyngeus is a relatively large pharyngeal muscle coming down from the styloid process and slipping between superior and middle constrictors. It joins the palatopharyngeus and runs with it deep to the inferior constrictor down to the pharyngeal wall and thyroid cartilage.
A third muscle of the pharynx—salpingopharyngeus—runs also joins the palatopharyngeus and stylopharyngeus. It arises near the auditory tube and you will see it in the next lab.
5. PHARYNGEAL CONSTRICTORS
On one side only, carefully dissect away the longitudinal pharyngeal musculature (palatopharyngeus and stylopharyngeus).
Remove the palatoglossus muscle, in order to display the inner surface of the pharyngeal constrictors.
Carefully examine the attachments of the three constrictors in front (anteriorly).
Investigate their attachments above and below.
The lateral and posterior walls of the pharynx are composed primarily of the three pharyngeal constrictor muscles: superior, middle, and inferior:
The superior constrictor arises from the posterior edge of the medial wall and hamulus of the pterygoid plate, as well as the pterygomandibular raphe (which is a narrow connective tissue band that runs from the pterygoid hamulus to the mandible posterior to the 3rd molar), and mandible.
The middle constrictor arises from the superior surface of the greater cornu of the hyoid as well as the stylohyoid ligament.
The inferior constrictor arises from the oblique line on the thyroid cartilage and the arch of the cricoid.
The three constrictors fan posteromedially to form a wide insertion at a midline posterior raphe that runs from occiput to the esophagus. This insertion is complex with overlapping fibers that fuse with those above at the midline leaving interesting gaps between constrictors:
The most superior fibers of the superior constrictor attach to the occiput at the pharyngeal tubercle.
The uppermost fibers of the middle constrictor fan out and overlap and fuse with the superior constrictor at the raphe.
There is a gap between the superior and middle contrictors through which passes: (1) the styloglossus muscle, (2) the glossopharyngeal nerve, (3) the stylopharyngeus muscle.
The uppermost fibers of the inferior constrictor overlap and fuse with the middle constrictor. The lowermost fibers of the inferior constrictor run down and blend with the esophagus. They are said to constitute a cricopharyngeus muscle.
The motor innervation of the pharyngeal constrictors is by branches of the vagus nerve and sensory innervation is carried by glossopharyngeal nerve.
6. HARD PALATE
Using a probe, remove the mucosa from the hard palate on the inferior surface where it was cut during the head bisection.
Grasp the mucosa with your fingers and peel it off from medial to lateral.
Detach the mucosa along the medial side of the alveolar process of the maxilla.
Identify the greater and palatine nerves and the greater palatinevessels where they emerge from the greater palatine foramen.
Use blunt dissection to follow the greater palatine nerve anteriorly.
FUNCTIONAL ANATOMY: The palatine nerves are branches of the maxillary nerve that pass through the pterygopalatine ganglion (which you will explore next lab). The greater palatine nerve emerges from greater palatine foramen to the roof of the mouth. There are also lesser palatine nerves passing through the small foramina and going to the soft palate. The nasopalatine nerve supplies the mucosa over the anterior part of the hard palate, passing through the incisive foramen. The palatine nerves carry parasympathetic, general sensory, and some taste fibers.
Posterior to the greater palatine nerve, identify the lesser palatine nerve.
Use blunt dissection to follow the lesser palatine nerve to the soft palate.
All three of these nerves (greater palatine, nasopalatine, and lesser palatine) are branches of CN V2.
Lab 24 - Bisected Head II
Suggested readings from Gray's Anatomy for Students, 2nd ed.
Ch. 8, 985-1012
Suggested readings from Langman's Medical Embryology, 11th ed.
The oral cavity is filled by the tongue when the mouth is closed. The mouth is usually closed in the cadaver because of rigor in the powerful masticatory muscles. You have already sectioned the tongue during the bisection of the head.
Examine the dorsal and midline surface of the tongue.
Note the various types of mucosal papillae covering the dorsal surface.
Identify the terminal sulcus.
NOTE: The tongue consists principally of muscles, which attach to each other, more than they attach to bone. You will want to dissect these carefully and consider the different directions of the fibers. You will look at the extrinsic muscles first and then look at the intrinsic and see how they all interact.
Identify the extrinsic muscles of the tongue:
Styloglossus - attaches the tongue to the skull
Genioglossus - attaches the tongue to the anterior of the mandible
Hyoglossus – attaches the tongue to the hyoid bone
You will look at these muscles now, and then examine them again with their innervation in the lab associated with masticatory muscles.
2. ROOT OF THE TONGUE
Look at the tongue in sagittal section. Of the extrinsic tongue muscles only the genioglossus muscle will be clear today.
Examine the genioglossus muscle. Look at the way it radiates from its attachment on the mandible.
Note that the lower portion of the genioglossus muscle is considered the root of the tongue.
Immediately inferior to the genioglossus, identify the geniohyoid muscle. The geniohyoid muscle runs from the inside of the jaw at the midline to the hyoid. Its fibers run anteroposteriorly.
Note how the geniohyoid muscle supports the root of the tongue.
Below the geniohyoid, identify the mylohyoid muscle. Its fibers run mediolaterally.
Thus, two muscles at right angles to each other (the geniohyoid and mylohyoid), support the floor of the mouth.
Note the intrinsic muscle fibers in the substance of the tongue running in three mutually perpendicular directions.
At the moment you can see the superior longitudinal band running under the mucous tissue of the tongue over the genioglossus muscle.
Remove some mucous tissue to expose the intrinsic muscle.
To see the remaining intrinsic muscles, make a coronal slice through the tongue on one side above the insertion of the genioglossus muscle, but not all the way down to it.
You can now see the transverse bands of the intrinsic fibers running from the median fibrous septum laterally, and the vertical intrinsic muscle fibers extending superiorly and blending with the superior longitudinal fibers.
The inferior longitudinal band running longitudinally between the genioglossus and hyoglossus muscles is hard to see and may be seen in the next lab associated with the masticatory muscles.
3. MUSCLES OF THE SOFT PALATE
Dissect the muscles that operate on the soft palate and upper pharynx:
levator veli palatini, which elevates the soft palate
tensor veli palatini, which tightens the soft palate (like the skin of a drum).
salpingopharyngeus, which helps to elevate the pharynx during swallowing.
To see these muscles, remove the mucosa around the opening of the auditory tube.
NOTE: These are very small muscles lying just under the mucosa. Make a small incision in the mucosa using your scissors or scalpel. Notice the muscle fibers just deep to the mucosa. Try to follow your incision making sure to cut only through the mucosa and avoiding the muscles as well as you can.
Display the origins of the levator and tensor veli palatini muscles.
NOTE: This takes time and patience but is well worth it.
The levator veli palatini muscle travels inferiorly into the soft palate. When it contracts, it elevates the palate. The tensor veli palatini muscle follows a similar path, but goes more laterally, passing around the hamulus of the medial pterygoid plate. Thus, it changes its direction of pull to a transverse direction. This muscle tenses the soft palate, making it more rigid so that the levator veli palatini muscle can raise it up. The salpingopharyngeus muscle does not insert directly into the soft palate, but it helps to elevate the palate by pulling on the pharynx below. When raised, the soft palate blocks off the nasal pharynx so that food or liquid passing from the oral cavity to the oral pharynx does not go up into the nasal cavity.
Similar to the salpingopharyngeus muscle, part of the tensor veli palatini muscle attaches to the cartilaginous portion of the pharyngotympanic tube (aka 'auditory tube' or 'Eustachian tube'). When these muscle contract, they pull on the tube, widening the opening, and allowing the pressure between the oropharynx and the middle ear to equalize.
4. NASAL SEPTUM
Note the uvula (which you already sectioned) hanging down from the back edge of the soft palate.
Distinguish the bony and cartilaginous portions of the nasal septum:
Perpendicular plate of the ethmoid
Identify the olfactory epithelium at the top of the nasal fossa by its distinctive yellow pigmentation.
The cartilaginous portion of the nasal septum represents the unossified part of the ethmoid bone. The vomer is a membrane bone. The nasopalatine nerve and the sphenopalatine artery pass diagonally down the nasal septum from the sphenopalatine foramen to the incisive canal. In addition to the nasal septum, the nasopalatine nerve and sphenopalatine artery supply a portion of the oral mucosa that covers the hard palate.
5. NASAL CAVITY
When you bisected the head, you should have gone slightly off the midline such that the nasal septum is intact on one side. Go through the steps below on the OPPOSITE site (the side that does not have the nasal septum).
Examine the lateral wall of the nasal cavity.
Identify the bony scroll-like structures called conchae on the lateral wall of the fossa (there are usually three conchae but sometimes there can be four of them)
Place a probe into the space overhung by each concha. Each space is called a meatus.
The upper two conchae are processes of the ethmoid bone. It is into their meatus that the ostia (mouths) of the ethmoidal, frontal, and maxillary air sinuses open. The sphenoid sinus opens through a hole in the sphenoid bone and into the posterior nasal cavity. The inferior conchae are their own separate bones. The nasolacrimal duct opens below the inferior concha. Consider these turbinates (conchae) in cross-section, and note how they influence the airflow for clearing debris, cooling/warming, and humidifying/dehumidifying.
Remove the inferior and middle conchae by gently fracturing them off with upward leverage
In the middle meatus, you will now be able to see openings to the paranasal sinuses.
Probe the openings of the paranasal sinuses into the middle meatus
Locate the opening of the nasolacrimal duct into the inferior meatus.