SEGMENT II: Gross Anatomy of the Head and Neck Dissection Sequence

 

TABLE OF CONTENTS

 

FRIDAY, APRIL 23: OSTEOLOGY OF THE NECK

 

FRIDAY, APRIL 23: POSTERIOR TRIANGLE OF THE NECK

 

WEDNESDAY, APRIL 28: ANTERIOR TRIANGLE OF THE NECK

 

THURSDAY, APRIL 29: ROOT OF THE NECK

 

MONDAY, MAY 3 AND WEDNESDAY, MAY 5: PAROTID REGION AND INFRATEMPORAL FOSSA

 

Neck

The neck is a region of transition between the head and the thorax. The major vessels that supply the head pass through the neck. Some nerves that innervate the organs within the thorax and abdomen pass through the neck. Portions of several systems are located in the neck: gastrointestinal system (pharynx and esophagus), respiratory system (larynx and trachea), cardiovascular system (major vessels to the head and upper limbs), central nervous system (spinal cord), and endocrine system (thyroid and parathyroid glands). Finally, nerves and vessels to the upper limbs pass through the inferior part of the neck.

 

Skeleton of the Neck

The bones of the neck were first studied when we did the dissection of The Back. Use an articulated skeleton and your atlas to recall several characteristics of the cervical vertebrae. Cervical vertebrae have:

Observe the following features of individual cervical vertebrae:

  1. Atlas (C1)
    1. Anterior arch and tubercle
    2. Transverse process with transverse foramen
    3. Groove for vertebral artery
    4. Posterior arch and tubercle
    5. Superior articular surface for occipital condyle
    6. Note that the atlas does not have a body.
  2. Axis (C2)
    1. Dens
    2. Body
    3. Transverse process with transverse foramen
    4. Lamina
    5. Spinous process
    6. Superior articular facet for atlas
  1. Vertebrae C3 to C7
    1. Body
    2. Transverse process with transverse foramen
    3. Groove for spinal nerve
    4. Lamina
    5. Spinous process
    6. Note that C7 has the longest cervical spinous process (vertebra prominens).

 

FRIDAY, APRIL 23: POSTERIOR TRIANGLE OF THE NECK

Dissection Overview

Study a transverse section through the neck (Fig. 7.1). The posterior part of the neck contains the cervical vertebral column and the muscles that move it. The anterior part of the neck houses the cervical viscera. The cervical viscera include:

The visceral part of the neck has the following boundaries:

Large vessels and nerves lie lateral to the cervical viscera (Fig. 7.1). The common carotid artery (internal carotid artery at more superior levels), internal jugular vein, and vagus nerve are contained within the carotid sheath.

 

The order of dissection will be as follows: The skin will be removed from the anterior and lateral neck. The platysma muscle will be studied and reflected. The external jugular vein will be identified. Several cutaneous branches of the cervical plexus (great auricular nerve, lesser occipital nerve, transverse cervical nerve, and supraclavicular nerves) will be dissected. The accessory nerve (cranial nerve XI) will be identified and followed from the sternocleidomastoid muscle to the trapezius muscle.

 

Dissection Instructions

Skin Removal

  1. The skin is thin on the neck. Be careful when removing it; remove only the skin, leaving as much subcutaneous tissue as possible.
  2. Refer to Fig. 7.2 and make an anterior midline skin incision from the jugular notch of the sternum (E) to the chin (F).
  3. Make a second skin incision along the margin of the mandible from point F to just below the ear lobe (G).
  4. Make a skin incision in the transverse plane from point G to the external occipital protuberance (H). If the back has been dissected, part of this incision has been made previously.
  5. Beginning at the anterior midline, reflect the skin in the lateral direction as far as the anterior border of the trapezius muscle. Detach the skin.

Posterior Triangle of the Neck For descriptive purposes the neck is divided into an anterior triangle and a posterior triangle (Fig. 7.3). The boundaries of the posterior triangle of the neck are:

  1. Anterior – the posterior border of the sternocleidomastoid muscle
  2. Posterior – the anterior border of the trapezius muscle
  3. Inferior – the middle one-third of the clavicle
  4. Superficial (roof) – superficial layer of the deep cervical fascia
  5. Deep (floor) – muscles of the neck covered by prevertebral fascia

The cutaneous nerves for the shoulder and anterior neck pass to the surface through the posterior triangle of the neck. Therefore, these structures are dissected with the posterior cervical triangle even though they may distribute over the shoulder or anterior triangle. At the outset, note that the structures in the following dissection lie within the superficial fascia of the neck except the accessory nerve, which lies deep to the superficial layer of the deep cervical fascia.

  1. Examine the platysma muscle in the superficial fascia (Fig. 7.4). The platysma muscle covers the lower part of the posterior triangle. At its inferior end, the platysma muscle passes superficial to the clavicle and attaches to the superficial fascia of the deltoid and pectoral regions. Superiorly, the platysma muscle is attached to the mandible, skin of the cheek, angle of the mouth, and orbicularis oris muscle. It is innervated by the facial nerve.
  2. Note that the supraclavicular nerves, the transverse cervical nerve, and the external jugular vein are in contact with the deep surface of the platysma muscle. Preserve them in the next dissection step.
  3. Near the clavicle, use a probe to raise the inferior border of the platysma muscle. Carefully use blunt dissection to free the platysma muscle from the vessels and nerves on its deep surface and reflect the muscle superiorly as far as the mandible. Leave the platysma muscle attached along the border of the mandible. This muscle is very thin and fragile; don’t be too concerned if it tears or falls apart completely.
  4. Identify the external jugular vein (Fig. 7.5). The external jugular vein is in the superficial fascia deep to the platysma muscle. The external jugular vein begins posterior to the angle of the mandible and crosses the superficial surface of the sternocleidomastoid muscle. About 3 cm superior to the clavicle, the external jugular vein pierces the superficial layer of the deep cervical fascia (roof of the posterior triangle) to drain into the subclavian vein. Follow the external jugular vein until it passes through the investing layer of deep cervical fascia. Note that these vessels may vary in size considerably from one side to the other, or in different cadavers.
  5. The skin of the neck and part of the posterior head is innervated by cutaneous nerves that are branches of the cervical plexus. The cutaneous nerves enter the superficial fascia near the midpoint of the posterior border of the sternocleidomastoid muscle (Fig. 7.5). Identify:
  6. The accessory nerve (XI) courses from slightly superior to the midpoint of the posterior border of the sternocleidomastoid muscle to the anterior border of the trapezius muscle (Fig. 7.5). The accessory nerve lies deep to the superficial layer the deep cervical fascia. Use blunt dissection to free the accessory nerve from the surrounding connective tissue. Note that branches of spinal nerves C3 and C4 join the accessory nerve in the posterior cervical triangle and these branches provide proprioceptive sensory innervation. The accessory nerve innervates the sternocleidomastoid muscle and the trapezius muscle. Confirm that the accessory nerve may be found on the deep surface of the trapezius muscle.
  7. The inferior portion of the posterior triangle will be dissected with the root of the neck.

IN THE CLINIC: Diaphragmatic Pain Referred to the Shoulder

The supraclavicular nerves and the phrenic nerve share a common origin from spinal cord segments C3 and C4. Irritation of the parietal pleura or parietal peritoneum covering the diaphragm (e.g., from an enlarged bladder) produces pain that is carried by the phrenic nerve and referred to the area supplied by the supraclavicular nerves (shoulder region).

Dissection Review

  1. Review Figure 7.1 and note that the platysma muscle, external jugular vein, and cutaneous nerves of the neck are all located in the superficial fascia. Of the structures just dissected, only the accessory nerve is located deep to the investing layer of deep cervical fascia.
  2. Use an illustration to review the relationship of the platysma muscle to the cutaneous branches of the cervical plexus. Note that the transverse cervical nerve crosses the neck deep to the platysma muscle but that its branches pass through the muscle to reach the skin of the anterior neck.
  3. Review the area of distribution of all cutaneous branches of the cervical plexus.
  4. Review the course of the accessory nerve. Note that the accessory nerve is superficial in the neck where it is vulnerable to injury by laceration or blunt trauma.
  5. Review the course of the occipital artery at the apex of the posterior triangle.

 

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WEDNESDAY, APRIL 28: ANTERIOR TRIANGLE OF THE NECK

 

Dissection Overview

The boundaries of the anterior triangle of the neck are (Fig. 7.3):

For descriptive purposes, the anterior triangle is divided by the digastric and omohyoid muscles into smaller triangles: muscular, carotid, submandibular, and submental (Fig. 7.3).

 

Bones and Cartilages

Use an illustration to identify the bony and cartilaginous landmarks that will be used as reference structures (Fig. 7.6):

The order of dissection will be as follows: The superficial veins of the anterior triangle will be studied. The contents of each subdivision of the anterior triangle will be dissected in the following order: muscular triangle, carotid triangle, submandibular triangle, and submental triangle.

 

Dissection Instructions

Superficial Fascia

  1. The platysma muscle has been removed, revealing the external jugular vein. Follow the external jugular vein superiorly and observe that it is formed by the joining of the posterior division of the retromandibular vein and the posterior auricular vein (Fig. 7.5).
  1. In the superficial fascia near the anterior midline, note the anterior jugular vein (Fig. 7.5). It courses inferiorly near the midline to the suprasternal region where it penetrates the superficial layer of the deep cervical fascia. The anterior jugular vein passes deep to the sternocleidomastoid muscle to join the external jugular vein in the root of the neck.

Muscular Triangle The contents of the muscular triangle of the neck are the infrahyoid muscles, the thyroid gland, and the parathyroid glands. The boundaries of the muscular triangle are (Fig. 7.3):

1.  In the midline of the neck, use a probe to break through the superficial layer of the deep cervical fascia and identify the sternohyoid muscle (Fig. 7.6). The inferior attachment of the sternohyoid muscle is the sternum and its superior attachment is the body of the hyoid bone. The sternohyoid muscle depresses the hyoid bone.

2.  Lateral to the sternohyoid muscle, identify the superior belly of the omohyoid muscle. The superior belly is attached to the inferior border of the hyoid bone. The inferior belly of the omohyoid muscle attaches to the superior border of the scapula near the suprascapular notch. The omohyoid muscle depresses the hyoid bone.

3.  Use blunt dissection to loosen the medial border of the sternohyoid muscle from the structures that lie deep to it. Use scissors to transect the sternohyoid muscle close to the hyoid bone and reflect the muscle inferiorly.

4.  Use a probe to raise the medial border of the superior belly of the omohyoid muscle and loosen it from deeper structures. Use scissors to transect the superior belly of the omohyoid muscle close to the hyoid bone and reflect it inferiorly.

5.  Identify the sternothyroid muscle (Fig. 7.6). The inferior attachment of the sternothyroid muscle is the sternum and its superior attachment is the oblique line of the thyroid cartilage. The sternothyroid muscle depresses the larynx.

6.  Identify the thyrohyoid muscle. The inferior attachment of the thyrohyoid muscle is the oblique line of the thyroid cartilage and its superior attachment is the hyoid bone. The thyrohyoid muscle elevates the larynx.

7.  The ansa cervicalis innervates the four infrahyoid muscles. It will be identified later.

8.  Gently retract the right and left sternothyroid muscles to widen the gap in the midline. Identify (Fig. 7.6):

a.  Laryngeal prominence

b.  Cricothyroid ligament

c.  Cricoid cartilage

d.  First tracheal ring

e.  Isthmus of the thyroid gland

IN THE CLINIC:Tracheotomy

Tracheotomy (tracheostomy) is the creation of an opening into the trachea. As an emergency operation, it must be rapidly performed in cases with sudden obstruction of the airway (e.g., aspiration of a foreign body, edema of the larynx, or paralysis of the vocal folds). The opening is made in the midline between the infrahyoid muscles of the neck.

 

Submandibular Triangle

  1. The contents of the submandibular triangle are the submandibular gland, facial artery, facial vein, stylohyoid muscle, part of the hypoglossal nerve (XII), and lymph nodes. The boundaries of the submandibular triangle are (Fig. 7.3):
  2. Refer to a skull. On the temporal bone, identify the mastoid process and the styloid process.
  3. Examine the inner aspect of the mandible and identify:
  4. On the cadaver, identify the submandibular gland and use a probe to define its borders. Note that a portion of the gland extends deep to the posterior border of mylohyoid muscle.
  5. Use blunt dissection to separate the facial artery and vein from the submandibular gland. Note that the facial vein passes superficial to the submandibular gland and the facial artery courses deep to the gland.
  6. Preserve the facial vessels and use scissors to remove the superficial part of the submandibular gland. Do not disturb the deep part of the gland.
  7. Use blunt dissection to clean the superficial surface of the anterior and posterior bellies of the digastric muscle (Fig. 7.6). The anterior attachment of the anterior belly is the digastric fossa of the mandible. The anterior belly of the digastric muscle is innervated by the mylohyoid nerve, a branch of the mandibular division of the trigeminal nerve (V3). The posterior attachment of the posterior belly is the mastoid process of the temporal bone and it is innervated by the facial nerve (VII). The two bellies attach to each other by an intermediate tendon. The intermediate tendon is attached to the body and the greater horn of the hyoid bone by a fibrous sling. The digastric muscle elevates the hyoid bone and depresses the mandible.
  8. Identify the tendon of the stylohyoid muscle, which attaches to the body of the hyoid bone by straddling the intermediate tendon of the digastric muscle. The stylohyoid muscle is innervated by the facial nerve and it elevates the hyoid bone.
  9. Use a probe to follow the hypoglossal nerve (XII) through the submandibular triangle. Observe that the hypoglossal nerve enters the submandibular triangle by passing deep to the posterior belly of the digastric muscle. It passes deep to the mylohyoid muscle within the submandibular triangle (Fig. 7.7).

Submental Triangle The contents of the submental triangle are the submental lymph nodes. The submental triangle is an unpaired triangle that crosses the midline. The boundaries of the submental triangle are (Fig. 7.3):

 

Carotid Triangle

1.   The contents of the carotid triangle are the carotid arteries (common, internal, and external), the branches of the external carotid artery, part of the hypoglossal nerve, and branches of the vagus nerve (X). The boundaries of the carotid triangle are (Fig. 7.3):

o  Inferomedial – superior belly of the omohyoid muscle

o  Inferolateral – anterior border of the sternocleidomastoid muscle

o  Superior – posterior belly of the digastric muscle

2.   Clean the anterior margin of the sternocleidomastoid muscle from its inferior end to its superior attachment. Use blunt dissection to free the anterior border of the muscle from the deep cervical fascia.

3.   Transect the sternocleidomastoid muscle about 5 cm superior to its attachments to the sternum and clavicle.

4.   Reflect the sternocleidomastoid superiorly. Attempt to conserve the cutaneous branches of the cervical plexus that radiate from the posterior border of the sternocleidomastoid muscle. Use your fingers to free the sternocleidomastoid muscle from the deep cervical fascia as far superiorly as the mastoid process. Release of the sternocleidomastoid up to the mastoid process is important to facilitate disarticulation of the head in a later dissection.

5.   Find the accessory nerve (XI) where it crosses the deep surface of the sternocleidomastoid muscle near the base of the skull. Trace the accessory nerve superiorly as far as possible. Note that the accessory nerve passes through the jugular foramen to exit the skull but this relationship is too far superior to be seen at this time.

6.   To allow better access to deeper structures cut the common facial vein where it empties into the internal jugular vein. Transect the digastric muscle at its intermediate tendon and reflect the posterior belly.

7.   Palpate the tip of the greater horn of the hyoid bone (Figs. 7.7 and 7.8). Find the hypoglossal nerve superior to the tip of the greater horn of the hyoid bone. Observe that a muscular branch of the occipital artery crosses superior to the hypoglossal nerve. The hypoglossal nerve carries axons of spinal nerve C1 that branch off as the nerve to the thyrohyoid muscle.

8.   Use blunt dissection to trace the hypoglossal nerve anteriorly. Verify that the hypoglossal nerve passes medial to the posterior belly of the digastric muscle and deep to the mylohyoid muscle (Fig. 7.7).

9.   The superior root of the ansa cervicalis travels with the hypoglossal nerve (Fig. 7.8). The superior root of the ansa cervicalis is mainly composed of fibers from C1. The inferior root of the ansa cervicalis (C2, C3) passes around the carotid sheath to join the superior root. Thus, a loop (L. ansa, handle) is formed.

10. Clean the ansa cervicalis and trace its delicate branches to the lateral borders of the infrahyoid muscles (Fig. 7.8).

11. Use a probe to raise the posterior border of the thyrohyoid muscle and identify the thyrohyoid membrane that extends between the thyroid cartilage and the hyoid bone (Fig. 7.9). Find the internal branch of the superior laryngeal nerve where it passes through the thyrohyoid membrane. The internal branch of the superior laryngeal nerve supplies sensory fibers to the mucosa of the larynx.

12. Follow the internal branch of the superior laryngeal nerve proximally. It joins the external branch of the superior laryngeal nerve to form the superior laryngeal nerve (Fig. 7.9). The superior laryngeal nerve may be too far superior to be seen at this stage of the dissection.

13. Trace the external branch of the superior laryngeal nerve distally and observe that it innervates the cricothyroid muscle. It also innervates part of the inferior pharyngeal constrictor muscle.

14. While preserving the ansa cervicalis, use scissors to open the carotid sheath. The carotid sheath contains the common carotid artery, internal carotid artery, internal jugular vein, and vagus nerve (X).

15. Observe that the internal jugular vein is located lateral to the common carotid or internal carotid artery in the carotid sheath. Use an illustration to study its largest tributaries: common facial vein, superior thyroid vein, and middle thyroid vein. Use blunt dissection to separate the internal jugular vein from the common and internal carotid arteries.

16. At the level of the superior border of the thyroid cartilage, find the origin of the external carotid artery (Fig. 7.9). Use blunt dissection to follow the external carotid artery superiorly until it passes on the medial side of (deep to) the posterior belly of the digastric muscle. Temporarily replace the posterior belly of the digastric muscle in its correct anatomical position to confirm this relationship.

17. The external carotid artery has six branches in the carotid triangle (Fig. 7.9). Each branch has a companion vein that may be removed to clear the dissection field. At this time, identify five branches:

o  Superior thyroid artery – arises from the anterior surface of the external carotid artery at the level of the superior horn of the thyroid cartilage. The superior thyroid artery descends to the superior pole of the lobe of the thyroid gland. The superior laryngeal artery is a branch of the superior thyroid artery, which pierces the thyrohyoid membrane together with the internal laryngeal nerve.

o  Lingual artery – arises from the anterior surface of the external carotid artery at the level of the greater horn of the hyoid bone (Fig. 7.9). It passes deeply into the muscles of the tongue. Do not follow it at this time.

o  Facial artery – arises from the anterior surface of the external carotid artery immediately superior to the lingual artery (Fig. 7.9). The facial artery crosses the inferior border of the mandible to enter the face. In 20% of cases, the lingual and facial arteries arise from a common trunk.

o  Occipital artery – arises from the posterior surface of the external carotid artery and supplies part of the scalp (Fig. 7.9).

o  Posterior auricular artery – arises from the posterior surface of the external carotid artery and passes posterior to the ear to supply part of the scalp.

18. Use blunt dissection to clean the bifurcation of the common carotid artery. Identify the carotid sinus, a dilation of the origin of the internal carotid artery. The wall of the carotid sinus contains pressoreceptors that monitor blood pressure. The carotid sinus is innervated by the glossopharyngeal nerve (IX) and the vagus nerve (X).

19. The carotid body is a small mass of tissue located on the medial aspect of the carotid bifurcation. The carotid body monitors changes in oxygen and carbon dioxide concentration of the blood. The carotid body is innervated by the glossopharyngeal nerve (IX) and the vagus nerve (X)

20. Identify the internal carotid artery and note that it has no branches in the neck.

21. The ascending pharyngeal artery is the sixth branch of the external carotid artery. It branches from the medial surface of the external carotid artery. Use your fingers to raise the external carotid artery and look for the origin of the ascending pharyngeal artery close to the carotid bifurcation.

22. Identify the vagus nerve (X) within the carotid sheath where it lies between and posterior to the vessels (Fig. 7.9). To see the vagus nerve, retract the internal jugular vein laterally and the common carotid artery medially.

Dissection Review

  1. Replace the sternocleidomastoid muscle and the infrahyoid muscles in their correct anatomical positions. Review the attachments and actions of the infrahyoid muscles.
  2. Review the cutaneous branches of the cervical plexus. Review the ansa cervicalis.
  3. Use the dissected specimen to review the positions of the common carotid and internal carotid arteries, internal jugular vein, and vagus nerve within the carotid sheath.
  4. Follow each branch of the external carotid artery through the regions dissected, noting their relationships to muscles, nerves, and glands.
  5. Trace the branches of the superior laryngeal nerve distally and note their distribution.
  6. Review the course of the hypoglossal nerve.
  7. Review the ansa cervicalis and its relationship to the hypoglossal nerve and carotid sheath.
  8. Note that the superior laryngeal nerve passes medial to the internal and external carotid arteries and the hypoglossal nerve passes lateral to the internal and external carotid arteries.

 

Thyroid and Parathyroid Glands

 

Dissection Overview

The cervical viscera are the pharynx, esophagus, larynx, trachea, thyroid gland, and parathyroid glands. The thyroid gland and parathyroid glands lie between the infrahyoid muscles and the larynx and trachea, and these glands will be dissected now. The pharynx, esophagus, larynx, and trachea will be dissected after head disarticulation has been performed.

 

Dissection Instructions

  1. Once again, reflect the sternocleidomastoid and sternohyoid muscles.
  2. Observe the thyroid gland. The thyroid gland is located at vertebral levels C5 to T1. Laterally, the thyroid gland is in contact with the carotid sheath (Fig. 7.10B).
  3. Identify the right lobe and left lobe of the thyroid gland. The two lobes are connected by the isthmus, which crosses the anterior surface of tracheal rings 2 and 3 (Fig. 7.10A).
  4. Frequently, the thyroid gland has a pyramidal lobe that extends superiorly from the isthmus. The pyramidal lobe is a remnant of embryonic development that shows the route of descent of the thyroid gland.
  5. Identify the superior thyroid artery where it enters the superior end of the lobe of the thyroid gland. Recall that the superior thyroid artery is a branch of the external carotid artery. The inferior thyroid artery will be dissected later.
  6. The superior and middle thyroid veins are tributaries to the internal jugular vein. The right and left inferior thyroid veins descend into the thorax on the anterior surface of the trachea. The right and left inferior thyroid veins drain into the right and left brachiocephalic veins, respectively.
  7. Look for the thyroidea ima artery (L. ima, lowest). The thyroidea ima artery is a relatively rare (published reports place the incidence at 2% to 12% of the population) but clinically significant variant. When present, the thyroidea ima artery enters the thyroid gland from inferiorly, near the midline.
  8. Use scissors to cut the isthmus of the thyroid gland. Use blunt dissection to detach the capsule of the thyroid gland from the tracheal rings. Spread the lobes widely apart.
  9. On both sides of the cadaver, use blunt dissection to display the recurrent laryngeal nerve that passes immediately posterior to the lobe of the thyroid gland in the groove between the trachea and esophagus. Use an illustration to note the close relationship of the recurrent laryngeal nerve to the thyroid gland (Fig. 7.10B).
  1. Cut all blood vessels leading to or from the left lobe of the thyroid gland. Use a probe to free the left lobe from surrounding connective tissue and remove it.
  2. Examine the posterior aspect of the left lobe of the thyroid gland and attempt to identify the parathyroid glands. The parathyroid glands are about 5 mm in diameter and may be darker in color than the thyroid gland. Usually, there are two parathyroid glands on each side of the neck but the number can vary from one to three.

IN THE CLINIC: Recurrent Laryngeal Nerve

If a recurrent laryngeal nerve is injured by a thyroid tumor or during thyroidectomy (removal of the thyroid gland), paralysis of the laryngeal muscles will occur on the affected side. The result is hoarseness of the voice.

IN THE CLINIC: Parathyroid Glands

The parathyroid glands play an important role in the regulation of calcium metabolism. During thyroidectomy, these small endocrine glands are in danger of being damaged or removed. To maintain proper serum calcium levels, at least one parathyroid gland must be retained during surgery.

Dissection Review

  1. Review the relationship of the thyroid gland to the infrahyoid muscles, carotid sheaths, larynx, and trachea.
  2. Use an illustration and the dissected specimen to review the blood supply and venous drainage of the thyroid gland. Note that there are only two thyroid arteries (superior and inferior) but there are three thyroid veins (superior, middle, and inferior).
  3. Review the relationship of the parathyroid glands to the thyroid gland. Use an embryology textbook to review the origin and migration of the thyroid and parathyroid glands during development.

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THURSDAY, APRIL 29: ROOT OF THE NECK

 

Dissection Overview

The root (base) of the neck is the junction between the thorax and the neck. The root of the neck is an important area because it lies superior to the superior thoracic aperture. All structures that pass between the head and thorax or the upper limb and thorax must pass through the root of the neck.

The order of dissection will be as follows: The branches of the subclavian artery will be dissected. The course of the vagus and phrenic nerves will be studied. The muscles that form the floor of the posterior cervical triangle will be studied. Some of these structures will be followed superiorly or inferiorly beyond the root of the neck.

 

Dissection Instructions

  1. The clavicle has been cut at its midlength and the thoracic wall was removed during dissection of the thorax. Remove the anterior thoracic wall and set it aside.
  2. Reflect superiorly the sternocleidomastoid muscle, sternohyoid muscle, and sternothyroid muscle.
  3. Clean the inferior belly of the omohyoid muscle. Note that its inferior belly and superior belly are joined by an intermediate tendon. Review its attachments and action.
  4. Use scissors to cut the fascial sling that binds the intermediate tendon of the omohyoid muscle to the clavicle.
  5. Follow the external jugular vein through the investing layer of deep cervical fascia. Note that the external jugular vein is the only tributary of the subclavian vein. To expose the blood vessels in the root of the neck, remove the investing layer of deep cervical fascia that forms the roof of the lower part of the posterior cervical triangle.
  6. Identify the subclavian vein (Fig. 7.11). Use blunt dissection to loosen the subclavian vein from structures that lie deep to it.
  7. Follow the subclavian vein inferiorly to the point where it is joined by the internal jugular vein to form the brachiocephalic vein.
  8. Identify the subclavian artery. Observe that the right subclavian artery is a branch of the brachiocephalic trunk and the left subclavian artery is a branch of the aortic arch.
  9. The subclavian artery has three parts that are defined by its relationship to the anterior scalene muscle (Fig. 7.12):
  10. The first part of the subclavian artery has three branches:
  11. The second part of the subclavian artery has one branch, the costocervical trunk (Fig. 7.12). The costocervical trunk arises from the posterior surface of the second part of the subclavian artery. Use your fingers to elevate the subclavian artery from the surface of the first rib and use blunt dissection to look for the costocervical trunk passing posteriorly above the cupula of the pleura. The costocervical trunk divides into the deep cervical artery and the supreme intercostal artery [YOU DO NOT HAVE TO FIND THESE BRANCHES]. The supreme intercostal artery gives rise to posterior intercostal arteries 1 and 2.[DO NOT SPEND AN INORDINATE AMOUNT TIME LOOKING FOR THE COSTOCERVICAL TRUNK]
  1. The third part of the subclavian artery has one branch, the dorsal scapular artery. The dorsal scapular artery passes between the superior and middle trunks of the brachial plexus to supply the muscles of the scapular region (Fig. 7.13). In about 30% of cases the dorsal scapular artery arises from the transverse cervical artery instead of from the subclavian artery.
  2. Find the thoracic duct, which ascends from the thorax into the neck. The thoracic duct is posterior to the esophagus at the level of the superior thoracic aperture, and then it arches anteriorly and to the left to join the venous system near the junction of the left subclavian vein and the left internal jugular vein (Fig. 7.13). The thoracic duct is usually a single structure, which has the diameter of a small vein, but it may be represented by several smaller ducts.
  3. On the right side of the neck, several small lymphatic vessels join with lymph vessels from the right upper limb and right side of the thorax to form the right lymphatic duct. The right lymphatic duct drains into the junction of the right subclavian and right internal jugular veins.
  4. On both sides of the neck, find the vagus nerve in the carotid sheath and follow it into the thorax. Note that the vagus nerve passes posterior to the root of the lung.
  1. The right vagus nerve passes anterior to the subclavian artery, where it gives off the right recurrent laryngeal nerve. The left vagus nerve passes on the left side of the aortic arch, where it gives off the left recurrent laryngeal nerve.
  2. Follow the right and left recurrent laryngeal nerves superiorly along the lateral surface of the trachea and esophagus. Trace them as far as the first tracheal ring. Do not follow them into the larynx at this time.
  3. Verify that the phrenic nerve crosses the anterior surface of the anterior scalene muscle (Fig. 7.13) postesrior to the transverse cervical and suprascapular arteries. Follow the phrenic nerve into the thorax and confirm that it passes anterior to the root of the lung.
  4. Identify the cervical portion of the sympathetic trunk. Note that the inferior cervical sympathetic ganglion is located in the root of the neck. Verify that the cervical sympathetic trunk is continuous with the thoracic sympathetic trunk.
  5. Examine the muscles that form the floor of the posterior cervical triangle. Identify the splenius capitis, the levator scapulae, and the anterior, middle, and posterior scalene muscles.
  6. Use blunt dissection to define the borders of the anterior scalene and middle scalene muscles. The anterior and middle scalene muscles attach to the first rib. The first rib and the adjacent borders of the anterior and middle scalene muscles form the boundaries of the interscalene triangle. Observe (Fig. 7.13):
  7. Use blunt dissection to clean the roots of the brachial plexus at the level of the interscalene triangle. Identify the parts of the supraclavicular portion of the brachial plexus: roots, trunks, and divisions.
  8. If the upper limb has been dissected previously, follow the suprascapular nerve as far laterally as the suprascapular notch of the scapula where it is joined by the suprascapular artery.

IN THE CLINIC; Interscalene Triangle

The interscalene triangle becomes clinically significant when anatomical variations (additional muscular slips, an accessory cervical rib, or exostosis on the first rib) narrow the interval. As a result, the subclavian artery and/or roots of the brachial plexus may be compressed, resulting in ischemia and nerve dysfunction in the upper limb.

Dissection Review

  1. Replace the anterior thoracic wall in its correct anatomical position. Replace the infrahyoid muscles and sternocleidomastoid muscle in their correct anatomical positions. Review the boundaries of the posterior cervical triangle. Review the attachments of the infrahyoid muscles. Review the distribution of the cutaneous branches of the cervical plexus.
  2. Remove the anterior thoracic wall. Review the origin and course of the brachiocephalic artery, left common carotid artery, and left subclavian artery in the superior mediastinum.
  3. Review the three parts and the branches of the subclavian artery.
  4. Review the distribution of the transverse cervical, suprascapular, and dorsal scapular arteries to the superficial muscles of the back and scapulohumeral muscles.
  5. Use an illustration to review the course of the vertebral artery from its origin on the first part of the subclavian artery to the cranial cavity.

 

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MONDAY, MAY 3 AND WEDNESDAY, MAY 5: PAROTID REGION AND INFRATEMPORAL FOSSA

 

Dissection Overview

The temporal region consists of two fossae: temporal and infratemporal. The temporal fossa is located superior to the zygomatic arch and it contains the temporalis muscle. The infratemporal fossa is inferior to the zygomatic arch and deep to the ramus of the mandible. The infratemporal fossa contains the medial and lateral pterygoid muscles, branches of the mandibular division of the trigeminal nerve (V3), and the maxillary vessels and their branches. The infratemporal and temporal fossae are in open communication with each other through the area between the zygomatic arch and the lateral surface of the skull.

The dissection will proceed as follows: The masseter muscle will be studied. The zygomatic arch will be detached and the masseter muscle will be reflected with the arch attached to it. The temporalis muscle will be studied. The coronoid process of the mandible will be detached and the temporalis muscle will be reflected with the coronoid process attached to it. The superior part of the ramus of the mandible will then be removed and the maxillary artery will be traced across the infratemporal fossa. The branches of the mandibular division of the trigeminal nerve will be dissected. The medial and lateral pterygoid muscles will be studied and the temporomandibular joint will be dissected.

 

Skeleton of the Temporal Region

Refer to a lateral view of the skull and identify the following (Figs. 7.15 and 7.24):

  1. Temporal fossa – formed by parts of four cranial bones: parietal, frontal, squamous part of temporal, and greater wing of sphenoid. Review the location of the pterion.
  2. Zygomatic arch – formed by the zygomatic process of the temporal bone and the temporal process of the zygomatic bone.
  3. Mandibular fossa and articular tubercle – on the temporal bone (Fig. 7.24).

From a lateral view of the mandible, identify (Fig. 7.31A):

  1. Head
  2. Neck
  3. Mandibular notch
  4. Coronoid process
  5. Ramus
  6. Angle

On the internal surface of the mandible, identify (Fig. 7.31B):

  1. Lingula – for the attachment of the sphenomandibular ligament
  1. Mandibular foramen – for the inferior alveolar nerves and vessels
  2. Mylohyoid groove – for the mylohyoid nerve and vessels

Remove the mandible from the skull and view the bones of the infratemporal fossa from the lateral perspective. Identify (Fig. 7.32):

  1. Pterygomaxillary fissure – between the lateral plate of the pterygoid process and the maxilla
  2. Inferior orbital fissure – between the greater wing of the sphenoid bone and the maxilla
  3. Infratemporal surface of the maxilla
  4. Greater wing of the sphenoid bone – contains the foramen ovale and the foramen spinosum
  5. Lateral plate of the pterygoid process of the sphenoid bone
  6. Pterygopalatine fossa – at the superior end of the pterygomaxillary fissure
  7. Sphenopalatine foramen – an opening in the medial wall of the pterygopalatine fossa that enters the nasal cavity

Reposition the mandible on the skull and identify the boundaries of the infratemporal fossa:

  1. Lateral – ramus of the mandible
  2. Anterior – the infratemporal surface of the maxilla
  3. Medial – lateral plate of the pterygoid process
  4. Roof – greater wing of the sphenoid bone

 

Dissection Instructions - Masseter Muscle and Removal of the Zygomatic Arch

  1. Reflect the facial nerve branches and the parotid duct anteriorly.
  2. Clean the lateral surface of the masseter muscle. The superior attachment of the masseter muscle is the inferior border of the zygomatic arch and its inferior attachment is the lateral surface of the ramus of the mandible. The masseter muscle elevates the mandible (closes the jaw) and protrudes the mandible. It is innervated by the masseteric branch of the mandibular division of the trigeminal nerve (V3).
  1. Cut the temporal fascia along the superior temporal line and use a scalpel to peel it inferiorly. Observe that the temporalis muscle is attached to the deep surface of the temporal fascia. Cut the temporal fascia along the superior border of the zygomatic arch and remove the fascia completely.
  2. Insert a probe deep to the zygomatic arch as close to the orbit as possible (Fig. 7.33, arrow 1). Use a saw to cut through the zygomatic bone to the probe.
  3. Insert the probe deep to the zygomatic arch near the anterior border of the head of the mandible (Fig. 7.33, arrow 2). Use a saw to cut through the zygomatic arch to the probe.
  4. Reflect the masseter muscle and the attached portion of the zygomatic arch in the inferior direction. Use a scalpel to detach the masseter muscle from the superior part of the ramus of the mandible, but leave the masseter muscle attached to the inferior part of the ramus, near the angle. During reflection, the masseteric nerve and vessels will be cut.

 

Temporal Region

  1. On a skull, identify the boundaries of the temporal fossa (Fig. 7.15):
  2. Note that the temporal vessels and the auriculotemporal nerve are located in the scalp, superficial to the temporal fascia. The primary content of the temporal fossa is the temporalis muscle.
  3. Identify the temporalis (temporal) muscle. Observe:

 

Infratemporal Fossa

  1. On a skull, identify the boundaries of the infratemporal fossa:

Superior:

Zygomatic arch superficially

Infratemporal crest of the sphenoid bone deeply

Anterior – alveolar border of maxilla

Lateral – ramus of mandible

Medial – lateral plate of the pterygoid process

  1. Wear eye protection for all steps that require the use of bone cutters.
  2. The ramus of the mandible must be removed to view the contents of the infratemporal fossa. The ramus of the mandible must be removed on both sides of the head to permit the head to be bisected in a later dissection step.
  3. The first step is to remove the coronoid process and reflect the temporalis muscle with the coronoid process attached. Insert a probe through the mandibular notch and push it anteroinferiorly toward the third mandibular molar tooth (Fig. 7.34, arrow 1). Keep the probe in close contact with the deep surface of the mandible. Use a saw to cut through the coronoid process to the probe.
  4. Reflect the coronoid process together with the temporalis muscle in the superior direction. Use blunt dissection to release the temporalis muscle from the skull and note that the deep temporal nerves (branches of the mandibular division of the trigeminal nerve) enter the muscle from its deep surface (Fig. 7.35B). The deep temporal nerves provide motor innervation to the temporalis muscle and they are accompanied by deep temporal arteries.
  5. Insert a probe medial to the neck of the mandible (Fig. 7.34, arrow 2). Use a saw to cut through the neck of the mandible.
  6. Place a probe horizontally deep to the ramus of the mandible, at the level of cuts 1 and 2. Scrape the probe against the inside of the mandible until you’re certain that the probe lies directly against the bone. Then, keeping the probe horizontal, move the probe inferiorly until it is stopped by the lingula of the mandible. Keeping the probe in place to protect deeper structures, make a horizontal saw cut through the mandible (approximately at dotted line #3 in Figure 7.34).
  7. Deep to the mandible, identify the inferior alveolar nerve and vessels (Fig. 7.35B). Clean the inferior alveolar nerve and follow it to the mandibular foramen (Fig. 7.31B). Note that the mylohyoid nerve arises from the inferior alveolar nerve just before it enters the mandibular foramen and runs in the mylohyoid groove (Fig. 7.31B).
  8. The inferior alveolar nerve and vessels enter the mandibular foramen and pass distally in the mandibular canal. Note that the inferior alveolar nerve provides sensory innervation to the mandibular teeth. The mental nerve is a branch of the inferior alveolar nerve, which passes through the mental foramen to innervate the chin and lower lip.
  9. Identify the lingual nerve (Fig. 7.35B). The lingual nerve is located just anterior to the inferior alveolar nerve. The lingual nerve passes medial to the third mandibular molar tooth and it innervates the mucosa of the anterior two-thirds of the tongue and floor of the oral cavity.
  10. Identify the maxillary artery where it arises from the bifurcation of the external carotid artery (Fig. 7.35A). The maxillary artery crosses either the superficial surface (two-thirds) or the deep surface (one-third) of the lateral pterygoid muscle. If the maxillary artery in your specimen passes deep to the lateral pterygoid muscle, perform step 15 first and then return to step 12.
  11. Use blunt dissection to trace the maxillary artery through the infratemporal fossa. The maxillary artery has 15 branches. Identify only the following five (Fig. 7.35A):

                               i.   Middle meningeal artery – arises medial to the neck of the mandible and courses superiorly, passing through a split in the auriculotemporal nerve. The middle meningeal artery passes through the foramen spinosum, enters the middle cranial fossa, and supplies the dura mater.

                             ii.   Deep temporal arteries (anterior and posterior) – pass superiorly and laterally across the roof of the infratemporal fossa at bone level and enter the deep surface of the temporalis muscle.

                            iii.   Masseteric artery (cut in a previous dissection step) – courses laterally and passes through the mandibular notch to enter the deep surface of the masseter muscle.

                            iv.   Inferior alveolar artery – enters the mandibular foramen with the inferior alveolar nerve.

                             v.   Buccal artery – passes anteriorly to supply the cheek.

  1. Identify the lateral pterygoid muscle (Fig. 7.35A). The lateral pterygoid muscle has two heads. The anterior attachment of the superior head is the infratemporal surface of the greater wing of the sphenoid bone. The anterior attachment of the inferior head is the lateral surface of the lateral plate of the pterygoid process. The posterior attachments of the lateral pterygoid muscle are the articular disc within the capsule of the temporomandibular joint and the neck of the mandible. The lateral pterygoid muscle depresses the mandible (opens the jaw).
  2. Inferior to the lateral pterygoid muscle, identify the medial pterygoid muscle (Fig. 7.35B). The lingual nerve and inferior alveolar nerve pass between the inferior border of the lateral pterygoid muscle and the medial pterygoid muscle and can be used as guides to separate the two muscles. The proximal attachments of the medial pterygoid muscle are the maxilla and the medial surface of the lateral plate of the pterygoid process. The distal attachment of the medial pterygoid muscle is the inner surface of the ramus of the mandible. The medial pterygoid muscle elevates the mandible (closes the jaw).
  3. [ON ONE SIDE ONLY] Remove the lateral pterygoid muscle to see the deeper part of the infratemporal fossa. Define the inferior border of the lateral pterygoid muscle by inserting a probe between it and the medial pterygoid muscle. Use scissors to cut the lateral pterygoid muscle close to its posterior attachments to the neck of the mandible and the articular disc. Remove the muscle in a piecemeal fashion to preserve superficially positioned nerves and vessels.
  4. Use a probe to follow the inferior alveolar nerve and the lingual nerve to the foramen ovale in the roof of the infratemporal fossa (Fig. 7.35B). Identify the chorda tympani, which joins the posterior side of the lingual nerve (Fig. 7.35B).
  5. Follow the maxillary artery toward the pterygopalatine fossa. Before entering the pterygopalatine fossa the maxillary artery divides into four branches: posterior superior alveolar artery, infraorbital artery, descending palatine artery, and sphenopalatine artery. At this time, identify only the posterior superior alveolar artery, which enters the infratemporal surface of the maxilla (Fig. 7.35A). The other branches will be dissected later.

IN THE CLINIC; Dental Anesthesia

A mandibular nerve block is produced by injecting an anesthetic agent into the infratemporal fossa. Understand from your dissection that the mandibular nerve block will anesthetize not only the mandibular teeth, but also the lower lip, the chin, and the tongue.

 

Dissection Review

    1. Review the attachments and actions of the four muscles of mastication (masseter, temporalis, medial pterygoid, and lateral pterygoid).
    2. Use an atlas illustration to study the origin of the mandibular division of the trigeminal nerve (V3) at the trigeminal ganglion and trace it to the foramen ovale. Follow the mandibular division of the trigeminal nerve through the foramen ovale into the infratemporal fossa. Review the sensory and motor branches of the mandibular division.
    3. Follow the external carotid artery from its origin near the hyoid bone to the infratemporal fossa.
    4. Review the course of the superficial temporal artery and the maxillary artery. Follow the branches of the maxillary artery that were identified in dissection to their regions of supply.
    5. Note the relationship of the middle meningeal artery to the auriculotemporal nerve.
    6. Use an illustration and the dissected specimen to preview the terminal branches of the maxillary artery.

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