Anatomy 514E
Human Anatomy of the Head and Neck
Unit IV
Instructors:
Julie Rosenheimer, Ph.D.
Scott Lozanoff, Ph.D.
Beth Jones, MS, PT
Sandy Tsuhako, M.D.
Fridays at 1:30 PM
BioMed B-206 / A-206
Sept. 8 - Dec. 1, 2000
1. SUPERFICIAL DISSECTION OF THE NECK
OBJECTIVES RESOURCES
1. Deep cervical fascia Netter, Plate 30;
Discussion 1-1.
Study and understand the principal
layers of cervical fascia, without
memorizing detailed attachments.
- superficial (cervical investing)
layer
- visceral (pretracheal) fascia
- prevertebral fascia
- carotid sheath
2a. Demonstrate on cadaver Discussion 1-2.
the boundaries of the
anterior and posterior
cervical triangles.
b. Identify the sensory nerves of Netter, Plates 18, 123.
the neck:
great auricular
lesser occipital
transverse cervical
supraclavicular
3a. 1) Identify and describe actions Netter Plate 24.
of the infrahyoid muscles:
- sternohyoid
- omohyoid
- sternothyroid
- thyrohyoid
2) and of the suprahyoid muscles: Netter, Plate 47.
- geniohyoid
- mylohyoid
- digastric
- stylohyoid
b. Know the nerve supply of each
of the above-named muscles.
(We will deal with the innervation
of the last 3 suprahyoid muscles
in a later dissection.)
OBJECTIVES RESOURCES
4a. Identify the ansa cervicalis Netter, Plates 27, 123;
and demonstrate its branches. Discussion 1-3.
b. Understand the relationship
between the ansa cervicalis and
the hypoglossal nerve (CN XII).
5. Analyze actions of the sterno-
cleidomastoid muscle.
6. Demonstrate the spinal Discussion 1-4.
accessory nerve (CN XI) and
discuss the location of the
nerve cell bodies which
contribute fibers to it.
7. Although we will not be dissecting Discussion 1-5.
this region of the neck, appreciate the
relationship of the brachial plexus and
the subclavian artery to the anterior
scalene muscle and the clavicle.
8. Identify the external and Netter, Plate 26.
anterior jugular veins and Note that the superficial
indicate where they terminate. veins of the neck (external and
anterior jugular) are somewhat
variable. The external jugular
usually enters the subclavian
vein low in the neck. The
anterior jugular is quite
variable and you may not be able
to find it but, if present, it
usually joins the external
jugular vein deep to the
sternocleidomastoid muscle.
DISSECTION
1. Begin skinning the neck by making one very shallow incision over the body of the mandible and another cut down the midline of the neck from the mandible to the sternal notch. The shallow incision is necessary to preserve the platysma muscle which is located within the superficial fascia just deep to the skin. Now skin in an inferior and lateral direction, i.e., parallel to the platysma muscle fibers, from the cut at the mandible towards the clavicle. This will ensure that you do the least damage to the underlying platysma. Your task is to remove the skin from both sides of the neck and as far posterior as the trapezius muscle at this time.
2. Now separate both platysma muscles from the clavicle and underlying muscles, working from medial to lateral all the way up to the mandible. Leave their superior attachments to the mandible intact. Appreciate the thin anterior jugular vein running vertically on either side of the midline, and the external jugular vein and transverse cervical nerves crossing the sternocleidomastoid muscle just deep to the platysma. You will probably find branches of this cutaneous nerve closely associated with the under surface of the platysma. Why? What is their function? Try to peel them off the platysma and follow them laterally to the posterior border of the sternocleidomastoid muscle. [The transverse cervical nerve is 1 of 4 sensory branches of the cervical nerve plexus. The other 3 branches will be dissected in the next 2 steps.] The anterior triangle of the neck is now exposed. We will return to this triangle in step #7.
3. In the posterior triangle of the neck, using blunt dissection, break the very tough cervical investing fascia along the posterior border of the sternocleidomastoid muscle, splitting the tissue parallel to the muscle fibers. This should expose the large cutaneous great auricular nerve that crosses the muscle on its way toward the skin of the auricle. Use a probe to find another cutaneous nerve-the lesser occipital nerve, under the same tough fascia and in the vicinity of the emergence of the great auricular nerve from behind the sternocleidomastoid. The lesser occipital nerve travels parallel to the posterior border of the sternocleidomastoid muscle towards the skin in the region of the occipital bone.
4. Still using the probe, break the remaining fascia in the posterior triangle and remove any fat with a forceps to find the large spray of supraclavicular nerves, the lowest of the cutaneous nerves of the cervical plexus, which descend from behind the sternocleidomastoid towards the clavicle. Preserve any arteries found in the triangle if you encounter them. Both the transverse cervical artery, heading straight back towards the trapezius muscle, and suprascapular artery, heading back and laterally towards the scapular notch, are in this triangle. Usually the suprascapular is larger and more likely to be found. These arteries will not be followed to their targets in this course so do not spend time today looking for them.
Drawing
5. Cut the attachments of the sternocleidomastoid from the sternum and clavicle and reflect this muscle until you find the large spinal accessory nerve (CN XI) where it enters the deep surface of this muscle high in the neck near the muscle's attachment to the mastoid process. Then follow CN XI from the sternocleidomastoid through the cranial half of the posterior triangle to its other target, the trapezius muscle, by using a probe to break the tough fascia.
6. Clean up the floor of the triangle to simply note the nerves of the brachial plexus, the subclavian vessels and the scalene muscles. These will not be dealt with in more detail in this course, but their relative locations are of clinical significance (Discussion 2-5).
7. In the anterior triangle of the neck strip away the fascia over the surfaces of the suprahyoid muscles. Cut and reflect the anterior belly of the digastric from its attachment to the mandible to reveal the deeper mylohyoid. Next, divide the mylohyoid at the midline, separate it also from its attachment to the mandible, and retract the cut ends to show the deepest geniohyoid muscle. Now follow the digastric back to its posterior belly and note the relationship of the posterior belly of the digastric to the stylohyoid. Next clean and identify the infrahyoid muscles. First locate the more superficial sternohyoid and omohyoid muscles. Then split the sternohyoid to locate the deeper sternothyroid and thyrohyoid. Think about the function of these muscles.
8. With a probe, break the tough fascia of the carotid sheath covering the internal jugular vein to find the ansa cervicalis--a loop of nerves representing the motor component of the cervical plexus that partially lies in the carotid sheath and usually encircles the internal jugular. [An alternate way to find the ansa is to follow its branches from their targets back to their origin at the ansa!] What does the ansa innervate? Follow the superior limb of the loop to its approximation with the large hypoglossal nerve (CN XII) high in the neck; then follow the inferior limb of the ansa to its source from the cervical nerves.
Drawings
DISCUSSION
1-1
In this dissection you will encounter examples of deep or investing fascia. This term applies to all fasciae deep to the superficial fascia, which is also called subcutaneous tissue. Upon removal of the superficial fascia, one finds a complete fascial investment, the "investing fascia". This can invest such structures as muscles, aponeuroses, bones or glands. Investing fascia is given regional names such as the cervical investing fascia you are seeing today, the brachial fascia, the fascia lata, etc. Fascial spaces are also important clinically because infections or abscesses tend to spread along fascial planes. The most important such fascial plane in the neck lies between the visceral fascia investing the pharynx and the prevertebral fascia around the vertebral column muscles. This is referred to as the retropharyngeal space and abscesses can spread in this space all the way down to the posterior mediastinum!
1-2
The boundaries of the anterior cervical triangle are the mandible above, the midline of the neck medially and the anterior border of the sternocleidomastoid muscle laterally. The boundaries of the posterior cervical triangle are the clavicle below, the posterior border of the sternocleidomastoid muscle anteriorly and the anterior border of the trapezius muscle posteriorly.
1-3
Ansa Cervicalis. The ventral rami of the first four cervical nerves form the cervical plexus. Each of the first four cervical nerves is joined by a postganglionic communicating ramus from the superior cervical ganglion, hence, in addition to sensory and motor fibers, each nerve consists also of postganglionic sympathetic fibers. You will observe the four cutaneous branches of the cervical plexus in your dissection. The ansa cervicalis is the motor part of the cervical plexus and innervates primarily the infrahyoid muscle group. The word "ansa" means "loop" and refers to the loop formed between a superior limb and an inferior limb. The superior limb of the loop originates from the first and second cervical nerves. A nerve leaves this loop and runs along with the hypoglossal nerve (CN XII) for a few centimeters to ultimately innervate the geniohyoid (a suprahyoid muscle) and thyrohyoid (an infrahyoid muscle). The inferior limb of the loop or ansa is formed by branches of the second and third cervical nerves. The loop formed by the two limbs can be quite variable in the level at which it is formed. Despite the close association of the ansa cervicalis with the hypoglossal nerve, all of its nerve fibers are considered to derive from the upper three cervical nerves and not the hypoglossal nerve.
1-4
The spinal accessory nerve fibers in the neck are branchiomotor fibers whose nerve cell bodies are located in the branchiomotor column of the cervical spinal cord. This column of cells extends into the medulla where it is continuous with the column of cells that contribute branchiomotor fibers to the cranial portion of CN XI, the vagus nerve (CN X) and the glossopharyngeal nerve (CN IX).
1-5
Because the roots of the brachial plexus and the subclavian artery pass between the anterior and middle scalene muscles, they are subject to compression against the first rib when the anterior scalene muscle contracts. The resulting "numbness" produced in the upper limb has been variously called the "scalenus anticus syndrome," "cervical rib syndrome," or "thoracic outlet syndrome." A simple test, Adson's test, is helpful in diagnosis. The patient sits with hands on the knees, takes a deep breath, extends the neck as much as possible and turns the face to the affected side. The test is positive if the pulse is obliterated in that limb. False positives may occur, however.
2. DEEP DISSECTION OF THE NECK
OBJECTIVES RESOURCES
1. Vessels
a. Demonstrate these branches of Netter, Plates 29,
the external carotid artery: 130, 131.
- superior thyroid
- superior laryngeal
- lingual
- occipital
- facial
b. Appreciate anastomoses in the Discussion 2-1.
carotid system.
c. Demonstrate 2 of the 3 branches Netter, Plates 28,
of the subclavian artery at the 65, 69, 131.
root of the neck (vertebral, and the
inferior thyroid branch of the
thyrocervical trunk).
d. Appreciate the position of the Netter, Plates 64,
subclavian vein. 68, 186;
Discussion 2-2.
e. Locate the thoracic duct where Netter, Plate 220.
it enters the left venous angle
in the neck.
OBJECTIVES RESOURCES
2. Nerves
a. Locate on the cadaver cranial Netter, Plates 27, 29, nerves IX, X, XI and XII. 63, 65.
b. Understand in general the Discussion 2-3.
components contained in each of
these nerves and the tissues
innervated by each.
c. Locate in the cadaver the Netter, Plates 65,
two branches of the superior 68, 69, 70, 74.
laryngeal nerve, namely the internal and
external laryngeal nerves; and
the inferior (recurrent) laryngeal;
all branches of the vagus nerve.
Discuss their components.
d. Locate the superior cervical Netter, Plates 122,
ganglion and the cervical 124, 125.
sympathetic trunk on the
cadaver.
e. Discuss the outflow from the Discussion 2-4.
cervical sympathetic ganglia.
3. Locate the carotid sinus and Netter, Plate 119.
carotid body in the cadaver
and explain their innervations
and functions.
4. Thyroid and parathyroid glands.
a. Note the position and blood
supply of the adult thyroid gland.
b. Discuss the location of the
parathyroid glands and demonstrate
at least one of them on the cadaver.
c. Understand the development of the
thyroid, parathyroids and the thymus.
d. Explain thyroglossal duct cysts.
5. Understand the derivatives of pharyngeal
pouches I, II, III, IV and V.
6. Learn the muscles, their innervation
and skeletal derivatives of each branchial
(pharyngeal) arch.
DISSECTION
*Do this dissection on both sides of the neck. If the right side has been marred by embalming, a more complete dissection may have to be performed on the left side. Both sternocleidomastoid muscles should have been cut from their sternal and clavicular attachments.
1. With a scalpel, carefully cut the attachments of the trapezius, pectoralis major and deltoid muscles from the superior and inferior aspects of both clavicles. Then, with a rib cutter, or hand saw if necessary, cut the lateral aspects of the clavicles free from the acromion process of the scapula. Next cut through the sternoclavicular joints with a scalpel to free the clavicles from the sternum. Remove both clavicles being careful of the underlying subclavian arteries and veins and the roots of the brachial plexus.
2. Carefully clean away the tissues around the carotid bifurcation, which is the site of the carotid sinus and the very small carotid body. Maintain any possible nerve branches from CN IX supplying this area. What information is this portion of CN IX carrying?
3. Trace as far as possible the branches of the external carotid artery. The superior thyroid and its branch, the superior laryngeal, the lingual, facial and occipital should be able to be identified at this time. You may look for the ascending pharyngeal as well, but it is often difficult to locate. It often branches off the medial aspect of the external carotid. The remaining branches, namely the posterior auricular, maxillary and superficial temporal arteries will be identified during the upcoming dissections of the face.
4. Find the large vagus nerve (CN X) within the carotid sheath. On both sides in the neck, identify the inferior (recurrent) laryngeal nerves. Remember that on the left, the nerve loops around the aortic arch, and on the right the nerve loops around the right subclavian artery. Locate the recurrent laryngeal nerves as they ascend between the trachea and the esophagus to the larynx. What does the recurrent laryngeal nerve innervate?
Drawings
5. Identify the 2 branches of the superior laryngeal nerve of the vagus, namely the (sensory) internal branch of the superior laryngeal nerve entering a small hole in the thyrohyoid fascia with the superior laryngeal artery, and the (motor) external branch of the superior laryngeal nerve traveling next to the superior thyroid artery to its target, the cricothyroid muscle. What does the internal branch innervate?
6. With your fingers displace anteriorly the common carotid artery and vagus nerve. Clear away the fascia behind these structures to find the large swelling lying against the vertebrae. This lump is the superior cervical ganglion of the sympathetic trunk. Follow its thin trunk as far inferiorly as possible. The inferior ganglion and a loop around the subclavian artery known as the ansa subclavia may also be visible, but don't spend a lot of time searching for these. What do fibers of the superior cervical ganglion innervate?
7. Expose the thyroid gland by splitting the sternohyoid and sternothyroid muscles. Cut the isthmus of the gland and fold back each half of the gland to expose its posterior surface. Try to trace a small artery from the inferior thyroid to a small, discrete body lying on the deep surface of the thyroid gland. If you are so lucky to find one, this lump is a parathyroid gland; several may be present.
8. If you have not already done so, find the large hypoglossal nerve (CN XII) as it crosses the neck about at the level of the carotid bifurcation, and trace the nerve towards the tongue.
9. Clean the surfaces of the left internal jugular and the left subclavian vein located just deep to the clavicle. Remove the fat carefully with a forceps behind the junction of the left internal jugular and subclavian veins to find the terminal part of the thoracic lymph duct and its entrance into the veins at that junction. The duct may be filled with clotted blood so that it resembles a vein here. Ordinarily, the duct forms a distinct loop before joining the junction and is usually rather easily found, but ask for help if you are having problems. Do you remember what region of the body is drained by the thoracic duct?
10. On the left side only, cut the attachment of the anterior scalene muscle from the first rib, reflect it, and expose the left subclavian artery. Trace the branches coming off the subclavian artery to the best of your ability. In particular, the inferior thyroid artery, which is one of three branches of the thyrocervical trunk, and the vertebral artery should be found. Since the sternum is still in place, it may be difficult on some bodies to locate the vertebral artery. The internal thoracic artery and other 2 branches of the thryrocervical trunk are dissected during dissection of the thorax, and the costocervical trunk is quite variable.
Drawings
DISCUSSION
2-1
Branches of the external carotid arteries from the two sides anastomose quite freely across the midline. A useful generalization is that it would be possible to ligate one external carotid artery without depriving its side of the face and neck of a blood supply. Good examples of vessels anastomosing across the midline are the superior thyroids, the labial branches of the facial arteries and the superficial temporal arteries. There are others. This is a matter of practical importance to the surgeon and is introduced here to make you aware of these connections even though they are not demonstrable by gross dissection.
2-2
The exact positions of the subclavian and external jugular veins have clinical significance because of the common practice of vena puncture for central venous pressure measurement or intravenous feeding. Examine its position in your cadaver. Note that the subclavian vein passes anterior to the anterior scalene muscle, and that the apex of the parietal pleura is close to the vein. Plate 64 in Netter shows that the clavicle is directly anterior to the vein. The vein is "subclavian" in the sense that it passes behind the clavicle to get to the axillary fossa. Note also that the external jugular vein passes anterior to the SCM.
2-3
There is a discontinuous column of cells in the brainstem which can appropriately be called the branchiomotor column. These are nuclei which consist of clusters of motor neurons which are located intermediate in position between somatomotor nuclei and autonomic nuclei in the brainstem. The branchiomotor fibers which leave the brainstem exit with cranial nerves associated with the branchial arches. Thus, cranial nerves V, VII, IX, X and XI contain such fibers. It is important to point out that branchial arch-related nerves never convey somatomotor fibers.
2-4
The superior cervical ganglion sends postganglionic fibers to the majority of the structures in the head and neck which require sympathetic innervation. Its largest outflow is the internal carotid nerve which enters the skull with the internal carotid artery. It also sends communicating (postganglionic) rami to the upper four cervical nerves, it forms a postganglionic plexus which follows the external carotid artery to the face, it contributes postganglionic fibers to the pharyngeal plexus on the back wall of the pharynx and finally, it forms the postsynaptic superior cardiac nerve which descends to the cardiac plexus. Each of these outflows is nicely illustrated in Netter, Plate 124. The major function of the sympathetics is to regulate the size of the blood vessels, but fibers to the orbit also regulate the dilator of the pupil and the smooth muscle in the upper eyelid. Damage to this outflow will result in a condition called Horner's syndrome: constricted pupil, drooping eyelid, flushed face and lack of sweating on the face.
3. SUPERFICIAL DISSECTION OF FACE AND SCALP
OBJECTIVES RESOURCES
1. Skull (osteology)
a. Identify the following bones Netter, Plates 1-5;
on a skull: Discussion 3-1.
-frontal
-parietal
-occipital
-temporal bone and mastoid
and styloid processes
-sphenoid
-zygomatic
-maxilla
-lacrimal
-nasal
-ethmoid
-vomer
-palatine
-mandible
b. Identify the following
foramina on a skull:
-stylomastoid
-supraorbital
-infraorbital
-mental
2. Muscles of facial expression
a. Identify the following muscles Netter, Plate 20.
of facial expression: frontalis,
orbicularis oculi, levator labii superioris,
zygomaticus major, buccinator,
orbicularis oris, depressor anguli
inferioris and platysma.
b. Understand the general attach- Discussion 3-2.
ments of all muscles of facial
expression.
c. Describe the characteristics of a
patient with facial nerve paralysis.
d. Know the branchial (pharyngeal) arch
origin of this muscle group.
OBJECTIVES RESOURCES
3. Nerves
a. Locate the main trunk of the Netter, Plate 19.
facial nerve in the substance
of the parotid gland and follow
a few of its terminal branches.
b. Describe the components of the Consider this branch of the
facial nerve as it leaves the facial nerve to be entirely
stylomastoid foramen and branchiomotor to striated
determine the general location muscle.
of the nerve cell bodies.
c. Identify the following nerves Netter, Plates 18, 31.
and know the location of their
cell bodies: The nerve cell bodies of the
sensory branches of the
-supraorbital trigeminal nerve are located
-infraorbital in the trigeminal ganglion
-mental within the skull.
4. Identify the parotid gland and trace Netter, Plate 19. Note
the parotid duct from the gland to that much of the gland
where it enters the cheek. is behind the ramus
of the mandible.
5. Trace the path of the Netter, Plates 17, 63, 64.
facial artery from the submental In general, the facial artery
region to the angle of the eye. supplies blood to the super-
ficial face.
6. Identify a site of collateral Netter, Plate 80.
venous flow between a branch of the
facial vein and a vein entering the
skull, and appreciate the clinical
implications.
7. Identify the layers of the scalp
and describe the general composition
of each layer.
8. Try to identify lymph nodes which
receive lymph from the scalp and
face. Be aware of the cervical
collar of lymph nodes (occipital,
retroauricular, parotid, submandibular
and mental).
DISSECTION
2. For the scalp, remove the skin carefully by trying to find the separation between the
skin and the distinct membrane deep to it called the galea aponeurotica (which is
really the extensive aponeurosis for some of the facial muscles). Take off the skin
of the scalp to a coronal plane even with or slightly behind the ears. As you do this,
work through the layers of the scalp and name them from superficial to deep.
the suggested muscles to be shown are associated with these sphincter-like muscles.
(The eyelids should be carefully skinned.) In particular, in addition to the frontalis, orbicularis oculi, and orbicularis oris muscles, identify the levator labii superioris, zygomaticus major, buccinator (which lies deep to a fat pad in the cheek; this pad and the risorius muscle need to be removed first), depressor anguli oris and re-identify the platsyma, which you dissected and reflected in a previous lab. With your table-mates, demonstrate the expression that contraction of each of these muscles produces.
4. As you skin just anterior and superior to the auricle, identify and save the superficial temporal artery, the end-branch of the external carotid artery. (Feel the pulse of this artery on yourself.)
5. Next, identify the parotid gland and duct. Remove the fat around the borders of both gland and duct to show the structures in their entirety. Appreciate that the parotid gland and duct are covered superficially by the same tough investing fascia that covers the neck. (How do you think the parotid gland got its name? In a mirror, locate the parotid papilla surrounding the opening of the parotid gland duct (Stenson's duct) in your own buccal mucosa. It is just lateral to your upper back molars. You can actually feel it with your tongue.)
Drawing
6. Identify on at least one side the three prominent facial foramina used by cutaneous nerves to the face, namely, the supraorbital (may only be a notch), infraorbital, and mental foramina. Then, using the skull as a reference, locate each foramen on the cadaver by using a probe to separate the tissues in the vicinity of the foramen. Be sure to probe down to the bone and do not be afraid to separate the tissues with some force. Alternatively, you may prefer to begin the search for these nerves and foramina by making a bold vertical cut with a scalpel blade right down to the bone in the general location of each foramen and then turn to the probe. Either technique will expose the supraorbital, infraorbital, and mental nerves, each of which is a cutaneous end-branch of the trigeminal nerve (CN V). (Most people only have a supraorbital notch. Check yours. You can easily feel the notch.)
7. There is a fat pad at the corner of the mouth just lateral to the orbicularis oris muscle. If you have not already removed it in search of the buccinator muscle, probe in this fat to find the loop of the facial artery, which almost always lies in this region. The loop may be already cut from the skinning, in which case the cut ends of the artery can usually be easily identified at this time. Now follow the course of the facial artery and its major branches as completely as possible over the face. The best way to do this is to use a scissors to cut the muscles and other tissues that at times cover the arteries. (Why do you think the facial artery is so tortuous? Do you know how to locate the pulse of this artery?)
8. Finally, locate the branches of the facial nerve coursing to their target muscles through the parotid gland and follow these branches back to the main trunk exiting the stylomastoid foramen. The parotid gland is usually very tough so feel free to cut through this glandular material in conjunction with blunt dissection to trace the branches through the gland. Note on the enlarged inset that there are significant branches of the facial nerve on either side of, that is, superior and inferior to, the parotid gland duct. Four or five branches can usually be found.
Drawing
DISCUSSION
3-1
Knowledge of the skull is essential before one can thoroughly understand the soft structures in the head and neck. Time prohibits discussing the details of osteology with you but you are encouraged to ask questions following initial efforts on your own. You will find that the skull makes more sense when you begin associating the soft tissue parts with it and vice versa. This practice should continue throughout your work on the head and neck. A foramen or fissure assumes much greater significance when a nerve or vessel is seen traversing it. Skulls will be available during the study of the head and neck. USE THEM. If you would like to refer to a skull at the dissection table please ask.
3-2
In general, the muscles of facial expression arise from bone and insert into the skin. Their function is to move the skin of the face, thereby expressing emotional states. They are concentrated around the major openings in the facial skeleton - orbits, nose, and mouth. You are not required to learn all of these muscles, but the several listed in the objectives are to be considered as examples. The orbicularis oculi, buccinator and orbicularis oris are three which have important functions in addition to their role in expressing emotions.
4. INFRATEMPORAL FOSSA
OBJECTIVES RESOURCES
1. Infratemporal fossa Netter, Plates 2, 5, 9.
Identify the following structures: the infratemporal fossa is the
space bounded laterally by
-foramen ovale the ramus of the mandible,
-foramen spinosum medially by the lateral
-mandibular foramen pterygoid plate and superiorly
-pterygoid plates (medial and lateral) by the undersurface of the
-sphenoid spine greater wing of the sphenoid
-greater wing of the sphenoid bone bone.
2. Temporomandibular joint Netter, Plates 10, 11;
Discussion 4-1.
Study the temporomandibular
joint and define axes around
which movements occur:
-transverse axis - elevation/depression
-gliding (no axis) - protraction/retraction
-vertical axis - rotation
3. Muscles of mastication Netter, Plates 48, 49.
Identify the attachments and analyze
the function of the muscles of
mastication :
-masseter
-temporalis
-medial pterygoid
-lateral pterygoid
OBJECTIVES RESOURCES
a. Demonstrate the bifurcation of the Netter, Plates 35, 63,
external carotid artery into maxillary 95.
and superficial temporal arteries.
b. Follow the maxillary artery into In general one can say that
the infratemporal fossa and the maxillary artery supplies
identify the following branches: the deep face. This would
- middle meningeal artery entering include the structures found
foramen spinosum in the infratemporal fossa, the
- inferior alveolar artery entering pterygopalatine fossa, the
mandibular foramen nasal cavity, and the sur-
- example of a deep temporal artery rounding bones, which we
- sphenopalatine artery will see in later dissections.
5 . Nerves
a. Demonstrate the mandibular Note that the lesser
nerve exiting the foramen ovale petrosal nerve also exits
on the cadaver. the skull through the ovale.
b. Identify the sensory Netter, Plates 41, 116.
and motor branches of the
mandibular division of CN V:
Anterior division Note: you will not be held
- masseteric responsible for identifying
- temporal the motor branches of the
- lateral pterygoid anterior division.
- buccal
Note: the buccal nerve is the
Posterior division only sensory branch of the
- lingual anterior division and the
- inferior alveolar mylohyoid nerve is the only
- mylohyoid motor branch of the posterior
- auriculotemporal division of V3.
c. Discuss the functional Discussion 4-2.
components found in V3
as it enters the foramen ovale.
d. Identify the petrotympanic Netter, Plates 5, 41, 125.
fissure on a skull and the chorda
tympani in the cadaver.
e. Discuss the functional components Netter, Plates 116, 117;
in the chorda tympani and lingual Discussion 4-3.
nerve.
OBJECTIVES RESOURCES
f. Trace parasympathetic innervation Netter, Plate 119;
to the parotid gland. Discussion 4-4.
g. Discuss the location of the nerve Discussions 4-2, 4-3, 4-4.
cell bodies of nerves seen in this
dissection.
6. Muscles of the styloid process
a. Identify the muscles from the Netter, Plates 53,
styloid process (stylohyoid, 62.
styloglossus, stylopharyngeus)
on the cadaver.
b. Understand the innervation, embryonic Discussion 4-5.
origin and action of each of these muscles.
DISSECTION
*The dissection instructions on this page should be carried out on both sides of the head. Also, it is helpful to keep a skull at your table for frequent reference and orientation.
1. Palpate the zygomatic arch and then with a scalpel cut the overlying tissue down to the bone along the arch. Clean all soft tissue from the arch. Continue this incision anteriorly and then superiorly around the anterior, superior and finally posterior boundaries of the temporalis muscle being careful posteriorly to spare the superficial temporal artery that you dissected in an earlier lab. Now peel away the very thick flap of tough fascia covering the muscle to expose the temporalis muscle.
2. Cut the distal-most ends of the branches of the facial nerve, and the parotid gland duct, and then peel back these structures with the remnants of the parotid gland (all of which are lying inferior to the zygomatic arch) to expose the masseter muscle. You may need to remove some fat and fascia to adequately expose the masseter. Clean the entire surface of the zygomatic arch as well.
3. With a hand saw, saw a deep groove at either end of the zygomatic arch even with the superior attachment sites of the masseter muscle (see Figure). Be careful not to make the posterior cut too far back or you will cut into the temporomandibular joint. Make the anterior cut almost perpendicular to the direction of the muscle fibers, or you will cut into the maxillary sinus. Then break the bone at each groove, freeing the arch.
4. Pull down the cut segment of zygomatic bone and simultaneously peel the attached masseter away from the external surface of the ramus of the mandible. Appreciate that the branchiomotor innervation of the masseter travels with the mandibular division of CN V. You may see a sprig of this masseteric nerve as you peel back the muscle, but do not spend time trying to save it. The coronoid process of the mandible should now be exposed.
Look for the buccal nerve, which you should find along the anterior border of the temporalis muscle. The buccal nerve is often embedded in the investing fascia of the temporalis muscle so finding it can be a challenge. Ask for help if you need it! This nerve crosses the external surface of the buccinator muscle in the cheek and ultimately penetrates this muscle. (This is the nerve that lets you know you have bitten your cheek!)
5. Being careful to preserve the buccal nerve, use a rib cutter to carefully cut the coronoid process from the mandible. Reflect the coronoid process with the temporalis muscle still attached to it and peel the inferior aspect of the temporalis from the cranium, reflecting it upwards, so that it is out of the way of the remainder of the dissection. Again, the branchiomotor innervation to the temporalis muscle also runs with the mandibular division of CN V. If you see any of the deep temporal nerves traveling between the muscle and the skull as you peel back the muscle, save them. At this time you should be able to find an example of the deep temporal arteries, branches of the maxillary artery, as well. Save them also.
Drawings
6. With a hand saw, saw a transverse groove through the articular process of the mandible. Chip away the bone inferior to this cut with a bone cutter or chisel and remove the fragments of bone until you locate the large inferior alveolar nerve and artery, which lie deep to the ramus and enter the mandible through the mandibular foramen.
7. Remove the remaining fat in the area to show the large maxillary artery and much of the lateral pterygoid muscle (innervated by the mandibular division of CN V). Hint: if you are having difficulty locating the maxillary artery, and have saved at least a stump of the superficial temporal artery, you can follow the superficial temporal artery inferiorly to the bifurcation of the external carotid into the superficial temporal and maxillary arteries. Occasionally the maxillary artery travels deep to the lateral pterygoid muscle, making it more difficult to locate until the muscle is shredded.
8. Being careful not to damage the inferior alveolar nerve and artery traversing the mandible, carefully crack the remainder of mandibular ramus to the angle of the jaw with a hammer and chisel or bone cutter to further open up the infratemporal fossa. Make these cracks roughly parallel to the inferior alveolar nerve. Now extract the bone fragments just produced.
Next, remove all fat and fascia exposed to demonstrate the large lingual nerve, more of the maxillary artery (and branches) and the medial pterygoid muscle.
The dissection should be cleaned up on both sides, leaving the dissection in this state on one side for the study of the muscles of mastication and other relevant structures exposed.
9. On one side, proceed a step further by using a forceps to shred and remove the fibers of the lateral pterygoid muscle as completely as possible, but try to avoid damage to the nerves and vessels lying within and deep to the muscle. CAUTION: Remove the fibers bluntly--not by cutting.
With a scalpel carefully open the capsule of the temporomandibular joint to show the articular disk inside and the attachment of some of the lateral pterygoid fibers to this disk. Removal of this joint will provide more access to the deep regions of the infratemporal fossa. (Put light pressure on this joint as you open and close your own jaw and note how the mandibular condyle slides forward during opening and backwards during closure. This sliding motion is made possible by the articular disk.)
Drawings
Clean up the space once occupied by the lateral pterygoid muscle to expose the
infratemporal fossa and its contents: (Why is this name very descriptive for this
fossa?) On this side all of the following structures should be cleaned up:
- branches of the mandibular division of CN V, including the:
- buccal
- lingual
- inferior alveolar
- mylohyoid (a slender nerve lying on the medial side of the mandible, branching off branching off the posterior aspect of the inferior alveolar nerve, carrying
branchiomotor innervation to the mylohyoid muscle and the anterior
belly of the digastric muscle)
- auriculotemporal (see below)
- maxillary artery and the following branches (there are many more) of the maxillary artery; the middle meningeal (see below), deep temporals (find at least one) inferior alveolar and the end-branch of the maxillary artery, the sphenopalatine artery. (The maxillary vein and its tributaries should be removed for better exposure of the fossa.)
- posterior auricular artery off the external carotid artery
The auriculotemporal nerve is large and runs rather directly lateral from the foramen ovale towards the parotid gland. Because there is usually a fair amount of tough fascial material around this nerve, finding it often takes some patience and determination on the part of the dissector. It forms a loop through which the middle meningeal artery often runs on its way to enter the skull through the foramen spinosum. (How do you think the auriculotemporal nerve got its name?)
10. Now find the chorda tympani nerve of CN VII. This very slender nerve emerges from the skull through the petrotympanic fissure and runs anteriorly to join the lingual nerve on the latter's inferior surface. The chorda tympani may also be difficult to find at first, but blunt dissection along the inferior border of the lingual will generally reveal it.
11. On the other side, relocate the long styloid process in the tough tissues anterior and inferior to the auricle by identifying the stylohyoid muscle and following it posterior to this bony process. Clean and expose the process and identify its three attaching muscles: stylohyoid (outermost), styloglossus (middle) and the stylopharyngeus (innermost). What is the function of each of these three muscles?
12. Find the glossopharyngeal nerve (CN IX) by exposing the stylopharyngeus muscle and pulling it superiorly with a forceps; the nerve lies deep to the stylopharyngeus muscle before running anteriorly to cross this muscle's external surface near its attachment to the pharynx.
fig
DISCUSSION
4-1
On Netter, Plate 11, it can readily be seen that an articular disk divides the temporomandibular joint into two cavities, each of which is lined by a synovium. Movement of the joint is in the manner of a hinge, but--because of the disk--gliding is also present. Simple hinge-type movements of elevation/depression occur between the head of the mandibular condylar process and the disk. This movement separates the incisors to a great extent, but produces only a small opening between the molars. The interspace between upper and lower molars is enlarged by a forward gliding movement, which occurs in the joint cavity above the disk. Laxity of the joint capsule permits the disk and mandible to slide forward and downward to the articular tubercle of the mandibular fossa; the net result is to produce a downward movement of the mandible. Because of the gliding movements, therefore, the temporomandibular joint is actually a modified hinge joint. Pure gliding movements produce protraction/retraction. Side-to-side movements can also occur as in grinding. These are defined as rotational movements about a vertical axis through the joint on the side toward which the movement occurs.
4-2
The trigeminal nerve is the principal sensory nerve of the head. A general concept which might prove helpful is to imagine cutting off the back of the head in a coronal plane which extends between the external auditory meatuses. All structures anterior to this plane receive general somatic sensation via fibers in some branch of CN V whose nerve cell bodies are found in the trigeminal ganglion. Pain and general sensation, for example, from such diverse areas as the teeth, anterior 2/3 of the tongue, orbit, cornea, the anterior 1/2 of the scalp and the paranasal sinuses are mediated by CN V.
Motor fibers are present only in the mandibular division of the trigeminal nerve. They have their cell bodies in the motor nucleus of V in the pons. These motor nerves innervate all of the muscles of mastication plus four more (tensor tympani, tensor veli palatini, mylohyoid and anterior belly of the digastric). [Note that in addition to the motor branches to the tensor tympani and tensor veli palatini muscles, a motor branch to the medial pterygoid and a sensory branch to the dura, the meningeal nerve, leave the mandibular division of CN V just distal to its emergence through the foramen ovale (Netter, Plates 41 and 116].
No autonomic fibers arise in association with the trigeminal nerve. Although all autonomic fibers do travel with branches of CN V, they arise either with CNs III, VII or IX (pre-parasympathetics) or the superior cervical ganglion (post-sympathetics). This can make study of the head and neck very confusing unless one constantly keeps in mind that parasympathetics arise with other cranial nerves. Parasympathetic fibers traveling to the otic and submandibular ganglia are covered in this dissection.
4-3
The chorda tympani is a branch of the facial nerve (CN VII) and contains both taste (special sensory; cell bodies located in the geniculate ganglion, to be encountered later) and secretomotor (preganglionic parasympathetic; cell bodies in the superior salivatory nucleus in the pons) fibers. These components travel with the main trunk of the facial nerve until the level of the middle ear, where they branch as the "chorda tympani nerve" and exit the skull via the petrotympanic fissure. The nerve crosses the infratemporal fossa and joins the lingual nerve, a branch of CN V. The lingual nerve travels forward to the inferior aspect of the tongue where it supplies general sensation (cell bodies in trigeminal ganglion) to the anterior 2/3 of the mucosa of the tongue.
The special sensory fibers of the chorda tympani travel with the lingual nerve (these fibers can no longer be distinguished as a separate entity) to supply taste to the anterior 2/3 of the tongue. The preganglionic parasympathetic fibers of the chorda tympani travel with the lingual nerve only as far as the submandibular gland. At this point they branch from the lingual nerve and travel to the submandibular ganglion where they synapse. Postganglionic fibers innervate both submandibular and sublingual glands as well as small glands in the oral cavity. (Postganglionic fibers to the sublingual gland may rejoin the lingual nerve to reach this gland.)
4-4
The lesser petrosal nerve is a branch of the glossopharyngeal nerve (CN IX) and contains secretomotor (preganglionic parasympathetic) fibers. It exits the skull through the foramen ovale. Just outside this foramen it enters the otic ganglion which is found in relation to the mandibular nerve. (How do you think the otic ganglion got its name?) From the otic ganglion, postganglionic parasympathetic fibers travel to the parotid gland by joining the auriculotemporal nerve in the infratemporal fossa. As the auriculotemporal nerve courses through the upper part of the parotid gland, the postganglionic parasympathetic fibers enter and innervate the gland. The gland receives its sensory innervation from the auriculotemporal nerve itself.
Although you will not be held responsible for the following details, for those who are interested, the parasympathetic fibers arising with CN IX (cell bodies in the inferior salivatory nucleus in the medulla) travel with the main trunk of the nerve through the jugular foramen. After exiting the skull, these parasympathetic fibers along with some sensory fibers (cell bodies in sensory ganglion of glossopharyngeal nerve) branch from the main trunk and re-enter the skull through the tympanic canaliculus just posterior to the carotid canal. This mixed nerve (called the "tympanic nerve") enters the middle ear cavity and forms a plexus on the promontory of the medial wall. The sensory fibers of the plexus distribute throughout the middle ear cavity and mediate general sensory innervation to the mucosal lining. The parasympathetic fibers reassemble to form a nerve (the lesser petrosal). This is discussed again under Discussion 8-3 so you will encounter this again!
4-5
The three muscles arising from the styloid process have different nerve supplies. The stylohyoid is innervated by CN VII, the styloglossus is innervated by CN XII, and the stylopharyngeus is innervated by CN IX. By noting their fixed attachment sites you should be able to determine what these muscles do. Very simply put, the styloid process is not a movable structure so that contraction of any of these muscles would help to move whatever they insert into (hyoid bone, tongue and pharynx) upward and backward toward the styloid process.
5. CONTENTS OF THE CRANIAL CAVITY
OBJECTIVES RESOURCES
1. Meninges and meningeal spaces Discussion 5-1.
a. Identify differences and
similarities between cranial
and spinal dura, and between
their epidural spaces.
Consider functional implications.
b. Describe relationships of Netter, Plates 94, 96.
arachnoid to dura and pia.
c. Identify meningeal spaces.
1) List their contents.
2) Explain their clinical Discussion 5-1.
significance.
d. Locate sites of elaboration, Discussion 5-2.
route to subarachnoid space
and sites of drainage of
cerebrospinal fluid.
1) Explain hydrocephaly.
2) Explain mechanical role
of CSF.
2. Dural sinuses and cranial dural structures
a. Locate the following dural Netter, Plates 97,
venous sinuses: 98, 100, 103;
superior, inferior sagittal Discussion 5-2.
straight
transverse
sigmoid
cavernous
b. Discuss the clinical significance Discussion 5-3.
of the connections to the cavernous
sinus.
c. Identify falx cerebri, tentorium Netter, Plates 97, 98,
cerebelli and tentorial notch and the 103.
cisterna magna (cerebellomedullary
cistern).
OBJECTIVES RESOURCES
d. Note that the middle meningeal Netter, Plate 95.
artery supplies blood to
dura and bone.
3. Cranial nerves and brain
a. Identify all cranial nerves at Netter, Plate 98.
sites where they pierce the
dura.
b. Relate all cranial nerves to Netter, Plates 6, 7.
foramina in skull through which Discussion 5-4.
they exit. Locate in skull the
anterior, middle and posterior
cranial fossae.
c. Identify the vertebral artery, Netter, Plates 98,
the basilar artery and the 132, 133;
internal carotid artery. Identify Discussion 5-5.
the following branches
associated with the arterial
Circle of Willis:
- anterior, middle and
posterior cerebral arteries
- anterior and posterior
communicating arteries
DISSECTION
1. You can start this dissection with the body in a supine or prone position. If you begin with the cadaver supine, you will eventually need to turn it for completion of the dissection. Peel the frontalis muscle inferiorly and the temporalis muscle superiorly (to save them) and remove all loose scalp material from the calvaria to a point just inferior to the superior aspects of the ears. It is best to place a block under the shoulders to elevate the calvarial region if you are beginning with the cadaver in a supine position, or to have the block under the chin if you are beginning with the cadaver in a prone position, for a better angle of dissection.
2. With the stryker saw, saw a groove around the calvaria, approximately in the position of the cut on the prepared skulls (i.e., above the eye brows and auricles but fairly low around the occipital bone). While sawing, try to avoid cutting completely through the bone and into the dura mater. Using a chisel as a lever, complete the job of breaking through the cranial bone. Now saw a groove in the loosened calvaria from the frontal bone back to the occipital bone, following the midsagittal line. You can then pry up the two pieces of calvaria by degrees, separating them from the dura with your fingers at the same time.
3. When the skull cap has been removed make a vertical midsagittal cut through the skin with a sharp scalpel from the seventh cervical vertebral spinous process and continue it in a superior direction until you meet the point where the calvaria was removed. The depth of this cut should extend to the spinous processes inferiorly and to the occipital bone superiorly. (If you stand at the cadaver’s head, you will more likely make this a true midsagittal cut, which is quite important!) Now, with a scalpel and chisel, carefully and cleanly reflect the skin and muscles AS A UNIT from the underlying vertebrae and occipital bone, working from your midline cut laterally. Be sure to clean TO THE BONE. Continue this procedure until you have reflected a wedge-shaped flap of skin and attached underlying muscle such that you can see the transverse processes of the first cervical vertebrae superiorly and about down to the fifth cervical spinous process inferiorly. Now saw a broad wedge-shaped cut in the occipital bone, from the point where the calvaria was removed to the foramen magnum. Pry off this piece of bone. Then, with the saw aimed slightly medially, cut through the laminae of the first four to five cervical vertebrae. Pry off these spinous processes. With these bony pieces removed, the cranial and spinal dura mater should now be exposed.
Drawings
6. This is a good time to locate and identify the cisterna magna, also known as the cerebellomedullary cistern, between the cerebellum and the medulla. This is where CSF drains from the ventricles into the subarachnoid space. This cistern is the site of choice for removal of CSF in infants. Why do you think this is so? Next, with a fresh scalpel blade, transect the spinal cord at about C4 and then sever the upper cervical rootlets and the spinal component of CN XI.
7. Now fold the cut portion of the spinal cord towards the brain, thereby exposing the following (for the most part paired) structures which should be cut with the sharp blade or scissors (cut one side close to the cranium and the other side close to the brain): (beginning inferiorly and working rostrally) the vertebral arteries, spinal parts of cranial nerve XI, paired rootlets of nerve XII, paired rootlets (in a row) of nerves XI (cranial parts), X and IX, nerves VIII and VII, paired nerve VI (which enter the floor of the cranium most medially of all cranial nerves), the pair of nerve V (the largest), the pair of nerve IV (the thinnest; it actually exits the brain stem from its dorsal surface and wraps around it), the internal carotid arteries, the pair of nerve III, the midline stalk of the pituitary gland, the paired optic nerves (II).
After the two optic nerve stalks have been severed, lift out the brain carefully, which will automatically break off the slender fibers of nerve I (which join the olfactory bulbs).
Drawings
DISCUSSION
5-1 - Meninges
In the cranium, the periosteum (the connective tissue lining the skull) and the dura are fused for the most part. However, in some areas, spaces intervene between them, e.g., dural venous sinuses (homologue of the epidural venous plexus within the vertebral canal).
The dura itself has a significant blood and sensory nerve supply. Several nerves and blood vessels are involved. Chief among them are the middle meningeal vessels and nerve, both of which enter the skull through the foramen spinosum. The branches of the middle meningeal artery are embedded in the dura-periosteal layer of the middle cranial fossa and deeply groove the inner bony table of the skull. Indeed, lateral fracture of the skull often ruptures this artery. The resulting hemorrhage, and the ensuing epidural hematoma, produces an acute emergency situation because arterial blood pressure is approximately 13 times greater than normal intracranial pressure. The resulting increased intracranial pressure depresses the medullary respiratory and vasomotor centers, and coma and death may ensue unless the cranial cavity is entered and the bleeding arrested. The meningeal nerve accompanies the artery and provides sensory innervation to the dura. The other vessels and nerves supplying the dura have a generally similar anatomical pattern.
Deep to the dura is the arachnoid which is held tightly against the dura by the CSF in the subarachnoid space. Therefore, the subdural space between arachnoid and dura is only a potential space which is traversed by the superior cerebral veins as they empty into the superior sagittal sinus. Head injuries may result in shearing of these veins which can produce hemorrhage into this space (subdural hematoma), and large accumulations of subdural fluid may occur (subdural cyst). Processes of the arachnoid, the arachnoid villi, also project through the dura into the superior sagittal sinus and its adjacent venous lacunae. These villi provide the principal pathway for the return of the cerebrospinal fluid from the subarachnoid space into the blood stream.
Beneath the arachnoid is the subarachnoid space which contains cerebrospinal fluid (CSF). CSF is produced in the ventricles and enters the subarachnoid space through apertures in the roof and sides of the fourth ventricle (the 2 (lateral) Foramina of Luschka and a single (medial) Foramen of Magendie). The CSF is absorbed into the venous sinuses through the arachnoid villi as indicated above. The fluid provides an appropriate chemical and physical environment for the CNS. Because of the absence of lymphatics, the tissue fluid of the brain is returned to the blood stream via the CSF. Physically the CSF provides a buoyant effect on the CNS. In effect, it reduces the weight of the brain from 1500 grams in air to 50 grams in situ. This is essential in reducing the pressure of the brain against the bony floor of the skull. There is an appreciable amount of CSF (estimated at about 500 ml) exchanged daily. Failure of normal absorption produces hydrocephalus which is caused by retention of the CSF within the cranial cavity. If this condition is congenital, it leads to striking enlargement of the head.
5-2 - Dural Venous Sinuses
A prominent feature of the cranial dura is the dural venous sinuses. These are endothelial lined spaces situated between the two layers of the dura which convey the venous blood from the brain to the jugular foramen where it enters the internal jugular vein. The sinuses contain no valves. Blood from the brain basically drains into the nearest sinus.
5-3 - Cavernous Sinus
Through the angular vein and veins in the orbit, the facial vein is connected to the cavernous sinus. Because none of these veins is valved, blood from the face may drain through the facial vein or may run through the connections of this vein to the cavernous sinus. If infectious material from the face (i.e., by squeezing pimples around the lower lip to side of the nose) is carried along the latter course, thrombosis of the cavernous sinus and ensuing meningitis are likely to occur, particularly because blood flow through this sinus is very slow. Serious neurological damage or even death may result. Suppuration in the upper nasal cavities and paranasal sinuses may also lead to septic thrombosis of the cavernous sinuses, with subsequent meningitis.
An arteriovenous communication may occur between the cavernous sinus and the internal carotid artery, giving rise to a pulsating swelling in the orbit. This may result from various injuries such as a bullet wound, or a fracture of the base of the skull. Ligation of the internal or common carotid artery has sometimes been performed in these cases.
5-4 - Cranial Nerves and the Dura
It is quite apparent from inspection of the posterior cranial fossa after the brain has been removed that the majority of the cranial nerves pierce the dura in this fossa. This includes cranial nerves III through XII. This is because the main portion of the brainstem, from which these nerves exit, lies below the tentorium cerebelli. Cranial nerves VII through XII (ie., the last 6 cranial nerves) enter their respective foramina directly at the sites where they pierce the dura. These nerves thus exit the skull from the posterior cranial fossa as well. However, cranial nerves III through VI (ie., the true cranial nerves of the first 6 cranial nerves) run an epi- or intradural course from the sites through which they pierce the dura in the posterior cranial fossa into the middle cranial fossa where they ultimately exit the skull. Upon inspection of the middle cranial fossa with the dura of its floor still intact one notes that the foramina are not visible. The structures which exit or enter these foramina (spinosum, ovale, superior orbital fissure, rotundum, carotid canal and a few more subtle ones) run an epidural course for at least a portion of their intracranial route. The middle meningeal artery and vein and their accompanying sensory nerve, as exceptions, remain epidural.
5-5 -Blood Supply to the Brain
You may wish to inspect Netter Plates 133, 134, 135 and 136 for an overview at this time. We want you to see the Circle of Willis and its formation from branches of the internal carotid and vertebral arteries. You may also wish to inspect Netter, Plate 157, which is an elegant illustration of the blood supply to the spinal cord. Unfortunately this is difficult to demonstrate on a dissection and this very important part of the blood vascular system is often neglected.
6. THE ORBIT AND ITS CONTENTS
OBJECTIVES RESOURCES
1. Osteology
a. Identify the bones forming the Netter, Plate 1.
orbit.
b. Identify the following structures:
- optic canal
- superior orbital fissure
- inferior orbital fissure
- fossa for lacrimal sac
- anterior and posterior
ethmoidal foramina
2. Fascia
a. Describe the relationship of Discussion 6-1.
dura to the periorbita and to
the optic nerve and sclera.
b. Describe the location and function Discussion 6-2.
of the suspensory ligament and
medial and lateral check ligaments
of the eye. Note the relationship
of these structures to the fascia of the
medial and lateral recti and inferior
oblique muscles.
3. Muscles
a. Identify the sites of origin Netter, Plates 78,
and insertion of the extra- 79.
ocular muscles.
b. Understand the function of the
levator palpebrae and smooth
muscle of the upper eyelid.
c. Define the primary movements of the Discussion 6-3.
eyeball using the 3 axes of rotation
(transverse axis-elevation, depression;
vertical axis-abduction, adduction;
A-P axis-medial, lateral rotation).
OBJECTIVES RESOURCES
d. Analyze the actions of the Discussion 6-3.
extraocular muscles using
the appropriate axes.
e. Discuss principles for testing Discussion 6-4.
integrity of these muscles.
4. Nerves
a. Locate the following branches Netter, Plate 81.
of the ophthalmic nerve, trace
their distribution and know
their function:
-frontal
-lacrimal
-nasociliary
b. Identify the motor nerves of the Netter, Plate 81.
orbit & trace their distribution.
-abducens
-trochlear
-oculomotor
c. Identify the ciliary ganglion Netter, Plate 81.
and the nerve fibers which
enter and leave it.
d. Trace the route by which parasym- Discussion 6-5.
pathetic &/or sympathetic fibers go to:
- smooth muscle of the eyelids
- ciliary muscle
- dilator of pupil
- sphincter of pupil
- lacrimal gland
e. Examine an illustration of a Netter, Plate 98.
coronal section through the
cavernous sinus and identify the
5 nerves and the artery within it.
OBJECTIVES RESOURCES
5. Blood Vessels
a. Identify the ophthalmic artery Netter, Plate 80.
on the cadaver and appreciate the
position of the central artery to the retina.
The branches of the ophthalmic
artery generally follow the nerves.
b. Using the bottom figure on Netter,
Plate 80, note the 3 routes of venous
drainage from the extraocular tissues
of the orbit.
6. Eyelids and lacrimal apparatus
a. Define the conjunctival sac.
b. Locate the lacrimal gland and Netter, Plate 77.
appreciate the flow of its
secretions to the nasolacrimal
duct and inferior nasal meatus.
7. Time permitting, section the eyeball Netter, Plate 82.
vertically (in the frontal plane) and identify:
- retina - lens
- choroid - ciliary body
- sclera - cornea
DISSECTION
*Keep the body in the prone position; it is best to place a block under the chin to put the orbital region fairly horizontal for dissection.
1. Strip off the dura from the roof of each orbit.
2. With a chisel and mallet, crack the bone of the orbital roofs into small fragments. The bone is thin and is easily broken this way. Continue to crack bone all the way to the rear of the orbit, including that over the optic canal and the superior orbital fissure. This is a good time to locate the ophthalmic artery. Off of what artery does this artery branch? Now remove the pieces of bone, exposing the rather smooth periorbital fascia enclosing the periorbital fat. Finally, enlarge the anterior exposure to the orbit with a hand saw by sawing grooves into the frontal bone near the anterior midline and as far laterally as convenient (see arrows on figure; the lacrimal gland will thus be exposed). Break off this segment of frontal bone; usually this is best done with blows from a mallet.
3. With a probe or other blunt tool, break the periorbital fascia for the length of the orbit. Strip away this fascial material exposing the fat. Identify the large nerve lying superiorly in the fat. This is the frontal nerve of V1. Does the frontal nerve innervate anything in the orbit? To what does the frontal nerve change its name before it actually innervates something? What does this nerve innervate?
4. With the frontal nerve now exposed, continue to remove fat until you have exposed the levator palpebrae superioris muscle, the lacrimal gland (lateral in the orbit), the lacrimal nerve traveling to the gland, and the superior oblique muscle (medial in the orbit). What modality (modalities?) does the lacrimal nerve carry?
Now, with a scissors, bluntly separate the levator palpebrae muscle from the underlying superior rectus muscle. Then cut the attachment of the levator close to the eyelid and fold back the muscle to reveal the superior rectus muscle beneath. You should be able to locate the branch of CN III innervating the underside of the levator. Discuss the functions of these muscles as you dissect them.
Drawings
5. Cut the anterior attachment of the superior rectus muscle and reflect it back as you did the levator to gain exposure to this area. The dissection from now on largely consists of removing the fat piecemeal with a forceps. This is slow, patient work and cannot be rushed. Muscles and all nerves or nerve-like strands must be carefully saved.
6. In the rear of the orbit identify the nerves entering the dura that are destined for the orbit: CN's II, III, IV, V (V1, the ophthalmic division, is the division that actually enters the orbit), and VI. With a scalpel, carefully cut the dura over the nerves.
7. Follow the (somato)motor nerves (III, IV and VI) to their respective muscles. Then, very carefully, remove the fat lying between the lateral rectus muscle and CN II, identifying as you do so the sinuous, slender short ciliary nerves from CN V1 serving the posterior surface of the eyeball. Then, trace these ciliary nerves posteriorly to a small lump (about the size of a pin head) that lies just lateral to CN II and is fed by a nerve from CN III and from the nasociliary branch of V1. This small, discrete swelling is the ciliary ganglion. Be sure to loosen up the rear of the orbit sufficiently to dissect posteriorly enough to find the ganglion. Using an atlas figure as a guide is helpful here.
8. On one side, make a slit through the inferior aspect of the orbicularis oculi muscle. Clean under the eyeball to find the inferior oblique and inferior rectus muscles. Dissection of the lacrimal sac and nasolacrimal duct is difficult and not very rewarding, but do appreciate where they are located.
9. Remove and section an eyeball in the frontal (coronal) plane and, using Netter, Plate 82 as a guide, identify the retina, lens, choroid, ciliary body, sclera and cornea.
Drawings
DISCUSSION
6-1 Dura, Periorbita and Sclera
The optic nerve (CN II) is actually an extension of the brain rather than a cranial nerve. All three meningeal layers invest this "nerve". At the optic canal the periosteal layer of the dura separates to become continuous with the periorbita which is nothing more than a rather stout periosteal lining of the bones which delineate the orbit. The subarachnoid space with its cerebrospinal fluid extends along the optic nerve to the sclera where it abruptly ends. One can say that at this site the dura becomes continuous with the sclera or simply that the dura fuses to the sclera. Increased cerebrospinal fluid pressure would indent the back of the eyeball along the perimeter where the optic nerve fibers penetrate the sclera. On ophthalmoscopic examination of a patient with increased intracranial pressure the clinician would observe a raised rim around the optic disk. This is referred to as "choked disk" or "papilledema".
6-2 Check and Suspensory Ligaments
The check ligaments are simply fascial thickenings from the medial and lateral rectus muscles to the medial and lateral orbital walls, respectively. Plate 78 in Netter illustrates them on horizontal section. Because they attach to bone, these ligaments serve to limit the amount of distance each muscle can move the eyeball toward its side. This is because they would restrain further shortening of the muscle on that side when tightened.
The suspensory ligament is a fascial thickening in the region of the inferior oblique and inferior rectus muscles which continues to either side to invest the medial and lateral rectus muscles. The resulting fascial sling serves to support the eyeball from below. The sketch below illustrates it from a frontal view.
(fig.)
6-3 Extraocular Muscles
The following table summarizes the isolated actions of each muscle around each of the three classical axes of rotation:
Vertical axis
abduction: lateral rectus, superior oblique, inferior oblique
adduction: medial rectus, superior rectus, inferior rectus
Horizontal axis
elevation: superior rectus, inferior oblique
depression: inferior rectus, superior oblique
Anteroposterior axis
medial rotation: superior rectus and superior oblique
lateral rotation: inferior rectus and inferior oblique
In normal composite eye movements, starting from a neutral position (looking straight ahead), the eyeball moves as follows:
medial rectus medial movement
lateral rectus lateral movement
superior rectus up and in (simultaneously)
inferior rectus down and in (simultaneously)
superior oblique down and out (simultaneously)
inferior oblique up and out (simultaneously)
Unlike the medial and lateral rectus muscles, which are truly perpendicular to the vertical axis of rotation, the only axis around which they can perform movements, none of the superior and inferior muscles is exactly perpendicular to an axis of rotation so that contraction of any of these muscles results in movements around all axes of rotation simultaneously.
figure
6-4 - Testing Extraocular Muscles
Extraocular muscles can be readily tested. For example, if a person has difficulty in depressing the eyeball, either the superior oblique (trochlear nerve) or inferior rectus (oculomotor nerve) could be involved. Note that these 2 muscles are perpendicular to each other. Therefore, one or the other can be eliminated to test them (and their innervation) individually. Asking the patient to look out (abducted position) will cause the line of pull of the inferior rectus to become perfectly perpendicular to the (new location of the) transverse axis, whereas the line of pull of the superior oblique becomes parallel to that same axis and therefore incapable of moving the eyeball. As the patient looks down (depresses the eyeball) from this lateral position, s/he can do so only with the inferior rectus. BE SURE THAT YOU UNDERSTAND THIS. YOU DO NOT NEED TO MEMORIZE THIS. IT MAKES SENSE. YOU CAN REFER TO IT FOR THE REST OF YOUR CAREER! Likewise, placing the eye in an adducted position (looking in) eliminates the inferior rectus because its fibers are now parallel to the transverse axis and can, therefore, produce no movement on this axis when looking down (depressing the eyeball). Only the superior oblique muscle can now perform this movement (looking down).
Utilizing principles such as these, any of the extraocular muscles and the integrity of their nerves can be tested individually. Thus, for clinical testing, the table below indicates the movement that each muscle can perform in its position of greatest efficiency. It must be understood that in the given position only the muscle tested can perform the movement in question. The category "Position of eyeball" indicates the first movement the patient performs. The category "Movement" indicates the second movement the patient is asked to perform. Under the category "Final position", the movement in normal type is the colloquial name for the first position (you don't ask your patient to "adduct" his/her eye, you ask him/her to "look in") and the bolded movement is the colloquial name for the second movement performed by the muscle being tested ("Muscle tested") from this new position.
Position of eyeball Movement Muscle tested Final position
adducted depression superior oblique in and down
adducted elevation inferior oblique in and up
abducted elevation superior rectus out and up
abducted depression inferior rectus out and down
neutral abduction lateral rectus out
neutral adduction medial rectus in
6-5 - Autonomics to Structures within the Orbit
Parasympathetics
Iris and Lens
a) Preganglionic cell bodies are located in the Edinger-Westphal nucleus in the midbrain.
b) Preganglionic parasympathetic axons travel with CN III to the orbit where they follow the inferior division of CN III to synapse in the ciliary ganglion.
c) Postganglionic axons reach the eye ball via short ciliary nerves from the ophthalmic division of CN V (V1) and innervate the sphincter pupillae and ciliary muscles.
Lacrimal Gland
Note: Innervation of the lacrimal gland will make much more sense to you after the last dissection, dissection #10, so read this now, but please return to it after dissection #10.
a) Preganglionic cell bodies are located in the pons in the superior salivatory and lacrimal nucleus.
b) Preganglionic axons leave the brainstem with the CN VII and are carried via the greater petrosal nerve of CN VII to the middle cranial fossa. Here they join the deep petrosal n. (postganglionic sympathetic axons from the superior cervical ganglion traveling in the internal carotid plexus). The joining of the above two nerves forms the nerve of the pterygoid canal which reaches the pterygopalatine fossa via the pterygoid canal.
c) In the pterygopalatine fossa, the preganglionic parasympathetic axons synapse in the pterygopalatine ganglion. The postganglionic axons together with the sympathetic fibers, distribute to the lacrimal gland, as well as nasal, and palatine glands via various branches of the maxillary division of CN V (V2). Those axons that travel to the lacrimal gland may also join the lacrimal nerve of V1.
Sympathetic
a) Postganglionic fibers from the superior cervical ganglion travel via the internal carotid plexus to the cavernous sinus. Within the sinus, many fibers leave the artery and pass to CN III, IV, V1, and V2 that are located in the wall of the sinus.
b) Those traveling with the nasociliary branch of V1 go to the ciliary ganglion to reach the eye ball via short ciliary nerves. They may also take a direct route from the cavernous sinus to the ciliary ganglion. They pass through the ciliary ganglion without synapsing (remember, they are postganglionic sympathetic) and they innervate the dilator pupillae muscle.
c) The superior tarsal muscle also receives sympathetic innervation. These fibers join CN III within the cavernous sinus and distribute with the superior branch of this nerve that is restricted to the innervation of the superior rectus and levator palpebrae muscles.
7. PHARYNX, PALATE, MOUTH AND CERVICAL MUSCLES
OBJECTIVES RESOURCES
1. Pharynx and Palate
a. With regard to the pharyngeal The pharyngeal
plexus, what is the major sensory and branch of CN IX is
motor nerve supply to the pharynx. sensory to the lining of
What is the role of the sympathetics? the pharynx and of CN X is branchiomotor
to the muscles of the
pharynx. Sympathetics innervate blood vessels.
b. Note the approximate attachments Netter, Plates 59,
of the three pharyngeal constrictors. 61, 62, 69.
c. Identify & describe the Netter, Plate 60.
boundaries of the naso-, oro-
and laryngopharynges.
d. Identify the retropharyngeal space Netter, Plate 30, 57, 59
and appreciate its significance in the
spread of infection.
e. Describe the attachments of the Netter, Plates 58, 59.
levator and tensor veli palatini Consider the tensor
muscles. Give their actions and to be the main opener
innervation. of the auditory tube.
f. Identify the hamulus of the medial Netter, Plate 5.
pterygoid plate.
g. Identify the site of the torus tubarius Netter, Plates 58, 59.
& the opening of the auditory tube.
h. Locate the palatine and pharyngeal Netter, Plates 45,
tonsils. 52, 54, 57, 58.
i. Identify the palatopharyngeus, Netter, Plates 46, 53,
salpingopharyngeus and 58, 59.
palatoglossus muscles.
OBJECTIVES RESOURCES
2. Mouth and Tongue
a. Find the buccinator muscle. Netter, Plates 49, 59, 62.
b. Locate the submandibular Netter, Plates 19,
and sublingual glands and 47, 55.
their ducts.
c. Note that the lingual and Netter, Plates 53, 55.
hypoglossal nerves, and the
submandibular duct and sublingual
gland lie on the lateral surface
of the hyoglossus muscle but
the lingual artery lies medial
to this muscle.
d. Analyze the actions of the intrinsic Netter, Plates 53, 54.
and extrinsic muscles of the
tongue.
3. Cervical Muscles
Observe the deep muscles of the Netter, Plate 25.
neck to appreciate their complexity.
DISSECTION
the neck is flexed. Reflect the flaps of skin and posterior neck muscles laterally and
begin the separation of the vertebral column from the skull and pharynx by carefully
cutting the ligaments and joint capsules associated with the atlanto-occipital joint.
You will find it helpful for orientation to palpate the transverse processes of the atlas
(look at a skeleton) and be sure that you are cutting between these processes and the
occipital bone. Again, be sure that you keep your cut above C1. Keep your scalpel
blade right up against the occipital bone.
Think about the anatomy of this region and have an atlas open as you cut through joint capsules laterally, and the superior extensions of the posterior and then the anterior longitudinal ligaments more medially. You will find it useful to keep the neck in a flexed position and to flex, extend and rotate the head frequently to assess progress, and to periodically insert a chisel to pry apart C1 and the occiput.
When the ligaments have all been severed you should be able to insert your fingers into the retropharyngeal space and push the skull--with attached pharynx–away from the cervical vertebral column. Now turn the body over and carefully cut any ligaments or muscles that are still connecting the pharynx to the lateral and anterior aspects of the vertebral column, allowing you to continue to push the skull and pharynx away from the vertebrae until you can clearly see the posterior aspect of the pharynx.
2. Take a moment to identify the remaining cervicospinal muscles still associated with the anterior and lateral aspects of the vertebral column, for example the scalenes and longus colli.
3. Clean up one side of the cheek by removing fat to show the buccinator muscle. Note that the posterior border of the buccinator is separated from the anterior border of the superior pharyngeal constrictor by a connective tissue raphe. Insert and extend your finger posteriorly between the mandible and buccinator and appreciate this relationship. Now, from a posterior approach, remove the outer fascial lining of the pharynx to show the fibers of the three pharyngeal constrictors. The muscle fibers of the constrictors generally run horizontally. The constrictors will only vaguely resemble the illustration in an atlas. As you dissect away the outer fascia, identify the pharyngeal nerve plexus, lying primarily on the middle pharyngeal constrictor and including the pharyngeal branches of CN IX and X, and fibers from the superior cervical ganglion. These may be difficult to find so do your best. What does each nerve innervate?
4. With a hand saw, divide the cranium as closely to the midline in the sagittal plane as possible. The nasal septum should ideally remain on one side. Make this saw cut entirely through the palate and upper jaws. Then bisect the tongue with a scalpel. Next, saw through the midline of the mandible.
Drawings
5. Lay open the split halves of the head, and then with a scissors, cut along the posterior raphe of the pharynx to open it. Now strip away the inner mucous membrane lining of the pharynx and palate to identify the deeper and more vertical muscles of the pharynx, namely the stylopharygeus, palatopharyngeus and salpingopharyngeus muscles. Netter Plates 58 and 61 are helpful here. Identification of the torus tubarius, the cartilagenous bulge around the opening of the auditory tube, will be helpful before identifying the salpingopharyngeus muscle. How would the function of these muscles differ from that of the constrictors? Finally look for the palatine tonsils between the palatopharyngeal and palatoglossal arches, and then peel away the mucous membrane covering the same-named muscles forming these arches (see Netter, Plate 58).
6. Peel off the nasopharyngeal mucous membrane lining on one side directly inferior to, and anterior to the torus tubarius to expose the levator and tensor veli palatini muscles, respectively, of the palate. These are difficult muscles for a first-timer to find so refer to Netter Plates 49 and 59 for orientation but also do not hesitate to ask for assistance. Now look for the pharyngeal tonsils along the roof of the nasopharynx.
7. Peel away the mucosa from the floor of the mouth lateral to the tongue, and beneath the tongue on one side to expose the muscles of the tongue. Identify the styloglossus, hyoglossus and genioglossus muscles and re-identify the palatoglossus, all extrinsic muscles of the tongue, and follow their fibers into the substance of the tongue. Refer to Netter, Plates 53 and 55. Think about the actions of each of these muscles. Now identify the submandibular gland duct and follow it to the sublingual gland; locate also the lingual nerve and the associated submandibular ganglion; and CN XII on the lateral surface of the hyoglossus muscle. Identify the lingual artery at the anterior and posterior edges of this muscle and note that the rest of the artery is hidden from view because it passes medial to the hyoglossus muscle.
8. NASAL CAVITY, MAXILLARY NERVE, LARYNX
OBJECTIVES RESOURCES
1. Nasal Cavity
a. Identify on a skull and on your Netter, Plates 42,43.
cadaver the paranasal sinuses:
- frontal
- sphenoid
- maxillary
- ethmoid (air cells)
Probe the ostia of these sinuses.
b. Identify the nasal conchae, Netter, Plates 32, 33,
meatuses, ethmoidal bulla 34, 38, 42.
and semilunar hiatus,
and know the composition of the
nasal septum.
c. Demonstrate the pterygopalatine Netter, Plates 37, 38, ganglion in the cadaver and 39, 40; Discussion
identify and discuss the 8-1.
entering and exiting fibers.
d. Identify the pterygomaxillary fissure Netter, Plates 2, 9.
the pterygopalatine fossa and the
sphenopalatine foramen on a skull.
e. Discuss the general distribution Netter, Plate 40.
of the maxillary nerve (V2) with
emphasis on the infraorbital,
superior alveolar and palatine
branches.
2. Larynx
a. Locate on the cadaver the Netter, Plate 71.
following skeletal elements
(cartilages) associated with
the larynx:
- thyroid
- epiglottis
- arytenoid
muscular and vocal processes
- cricoid
OBJECTIVES RESOURCES
b. Define the piriform recess, vestibule, Netter, Plates 60, 75.
ventricles, true and false vocal folds,
and glottis.
c. Understand the general actions of Netter, Plates 72, 73.
the following muscles:
- cricothyroid
- posterior & lateral
cricoarytenoids
- vocalis
- transverse & oblique
arytenoids
- thyro- and aryepiglottics
d. Understand the valve action of
the larynx during swallowing
and the action of the various
muscles during vocalization.
e. Know the distribution of Netter, Plate 74.
the superior and inferior
(recurrent) laryngeal nerves.
DISSECTION
*The sagittal cut through the head on the preceding dissection may or may not have destroyed the nasal septum. If the septum is still intact, see if you can identify the olfactory nerves which comprise CN I along the superior portion of the septum. Then excise the septum as a complete plate. This will expose the lateral wall of the nasal passage on that side. Along the superior aspect of the lateral wall as well, first try to find nerves associated with CN I. Netter, Plate 38 can be used to assist you.
1. On both sides, cut the base of the inferior and middle nasal conchae and remove them to expose the respective meatuses. In these meatuses, locate the openings of the paranasal sinuses, most of which are associated with the ethmoidal bulla and semilunar hiatus (under the middle concha), and the opening of the nasolacrimal duct (under the inferior concha). On one side go one step further and, with a chisel, open the internal wall of the maxillary sinus by breaking through the lateral nasal wall so that its large cavity is well-exposed. Locate the frontal sinus, the ethmoidal air cells and the sphenoid sinus as well.
2. On the side with the exposed maxillary sinus, continue to CAREFULLY chip away bone in a posterior direction working from the opening you have created into the maxillary sinus towards the torus tubarius to find the greater and lesser palatine nerves. Follow them down to the greater palatine foramen. Netter, Plate 38 may prove helpful here. Note that these nerves are anterior to the torus.
3. The next step is to find the pterygopalatine (parasympathetic) ganglion. To do this, continue to crack the palatine bone around the palatine nerves in a superior direction until you see the union of these nerves with the ganglion and the maxillary (V2) division (you have to destroy the sphenopalatine foramen to access the ganglion). The ganglion will ordinarily be seen as a small swelling near the junction of the two nerves. Next, very carefully crack away the bone directly posterior to the ganglion to find the small nerve of the pterygoid canal that joins the ganglion.
4. If you now look into the opened maxillary sinus, you should be able to identify the infraorbital branches and the superior alveolar branches of V2 traveling along its roof and lateral walls, respectively. The other branches of the maxillary division are quite a bit smaller and more difficult to follow. Keep an intact skull handy, because orientation of the skull during dissection is difficult.
5. Now clean up the surface of the larynx anteriorly and posteriorly. Posteriorly, identify the piriform recesses lateral to the cricoid cartilages. Then peel away the fascia to expose the large posterior cricoarytenoid and the arytenoid muscles. Anteriorly, re-identify the cricothyroid muscles. Then, carefully make a midline vertical cut between the two plates of the thyroid cartilage and pry off one thyroid plate from the deeper tissues, exposing the lateral cricoarytenoid and thyroarytenoid muscles. Finally, with a scissors, cut the posterior midline of the larynx and continue this cut inferiorly into the trachea. The larynx can now be opened like a book to expose the cavity of the larynx. Identify the cartilagenous epiglottis, inferior to it the vestibule (entrance to the larynx), the ventricle, ie., the depression between the true and false vocal folds and the glottis (floor of the larynx).
DISCUSSION
8-1
Pterygopalatine Ganglion
a) Preganglionic parasympathetic cell bodies are located in the pons in the superior salivatory and lacrimal nuclei.
b) Preganglionic fibers leave the brainstem with CN VII and are carried via the greater petrosal nerve to the middle cranial fossa where they join the deep petrosal nerve (postganglionic sympathetic fibers from the superior cervical ganglion via the internal carotid plexus). The joining of the above two nerves forms the nerve of the pterygoid canal which reaches the pterygopalatine fossa via the pterygoid canal in the sphenoid bone. (Remember that the cell bodies of the preganglionic parasympathetic fibers of the chorda tympani are also located in the superior salivatory and lacrimal nucleus. See Discussion 5-3.)
c) In the pterygopalatine fossa, the preganglionic parasympathetic fibers synapse in the pterygopalatine ganglion and the postganglionic paras together with the postganglionic sympathetic fibers, distribute to the lacrimal, nasal, and palatine glands via various sensory branches of the maxillary division of the trigeminal. Those to the lacrimal gland may also join the lacrimal nerve.
Note: A complete description of the innervation of the lacrimal gland can be found under Discussion 6-5. Please reread this discussion at this time.
9. Dissection of the Brain / Demo of the Ear
Part 1
In today's lab you will first examine structures associated with the lateral, basal and medial surfaces of the brain. Before sectioning, identify those structures on the lateral and basal surfaces. Then cut the brain between the two hemispheres to identify structures associated with the medial surface. Finally, slice one half of the brain into sections in the horizontal plane and the other half into sections in the frontal plane for identification of structures in those planes. Cut each section approximately 1 cm thick. It will be advantageous for you to bring along an atlas of the human brain in addition to the Netter atlas.
Structures to be identified:
Lateral surface
cerebrum
cerebellum
brainstem (medulla, pons and midbrain)
longitudinal fissure
central sulcus (of Rolando)
precentral gyrus
postcentral gyrus
lateral fissure (of Sylvius)
frontal lobe
parietal lobe
occipital lobe
temporal lobe
Basal surface
cerebrum
cerebellum
vasculature:
Circle of Willis:
internal carotid arteries
cerebral arteries (anterior, (middle; not part of Circle) and posterior)
anterior and posterior communicating arteries
vertebral arteries
basilar artery
cerebellar arteries (superior, anterior and posterior inferior arteries)
anterior and posterior spinal arteries
cranial nerves I and II (associated with the tel- and diencephalon, respectively), including:
olfactory bulb and tract
optic nerve, chiasm and tract
cranial nerves III - XII (associated with the brainstem)
midbrain
cerebral peduncles
pons
medulla
inferior olive
pyramids and pyramidal decussation
cervical spinal cord
parahippocampal gyrus
uncus
hypothalamus
mammillary bodies
infundibulum
Medial surface
cerebrum third ventricle
cerebellum cerebral aqueduct
corpus callosum fourth ventricle
cingulate gyrus pineal body
central sulcus brainstem
parieto-occipital fissure medulla
calcarine fissure pons
parahippocampal gyrus midbrain
uncus superior colliculus
lateral ventricle inferior colliculus
septum pellucidum hypothalamus
thalamus
medial geniculate body
lateral geniculate body
Frontal and horizontal sections
corpus callosum
internal capsule (anterior and posterior limbs and genu)
optic radiations (retrolenticular part of internal capsule; seen best on horizontal section)
auditory radiations (sublenticular part of internal capsule; seen best on frontal section)
caudate nucleus
lenticular nucleus (putamen and globus pallidus)
thalamus
hypothalamus (supraoptic, tuberal and mammillary regions if possible; frontal section)
fornix
mammillothalamic tract (if possible; seen best on frontal section)
hippocampus
lateral ventricles
third ventricle
substantia nigra (midbrain)
amygdala
Upon completion of the dissection, please wrap the brain slices in the proper sequence in a paper towel and put them in your brain bucket.
Part 2
The Ear
You should look at a prosected dissection and a commercial model. Netter Plate #118 is helpful for orientation.
OBJECTIVES RESOURCES
1. External Ear
a. Identify the: Netter, Plate 88.
- helix
- lobule
- tragus
b. Demonstrate the external acoustic
meatus on the decalcified specimen.
c. Describe the position and composition
of the tympanic membrane.
d. Know the embryonic precursors
for the:
- lining of the tympanic cavity and
auditory tube
- ossicles
- tympanic membrane
2. Osteology
a. Identify the petrous, mastoid, Netter, Plates 2,3,5,
squamous and tympanic portions 6 (the red bone on
of the temporal bone on a skull. these plates).
b. Identify on a skull or on one of Netter, Plate 89
the commercially prepared temporal should be studied for
bones the: orientation to the
medial wall of the
- oval window middle ear.
- round window Discussion 9-1.
- semicanal for the auditory tube
c. Describe the following air- Netter, Plate, 89;
containing spaces of the middle temporal bone preparations.
ear cavity: Discussion 9-2.
- auditory tube
- tympanic cavity
- mastoid air cells
OBJECTIVES RESOURCES
3. Middle Ear Cavity
a. Demonstrate on the decalcified Netter, Plates 41, 88, 89.
temporal bone the:
- malleus
- incus
- stapes
- tensor tympani muscle
- chorda tympani
b. Know the function and
innervation of the tensor
tympani and stapedius muscles.
c. Know the sensory innervation Discussion 9-3.
of the middle ear.
d. Locate the auditory tube in Netter, Plate 93.
a frontal section through
the anterior portion of the
decalcified specimen.
4. Facial Nerve Netter, Plates 41, 89, 92,
118.
a. Demonstrate the path of the
facial nerve on the decalcified
temporal bone.
b. Identify on this bone the:
- geniculate ganglion
- greater petrosal nerve
- chorda tympani
- main trunk of the facial nerve
at the stylomastoid foramen
5. Inner Ear
a. Demonstrate on a decalcified Netter, Plates 87,
specimen the position of the cochlea 91, 92.
and vestibular apparatus. Note that
the facial nerve passes between them
to the geniculate ganglion.
b. Follow CN VIII through the Netter, Plates 87,
internal meatus and identify the 90, 92.
cochlear and vestibular portions.
c. Trace the vibrations from eardrum Discussion 9-4.
to round window.
DISCUSSION
9-1
The term "semicanal" is not used in Netter. The auditory tube, as well as the tensor tympani muscle, lies in a semicanal of bone. We, thus, speak of the semicanal for the auditory tube and the semicanal for the tensor tympani because they are shaped like canals (cylinders) of bone with their lateral walls removed, hence, semi- or half-canals.
9-2
For purposes of conceptualizing the air-containing spaces of the middle ear, one must understand that the only way air can get into the middle ear cavity is through the auditory tube. Beginning at the pharyngeal end of the auditory tube, air can travel to the middle ear, then through the opening (aditus) into the mastoid antrum and then into the mastoid air cells. The volume of the air-containing space contributed by the mastoid air cells probably acts as a buffer so that reabsorption of air during blockage of the auditory tube occurs less rapidly.
9-3
Without belaboring a description of the innervation of the middle ear, suffice it to say that the first branch off CN IX after this cranial nerve exits the jugular foramen is the tympanic nerve. This nerve re-enters the skull through a little canal of bone between the jugular foramen and the carotid canal called the tympanic canaliculus. This little canal leads into the floor of the middle ear cavity just below the promontory and the nerve divides into the tympanic plexus within the mucous membrane covering the promontory (see Netter Plate #89). The sensory fibers to the entire air-containing space described in 9-2 can be considered to come from this tympanic branch of CN IX although a purist would be quick to point out that CN VII and CN X also participate to a smaller degree. Sympathetic fibers join the plexus from the internal carotid nerve through tiny openings in the carotid canal. They innervate the blood vessels in the middle ear. Parasympathetic fibers from the tympanic branch of CN IX destined for the otic ganglion sort out from the tympanic plexus to form the lesser petrosal nerve which exits the middle ear through a tiny crevice in the roof of the petrous portion of the temporal bone. It runs a course parallel to but below the greater petrosal nerve, a branch off CN VII, within the middle cranial fossa. It exits the foramen ovale to enter the otic ganglion. At this time you may want to look back at Discussion 5-4.
9-4
Sound pressure waves striking the tympanic membrane cause it to move inward, compressing the air of the middle ear chamber. If the middle ear were full of liquid, this mechanism would not work because liquid cannot be compressed. The primary role of the auditory tube is to maintain the air in the middle ear cavity. If it is plugged by a tumor or the like, the air is gradually absorbed and eventually replaced by serous fluid.
The malleus and incus form a simple lever system to increase the force at the oval window where the footplate of the stapes sits. The muscles of the middle ear (tensor tympani attached to the malleus and stapedius attached to the stapes) modulate sound transmission. The stapes footplate transfers the vibrations to the fluids in the inner ear which in turn causes the basilar membrane to vibrate. Motion of the basilar membrane and the attached organ of Corti leads to excitation of the receptor (hair) cells and neural transmission in CN VIII. When the stapes is pushed inward against the incompressible liquid of the cochlea, the membrane covering the round window bulges outward into the air space of the middle ear. The round window is needed to allow free movement of the footplate of the stapes. It is necessary that this membrane be present to allow sound energy to enter the inner ear.
Because the tympanic membrane is about 15X the area of the oval window, and the arm of the malleus is longer than that of the incus, there is about a 20X increase in pressure (15 X 1.3) at the oval window. This multiplier effect of the middle ear mechanism overcomes the acoustic impedance mismatch between air and the fluid of the inner ear. In middle ear (conductive) deafness this pressure multiplier effect is greatly reduced or lost.