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:
- Small bodies
- Relatively large vertebral foramina
- Bifid spinous processes
- Transverse processes that contain a transverse foramen (foramen
transversarium)
Observe the following features of
individual cervical vertebrae:
- Atlas (C1)
- Anterior arch and tubercle
- Transverse process with transverse foramen
- Groove for vertebral artery
- Posterior arch and tubercle
- Superior articular surface for occipital condyle
- Note that the atlas does not have a body.
- Axis (C2)
- Dens
- Body
- Transverse process with transverse foramen
- Lamina
- Spinous process
- Superior articular facet for atlas
- Vertebrae C3 to C7
- Body
- Transverse process with transverse foramen
- Groove for spinal nerve
- Lamina
- Spinous process
- 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:
- Pharynx and esophagus the superior parts of the
digestive tract
- Larynx and trachea
the superior parts of the respiratory tract
- Thyroid gland and parathyroid glands
The visceral part of the neck has the following
boundaries:
- Posterior the cervical vertebrae
- Posterolateral the scalene muscles (not
labeled in Fig. 7.1)
- Lateral the sternocleidomastoid muscle
- Anterior the infrahyoid muscles (e.g., sternohyoid,
sternothyroid)
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
- The skin is thin on the neck. Be careful when removing it; remove
only the skin, leaving as much subcutaneous tissue as possible.
- Refer to Fig. 7.2 and make an anterior midline skin
incision from the jugular notch of the sternum (E) to the chin (F).
- Make a second skin incision along the margin of the mandible from
point F to just below the ear lobe (G).
- 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.
- 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:
- Anterior the posterior border of the sternocleidomastoid
muscle
- Posterior the anterior border of the trapezius muscle
- Inferior the middle one-third of the clavicle
- Superficial (roof) superficial
layer of the deep cervical fascia
- 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.
- 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.
- 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.
- 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; dont be too concerned if it tears or falls apart
completely.
- 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.
- 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:
- Great auricular nerve crosses the
superficial surface of the sternocleidomastoid muscle parallel to the
external jugular vein. The great auricular nerve supplies the skin of the
lower part of the ear and an area of skin extending from the angle of the
mandible to the mastoid process.
- Lesser occipital nerve passes
superiorly along the posterior border of the sternocleidomastoid muscle.
The lesser occipital nerve supplies the part of the scalp that is
immediately behind the ear.
- Transverse cervical nerve passes transversely
across the sternocleidomastoid muscle and neck. It supplies the skin of
the anterior triangle of the neck. If you have trouble identifying the
transverse cervical nerve, it may have been removed with the platysma
muscle.
- Supraclavicular nerves pass inferiorly
to innervate the skin of the shoulder.
- 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.
- 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
- 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.
- 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.
- Review the area of distribution of all cutaneous branches of the
cervical plexus.
- 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.
- Review the course of the occipital artery at the apex of the
posterior triangle.
RETURN TO TABLE OF CONTENTS
WEDNESDAY, APRIL 28: ANTERIOR TRIANGLE OF THE NECK
Dissection Overview
The boundaries of the anterior triangle
of the neck are (Fig. 7.3):
- Anterior the median line of the neck
- Posterior the anterior border of the sternocleidomastoid
muscle
- Superior the inferior border of the mandible
- Superficial (roof) superficial
layer of the deep cervical fascia
- Deep (floor) prevertebral fascia
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):
- Hyoid bone (Gr. hyoideus,
U-shaped) at the angle between the floor of the mouth and the superior end
of the neck
- Thyrohyoid membrane stretching
between the thyroid cartilage and hyoid bone
- Thyroid cartilage (Gr. thyreoeides,
shield) in the anterior midline of the neck
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
- 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).
- 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):
- Superolateral superior belly of the
omohyoid muscle
- Inferolateral anterior border of the
sternocleidomastoid muscle
- Medial median plane of the neck
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
- 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):
- Superior inferior border of the mandible
- Anteroinferior anterior belly of the
digastric muscle
- Posteroinferior posterior belly
of the digastric muscle
- Superficial (roof) superficial
layer of deep cervical fascia
- Deep (floor) mylohyoid and hyoglossus
muscles
- Refer to a skull. On the temporal bone, identify the mastoid
process and the styloid process.
- Examine the inner aspect of the mandible and identify:
- Digastric fossa
- Mylohyoid line
- Submandibular fossa
- Mylohyoid groove
- 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.
- 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.
- 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.
- 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.
- 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.
- 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):
- Right and left anterior bellies of the
right and left digastric muscles
- Inferior hyoid bone
- Superficial (roof) superficial
layer of the deep cervical fascia
- Deep (floor) mylohyoid muscle
- Use a probe to clean the superficial fascia from the surface of the
right and left mylohyoid muscles. Each mylohyoid muscle has a proximal
attachment on the mylohyoid line of the mandible and distal attachments on
the hyoid bone and the mylohyoid raphe. The mylohyoid muscle supports the
floor of the oral cavity.
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
- Replace the sternocleidomastoid muscle and the infrahyoid muscles
in their correct anatomical positions. Review the attachments and actions
of the infrahyoid muscles.
- Review the cutaneous branches of the cervical plexus. Review the
ansa cervicalis.
- 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.
- Follow each branch of the external carotid artery through the
regions dissected, noting their relationships to muscles, nerves, and
glands.
- Trace the branches of the superior laryngeal nerve distally and
note their distribution.
- Review the course of the hypoglossal nerve.
- Review the ansa cervicalis and its relationship to the hypoglossal
nerve and carotid sheath.
- 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
- Once again, reflect the sternocleidomastoid and sternohyoid
muscles.
- 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).
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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).
- 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.
- 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
- Review the relationship of the thyroid gland to the infrahyoid
muscles, carotid sheaths, larynx, and trachea.
- 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).
- 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.
RETURN TO TABLE OF CONTENTS
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
- 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.
- Reflect superiorly the sternocleidomastoid muscle, sternohyoid
muscle, and sternothyroid muscle.
- 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.
- Use scissors to cut the fascial sling that binds the intermediate
tendon of the omohyoid muscle to the clavicle.
- 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.
- Identify the subclavian vein (Fig. 7.11). Use blunt dissection to loosen
the subclavian vein from structures that lie deep to it.
- Follow the subclavian vein inferiorly to the point where it is
joined by the internal jugular vein to form the brachiocephalic
vein.
- 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.
- The subclavian artery has three parts that are defined by its
relationship to the anterior scalene muscle (Fig. 7.12):
- First part from its origin to the
medial border of the anterior scalene muscle
- Second part posterior to the anterior
scalene muscle
- Third part between the lateral border
of the anterior scalene muscle and the lateral border of the first rib
- The first part of the subclavian artery has three branches:
- Vertebral artery courses
superiorly between the anterior scalene muscle and the longus colli muscle
(Fig. 7.12). Trace the vertebral artery
superiorly until it passes into the transverse foramen of vertebra C6.
- Internal thoracic artery arises from the
anteroinferior surface of the subclavian artery and passes inferiorly to
supply the anterior thoracic wall (Fig. 7.12).
- Thyrocervical trunk arises from the anterosuperior surface of the subclavian artery
(Fig. 7.12). The thyrocervical trunk has three
branches:
- Transverse cervical artery crosses the
root of the neck about 2 to 3 cm superior to the clavicle and deep to
the omohyoid muscle (Fig. 7.13). It supplies the trapezius
muscle.
- Suprascapular artery passes
laterally and posteriorly to the region of the suprascapular notch (Fig. 7.13). It passes superior to the
transverse scapular ligament and supplies the supraspinatus and
infraspinatus muscles.
- Inferior thyroid artery passes medially
toward the thyroid gland. Trace the inferior thyroid artery toward the
thyroid gland. Usually, the inferior thyroid artery passes posterior to the
cervical sympathetic trunk. The ascending cervical artery
is a branch of the inferior thyroid artery.
- 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]
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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):
- The subclavian artery and the roots of the brachial
plexus pass between the anterior and middle scalene muscles (through
the interscalene triangle).
- The subclavian vein, transverse cervical artery, and
suprascapular artery cross the anterior surface of the anterior
scalene muscle.
- The phrenic nerve descends vertically across the anterior
surface of the anterior scalene muscle.
- 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.
- 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
- 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.
- 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.
- Review the three parts and the branches of the subclavian artery.
- Review the distribution of the transverse cervical,
suprascapular, and dorsal scapular arteries to the superficial muscles of
the back and scapulohumeral muscles.
- 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.
RETURN TO TABLE OF CONTENTS
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):
- 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.
- Zygomatic arch formed by the zygomatic
process of the temporal bone and the temporal process of the
zygomatic bone.
- Mandibular fossa and articular tubercle on the temporal bone (Fig. 7.24).
From a lateral view of the mandible,
identify (Fig. 7.31A):
- Head
- Neck
- Mandibular notch
- Coronoid process
- Ramus
- Angle
On the internal surface of the mandible,
identify (Fig. 7.31B):
- Lingula for the attachment of the sphenomandibular
ligament
- Mandibular foramen for the
inferior alveolar nerves and vessels
- 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):
- Pterygomaxillary fissure between the
lateral plate of the pterygoid process and the maxilla
- Inferior orbital fissure between the
greater wing of the sphenoid bone and the maxilla
- Infratemporal surface of the maxilla
- Greater wing of the sphenoid bone contains
the foramen ovale and the foramen spinosum
- Lateral plate of the pterygoid process of the sphenoid bone
- Pterygopalatine fossa at the superior
end of the pterygomaxillary fissure
- 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:
- Lateral ramus of the mandible
- Anterior the infratemporal surface of the maxilla
- Medial lateral plate of the pterygoid process
- Roof greater wing of the sphenoid bone
Dissection Instructions - Masseter Muscle and Removal of the Zygomatic Arch
- Reflect the facial nerve branches and the parotid duct anteriorly.
- 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).
- 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.
- 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.
- 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.
- 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
- On a skull, identify the boundaries of the temporal fossa (Fig. 7.15):
- Superior and posterior superior
temporal line
- Anterior frontal and zygomatic bones
- Inferior zygomatic arch superficially and infratemporal
crest of the sphenoid bone deeply
- Deep parts of the frontal, parietal, temporal, and
sphenoid bones
- Superficial temporal fascia
- 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.
- Identify the temporalis (temporal) muscle. Observe:
- The temporalis muscle was attached to the deep surface of the
temporal fascia.
- The inferior attachment of the temporalis muscle is the coronoid
process of the mandible.
- Fibers of the anterior portion of the temporalis muscle have a
vertical direction (elevation of the mandible).
- Fibers of the posterior portion of the temporalis muscle have a
more horizontal direction (retraction of the mandible).
Infratemporal Fossa
- 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
- Wear eye protection for all steps that require the use of bone
cutters.
- 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.
- 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.
- 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.
- 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.
- 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 youre 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).
- 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).
- 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.
- 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.
- 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.
- 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.
- 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).
- 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).
- [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.
- 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).
- 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
- Review the attachments and actions of the four muscles of mastication
(masseter, temporalis, medial pterygoid, and lateral pterygoid).
- 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.
- Follow the external carotid artery from its origin near the hyoid
bone to the infratemporal fossa.
- 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.
- Note the relationship of the middle meningeal artery to the auriculotemporal
nerve.
- Use an illustration and the dissected specimen to preview the
terminal branches of the maxillary artery.
RETURN TO TABLE OF CONTENTS