Lab 1B
Anterior Neck

Use a scalpel to make the following skin incisions on both sides of the neck. Make the scalpel cuts shallow because the facial skin 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.

• 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.

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.

If the platysma is carefully reflected from the neck, you may be looking at the investing fascia (the most superficial layer of the deep cervical fascia). The investing fascia (if it is intact, which it may not be) will look like a layer of plastic wrap lying on top of the muscles in the neck.

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).

Emerging from the posterior border of the SCM you may (or may not) see cutaneous branches of the cervical plexus. They look large in the figures but in fact are somewhere between dental floss and hair in thickness. These are sensory branches of the C1 through C5 ventral rami, and they provide sensory input from the skin over the anterior region of the neck. There are several named sensory nerves of the cervical plexus; the lesser occipital n., great auricular n., transverse cervical n., and supraclavicular nn.

These are cutaneous nerves, so they are very thin and structurally they will be similar to other cutaneous nerves you have seen going to the skin. We don’t expect that you’ll be able to identify individual branches, but you should know the general regions they carry sensation from, and that they are the sensory part of the cervical plexus.

You’ll have to dissect away the investing fascia to clearly see the structures deep to it. Use forceps, scissors, a sharp scalpel, whatever works best for you. You’ll probably have to remove the investing fascia piecemeal (small bits at a time) because it’s typically very tough. Just don’t get overzealous and destroy the structures deep to the fascia.

One structure you may see, at about the same level as the cutaneous nerves, is the external jugular v. The external jugular vein brings venous blood back from the superficial face and neck. It is a fairly superficial structure, lying on the surface of the SCM, so it may be damaged or destroyed when the skin or investing fascia is removed.

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

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. However you will not be held responsible (on an exam) for the borders or contents or even names of any of the triangles. If they’re helpful as an organizing or study tool please do use them, but we will not test you on them.

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

• The cervical viscera, 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.

• 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).

Reflection of SCM, 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 aa., 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.

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.

• We saw the inferior belly of the omohyoid m. in the posterior triangle of the neck. 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.

• 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 sternothyroid 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 that is superficial to the larynx,

We’ll get to the suprahyoid muscles when we move superiorly, but for now we’ll continue to dissect and identify structures in the infrahyoid region. Notice that the infrahyoid muscles are named according to their attachments. The actions of those muscles may vary, depending on the state of other muscles. For example the thyrohyoid may depress the hyoid or elevate the thyroid cartilage, depending on whether or not other other strap muscles are contracting at the same time.

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 sternothyroid on one side of the neck. Do not reflect the sternothyroid on the other side, but work to loosen it 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.

• There are three veins that drain the thyroid gland, the superior, middle, and inferior thyroid vv. The veins drain into the internal jugular vv. and the brachiocephalic vv.

• Having found the inferior thyroid artery you’ll be able to trace it inferiorly to the thyrocervical trunk, which branches directly from the subclavian a.

• 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. The two vertebral aa. run to the cervical vertebrae and pass through the successive transverse foramina 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.

Structures deep to the thyroid gland:

• Now cut through the isthmus of the thyroid gland in the midline and reflect the two lobes laterally. The parathyroid glands (typically 4 small patches of tissue) lie on the deep surface of the thyroid gland. Sometimes the parathyroids are a different color than the thyroid gland and you’ll be able to distinguish them, but often you can’t distinguish between the parathyroid and thyroid glands.

• As you dissect even further posteriorly, along the lateral surface of the trachea and towards the esophagus you will find the recurrent laryngeal nn. These are branches of the vagus n. that ascend to the larynx and innervate the muscles of phonation (speech). The terminal (most superior) part of the recurrent laryngeal n. is called the inferior laryngeal n.

Open the carotid sheath:

• Taking care to preserve the ansa cervicalis as much as possible, 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 that serve to monitor blood pressure.

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.

• Lingual a. is usually the next artery. It passes anteriorly and medially toward the tongue.

• 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.

• Occipital a. and posterior auricular a. branch from the deep surface of the external carotid a. and ascend to the region of the head posterior to the ear.

continue to uncover them as dissection of the head and neck progresses.

The submental triangle contains the suprahyoid mm.:

Deep Neck Space Infections:

Understanding the anatomy of the fascial planes of the neck is important because some of these spaces extend from the base of the skull into the mediastinum. Therefore an infection in the neck may spread into the mediastinum and involve structures there.

These infections most commonly arise from infection of mandibular teeth, tonsils, parotid glands, lymph nodes, middle ear, or sinuses. Penetrating trauma and surgical complications are also causes.

• Pretracheal space infections most commonly occur due to perforation of the anterior esophageal wall, but may also be caused by a prolonged tracheostomy. The pretracheal fascia encloses the esophagus, trachea, and thyroid gland, and thus defines the pretracheal space. The first symptom may be hoarseness, then dyspnea (difficulty breathing), and difficulty swallowing. The most dangerous potential complications are airway obstruction and extension of the infection into the mediastinum.

• Prevertebral space infections typically begin by spread of a cervical spine infection (discitis or vertebral osteomyelitis), or as a surgical complication. The prevertebral space is the potential space between the alar fascia (red) and the prevertebral fascia (orange). It is also called the “danger space” because it passes from the base of the skull all the way to the coccyx. 75% of patients present with back or neck pain, 50% with fever, and 1/3 with varying neurologic deficits. The most dangerous potential complication is compression of the spinal cord due to epidural pressure from the infective process, with irreversible paralysis occurring in 4 to 22% of patients. The prevertebral space is continuous from the base of the skull to the coccyx, so infection may spread widely.

• Parapharyngeal space (also called carotid space) infections are especially dangerous because they involve the carotid artery, jugular vein, and the vagus nerve, and due to their location they also carry the potential for airway impingement. This is the space surrounded by the carotid sheath. Dental infections are the most common cause of parapharyngeal infections, followed by tonsilar abscess. Symptoms include trisumus (reduced opening of the jaw), swelling below the angle of the mandible, and medial bulging of the pharyngeal wall.

• Retropharyngeal and danger space infections are the most dangerous because those spaces communicate with the mediastinum, so infection can easily spread into the thoracic cavity. They may be caused by penetrating trauma, or may spread from dental or tonsilar infections. The most serious complication of retropharyngeal and danger space infections is acute necrotizing mediastinitis, which has a mortality rate of 25%. The retropharyngeal space is the potential space between the alar (red) and pretrachial (purple) fascia.

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.

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.

Hashimoto’s disease (Hashimoto’s thyroiditis) is an autoimmune disease in which the immune system attacks the cells of the thyroid gland. It was first described in 1912, and was the first disease to be described as an autoimmune disease. Hashimoto’s disease sometimes causes an enlarged thyroid, but not always.

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:

1. 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.

2. 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.

3. 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:

1. 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.

2. 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.

3. 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.

Nerves in the Neck:

You may have noticed that the neck connects the head to the rest of the body. Therefore it should come as no surprise that many nerves pass through the neck.

• Sympathetic nerves. Horner’s syndrome was discussed in a previous correlation.

• Some other nerves passing through the neck – hypoglossal (CN XII), vagus (CN X) and its branches (superior laryngeal (external and internal laryngeal), recurrent laryngeal (inferior laryngeal), cervical plexus (C1-C5 ventral rami, motor (ansa cervicalis) and sensory branches), and phrenic.

• During carotid endarterectomy (described in a previous correlation) the surgeon must take care to isolate the common, internal and external carotid arteries from the surrounding nerves (Hypoglossal, Vagus, and Ansa cervicalis) to insure those nerves are not inadvertently clamped with the arteries.

• Damage to branches of the vagus. The inferior laryngeal nerves (terminal branch of the recurrent laryngeal) are susceptible to damage during surgery in the anterior triangle of the neck (e.g. thyroidectomy). Unilateral lesion results in a poor voice because the vocal fold on that side cannot adduct to meet the fold on the opposite side. Bilateral lesion results in almost no voice at all. Lesion of the external laryngeal branch of the superior laryngeal nerve results in a monotonous voice because the cricothyroid muscle is unable to contract and thereby modulate the tension of the vocal fold.

• The phrenic nerve has a fairly long course, and can be damaged in the neck or within the thoracic cavity. Unilateral lesion of the phrenic nerve leads to paralysis of the nerve on that side, but as long as the other side is intact ventilation will still occur (though reduced). During certain thoracic surgeries (e.g. a lung surgery) a local anesthetic agent may be injected around the phrenic nerve (phrenic block) where it lies on the anterior surface of the anterior scalene muscle to cause temporary paralysis of the diaphragm on that side.