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Human Anatomy
Head and Neck
FOR MEDICAL AND DENTAL STUDENTS
Revised by: Dr. Mohammad Reza Nikravesh
Professor of Anatomy
(With grateful to web site of Minnesota University for this animation)
IN THE NAME OF GOD
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INTRODUCTION
In most vertebrates, specializations have occurred at the cranial end. As a result, the organs of special sense concerned with vision, hearing, balance, olfaction and taste are found in the head. The inflow of sensory input from these sense organs has caused an enormous development of the cranial end of the nervous system, resulting in the formation of the brain. The brain and special sense organs are in close association with one another and have become enclosed in a common protective covering known as the skull. The part of the skull protecting the brain, internal ear, eye and nose is called the neurocranium while that part of the skeleton forming the face and lodging the organs of taste is known as the viscerocranium.
These two components have fused to form the skull. However, the lower jaw or mandible, which is a part of the viscerocranium is still free to move at the temporomandibular joint. The two different parts of the skull have different developmental origins. Thus the neurocranium is developed from paraxial mesoderm whereas the viscerocranium is derived from branchial mesoderm. The branchial mesoderm not only forms the facial skeleton but also gives rise to the branchial musculature which is concerned with mastication and facial expression. In addition, it gives rise to the pharyngeal and laryngeal musculature concerned with deglutition and phonation. Owing to the dominance of vision in higher primates, including humans, the olfactory sense organs have undergone recession. Thus the facial region has become more flattened and the eyes have shifted to the front so that there is a considerable increase in the range of binocular stereoscopic vision. Moreover, the erect posture of humans and their bipedal mode of progression have resulted in the shifting of the foramen magnum towards the centre of the base of the skull. This enables the skull to be balanced more easily without having recourse to the development of the large postvertebral neck muscles so characteristic of lower mammals. Though the arrangement of structures in the neck appears complicated at first sight, it is built on the same general pattern as the trunk. However, the coelom represented by the pleural, pericardial and peritoneal cavities in the trunk is absent in the neck. In addition, both branchial and somatic musculature are found in the neck. The somatic muscles are concentrated in front of, and behind the vertebral column as the pre- and postvertebral muscle groups, as well as in the ventral midline as the superior continuation of the rectus abdominis mass. The branchial musculature is found around the pharynx and larynx. In the peripheral parts of the neck, it is represented by the trapezius, sternocleidomastoid and platysma.
The entire framework of the human body is known as the skeleton. It is made up of two primary groups of bones;
1. Axial skeleton - This group of bones are the ones that make up the head and actual trunk of the body.
2. Appendicular skeleton - This is the group of bones that make up the body’s extremities (arms and legs).
The entire group of bones that make up the head is called the skull and it too can be divided into two primary groups:
1. Cranium- This is the rounded area that houses and protects the brain. It is comprised of eight distinct bones that are fused together in an adult.
2. Facial area - The facial area is made up of fourteen separate bones, which include those that make up the jaws, cheeks and nasal area.
The eight bones that make up the cranium are:
1. Frontal bone-The frontal bone is the one that comprises the forehead, the upper orbit of the eye and the forward parts of the cranium. The frontal bone also contains two air spaces that are called sinuses. The frontal bone is fused with the parietal bones at the top, the sphenoid bones, maxilla and nasal bones.
2. Parietal bones- There are two parietal bones which form the largest portion of the top and sides of the cranium. They are fused down the middle at the top of the skull. In addition to the frontal bones, the parietal bones are also fused to the sphenoid, temporal and occipital bones.
3. Temporal bones-Like the parietal bones, there are two distinct temporal bones. These bones are what form the lower, central sides of the skull. The temporal bones also hold the mastoid sinuses as well as parts of the ears.
4. Ethmoid bone- There is only one ethmoid bone and it has a different consistency that the other bones in the head. Where they are hard and dense, the ethmoid bone is a delicate, spongy bone that is located between the eyes. It also forms a part of the frontal floor area of the cranium.
5. Sphenoid bone-There are two sphenoid bones and they sit behind the eyes and run back towards the temporal bones.
6. Occipital bone-There is only one occipital bone. It forms the back base of the skull.
The facial bones include:
1. Mandible- The mandible (lower jaw) is the only moveable bone in the skull.
2. Maxillae – There are two maxillae bones. They are what comprise the upper jaw and each one of them contains a large “maxillary” sinus.
3. Zygomatic-There are two zygomatic bones and they sit on either side of the skull and comprise the higher area of the cheek.
4. Lacrimal bones- There are two small lacrimal bones that sit at the inside corner of each eye.
5. Vomer-There is only one vomer bone and it is what forms the lower portion of the nasal septum.
6. Nasal bones- The nasal bones are a pair of small, slender bones that support the actual bridge of the nose. They are fused at the top to the frontal bone and to the maxillae at the area that completes the inside orbit of the eye.
In addition to the previous bones, there are six tiny bones (three pairs) called ossicles that are located in the ears. These are joined in such a way as to amplify the sound waves received by the eardrum (tympanic membrane).
1. Malleus- Often called the hammer are the two malleus bones (one in each ear).They are the first bones in the inner ear and appear similar in shape to a hammer. The “handle” part attaches to the tympanic membrane and the “head” portion attaches to the incus.
2. Incus-The incus is also called the anvil and it is the middle of the three bones.
3. Stapes- The stapes is the innermost bone and it appears very similar to a tiny stirrup. In fact, the stapes is often called the stirrup.
There is one final bone of the human skull. It is a single U-shaped bone to which the tongue attaches to and it is called the hyoid bone. The hyoid is located underneath the skull and forms the boundary between the head and neck.
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SOFT TISSUE OF THE HEAD AND NECK AND REGIONAL DISSECTION
POSTERIOR TRIANGLE OF THE NECK
Relevant skeletal features:
temporal bone - mastoid process;
mandible - angle; lower border; symphysis menti;
sternum - jugular notch;
clavicle - medial end; shaft; lateral end.
Subcutaneous structures:
platysma muscle; external jugular vein; lesser occipital nerve; great
auricular nerve; transverse cutaneous nerve of neck; supraclavicular nerves.
Deep fascia:
superficial layer of deep cervical fascia (forming roof of posterior
triangle); prevertebral fascia (covering floor of posterior triangle); axillary sheath.
Muscles:
platysma; sternocleidomastoid; trapezius; inferior belly of omohyoid;
scalenus anterior; scalenus medius; levator scapulae; splenius capitis;
semispinalis capitis.
Nerves:
accessory nerve;
brachial plexus; roots; trunks;
suprascapular; long thoracic;
cervical plexus: cutaneous branches; phrenic nerve.
Arteries:
occipital; suprascapular; subclavian.
Veins:
suprascapular; external jugular; anterior jugular; subclavian.
Lymph nodes:
superficial cervical nodes along external jugular vein.
POSTERIOR TRIANGLE OF THE NECK
DISSECTION:
1. With the body in the supine position make the following incisions:
(a) from the jugular notch along the clavicle as far as the acromion
process of the scapula (if not already made);
(b) from the mastoid process of the temporal bone along the lateral
side of the neck to the acromion process (if not already made); and
(c) from the mastoid process along the anterior margin of the
sternocleidomastoid muscle to the jugular notch, taking care not to cut too deep.
Remove the skin flap.
2. Try to identify the fibres of the platysma muscle in the superficial fascia.
This muscle passes from the skin and fascia below the clavicle towards
the lower border of the mandible. Divide the muscle about 2 cm above
the clavicle and reflect the divided portions. Avoid damaging the
underlying external jugular vein and the supraclavicular nerves.
3. Identify the external jugular vein as it descends from behind the angle
of the mandible towards the middle of the clavicle where it pierces the
deep fascia to enter the subclavian vein. Note its formation by the
union of the posterior auricular vein with the posterior branch of
the retromandibular vein, which emerges from the inferior margin
of the parotid gland. The external jugular vein receives the transverse
cervical, suprascapular and anterior jugular veins, which will be seen later.
4. Try to dissect the following cutaneous branches of the cervical plexus
radiating like the spokes of a wheel from about the middle of the
posterior border of the sternocleidomastoid muscle:
(a) lesser occipital nerve (C2) ascending along the posterior border
of the sternocleidomastoid to reach the back of the scalp and the
adjoining posterior surface of the auricle;
(b) great auricular nerve (C2, 3) passing superficial to the
sternocleidomastoid towards the parotid gland and ear;
(c) transverse cutaneous nerve of neck (C2, 3) crossing the
sternocleidomastoid superficially towards the midline of the neck; and
(d) medial, intermediate and lateral supraclavicular nerves
(C3, 4) descending over the corresponding portions of the clavicle.
5. Now turn your attention to the boundaries of the posterior triangle
formed in front by the sternocleidomastoid, behind by the trapezius
and below by the intermediate third of the clavicle. Observe that the
lower boundary forms the base while the meeting point of the two
muscles constitutes the apex of the triangle.
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6. Identify the superficial layer of deep cervical fascia stretching between
the sternocleidomastoid and trapezius and forming the roof of the
triangle. Secure the accessory nerve (spinal part) lying in this deep
fascia and issuing out from the posterior border of the
sternocleidomastoid at the junction of the upper and middle third of
the muscle. Trace the nerve downwards and note that it disappears
under the anterior border of the trapezius. Note that the accessory
nerve is the uppermost nerve running across the triangle. It supplies
both the sternocleidomastoid and trapezius muscles on their deep surface.
7. Cut the clavicular head of the sternocleidomastoid and displace the
cut portion of the muscle medially to expose the underlying scalenus
anterior muscle close to the root of the neck. Note that this muscle
and the muscles forming the floor of the posterior triangle are covered
by the prevertebral layer of deep cervical fascia. Identify the phrenic
nerve (C3, 4, 5) as it descends in front of the scalenus anterior deep to
the prevertebral fascia. Now clean the structures crossing in front of
the muscle. From above downwards, these are:
(a) inferior belly of omohyoid, a slender muscle running obliquely
downwards and backwards across the posterior triangle about
1-2 cm above the middle third of the clavicle;
(b) suprascapular artery running below the omohyoid and passing
towards the middle of the clavicle to accompany the suprascapular
nerve. The suprascapular artery is a branch of the thyrocervical
trunk from the first part of the subclavian artery;
(c) anterior jugular vein running deep to the sternocleidomastoid in
the lower part of the neck and draining into the external jugular vein; and
(d) subclavian vein which receives the external jugular vein.
8. Behind the scalenus anterior, identify the scalenus medius muscle.
Between these two muscles look for the roots of the brachial plexus
formed by the anterior rami of C5-T1 and the subclavian artery.
Secure the upper trunk formed by the union of C5, 6 and the middle
trunk formed by the anterior ramus of C7, both of which lie above the
omohyoid muscle. Clean the lower trunk formed by C8, T1 below the muscle.
Trace the following branches of the brachial plexus:
(a) suprascapular nerve C5, 6 arising from the upper trunk of the
brachial plexus and running along the upper border of the omohyoid muscle; and
(b) long thoracic nerve from the back of C5, 6, 7 roots descending
behind the brachial plexus. Note that roots C5, 6 pierce the scalenus medius.
9. Identify the muscles which form the floor of the posterior triangle.
From above downwards, these are:
(a) semispinalis capitis near the apex;
(b) splenius capitis;
(c) levator scapulae; and
(d) scalenus medius and posterior lying behind the brachial plexus;
10. Elevate the cut end of the clavicle on the right side and clean the third
part of the subclavian artery. This part of the artery begins at the lateral
border of the scalenus anterior and ends at the outer border of the first
rib where it continues as the axillary artery. Observe the relationship
of the lower trunk of the brachial plexus to the artery. Clean the
subclavian vein which lies below and in front of the artery and is
separated from it by the scalenus anterior. Note that the artery, vein
and trunks of the brachial plexus are enclosed within a prolongation of
the prevertebral fascia which is continued into the axilla as the axillary
sheath. You will see that the subclavian artery arches above the clavicle
while the vein is completely hidden by the bone.
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Summary
It must be noted that the deep cervical fascia covers the neck like a collar.
This superficial layer of deep cervical fascia splits to enclose the
sternocleidomastoid and trapezius muscles. These muscles have possibly
developed from the same premuscle mass. Consequently, it is not surprising
that they are innervated by the same nerve, i.e. accessory nerve, which
provides their motor supply. However, additional branches from the cervical
plexus passing to these muscles are said to provide them with proprioceptive
(sensory) fibers.
Other important structures in the posterior triangle are the brachial plexus
and subclavian vessels. The plexus descends behind the lower fourth of the
sternocleidomastoid muscle to enter the posterior triangle. Here the trunks
of the brachial plexus course downwards and outwards behind the middle
third of the clavicle. The lower trunk of the brachial plexus is closely related
to the subclavian artery so that the groove behind the scalene tubercle of
the first rib is produced not only by the subclavian artery but also by the
lower trunk of the plexus. This close relationship of the vessel and the lower
trunk (or more particularly the ventral ramus of the first thoracic nerve)
implies that both of them can be affected when a cervical rib is present.
Such a condition may lead to a compression of the subclavian artery, which
is the only vessel supplying the upper limb. It can also produce symptoms
of compression of the anterior ramus of the first thoracic nerve.
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POSTERIOR TRIANGLE OF THE NECK
Objectives:
1. Define the boundaries, roof and floor of the posterior triangle.
2. Describe the relationship of the superficial layer of deep cervical fascia
to (a) the trapezius and sternocleidomastoid muscles; and (b) the accessory nerve.
3. Describe the relationship of the prevertebral fascia to the axillary sheath.
4. Test the integrity of the trapezius and sternocleidomastoid muscles.
5. Define the posterior border of the sternocleidomastoid muscle and use
it to surface mark the accessory nerve and brachial plexus.
6. Identify the scalenus anterior muscle and the structures passing in front
of and behind this muscle.
7. Illustrate: (a) the formation of the trunks of the brachial plexus from
nerve roots C5-T1; and (b) the branches given off by the roots and trunks.
8. Trace the course of the neurovascular bundle of the upper limb from
the posterior triangle into the axilla.
9. Enumerate the contents of the posterior triangle.
10. Identify, on a living subject, the surface landmarks of this region.
11. Discuss the anatomical basis of:
(a) injuries to the subclavian artery and lower trunk of the brachial plexus;
(b) damage to the accessory nerve during surgery in the posterior
triangle; and
(c) the cervical rib syndrome.
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QUESTIONS FOR STUDY:
1. What areas are drained by the superficial cervical lymph nodes?
2. The posterior triangle can be divided into a “carefree” portion above
the accessory nerve and a “careful” portion below the accessory nerve.
Why? What important structures may be damaged during surgery in
the inferior portion of the posterior triangle?
3. What is torticollis?
ANTERIOR TRIANGLE OF THE NECK
Relevant skeletal features:
occipital bone - superior nuchal line;
temporal bone - mastoid process;
mandible - lower border; symphysis menti;
hyoid bone - body; lesser and greater horns;
thyroid cartilage - thyroid notch; oblique line;
cricoid cartilage - arch;
trachea - cartilaginous rings;
manubrium of sternum - jugular notch.
Subcutaneous structures:
platysma; anterior jugular vein; cervical branch of facial nerve; transverse
cutaneous nerve of neck; submental lymph nodes.
Deep fascia:
superficial layer of deep cervical; pretracheal; prevertebral; carotid sheath.
Ligaments:
median thyrohyoid; cricothyroid; cricotracheal.
Glands:
parotid; thyroid; parathyroid.
Trachea:
cervical part.
Oesophagus. (cervical part).
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Muscles:
sternocleidomastoid; digastric; mylohyoid; sternohyoid; superior belly
of omohyoid; sternothyroid; thyrohyoid; cricothyroid; inferior constrictor of pharynx.
Nerves:
external laryngeal; internal laryngeal; recurrent laryngeal; hypoglossal;
ansa cervicalis; vagus; sympathetic trunk.
Arteries:
common carotid; internal carotid; external carotid; superior thyroid;
lingual; facial; occipital; posterior auricular; inferior thyroid.
Veins:
internal jugular; superior thyroid; middle thyroid; inferior thyroid;
brachiocephalic.
Lymph nodes:
anterior cervical; jugulodiagastric; juguloomohyoid.
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ANTERIOR TRIANGLE OF THE NECK
DISSECTION:
1. Make the following incisions:
(a) from the mandibular symphysis along the midline to the jugular
notch; and
(b) from the mandibular symphysis along the lower border of the
mandible to the mastoid process.
Remove the skin flap.
2. Again note the fibres of the platysma muscle in the superficial fascia
which pass obliquely upwards and medially from the skin and fascia
below the clavicle to gain insertion into the lower border of the
mandible. The cervical branch of the facial nerve passes from the
lower end of the parotid gland to supply the platysma.
3. Clean the anterior jugular vein which arises from below the chin and
descends close to the midline. Observe that this vein passes deep to the
lower part of the sternocleidomastoid to drain into the external jugular vein.
4. Clear away the remains of the superficial fascia of the anterior triangle
which is bounded by the sternocleidomastoid, the lower border of the
mandible and the midline of the neck. Trace the superficial layer of
deep cervical fascia passing forwards from the anterior border of the
sternocleidomastoid to gain attachment to the lower border of the
mandible, the hyoid bone and the upper border of the sternum.
5. Clean the sternal head of the sternocleidomastoid arising from the
sternum. The clavicular origin of the muscle has already been seen.
Follow the muscle to its insertion into the mastoid process of the
temporal bone and the superior nuchal line of the occipital bone.
Preserve the parotid gland which overlaps this muscle near its insertion.
The muscle is supplied by the accessory nerve and C2 (3) nerves.
6. Remove the remnants of the deep fascia and note the structures lying
along or close to the midline of the neck. From above downwards these are:
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(a) the mandibular symphysis;
(b) the anterior bellies of the digastric muscles arising from the lower
border of the mandible close to the mandibular symphysis and
passing posterolaterally;
(c) the mylohyoid muscles which are visible between the two
diverging anterior bellies of the digastric;
(d) the hyoid bone;
(e) the median thyrohyoid ligament attached to the upper border of
the thyroid cartilage and passing upwards behind the body of
the hyoid to be attached to its upper border;
(f) the V-shaped notch of the thyroid cartilage below the hyoid bone;
(g) the cricothyroid ligament connecting the upper border of the arch
of the cricoid cartilage to the lower border of the thyroid cartilage;
(h) the isthmus of the thyroid gland below the cricoid cartilage; look
for the pyramidal lobe on the upper border of the isthmus. The
pyramidal lobe may be continued upwards as a fibromuscular band
known as the levator glandulae thyroideae. This is attached to the hyoid bone;
(i) trachea; and
(j) jugular notch.
7. Detach the sternocleidomastoid muscle from its sternal origin (if not
already detached) and turn it upwards. Clean the underlying infrahyoid
muscles:
(a) the sternohyoid which arises from the back of the manubrium
of the sternum and medial end of the clavicle to be inserted into
the lower border of the body of the hyoid bone;
(b) the superior belly of the omohyoid which is attached to the body
of the hyoid bone lateral to the insertion of the sternohyoid. Identify
the intermediate tendon connecting the superior and inferior bellies
of the muscle; and
(c) Cut the sternohyoid in the middle and reflect the two halves to
study the attachments of the sternothyroid and thyrohyoid which
lie deep to the sternohyoid. The sternothyroid muscles arises from
the back of the manubrium of the sternum and first costal cartilage
and is inserted into the oblique line of the thyroid cartilage; the
thyrohyoid muscle arises from the oblique line of the thyroid
cartilage and is inserted into the lower border of the of the greater horn of the hyoid bone.
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8. Trace the nerve supply of the infrahyoid muscles from a slender nerve
loop called the ansa cervicalis which lies on the internal jugular vein
at the level of the cricoid cartilage. The thyrohyoid muscle is supplied
by a twig from the hypoglossal nerve carrying C1 fibres.
9. Detach the sternothyroid from its origin and turn it upwards to expose
the thyroid gland. Note that the thyroid gland is enveloped by a sheath
of pretracheal fascia which is attached to the cricoid cartilage and the
oblique line of the thyroid cartilage. Remove this fascia and study the
gland which consists of two lobes connected by an isthmus. Note that
each lobe extends from the oblique line of the thyroid cartilage above
to the level of the fifth or sixth tracheal ring below. The isthmus lies on
the second, third and fourth tracheal rings.
Identify the superior and middle thyroid veins draining into the
internal jugular vein and the inferior thyroid veins of the two sides
draining into the left brachiocephalic vein. Near the upper end of
the lateral lobe, secure the superior thyroid artery, a branch of the
external carotid artery. Near the lower end of the lateral lobe, identify
the inferior thyroid artery, a branch of the thyrocervical trunk of
the subclavian artery. Occasionally there is a thyroidea ima artery,
a branch of the brachiocephalic trunk, supplying the isthmus. Above
identify the external laryngeal nerve accompanying the superior
thyroid artery. Below follow the recurrent laryngeal nerve running
upwards in the groove between the trachea and oesophagus and
disappearing behind the lateral lobe of the thyroid gland. Divide the
isthmus and remove one lobe by cutting the blood vessels close to the
gland taking care not to damage the nerves. Trace the external
laryngeal nerve distally into the cricothyroid muscle. Follow it
proximally to its origin from the superior laryngeal nerve. Trace
the other branch of the superior laryngeal nerve, the internal
laryngeal nerve, which pierces the thyrohyoid membrane to enter
the larynx. Now trace the recurrent laryngeal nerve passing beneath
the lower border of the inferior constrictor muscle of the pharynx
to enter the larynx. Both the superior laryngeal and the recurrent
laryngeal nerves are branches of the vagus nerve.
10. Next clean and examine the cervical portion of the trachea and
oesophagus. Note that the trachea commences at the lower border of
the cricoid cartilage (at C6) and lies in the midline. Anterior to the
trachea are: (a) the infrahyoid muscles; (b) the isthmus of the thyroid
gland; (c) the inferior thyroid veins; and (d) in the thorax the thymus
and the left brachiocephalic vein.
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Note that the oesophagus, which lies behind the trachea, also
commences at the level of the lower border of the cricoid cartilage.
11. Detach the superior belly of the omohyoid from the hyoid bone. Next
clean the common carotid artery and the internal jugular vein which
are enclosed in a fascial sheath known as the carotid sheath. Look for
the deep cervical lymph nodes that are distributed along the length of
the internal jugular vein. Remove these lymph nodes and clean the
contents of the carotid sheath. As you do so, preserve the superior
root of the ansa cervicalis (from C1), a branch of the hypoglossal
nerve which runs down on the common carotid artery to join the
inferior root (C2,3) to form the ansa cervicalis which has already
been dissected. Separate the internal jugular vein from the common
carotid artery and identify the vagus nerve which lies between the two
vessels. Clean the common carotid artery up to its bifurcation into
external and internal carotid arteries. Note that the bifurcation occurs
at the level of the upper border of the thyroid cartilage, where the
external carotid artery lies anteromedial to the internal carotid artery.
Observe that the internal carotid artery, internal jugular vein and vagus
nerve lie within the carotid sheath.
12. Clean the lower branches of the external carotid artery. From below
upwards these are:
(a) superior thyroid artery which descends beneath the superior belly
of the omohyoid to supply the thyroid gland;
(b) ascending pharyngeal artery which ascends medial to the internal
carotid artery. This will be seen later;
(c) lingual artery which arises near the tip of the greater horn of the
hyoid bone; observe that it describes an upward loop which is
crossed by the hypoglossal nerve;
(d) facial artery ascending deep to the posterior belly of the digastric;
(e) occipital artery running backwards along the lower border of the
posterior belly of the digastric; and
(f) posterior auricular artery running along the upper border of the
posterior belly of the digastric.
13. Identify the sympathetic trunk lying posterior to the common and
internal carotid arteries.
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Summary
The neck, like the trunk, is derived from the three primary germ layers-
ectoderm, endoderm and mesoderm. The gut, which is developed from
endoderm, is the deepest structure and is surrounded by visceral or branchial
mesoderm forming the musculature of the gut. Outside this is the somatic
mesoderm, giving rise to the various muscles. Thus in the neck, the gut and
its derivatives are represented by the pharynx and oesophagus running in
the midline as well as by the larynx and trachea which arise as a ventral
outgrowth from the foregut. The thyroid gland, which itself is formed from
a median diverticulum arising from the ventral aspect of the primitive
pharynx, (foregut) lies ventral (anterior) to the lower part of the larynx and
upper part of the trachea. Dorsolateral to the gut in the embryo are the
neurovascular structures. Thus the carotid sheath containing the common
carotid artery, internal jugular vein and vagus nerve occupy a posterolateral
position to the gut. These are overlapped by the lateral lobes of the thyroid
gland. Lying anterior to the thyroid gland are the infrahyoid muscles which
appear to be a cranial continuation of the same muscle mass as the rectus
abdominis. The upward continuation of the infrahyoid muscles are the
geniohyoids. All these muscles are supplied by segmental nerves just like
the rectus abdominis. Superficial to the infrahyoid muscles are the
sternocleidomastoid muscles which run obliquely upwards from the sternum
and clavicle towards the lateral side of the neck to be inserted into the skull.
The exact developmental status of the sternocleidomastoid is uncertain,
although on the basis of its innervation by the accessory nerve, it may be of
branchial origin. Lying superficial to the sternocleidomastoid and running
in the opposite direction to it is the platysma, which is also of branchial
origin. It is supplied by the facial nerve which is the nerve of the second branchial arch.
The pretracheal fascia which envelopes the thyroid gland is attached to
the cricoid cartilage and to the oblique line of the thyroid cartilage. Because
of this attachment, the gland moves up and down together with the larynx
during deglutition. The close relationship of the external laryngeal and
recurrent laryngeal nerves to the superior and inferior thyroid arteries must
be borne in mind during thyroid surgery so as to avoid damage to the nerves.
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ANTERIOR TRIANGLE OF THE NECK
Objectives:
1. Define the boundaries of the anterior triangle of the neck.
2. Describe the arrangement of the superficial layer of deep cervical fascia,
the pretracheal fascia and the carotid sheath, and explain why a
knowledge of this anatomy is of great importance.
3. Enumerate the midline structures of the neck from the mandible to the
jugular notch.
4. Review the attachments and actions of the infrahyoid muscles and the
formation and distribution of the ansa cervicalis.
5. Give an account of the topographical relationships of the thyroid gland.
6. Discuss the clinical importance of:
(a) the blood supply and lymphatic drainage of the thyroid gland; and
(b) the nerves in relation to this gland.
7. Outline the course, branches and area of distribution of the external
carotid artery.
8. Outline the course of the vagus nerve and the sympathetic trunk in the
anterior triangle.
9. Review the attachments and relations of the sternocleidomastoid muscle.
10. Demonstrate the pulsations of the common carotid artery and the
internal jugular vein in the living and explain their clinical importance.
11. Identify the vertebral levels at which the following structures can be
found: the thyroid cartilage, the cricoid cartilage, the beginning of the
trachea, and the isthmus of the thyroid gland.
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QUESTIONS FOR STUDY:
1. What vascular structures in the lower neck are at risk during the
performance of a tracheotomy?
2. What are the relationships of the external laryngeal nerve and the
recurrent laryngeal nerve to the thyroid gland? What functional deficits
would result from injury to these nerves during thyroid surgery?
FACE
Relevant skeletal features:
nasal bone -root of nose;
maxilla - body; processes; infraorbital foramen;
zygomatic bone - arch, zygomaticoorbital foramen;
mandible - ramus; angle; body; symphysis; mental foramen.
Subcutaneous structures:
palpebral branch of lacrimal, infratrochlear, external nasal, infraorbital,
zygomaticofacial, buccal, mental and great auricular nerves.
Muscles:
orbicularis oculi; orbicularis oris; buccinator and other muscles of facial expression.
Nerves:
temporal; zygomatic; buccal; mandibular; cervical and posterior auricular branches of facial nerve.
Arteries:
facial; transverse facial; buccal and infraorbital branches of maxillary artery.
Veins:
facial; deep facial.
page 16:
SCALP
Relevant skeletal features:
skull - vault; base;
individual bones - frontal; parietal; temporal; occipital;
sutures - sagittal; coronal; lambdoid;
meeting point of sutures - bregma; lambda;
eminences - frontal; parietal;
landmarks - nasion; superior orbital margin;
supraorbital - notch; temporal lines; mastoid process;
inion - (external occipital protuberance); superior
nuchal line;
emissary foramina - parietal; mastoid; condylar.
Subcutaneous structures:
supratrochlear nerve and vessels; supraorbital nerve and vessels;
zygomaticotemporal nerve; auriculotemporal nerve and superficial
temporal vessels; great auricular nerve; lesser occipital nerve; greater
occipital nerve and occipital vessels; third occipital nerve; posterior
auricular vessels.
Deep fascia:
temporalis fascia.
Muscles:
occipitofrontalis muscle; epicranial aponeurosis.
Five layers of the scalp.
Nerves:
posterior auricular and temporal branches of facial.
Lymph nodes:
occipital; mastoid.
page 17:
FACE
1. Make the following incisions:
(a) from the root of the nose (nasion) to the symphysis menti;
(b) from the angle of the mouth towards the intertragic notch;
(c) incisions encircling the orbital, nasal and oral apertures;
(d) from nasion to inion (external occipital protuberance) along
the midline; and
(e) frontal incision from ear to ear passing through the vertex.
Reflect all the skin flaps.
2. Note that the muscles of the face are attached to overlying skin and
fascia and consequently function as muscles of facial expression.
Moreover, the muscles surrounding the mouth and eyes have a
sphincteric function.
3. Try to identify the following sensory nerves which are derived chiefly
from the three divisions of the trigeminal nerve:
(a) palpebral branch of the lacrimal nerve, supplying the lateral part
of the upper eyelid;
(b) infratrochlear nerve, a branch of the nasociliary nerve emerging
along the medial angle of the eye to supply the upper half of the
nose, lower eyelid, lacrimal sac and lacrimal caruncle;
(c) external nasal nerve, the terminal part of the nasociliary nerve
supplying the lower half of the nose;
These three branches arise from the ophthalmic division.
(d) infraorbital nerve, emerging 1 cm below the lower margin of the
orbit, and supplying the lower eyelid, lower lateral part of the
nose and upper lip;
page 18:
(e) zygomaticofacial nerve, emerging from the zygomatic bone to
supply the skin over the upper part of the cheek;
These two nerves are from the maxillary division.
(f) buccal nerve, emerging in front of the ramus of the mandible to
supply the skin over the cheek;
(g) mental nerve, a branch of the inferior alveolar nerve emerging
from the mental foramen to supply the skin over the body of the
mandible and lower lip; and
These two nerves are from the mandibular division.
(h) great auricular nerve (C2, 3) supplying branches to the skin over
the angle of the mandible.
4. Clean the orbital and the palpebral portions of the orbicularis oculi
muscles arising from the medial side of the orbit. Note that the fibres
of the muscle encircle the orbit. A deep portion of the muscle, the
lacrimal part, will be identified later.
5. Clean the superficial facial muscles lying between the orbit and the
mouth. These are the levator labii superioris alaeque nasi, levator
labii superioris, zygomaticus minor and zygomaticus major, which
take origin in this order from the frontonasal region medially to the
zygoma laterally. Trace these muscles towards the upper lip. An
additional slip from the levator labii superioris alaeque nasi passes to
the ala of the nose.
Trace the risorius muscle passing from the fascia over the parotid
gland towards the angle of the mouth. Clean the depressor labii
inferioris and the depressor anguli oris, taking origin from the
mandible and passing upwards towards the lower lip and the angle of
the mouth respectively.
Identify the superficial part of the orbicularis oris surrounding the
oral aperture. This is formed by the fibres of the superficial muscles
which pass into the lips.
page 19:
6. Clean the fascia over the parotid gland and look for the parotid duct
arising from the anterior border of the gland and passing forward 2 cm
below the zygomatic arch.
7. Try to secure the following branches of the facial nerve as they emerge
from the anterior border of the parotid gland and radiate forwards to
supply the muscles of the face:
(a) temporal supplying the frontal belly of the occipitofrontalis (see later);
(b) zygomatic supplying the orbicularis oculi;
(c) buccal innervating the superficial group of muscles lying above
the mouth as well as the buccinator and part of the orbicularis
oris;
(d) mandibular supplying the lower part of the orbicularis oris, the
depressor labii inferioris, depressor anguli oris and the risorius;
(e) cervical supplying the platysma;
(f) posterior auricular passing behind the ear and supplying the
occipital belly of the occipitofrontalis.
Note that these nerves form plexuses with the sensory nerves
supplying the face.
8. Clean the following arteries which form a rich network in the face:
(a) facial artery, a branch of the external carotid, curving round the
lower border of the mandible in front of the masseter muscle and
giving off several branches to the face. It anastomes near the inner
angle of the eye with the dorsal nasal branch of the ophthalmic
artery, which is given off by the internal carotid artery inside the cranial cavity;
(b) transverse facial artery, a branch of the superficial temporal
artery, running above the parotid duct to supply the face; the
superficial temporal artery is one of the terminal branches of the
external carotid artery; and
(c) buccal and infraorbital branches of the maxillary artery,
accompanying the corresponding nerves; the maxillary artery is
the other terminal branch of the external carotid artery.
9. Identify the facial vein formed near the medial angle of the eye by the
union of the supratrochlear and supraorbital veins. It has
communications with the ophthalmic vein.
Observe that the branches of the facial nerve, artery and vein lie
between the superficial and deep facial muscles (neurovascular plane).
10. Identify the buccinator and the levator anguli oris muscles which
form the deep facial muscles. Note the origin of the buccinator muscles
from the maxilla above and mandible below and from the
pterygomandibular raphe posteriorly. The fascia covering the
buccinator is known as the buccopharyngeal fascia. Remove this fascia
and trace the fibres of the buccinator towards the lips where they form
the deep layer of the orbicularis oris muscle.
Identify the levator anguli oris lying deep to the zygomaticus minor
and partly overlapped by the levator labii superioris.
page 20:
SCALP
Now turn your attention to the scalp. The layers of the scalp are: (a) skin;
(b) subcutaneous tissue which is tough and loculated; (c) occipital and
frontal bellies of the occipitofrontalis muscle connected by the epicranial
aponeurosis; (d) loose areolar tissue; and (e) pericranium.
1. Try to identify the fibres of the frontal belly of the occipitofrontalis
muscle in the frontal region.
2. Note that the following structures lie in the anterior half of the scalp:
(a) supratrochlear nerve from the frontal branch of the ophthalmic
division ascending close to the medial margin of the orbit to supply
the medial part of the upper eyelid and forehead. It is accompanied
by the supratrochlear artery and vein; and
(b) supraorbital nerve also from the frontal branch of the ophthalmic
division emerging from the upper margin of the orbit about
2-3 cm lateral to the supratrochlear nerve. It runs from the frontal
region towards the lambda. It is accompanied by the supraorbital
artery and vein.
The supratrochlear and supraorbital arteries are branches of the
ophthalmic artery which is given off by the internal carotid artery inside the cranium;
(c) auriculotemporal nerve from the mandibular division ascending
in front of the root of the ear. This nerve is accompanied by the
superficial temporal artery and vein. Note that the superficial
temporal artery divides into anterior and posterior branches. The
superficial temporal vein disappears into the parotid gland where it
joins the maxillary vein to form the retromandibular vein; and
(d) temporal branches of the facial nerve crossing the zygomatic
arch to supply the frontal belly of the occipitofrontalis.
page 21:
3. Note that the following structures lie in the posterior half of the scalp:
(a) great auricular nerve (ventral rami of C2, C3) behind the ear;
(b) lesser occipital nerve (ventral ramus of C2) along the posterior border
of the sternocleidomastoid muscle. This nerve supplies the cranial
surface of the ear and the adjacent part of the scalp behind the ear;
(c) greater occipital nerve (dorsal ramus of C2) ascending 2–3 cm,
lateral to the external occipital protuberance. Note that the nerve
is accompanied by the occipital artery, a branch of the external
carotid artery;
(d) third occipital nerve (dorsal ramus of C3) medial to and below
the greater occipital nerve; and
(e) posterior auricular branch of the facial nerve behind the root
of the ear where it supplies the occipital belly of the
occipitofrontalis. This branch is accompanied by the posterior
auricular artery and vein. The vein joins the posterior branch
of the retromandibular vein to form the external jugular vein.
4. Note the lymph nodes in the region of the external occipital
protuberance and mastoid process.
5. Clean the frontal and occipital bellies of the occipitofrontalis and
the epicranial aponeurosis connecting them. The frontal belly
originates from the skin over the region of the eye brow and the occipital
belly from the lateral part of the highest nuchal line of the occipital
bone. Note that the epicranial aponeurosis is attached laterally to the
superior temporal line and that it also sends an extension to the
zygomatic arch.
6. Incise the aponeurosis longitudinally and introduce the handle of the
scalpel to explore the space deep to the aponeurosis containing loose
areolar tissue.
7. Note that the superficial layers of the scalp move as one unit over the
underlying pericranium.
8. Now examine the temporal region and note that the temporalis muscle
and the overlying temporalis fascia form additional layers of the scalp
in this region. These are situated between the lateral extensions of the
epicranial aponeurosis and the pericranium.
9. Remove the temporalis fascia attached to the superior temporal line
and note the origin of the temporalis muscle from the inferior temporal
line and temporal fossa. The insertion and other details regarding this
muscle will be seen later.
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Summary
The sensory innervation of the face is based on its development. The face is
developed from three primordia—frontonasal, maxillary and mandibular
processes. The frontonasal process gives rise approximately to the nasal
and frontal regions. The maxillary and mandibular processes form the
regions occupied by the maxilla and mandible respectively. Each process
has its own nerve supply. Thus the frontonasal process is supplied by the
ophthalmic division of the trigeminal nerve, while the maxillary and
mandibular divisions supply the areas developed from the corresponding
processes. Consequently branches from the three divisions of the trigeminal
nerve provide the sensory innervation of the face. However, a small area
over the angle of the mandible is supplied by C2 fibres of the great auricular nerve.
Although the facial muscles are known as the muscles of facial
expression, they are generally sphincters and dilators of the oral, nasal
and orbital cavities. Consequently the facial muscles are developed in
relation to these cavities. In humans, the sphincters and dilators are best
developed around the oral cavity and least around the nares. Many of the
muscles whose functions are alimentary and respiratory are used in speech,
e.g. the muscles of the lips and cheeks.
All the facial muscles are developed from the second branchial arch and
hence derive their motor supply from the facial nerve. During the course of
evolution, the facial musculature has gradually differentiated into a number
of components which are used for displaying different emotional states.
The sensory innervation of the scalp in front of the ear is by the three
divisions of the trigeminal nerve; both anterior and posterior rami of cervical
nerves supply the skin of the scalp behind the ear. These nerves run in the
subcutaneous tissue which is dense and loculated. Hence any infection of
the subcutaneous tissue is localised and painful. Scalp infections are also
dangerous since they could spread to the brain via emissary veins. The
arteries of the scalp also run in the subcutaneous tissue and anastomose
extensively. Injury to these vessels can cause profuse bleeding since their
walls are prevented from collapsing by the adherent subcutaneous tissue.
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FACE AND SCALP
Objectives:
1. Describe attachments of orbicularis oculi, orbicularis oris, buccinator
and occipitofrontalis muscles and their actions.
2. Describe the layers of the scalp.
3. Discuss the clinical importance of:
(a) the mobility of the superficial layers of the scalp on the pericranium; and
(b) the ‘dangerous area’ of the scalp.
General objective 2:
4. Outline areas of the face and scalp supplied by the three divisions of
the trigeminal nerve and the developmental basis for this arrangement.
5. Enumerate the nerves in each of the above areas.
6. Describe the relationship of the parotid gland to the motor branches of the facial nerve.
7. Discuss the anatomical basis for the clinical differentiation between
upper and lower motor neuron types of facial nerve paralysis.
5. Describe the blood supply of the face and scalp, emphasising the and richness of their vascularity.
QUESTIONS FOR STUDY:
1. At which points on the face can an arterial pulse be readily felt?
2. What is the arterial supply of the scalp? When the scalp is cut, between
what layers would blood tend to accumulate? How could bleeding be stopped?
3. By what pathway could an infection of the scalp reach the brain?
4. What is Bell’s palsy (facial paralysis)?
page 24:
CRANIAL CAVITY
Relevant skeletal features:
skull - vault; inner and outer tables; diploe.
Anterior cranial fossa:
ethmoid bone - cribriform plate; crista galli;
frontal bone - frontal crest; orbital plates;
sphenoid bone - lesser wing; anterior clinoid process;
foramina - olfactory; anterior and posterior ethmoidal; foramen caecum.
Middle cranial fossa:
sphenoid bone - body; hypophysial fossa; dorsum sellae; posterior
clinoid process; basisphenoid; groove for cavernous sinus; greater wing;
temporal bone - anterior surface of petrous part; squamous part;
groove for posterior branch of middle meningeal artery and vein;
parietal bone - anteroinferior angle; groove for anterior branch of
middle meningeal artery and vein;
foramina - optic canal; superior orbital fissure; foramen
rotundum; foramen lacerum; foramen ovale;
foramen spinosum; hiatuses for greater and lesser
petrosal nerves.
Posterior cranial fossa:
temporal bone - posterior surface of petrous part; squamous part;
mastoid part; groove for sigmoid sinus;
occipital bone - groove for superior sagittal sinus; internal occipital
protuberance; internal occipital crest; groove for transverse sinus;
parietal bone - posteroinferior angle; groove for sigmoid sinus;
foramina - internal acoustic meatus; jugular foramen;
hypoglossal canal; foramen magnum.
Duramater:
outer (endocranial) and inner (meningeal) layers; falx cerebri; falx
cerebelli; tentorium cerebelli; diaphragma sellae; trigeminal cave.
Dural venous sinuses:
superior sagittal sinus; inferior sagittal sinus; straight sinus; occipital
sinus sphenoparietal sinus; cavernous sinus (contents of its walls and its
venous connections); superior petrosal sinus; inferior petrosal sinus;
transverse sinus; sigmoid sinus.
Emissary foramina:
foramen caecum; parietal foramen; mastoid foramen; condylar canal.
page 25:
Cranial nerves:
olfactory; optic; oculomotor; trochlear; trigeminal; abducent; facial; nervus intermedius; vestibulocochlear; glossopharyngeal; vagus; accessory; hypoglossal.
Arteries:
middle meningeal (extradural); internal carotid and branches; vertebral; basilar.
CRANIAL CAVITY
DISSECTION:
1. Remove the remnants of the scalp. Mark a point 2 cm above the
superior orbital margin in front and another point 2 cm above the
external occipital protuberance behind. Connect these two points by
a line passing round the skull. With care, use a saw to cut the outer
table of the skull along this line. Observe the diploe and then with a
chisel and a hammer, gently break the inner table along the saw cut,
taking care not to damage the underlying brain and its coverings.
2. Carefully remove the vault of the skull and examine the outer layer of
dura mater covering the brain. This is the periosteum, the
endocranium, covering the inside of the skull. Deep to this is the inner
layer of dura, which is adherent to the outer layer except where dural
venous sinuses are situated.
page 26:
3. Observe the blood vessels seen on the surface of the dura. These are
the branches of the middle meningeal artery and accompanying veins.
Observe the grooves produced by these vessels on the inside of the vault.
4. Slit open the outer layer of dura in the midline longitudinally and
observe the superior sagittal sinus enclosed within the layers of the
dura. This venous sinus extends from the crista galli in front to the
internal occipital protuberance at the back. Correlate the course of
the sinus with the longitudinally running groove in the middle of the
inner aspect of the vault. Clean the sinus and note: (a) the openings of
the veins draining the cerebral hemispheres, and (b) projections of the
arachnoid called arachnoid granulations (draining cerebrospinal fluid, CSF).
5. Make an anteroposterior incision through the dura on either side of
the superior sagittal sinus and observe the median fold extending
between the two cerebral hemispheres. This dural fold, called the
falx cerebri, is formed by the inner layer of dura.
6. Cut the anterior attachment of the falx cerebri close to the crista galli
and pull it backwards. Observe that the falx cerebri is sickle shaped
and has a free inferior border enclosing the inferior sagittal sinus within its layers.
7. Make a frontal incision in the dura on both sides starting from above
the ear and passing towards the midpoint of the falx cerebri and reflect
the four triangular flaps downwards. Note that the inner surface of the
dura is smooth and that both outer and inner layers are fused.
8. Allow the head to hang over a wooden block to facilitate the removal
of the brain. Pull the frontal lobes of the cerebral hemispheres gently
backwards from the anterior cranial fossa. Identify and cut the
following structures on either side close to the brain:
(a) olfactory nerves from beneath the olfactory bulbs which lie on
the cribriform plate of the ethmoid bone on either side of the crista galli;
(b) optic nerves close to the optic canals;
(c) internal carotid arteries immediately behind the optic nerves;
(d) infundibulum of the pituitary gland near the centre of the
hypophysial fossa;
(e) oculomotor nerves behind and lateral to the infundibulum; and
(f) the slender trochlear nerves behind the oculomotor nerves.
Note that the occipital lobes of the brain rest on the tentorium cerebelli.
9. Release the temporal lobes from the middle cranial fossa. Pass the
knife along the upper surface of the tentorium cerebelli on one side
and cut across the midbrain up to the midline, also severing the
posterior cerebral artery. Repeat this procedure on the opposite side.
Now the cerebrum and part of the midbrain can be removed together.
The rest of the brain stem and the cerebellum are retained in the
posterior cranial fossa.
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10. Study the peripheral attachment of the tentorium cerebelli to the lips
of the groove for the transverse sinus laterally, the superior border of
the petrous part of the temporal bone anteriorly and the posterior
clinoid process medially. This is the attached border of the tentorium.
Now turn your attention to the free border of the tentorium and trace
its attachment to the anterior clinoid process. Examine the tentorial
notch occupied by the midbrain.
11. Cut the tentorium along its attached margin. Next identify and cut the
following structures close to the remaining part of the brain as they
are encountered:
(a) trigeminal nerves near the apices of the petrous part of the
temporal bones;
(b) abducent nerves behind and medial to the trigeminal nerves;
(c) facial and vestibulocochlear nerves near the internal acoustic meati;
(d) glossopharyngeal, vagus and accessory nerves opposite the
jugular foramina; and
(e) hypoglossal nerves close to the hypoglossal canals.
12. Gently push the pons backwards to bring into view the medulla
oblongata and the two vertebral arteries related to it. Divide the
arteries and the medulla oblongata at the level of the foramen
magnum. Now remove the brainstem and cerebellum.
Anterior Cranial Fossa
1. Note that the anterior cranial fossa, which contains the frontal lobe of
the brain, is at a higher level than the middle cranial fossa and is limited
posteriorly by the sharp free border of the lesser wings of the sphenoid
laterally and by the groove connecting the two optic canals in the middle.
2. In the midline of the fossa, observe the sharp keel of bone called the crista
galli as well as the raised ridge of bone running upwards on the frontal
bone called the frontal crest. These give attachment to the falx cerebri.
3. At the junction of the frontal crest and the crista galli note the presence
of the foramen caecum through which may pass an emissary vein
connecting the superior sagittal sinus and the veins of the nasal cavity.
4. Observe the numerous foramina in the cribriform plate on either side of
the crista galli. The olfactory nerves from the nose enter through these
foramina to end in the olfactory bulbs which rest on the cribriform plate.
5. Note that the anterior ethmoidal nerve, a branch of the nasociliary
nerve, and the anterior ethmoidal artery, a branch of the ophthalmic
artery, come from the orbit and run along the lateral margin of the
cribriform plate to enter the nasal cavity.
6. Note the sharp posterior border of the lesser wing of the sphenoid
overhanging the middle cranial fossa. The dura along the free border
contains a dural venous sinus called the sphenoparietal sinus.
7. Trace the posterior border of the lesser wing of the sphenoid medially
and identify the anterior clinoid process to which the free border of
the tentorium cerebelli is attached.
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Middle Cranial Fossa
1. Note that the middle cranial fossa, which contains the temporal lobe
of the brain, is at a higher level than the posterior cranial fossa. Observe
that the posterior boundary of this fossa is formed by the sharp superior
border of the petrous temporal bone posteriorly and the raised area
of the body of the sphenoid with its sella turcica medially.
2. Identify the optic canal through which the optic nerve and ophthalmic
artery, a branch of the internal carotid artery, pass into the orbit.
3. Observe the concave depression in the body of the sphenoid called the
sella turcica. Note the fold of dura mater covering this depression is
called the diaphragma sellae through which passes the stalk of the
pituitary gland. This gland lies in the hypothysial fossa within
the sella turcica. Dissect out the pituitary gland. The posterior wall of
the sella turcica is formed by the dorsum sellae.
4. Examine the cavernous sinus situated on either side of the body of the
sphenoid and dissect the following cranial nerves embedded in its lateral wall:
(a) oculomotor nerve which pierces the dura anterior to the crossing
of the attached and free borders of the tentorium, slightly lateral
to the posterior clinoid process;
(b) trochlear nerve which pierces the dura at the crossing of the free
and attached borders of the tentorium;
(c) ophthalmic division of the trigeminal nerve which lies below the
trochlear nerve; and
(d) maxillary division of the trigeminal nerve lying below the ophthalmic division.
Trace the latter two nerves backwards to the trigeminal ganglion
situated in a depression near the apex of the petrous part of the temporal
bone. Cut the fold of dura called the trigeminal cave to expose this ganglion.
5. Next expose the internal carotid artery and the abducent nerve lying
within the cavernous sinus.
6. Identify the mandibular division of the trigeminal nerve arising from
the lateral part of the ganglion and leaving the cranial cavity through
the foramen ovale.
7. Peel off the dura over the anterior surface of the petrous part of the
temporal bone and look for the greater petrosal nerve, a branch of
the facial nerve passing laterally beneath the trigeminal ganglion
towards the pterygoid canal. It is joined by the deep petrosal nerve
arising from the sympathetic plexus around the internal carotid artery.
8. Look for the lesser petrosal nerve, a branch of the glossopharyngeal
nerve, running anterolaterally to the greater petrosal nerve to leave the
cranial cavity through the foramen ovale.
9. Look for the middle meningeal artery, a branch of the maxillary
artery, which enters the middle cranial fossa through the foramen
spinosum and divides into anterior and posterior branches. The anterior
branch runs towards the pterion from where it ascends on to the vertex.
This branch overlies the motor area of the brain. The posterior branch
runs backwards and then upwards towards the lambda. This branch
runs over the auditory cortex.
10. Observe that the middle cranial fossa also lodges the ventral part of
the diencephalon in the middle.
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Posterior Cranial Fossa
1. The posterior cranial fossa lodges the cerebellum, a part of the hindbrain.
Identify the following structures in this cranial fossa:
(a) trigeminal nerve which will be seen evaginating the dura below
the tentorium near the apex of the petrous part of the temporal
bone. Look for the small motor root on its under surface;
(b) abducent nerve where it pierces the dura over the clivus;
(c) facial nerve along with the nervus intermedius, the
vestibulocochlear nerve and the labyrinthine artery, entering
the internal acoustic meatus;
(d) glossopharyngeal, vagus and accessory nerves passing through
the middle compartment of the jugular foramen. Note that the
anterior compartment transmits the inferior petrosal sinus while
the sigmoid sinus passes through the posterior compartment to
become the internal jugular vein;
(e) hypoglossal nerve lying more medial to the preceding nerves and
leaving the cranial fossa through the hypoglossal canal; and
(f) medulla oblongata, meninges, vertebral arteries, spinal roots
of the accessory nerves passing through the foramen magnum.
Note that the vertebral arteries lie in front of the first pair of
ligamenta denticulata while the spinal roots of the accessory
nerves run behind them.
2. Identify the following dural venous sinuses in the posterior cranial fossa:
(a) superior petrosal sinus along the superior border of the petrous
temporal bone commencing from the posterior end of the cavernous
sinus medially and draining into the junction of the transverse and
sigmoid sinuses laterally;
(b) inferior petrosal sinus in the groove between the basiocciput
and the petrous temporal bone. This vein commences from the
posterior end of the cavernous sinus and passes downwards through
the anterior compartment of the jugular foramen to drain into the
internal jugular vein;
(c) transverse sinus lying in the attached margin of the tentorium
and extending from the internal occipital protuberance to the base
of the petrous temporal bone. Note that the right transverse sinus
is a continuation of the superior sagittal sinus;
(d) straight sinus running in the junction of the falx cerebri and
tentorium cerebelli. Note that it is formed by the union of the
inferior sagittal sinus with the great cerebral vein. Trace its
continuity into the left transverse sinus. At the internal occipital
protuberance, verify if there is a communication between the
transverse sinuses of the two sides, i.e., confluence of the sinuses;
(e) sigmoid sinus, the continuation of the transverse sinus, entering
the posterior compartment of the jugular foramen to become the
internal jugular vein; and
(f) occipital sinus within the falx cerebelli, a fold of dura partially
separating the two cerebellar hemispheres and attached to the
internal occipital crest. Note that it connects the transverse and
sigmoid sinuses. This sinus also communicates with the vertebral
venous plexus.
Slit open these sinuses and verify their communications.
page 30:
Summary
The identification of the cranial nerves becomes easy if one remembers
that the nerves arising from the forebrain (olfactory nerves) are found in
the anterior cranial fossa, while the optic nerve (also from the forebrain)
and the nerves arising from the midbrain (oculomotor and trochlear nerves)
pierce the dura in the middle cranial fossa. The remaining cranial nerves
arise from the pons and medulla of the hind brain and pierce the dura in the
posterior cranial fossa. However, the trigeminal and abducent nerves after
piercing the dura in the posterior cranial fossa, enter the middle cranial
fossa where they are seen in relation to the cavernous sinus and internal
carotid artery.
The functional components appear to determine the site at which a particular cranial nerve pierces the dura. Thus somatic efferent nerves, e.g. oculomotor, trochlear, abducent and hypoglossal nerves, pierce the dura close to the midline whereas those having entirely special somatic afferent
components, e.g. the vestibulocochlear nerve, pierce the dura most laterally. The internal carotid artery which supplies the cerebral hemispheres, excepting the occipital lobe, has a sinuous course. It passes upwards and forwards through the carotid canal in the petrous part of the temporal bone
to enter the middle cranial fossa where it pierces the dura mater and enters the cavernous sinus. It then passes forwards and gives off the important hypophyseal arteries. Thereafter the internal carotid artery ascends medial to the anterior clinoid process and pierces the roof of the cavernous sinus. After giving off the ophthalmic artery, it curves backwards. This tortuous course of the artery, and its passage through the bone and cavernous sinus are possible mechanisms which tend to reduce the pressure in the artery and its branches supplying the brain.