Head and Neck Lab 1A

Lab 1A: Anterior Neck

Learning objectives:

Understand the organization of the neck into anterior and posterior triangles.
Describe the attachments of the strap muscles in the anterior region of the neck, and their actions of elevating, depressing, and stabilizing the hyoid bone and thyroid cartilage.
List the cartilages of the larynx and airway.
View the contents of the carotid sheath.

Short List of Structures - 1A

Removing the skin

Before you begin...

During dissection of the neck it will be helpful to place one or two wooden blocks under the cadaver’s shoulders. This will allow better access to the lateral aspects of the neck, and will also allow the head to fall back (the neck to extend), which will allow easier access to the anterior aspect of the neck.
Remove all the skin from the neck (anterior and posterior). This will ultimately make for an easier dissection.
You will encounter numerous cutaneous (= sensory) nerves as you remove the skin. These are cutaneous branches of the cervical plexus. You do not need to know the names of the cutaneous branches of the cervical plexus.
On the right side of the neck you’ll see the site used for embalming (a canula inserted into the right common carotid artery), some of the anatomy of that side will be messed up, do your best.

Removing the skin

Use a scalpel to make skin incisions on both sides of the neck by starting with a corner, do not just outline the cuts with a scalpel. Make the scalpel cuts shallow because the skin in the neck and face is thin:

Mandible (A) to the jugular notch (B), along the midline of the neck.

Then start your incision from A toward D, reflecting the skin as you go. The platysma muscle is very thin, superficial, and is connected to the skin of the neck. Try not to reflect it with the skin of the neck and try not to cut the external jugular vein that is immediately deep to it.
The facial a. passes over the mandible as it passes from the neck onto the face (deep to the platysma). Try to avoid cutting the facial artery as you reflect the skin. Continue to remove the skin as far posteriorly as you can.
You made the cut from point C to B to C during dissection of the thorax.

Skin incision lines to reflect the skin from the neck.

The facial a. passes over the mandible, deep to the platysma muscle. You will see it only if you inadvertently remove the platysma at this time (platysma removed in this image)

The external jugular veins sit on the surface of the SCM and the internal surface of the platysma m.

Identify the platysma muscle:

The platysma is located in the superficial fascia. It is one of the muscles of facial expression, so it is attached to the skin itself.
Because the muscle is so superficial and so intimately applied to the skin, you may inadvertently reflect the muscle with the skin. View it on a body with a preserved platysma if you inadvertently removed it.

If you successfully reflected the skin off of the platysma such that the platysma is still attached to the neck, now you need to cut away the platysma from the remains of the clavicle and reflect it up toward the mandible. You must reflect the platysma to see the deeper structures in the neck, but if possible, leave the platysma attached to the mandible.
The external jugular vein is a superficial vein in the neck that sits just superficial to the SCM. It drains venous blood from the face and neck, and eventually drains into the ipsilateral (= on the same side) subclavian vein. Because it is superficial, it is often taken with the skin.

Locate the sternocleidomastoid (SCM) m. on each side of the neck. These are strap muscles that run from the sternum and clavicle (= sternocleido) to the mastoid process of the skull (= mastoid).

Note the cutaneous nerves (cervical plexus):

The cervical plexus of nerves includes contributions from the C1 through C5 ventral rami, and provides somatic sensory and somatic motor innervation to the neck.

In the schematic, cutaneous sensory branches are shown on the left in yellow (lesser occipital, great auricular, transverse cervical, and suparaclavicular nn.). These cutaneous nerves are often taken with the skin and so are not seen. You are not responsible for identifying the cutaneous branches of the cervical plexus.

Emerging from the inferior angle of the mandible you’ll also see a body of glandular tissue, the submandibular salivary gland.

Right lateral view of the cutaneous branches of the cervical plexus emerging from the investing fascia. You do not need to know the names of the cutaneous branches of the cervical plexus

Triangles of the neck

The neck is often divided into a series of “triangles”. There is a lot of stuff in the neck, and dividing it into pieces and then subdividing those pieces even further may be a useful tool in order to “divide and conquer” the anatomy.

We will define and use the triangles of the neck as an organizational tool. You are likely to hear references to the triangles clinically, as many clinicians use the terminology e.g., to describe where a tumor is palpable or where an incision should be made.

The two main triangles of the neck are the anterior and posterior triangles. The anterior triangle is bordered by the midline of the neck, the anterior border of the SCM, and the inferior margin of the mandible. The posterior triangle is bordered by the posterior border of the SCM, the anterior border of the trapezius, and the clavicle.

Left lateral view of the anterior and posterior triangles of the neck.

Anterior triangle of the neck

There are two main groups of structures in the anterior triangle of the neck:

The cervical viscera, or the organs of the neck, which includes the larynx and trachea, esophagus, thyroid and parathyroid glands, carotid arteries and internal jugular veins, and the vagus nerves.
The strap muscles of the neck, a set of straight, flat muscles that connect the sternum, thyroid cartilage, and hyoid bone.

Locate the following structures in the anterior triangle of the neck:

In the midline, just superior to the manubrium (or where the manubrium used to be), between the sternal attachments of the SCM mm., find the cartilagenous rings of the trachea.
Somewhere along the trachea you’ll find a band of glandular tissue crossing it from left to right (or right to left). That tissue is the isthmus of the thyroid gland. The isthmus connects the left and right lobes of the thyroid gland, which are still hidden behind strap muscles.
The most superior and largest of the cartilagenous rings of the trachea is the cricoid cartilage. The cricoid cartilage is unique in that it is the only tracheal cartilage that forms a complete ring around the airway. The other cartilages are C-shaped, with the open part of the C facing posteriorly.
Superior to the cricoid cartilage is the large thyroid cartilage. The thyroid cartilage and cricoid cartilage together form the framework of the larynx (= voice-box), and contain the vocal cords (= vocal ligaments). The laryngeal prominence (= Adams apple) is the anterior projection of the thyroid cartilage. On its deep surface is the anterior attachment of the vocal cords.
Superior to the thyroid cartilage is the hyoid bone. This small C-shaped bone has no connection to other bones, and is held suspended in the neck by several muscles. You may not be able to see the hyoid bone, but you should be able to palpate the tips of the hyoid.
Between the cricoid cartilage and the thyroid cartilage is the cricothyroid membrane (or ligament), and between the thyroid cartilage and the hyoid bone is the thyrohyoid membrane (or ligament).

Superficial structures in the anterior triangle of the neck.

Reflection of SCM, carotid sheath, and the hunt for ansa cervicalis:

At this point it’s helpful to reflect the SCM muscles on both sides of the neck. Detach them from their sternal and clavicular attachments if they’re not already separated, and carefully reflect them superiorly. You may need to cut cutaneous branches of the cervical plexus to reflect the SCM. Take care as you reflect the muscle superiorly, and try to preserve the branch of the spinal accessory nerve that innervates the SCM. The nerve enters the deep surface of the muscle superiorly.

Just deep to the SCM is a dense connective tissue structure, the carotid sheath, which contains the carotid a., internal jugular v., and the vagus n. Don’t open the carotid sheath yet.

Lying on top of the carotid sheath and partially embedded in it is the ansa cervicalis. Ansa means “loop”, and the loop of the ansa cervicalis is formed from a superior and inferior root. The superior root is a branch of the C1 spinal nerve, and the superior part of the superior root runs next to a larger nerve, the hypoglossal n. (CN XII). The inferior root of the ansa is formed from branches of the C2 and C3 spinal nerves and emerges posterior to the carotid sheath and joins with the superior root on the anterior surface of the carotid sheath to complete the loop.

The ansa cervicalis provides somatic motor innervation to several of the strap muscles in the anterior neck. Note that the ansa cervicalis is a thin, delicate structure, and is often inadvertently removed during the dissection process since it lies on the top of the carotid sheath. See below for an image of an exceptionally large ansa cervicalis, highlighted in yellow.

Muscles in the anterior triangle of the neck:

The muscles in the anterior triangle of the neck may be divided into two groups. Muscles that are inferior to the hyoid bone are infrahyoid muscles, and muscles that are superior to the hyoid bone are suprahyoid muscles.

Infrahyoid muscles:

Take care as you dissect the infrahyoid muscles to look for branches of the ansa cervicalis, which innervates most of the infrahyoid muscles.
Note that the names of the infrahyoid muscles are composed of their origin and insertion. For example the sternohyoid m. originates from the sternum and inserts into the hyoid.

Locate the superior belly of the omohyoid m. in the anterior triangle. With the SCM removed you should also be able to dissect out the intermediate tendon of the omohyoid m. The inferior belly of the omohyoid m. originates from the scapula ("omo" = shoulder).

Strap muscles of the neck.

Medial to the superior belly of the omohyoid m. you’ll find another long strap muscle, the sternohyoid m. Other infrahyoid muscles are just deep to the sternohyoid. Carefully separate the sternohyoid from the deeper tissues so that you can move it around to see the deeper muscles.

The sternothyroid m. lies deep to the sternohyoid m., connecting the sternum to the thyroid cartilage.

The thyrohyoid m. also lies deep to the sternohyoid m. It connects the thyroid cartilage to the hyoid bone.

The cricothyroid m. is the smallest of the infrahyoid muscles, connecting the cricoid cartilage to the thyroid cartilage. This is the only muscle of phonation (speech) that is superficial to the larynx

Thyroid gland and associated structures:

You should be able to see the isthmus of the thyroid gland, lying on top of the trachea and medial to the two sternothyroid muscles. To get a full view of the thyroid gland cut and reflect the sternohyoid m. and the sternothyroid m. on one side of the neck. Do not reflect these muscles on the other side of the neck, but work to loosen them from the underlying tissue so you can pull it out of the way.
Get a clear view of the left and right lobes of the thyroid gland.
There are two arteries that serve the thyroid gland, the superior thyroid a. (a branch of the external carotid a.), and the inferior thyroid a. (a branch of the thyrocervical trunk). The inferior thyroid a. enters the deep surface of the gland, so is not easily visible.
Medial to the thyrocervical trunk is another branch of the subclavian a., the vertebral a. The vertebral a.may be difficult to find because it comes off the subclavian a. and runs posteriorly toward the vertebral column. You'll have to pull the subclavian a. medial or lateral to see the vertebral a. emerging from it. The two vertebral aa. run to the cervical vertebrae and pass through the transverse foraminae of the cervical vertebrae to the foramen magnum of the skull. They enter the foramen magnum and supply the posterior arterial supply of the brain.

Anterior view of the thyroid gland and associated structures with all overlying muscle removed.

The triangle of the vertebral artery.

Vertebral artery and thyrocervical trunk.

Open the carotid sheath:

Taking care to preserve the ansa cervicalis (if you found it), open the carotid sheath to expose its contents. Within the sheath you will find the common carotid a., the internal jugular v., and the vagus n. The common carotid a. has no branches until it splits to form the internal and external carotid aa.
In the superior region of the neck the common carotid a. splits to form the internal and external carotid aa., at the carotid bifurcation.
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).

The external carotid aa. supply blood to the neck and face. The branches of the external carotid a. are:

Superior thyroid a. is usually the most inferior branch, and descends to the superior pole of the thyroid gland. The superior laryngeal a. is a branch of the superior thyroid a., and passes through the thyrohyoid membrane along with the internal laryngeal n., which is one of the terminal branches of the superior laryngeal n.
Facial a. often branches from the external carotid a. posterior to the mandible, so its attachment to the external carotid a. can be difficult to find. However the facial a. will eventually pass over the inferior border of the mandible as it passes onto the face.

The submental triangle contains the suprahyoid mm.:

The submental triangle is one of the sub-triangles of the anterior triangles, it is bordered by the mandible and the hyoid bone, and contains the suprahyoid muscles.

Begin by finding the digastric m. ("di" = two, "gastr" = belly). Similar to the omohyoid m., the digastric m. has two bellies that are separated by an intermediate tendon. The posterior belly of the digrastric m. attaches to the mastoid process, the anterior belly of the digastric m. attaches to the anterior part of the mandible, and the intermediate tendon runs through a ring of connective tissue that attaches to the hyoid bone.

The submental triangle.

Running with the posterior belly of the digastric m. is the stylohyoid m. The stylohyoid m. attaches to the styloid process of the skull and to the hyoid bone (hence the name, stylo hyoid). The tendon of the stylohyoid m. splits near the hyoid, and the intermediate tendon of the digastric m. runs between the two parts of the stylohyoid tendon.

The mylohyoid m. forms the muscular “floor” of the oral cavity. It is attached laterally to the inner margin of the mandible, and its muscle fascicles run medially to attach to a median fibrous raphe. You'll see that the muscle fascicles of the mylohyoid run transversly, orthoganal (at a right angle, or perpendicular to) the anterior belly of the digastric m.

The glandular tissue you see in this region is the submandibular salivary gland.

Inferior view of the submental triangle

Clinical Correlations

Tracheotomy and Cricothyrotomy

These are procedures used to create an airway that by-passes the head and pharynx. Tracheostomy replaces an endotracheal tube when the patient is to be mechanically ventilated for more than 1 week.

In a tracheotomy a transverse incision is made in the skin of the neck. The infrahyoid muscles are retracted laterally, and the isthmus of the thyroid gland is either divided or retracted superiorly. An opening is made in the trachea, either between the 1st and 2nd tracheal rings, or though the 2nd through 4th tracheal rings. A tracheostomy tube is then inserted into the trachea and secured. Given the robust blood supply to the thyroid gland, the main risk of tracheostomy is bleeding from one of the thyroid vessels. A tracheotomy refers to the cut the surgeon makes, while a tracheostomy is the hole. Often these terms are used interchangeably.
A cricothyrotomy is quicker and easier to perform than a tracheotomy, but is only used as a last resort, and is a temporary measure to be used until a more permanent airway can be established. The skin and cricothyroid membrane are incised just inferior to the laryngeal prominence (Adam’s apple) and a tube is inserted into the airway. The approach to, and incision of, the cricothyroid membrane is superior to the isthmus of the thyroid gland, and so the risk of complication is much lower.

Sites for tracheotomy and cricothyrotomy.Red lines represent the incision sites in the trachea for cricothyrotomy and tracheostomy. You can see why the cricothyrotomy entails fewer complications.

Carotid Endarterectomy

Carotid endarterectomy is a surgical procedure to clear atheromatous plaque from the common and internal carotid arteries. It is most typically performed when a patient has 60-70% occlusion of the internal carotid artery. An incision is made through the skin following the anterior border of the sternocleidomastoid muscle, and the carotid sheath is exposed and opened. The extent of the plaque is determined by palpation of the artery, and the internal, common, and external carotid artery are clamped off. The wall of the artery is opened and the plaque removed, and then the vessels are closed. Alternatively, a catheter may be used to insert a stent and a balloon into the common and internal carotid arteries. After the stent is placed a balloon is inflated to push the plaque out of the lumen of the artery and into the wall (balloon angioplasty).

Left and right carotid angiograms of the same patient. You can see the common carotid at the bottom of the image, and its bifurcation into internal and external carotid arteries. The blue arrow shows that the left internal carotid artery is nearly completely occluded.

Carotid angiograms before (left) and after (right) balloon angioplasty and stenting. On the left you can see how occluded the base of the internal carotid artery is. On the right you can see the stent placed to maintain the opened internal carotid artery.

Diagrams of carotid endarterectomy (above) vs. balloon angioplasty (below). Endarterectomy removes the plaque, while angioplasty compacts the plaque into the wall of the artery.

Carotid angiograms

Before and after balloon angioplasty and stent.

Endarterectomy vs. balloon angioplasty.

Nerves in the neck

Horner’s syndrome:

Horner’s syndrome is caused by a lesion anywhere along the pathway that supplies sympathetic innervation to the head and neck. Loss of sympathetic innervation to the head and neck cause the 3 classic symptoms of Horner’s syndrome:

Miosis (not mitosis, cells do that) = constriction of the pupils. The constrictor pupillae smooth muscle is innervated by parasympathetic nerve fibers, while the dilator pupillae muscle is innervated by sympathetic nerve fibers. Loss of sympathetic innervation to the dilator pupillae muscle will lead to its inactivity, and therefore the sphincter pupillae muscle will be unopposed.
Ptosis = droopy eyelids. The superior tarsal muscles are smooth muscles in the upper eyelid that help hold the eyelid open. Loss of sympathetic innervation to the superior tarsal muscles lead to their inactivity and associated droopiness of the eyelid.
Anhidrosis = lack of sweating, in this case specifically a lack of sweating from the skin of the head and neck. Sweat glands are innervated by sympathetic neurons, so loss of sympathetic innervation leads to a lack of sweating.

The lesion that causes Horner’s syndrome can occur anywhere along the sympathetic pathway from the hypothalamus to the head and neck. The location of the lesion determines what specific “type” of Horner’s syndrome is present:

A first-order syndrome occurs due to damage in the nerve tracts that lead from the hypothalamus to the superior thoracic spinal cord, most commonly caused by infarction in the medulla.
A second-order syndrome occurs in the pre-ganglionic neurons that lead from the lateral horn of the spinal cord to the cervical ganglia, usually caused by trauma or surgery to the spinal cord, thoracic outlet, or lung apex.
A third-order syndrome occurs in the post-ganglionic neurons that lead from the cervical ganglia to the effectors in the head and neck, typically due to damage to the internal carotid artery, including carotid endarterectomy and stenting.

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