HEAD AND NECK - PART 1

The study of head and neck anatomy provides a considerable intellectual challenge because the region is packed with small, important structures. These structures are associated with the proximal ends of the respiratory and gastrointestinal systems, the cranial nerves, and the organs of special sense. Dissection of the head and neck provides a special problem in that peripheral structures must be dissected long before their parent structure can be identified. A complete understanding of the region cannot be gained until the final dissection is completed.
OSTEOLOGY
Learning Objectives: Upon completion of this session, the student will be able to:
  1. Identify the major bones of the skull
  2. Identify major sutures of the skull
  3. Identify major bony features of the skull
  4. Identify major surface features of the face and scalp
The dissection of the head is foremost a dissection of the course and distribution of the cranial nerves and the branches of the external carotid artery. All of the cranial nerves and many blood vessels pass through openings in the skull. Therefore, the skull is an important tool with which to organize the study of the soft tissues of the head and neck. We begin here with a general overview of its osteology. Parts of the skull will be studied as needed and details will be added as the dissection proceeds.

All parts of the skull are fragile, but the bones of the orbit are exceptionally delicate. The medial wall of the orbit is very easily broken. Never hold a skull by placing your fingers into the orbits.

Examine the skull from an anterior view and identify:
  • Frontal bone
    • Glabella
    • Superciliary arch
    • Supraorbital notch (foramen)
  • Nasal bone
  • Zygomatic bone
  • Maxilla
    • Frontal process of the Maxilla
    • Infraorbital foramen
    • Alveolar process of the Maxilla
  • Nasal septum
  • Mandible
    • Alveolar process of the Mandible
    • Mental foramen
    • Mental protuberance
Parts of several bones combine to form the following features: Lateral View of the Skull - Examine the skull from a lateral view and identify:

The calvaria (cranial vault) is the dome-like structure that houses the brain. It is formed by parts of the frontal, parietal, and occipital bones. Examine the external surface of the calvaria and identify:

Sutures:

Craniometric landmarks:
While you are looking at the superior aspect of the skull, see if you can find parietal emissary foramina. These foramina can be found at the back part of the parietal bone and close to the sagittal suture. These will not always be present (inconstant). These emissary formamina will transmit emissary veins that connect the extracranial veins with the dural venous sinuses of the cranial cavity.


SURFACE ANATOMY OF THE FACE AND SCALP
Palpate the following structures on the head of the cadaver:



THE FACE
Learning Objectives - Face and Scalp:
    Upon completion of this session, the student will be able to:
  1. Describe the location of the parotid salivary gland.
  2. Identify three main neurovascular structures that traverse the gland: the facial nerve, the retromandibular vein and external carotid artery.
  3. Identify the branches of the facial nerve in the face.
  4. Identify some exemplary muscles of facial expression.
  5. Trace the course of the facial artery and facial vein in the face.
  6. Define the scalp, its structural layers, muscles, nerves, and vessels.

Dissection Overview
The skin of the face receives sensory innervation from three divisions of the trigeminal nerve (Cranial Nerve V). Two cervical spinal nerves complete the cutaneous innervation of the head (Fig. 7.19).

A note about the 12 pairs of cranial nerves: they are numbered 1 - 12 but roman numerals are used exclusively to indicate their number. You may either refer to them by name (e.g., trigeminal nerve) or by roman numeral (e.g., Cranial Nerve V or CN-V in this case) but use of arabic numerals (e.g., Cranial Nerve 5 in this case) will result in your answer being marked wrong on practical exams.

In contrast to sensory innervation, all of the muscles of facial expression receive motor innervation from the facial nerve (Cranial Nerve VII).

The order of dissection will be as follows: The skin of the face will be removed to expose the muscles of facial expression. The parotid duct and gland will be identified. Branches of the facial nerve will be identified as they emerge from the anterior border of the parotid gland. Several facial muscles will be identified. Two important sphincter muscles will receive particular attention: the orbicularis oris (mouth) and the orbicularis oculi (eye). The terminal branches of the three divisions of the trigeminal nerve will be exposed where they emerge from openings in the skull.

Dissection Instructions - Skin Incisions
The skin of the face is very thin. It is firmly attached to the cartilage of the nose and ears but it is mobile over other parts of the face. This mobility permits the action of the muscles of facial expression. The muscles of facial expression are attached to the skin superficially and the bones of the skull deeply. They act as sphincters and dilators for the openings of the eyes, mouth, and nostrils.
  1. Place the cadaver in the supine position and refer to the skin incisions indicated in the adjacent figure.
  2. Remember that the skin of the face is thin. REMOVE ONLY THE SKIN. The muscles of facial expression insert into the skin and the branches of the facial nerve and vessels are very superficial.
  3. You do not need to make flaps as you do your skinning. You can remove the skin piecemeal and keep the dissected face covered with a moist towel.
  4. In the midline, make a skin incision that begins at the nasion (B) and continues to the mental protuberance (C). Encircle the mouth at the margin of the lips.
  5. Starting at the nasion (B), make an incision that encircles the orbital margin. Extend the incision from the lateral angle of the eye to the incision near the ear.
  6. Make an incision from the mental protuberance (C) just inferior to the inferior border of the mandible to point D.
  7. Remove the skin of the lower face, beginning at the midline and proceeding laterally. The superficial fascia of the face is thick and contains the muscles of facial expression.
  8. Detach the skin along the incision line from the midline to the angle of the mandible (B to D).


Superficial Fascia of the Face
The superficial fascia of the face contains the parotid gland, part of the submandibular gland, muscles of facial expression, branches of the facial nerve, branches of the trigeminal nerve, and the facial artery and vein. The muscles of facial expression are attached to the skin, and these attachments have been severed during skin removal. The goal of this stage of the dissection is to identify some of the muscles of facial expression and follow branches of the facial nerve posteriorly into the parotid gland.
  1. A small part of the platysma muscle extends into the face along the inferior border of the mandible. The inferior attachment of the platysma muscle is the superficial fascia of the upper thorax. It forms a sheet of muscle that covers the anterior neck. Use blunt dissection to define the superior attachment of the muscle on the inferior border of the mandible, skin of the cheek, and angle of the mouth.
  2. Identify the masseter muscle. It is a thick muscle of mastication, which will be dissected later.
  3. Identify the parotid duct. The parotid duct is approximately the diameter of a probe handle and it crosses the lateral surface of the masseter muscle about 2 cm inferior to the zygomatic arch. Use blunt dissection to follow the parotid duct anteriorly as far as the anterior border of the masseter muscle where the duct turns deeply, pierces the buccinator muscle of the cheek, and drains into the oral cavity.

Facial Nerve
Branches of the facial nerve will be studied as a prosection. Note that the facial nerve emerges from the base of the skull through the stylomastoid foramen, just deep to the inferior aspect of the ear. Branches of the facial nerve radiate from that point (see figure).

At the anterior border of the parotid gland, locate branches of the facial nerve. The facial nerve exits the stylomastoid foramen and forms a plexus within the substance of the parotid gland (parotid plexus). From this plexus branches of the facial nerve exit the anterior border of the gland. Identify the following:
Facial Artery and Vein
The facial artery and vein follow a winding course across the face and they may pass either superficial or deep to the muscles of facial expression.
  1. Find the facial artery where it crosses the inferior border of the mandible at the anterior edge of the masseter muscle. The facial vein should be located posterior to the facial artery. At this location, the facial artery and vein are covered only by skin and the platysma muscle. The facial artery is highly coiled and takes a more winding course, while the facial vein is straighter and takes a more direct course toward the nose.
  2. Preserve the facial vessels. Reflect the platysma muscle by cutting it from the inferior border of the mandible and detach it from the angle of the mouth. NOTE THIS MUSCLE IS THIN AND FRIABLE.
  3. Use a probe to trace the facial artery superiorly toward the angle of the mouth. Observe that the facial artery has several loops or bends in this part of its course. Near the angle of the mouth, the facial artery gives off the inferior labial and superior labial arteries.
  4. Continue to trace the facial artery as far as the lateral side of the nose, where its name changes to angular artery.
  5. The facial vein receives tributaries that correspond to the branches of the facial artery. The angular vein has a clinically important anastomotic connection with the ophthalmic veins in the orbit, which will be detailed when the orbit is dissected.


Muscles around the Orbital Opening
  1. At only 1 to 2 mm in thickness, the skin of the eyelids is the thinnest skin in the body. Carefully skin the upper and lower eyelids.
  2. Identify the orbicularis oculi muscle, which encircles the palpebral fissure (opening of the eyelid). Identify:
    • Orbital part - surrounds the orbital margin and is responsible for the tight closure of the eyelid
    • Palpebral part - a thinner portion, which is contained in the eyelids and is responsible for blinking of the eyelid
  3. Note that the medial attachment of the orbicularis oculi muscle is the medial orbital margin, the medial palpebral ligament, and the lacrimal bone. The lateral attachment of the orbicularis oculi muscle is the skin around the orbital margin. It is innervated by the temporal and zygomatic branches of the facial nerve.
Muscles around the Oral Opening
  1. Several muscles alter the shape of the mouth and lips. Use a probe to define some of these muscles:
    • Orbicularis oris muscle - has medial attachments to the maxilla, mandible, and skin in the median plane and a lateral attachment to the angle of the mouth. The orbicularis oris muscle is the sphincter of the mouth.
    • Buccinator muscle - has proximal attachments to the pterygomandibular raphe and the lateral surfaces of the alveolar processes of the maxilla and mandible. The distal attachment of the buccinator muscle is the angle of the mouth. It compresses the cheek against the molar teeth, keeping food on the occlusal surfaces during chewing.
  2. Use the prosection of the muscles of facial expression to study the following muscles:
    • Levator labii superioris muscle - has a superior attachment to the maxilla just below the orbital margin and an inferior attachment to the upper lip. It elevates the upper lip.
    • Zygomaticus major muscle - has a lateral attachment to the zygomatic bone and a medial attachment to the angle of the mouth. It draws the angle of the mouth superiorly and posteriorly.
    • Depressor anguli oris muscle - has an inferior attachment to the mandible and a superior attachment to the angle of the mouth (corner of the mouth). It depresses the angle of the mouth.


IN THE CLINIC: Facial Nerve Bell's palsy is a sudden loss of control of the muscles of facial expression on one side of the face. The patient presents with drooping of the mouth and inability to close the eyelid on the affected side.


Sensory Nerves of the Face
  1. Use an illustration to summarize the sensory nerves of the face:
    • Supraorbital nerve - a branch of the ophthalmic division of the trigeminal nerve (Cranial Nerve V1) that passes through the supraorbital notch (foramen). It be seen when the scalp is studied.
    • Infraorbital nerve - a branch of the maxillary division of the trigeminal nerve (Cranial Nerve V2) that passes through the infraorbital foramen. It supplies sensory innervation to the inferior eyelid, side of the nose, and upper lip.
    • Mental nerve - a branch of the mandibular division of the trigeminal nerve (Cranial Nerve V3) that passes through the mental foramen.
  2. On the side with the poorer looking muscles of facial expression and facial nerve branches, use blunt dissection to define the borders of the levator labii superioris muscle. Transect the muscle close to the infraorbital margin and reflect it inferiorly to expose the infraorbital nerve (Cranial Nerve V2).
  3. In the midline, make an incision through the entire thickness of the lower lip, extending as far inferiorly as the mental protuberance. On the side with the poorer looking muscles of facial expression and facial nerve branches, make a second incision parallel to the first. The second incision should begin at the angle of the mouth and end at the inferior border of the mandible.
  4. Reflect the flap of lip inferiorly using a probe to separate the mucous membrane from the mandible. Continue to use blunt dissection. Probe through the mucous membrane where it reflects from the lip to the gums. Use blunt dissection to peel the flap of lip from the bone and locate the mental foramen (L. mentum, chin). The mental foramen is located approximately 3 cm from the median plane.
  5. Observe that the mental nerve, mental artery, and mental vein emerge from the mental foramen. The mental nerve supplies sensory innervation to the lower lip and chin.
  6. There are several smaller branches of the trigeminal nerve that innervate the facial region (lacrimal, infratrochlear, zygomaticofacial, zygomaticotemporal, etc.). Do not dissect these branches. The auriculotemporal nerve (a branch of V3) will be dissected later.
IN THE CLINIC: Dental Anesthesia

Study the infraorbital foramen and infraorbital canal in the skull. For purposes of dental anesthesia, the infraorbital nerve may be infiltrated where it emerges from the infraorbital foramen. The needle is inserted through the oral mucosa deep to the upper lip and directed superiorly.


Dissection Review
  • Use the dissected specimen to trace the branches (temporal, zygomatic, buccal, mandibular and cervical) of the facial nerve from the parotid plexus to the muscles of facial expression.
  • Review the course of the parotid duct. The parotid duct enters the oral vestibule lateral to the second maxillary molar tooth.
  • Review the attachments, action, and innervation of each muscle that was identified in this dissection.
  • Use a skull and the dissected specimen to review the branches of the trigeminal nerve that were dissected and the openings in the bones that they pass through.
  • Use an illustration and the dissected specimen to review the origin and course of the facial artery and vein.


THE SCALP
Dissection Overview
Upon completion of this session, the student will be able to define the boundaries of the scalp, its structural layers, muscles, nerves, and vessels. The scalp consists of five layers that are firmly bound together: As an aid to memory, note that the first letters of the names of the five layers spell the word
S - C - A - L - P.

IN THE CLINIC: The connective tissue layer of the scalp contains collagen fibers that attach to the external surface of the blood vessels. When a blood vessel of the scalp is cut, the connective tissue holds the lumen open, resulting in profuse bleeding. If an infection occurs in the scalp, it can spread within the loose connective tissue layer. Therefore, the loose connective tissue layer is often called the "dangerous area." From the "dangerous area," the infection may pass into the cranial cavity through emissary veins.


Dissection Instructions
  1. These cuts should be made through the entire scalp and the scalpel should contact the bones of the calvaria.
  2. Refer to the adjacent figure and make a midline cut from the nasion (C) to the vertex (A). Extend this cut to the external occipital protuberance (G).
  3. Make a cut in the coronal plane from the vertex (A) to the ear (D). Duplicate this cut on the opposite side of the head.
  4. Beginning at the vertex, use forceps to grasp one corner of the cut scalp and insert a chisel between the scalp and the calvaria. Use the chisel to loosen the scalp from the calvaria.
  5. Once the flap of scalp is raised, grasp the flap with both hands and pull it inferiorly. You will be working in the plane of loose connective tissue.
  6. Reflect all four flaps of scalp down to the level that a hatband would occupy. Do not detach the flaps.
  7. Examine the cut edge of the scalp and identify the occipitalis and frontalis muscles. On the deep surface of the reflected scalp, scrape off some of the dense connective tissue to expose these muscles. The inferior attachment of the occipitalis muscle (a.k.a. occipital belly of the occipitofrontalis muscle) is the occipital bone and its superior attachment is the epicranial aponeurosis (galea aponeurotica). The superior attachment of the frontalis muscle (a.k.a. frontal belly of the occipitofrontalis muscle) is the epicranial aponeurosis and its inferior attachment is the skin of the forehead and eyebrows. Both muscles are innervated by the facial nerve (Cranial Nerve VII).
  8. Pull the anterior scalp flap inferiorly to expose the supraorbital margin. Identify the supraorbital nerve, artery, and vein where they exit the supraorbital notch and enter the deep surface of the scalp.
  9. Use an illustration to observe that nerves and vessels are contained within the flaps of the scalp. Note that the nerves and vessels enter the scalp from more inferior regions.
  10. On the lateral surface of the calvaria, note that the scalp has been separated from the fascia that covers the temporalis muscle (temporal muscle).



Dissection Instructions: Detachment of Nuchal Muscles
During the next dissection laboratory session, a wedge of bone must be removed from the occiput to allow the brain to be removed. To gain access to this bone, the muscles of the posterior neck (nuchal region) must first be detached. These muscles should be dissected carefully and preserved for the M1 students' study of the back muscles.
  1. Place the cadaver in the prone position and refer to Fig. 7.38. Place one or two blocks under the chest to flex the head and neck.
  2. Review the following landmarks on a skull (Fig. 7.38):
    • Mastoid process
    • External occipital protuberance
    • Lambdoid suture
  3. Using a scalpel, make a deep midline cut from the external occipital protuberance inferiorly to about the level of C4 or C5. Cut down to the cervical vertebrae.
  4. Using a scalpel, make a deep transverse cut from the external occipital protuberance laterally to the mastoid process, detaching all musculature from the occipital bone. Keep the scalpel close to the occipital bone. Repeat this on the other side.
  5. Reflect the musculature laterally en bloc until the C1 and C2 vertebrae are exposed.
  6. Detach any remaining skin flaps that prevent access to the occipital bone.
Dissection Review
  • Replace the flaps of scalp in their correct anatomical positions.
  • Use an illustration to review the course of nerves and vessels that supply the scalp.
  • Use a skull and the dissected specimen to review the course of the supraorbital nerve through the supraorbital notch.
  • Use an illustration to study the course of the greater occipital nerve from the cervical region to the posterior surface of the head.
  • Recall the attachments of the occipitofrontalis muscle and review its two bellies in the sagittal scalp cut.


INTERIOR OF THE SKULL
Learning Objectives - Interior of the Skull, Brain Removal, and Dural Venous Sinuses:
Upon completion of this session, the student will be able to:
  1. Identify the prominent landmarks on the internal surface of the skull base.
  2. Identify the major blood vessels of the brain, the specializations of cranial meninges, and cranial dural modifications.
  3. Identify the cranial nerves on the brain and their courses through the skull base.
  4. Identify the parts of the ventricular system and trace the flow of cerebrospinal fluid from production to reabsorption.

Dissection Overview
The bones of the calvaria provide a protective covering for the cerebral hemispheres. To view the internal features of the cranial cavity, the calva (skullcap) must be removed. In addition, a wedge of occipital bone will be removed to open the dissection field and make removal of the brain easier.

The order of dissection will be as follows: The calvaria will be cut with a saw and the calva will be removed. A wedge of occipital bone will be removed. The dura mater will be examined and then opened to reveal the arachnoid mater and pia mater.

Dissection Instructions: Removal of the Calva
  1. The cadaver should be in the supine position. Place 2 to 3 blocks under the scapulae to prop up the body to an angle of about 30 to 45 degrees. Reflect the scalp inferiorly.
  2. Use a scalpel to detach the temporalis muscle from the calvaria and reflect the temporalis muscle inferiorly. Fold it down over the reflected scalp (Fig. 7.37).
  3. Observe the pericranium that covers the calvaria. Use a scalpel or chisel to scrape the bones of the calvaria clean of periosteum and muscle fibers.
  4. Place a rubber band around the circumference of the skull (Fig. 7.37, dashed line). Anteriorly, the rubber band should be about 2 cm superior to the supraorbital margin. Posteriorly, the rubber band should be about 2 cm superior to the external occipital protuberance. Use the rubber band as a guide to mark the circumference of the calvaria with a pencil line.
  5. Refer to a skull. Remove the calva of the skull and note that the bones of the calvaria have three layers:
    • Outer lamina - compact bone
    • Diploë - spongy bone between the outer and inner laminae
    • Inner lamina - compact bone
  6. Use the electric autopsy saw to cut along the pencil line. The saw cut should pass through the outer lamina of the calvaria but not completely through the bone. Moist red bone indicates that the saw is within the diploë. Be particularly careful when cutting the squamous part of the temporal bone, which is very thin. If you saw through the inner lamina, you may damage the underlying dura mater or the brain.
  7. Work your way around the skull with the electric autopsy saw. After making a complete circumferential cut, break the inner lamina of the calvaria by repeatedly inserting a chisel at a 45 degree angle (this oblique placement of the chisel will prevent you from damaging the dura mater and brain) into the saw cut and striking the chisel gently with a mallet. Continue with this procedure until the calva can be pried loose using the Virchow skull cracker (supplied by your instructor).
  8. Remove the calva by prying it from the dura mater with the handle of a forceps or a chisel blade. Work from anterior to posterior and do not use more force than is necessary. Violent pulling may result in tearing of the dura mater and damage to the brain.


Dissection Instructions: Removal of an Occipital Wedge
  1. Use a scalpel or chisel to scrape the occipital bone clean of remaining muscle fibers and pericranium.
  2. On a skull, note the point where the lambdoid suture meets the saw cut where the calva was removed. Transfer this point to the cadaver specimen and mark the location with a pencil (Fig. 7.39, point A).
  3. On a skull, examine the internal surface of the occipital bone and identify (Fig. 7.40):
    • Foramen magnum
    • Groove for the superior sagittal sinus
    • Grooves for the transverse sinuses
    • Fossae for the cerebellum - inferior to the grooves for the transverse sinuses
    • Fossae for the occipital poles of the cerebral hemispheres - superior to the grooves for the transverse sinuses
  4. Using a blunt probe, separate the dura mater from the wedge of occipital bone that you will remove.
  5. On the external surface of the skull, identify the lateral margin of the foramen magnum and transfer this point to the cadaver specimen (Fig. 7.39, point B). On the right and left sides of the cadaver, connect points A and B with a pencil line to define the wedge of occipital bone that will be removed in the cadaver.
  6. Use an electric autopsy saw to cut along the pencil lines. As in removing the calva, do not cut through the inner lamina of compact bone. Extend the saw cut into the foramen magnum but preserve the vertebral arteries. Loosen the wedge of bone with chisel and mallet and remove it, leaving the dura mater intact (Fig. 7.41).
  7. The wedge of bone will be held in place by the posterior atlanto-occipital membrane (link to Fig. 7.38 below). Identify the posterior atlanto-occipital membrane, which spans the interval between the atlas (C1) and the occipital bone. Use a scalpel to incise the posterior atlanto-occipital membrane transversely.
  8. Identify the vertebral arteries. Sever the two vertebral arteries where they enter the skull through the foramen magnum. Use a scalpel to cut the cervical spinal cord in the cervical vertebral canal between the atlas and the occipital bone.

Link to Fig. 7.38


Cranial Meninges
  1. Turn the cadaver to the supine position for the next sequence of instructions. Place 2 to 3 blocks under the scapulae to prop up the body to an angle of about 30 to 45 degrees.
  2. The brain is covered with three membranes called meninges (Gr. meninx, membrane). From outside to inside they are (Fig. 7.42):
    • Dura mater - the outer tough membrane
    • Arachnoid mater - the intermediate membrane
    • Pia mater - a delicate membrane that is closely applied to the surface of the brain
  3. The dura mater (L. dura mater, hard mother) consists of two layers, an external periosteal layer and an internal meningeal layer (Fig. 7.42). The two dural layers are indistinguishable except where they separate to enclose the dural venous sinuses.
  4. Identify the superior sagittal sinus and the right and left transverse sinuses (Fig. 7.41).
  5. Use scissors to make a longitudinal incision in the superior sagittal sinus (Fig. 7.43) and verify that:
    • Its inner surface is smooth because it is lined by endothelium.
    • Its caliber increases from anterior to posterior (direction of venous blood flow).
    • It has lateral expansions called lateral venous lacunae.
    • Arachnoid granulations may be seen in the lateral venous lacunae (Fig. 7.42). The arachnoid granulations return cerebrospinal fluid (CSF) to the venous system.
  6. Examine the surface of the dura mater that covers the cerebral hemispheres and observe the branches of the middle meningeal artery (see figure. The middle meningeal artery supplies the dura mater and adjacent calvaria. Note that the anterior branch of the middle meningeal artery crosses the inner surface of the pterion, where it may tunnel through the bone. Fractures through the pterion may result in tearing of the middle meningeal artery.
  7. Examine the inner surface of the removed calva. Identify the following features:
  8. Use scissors to make a parasagittal cut through the dura mater about 2 cm lateral to the midline (Fig. 7.43). Cut only the dura mater, not the arachnoid mater. This cut should be lateral and parallel to the lateral edge of the superior sagittal sinus. Extend the cut to the frontal bone anteriorly and to the transverse sinus posteriorly. Duplicate the parasagittal cut on the opposite side of the cadaver.
  9. Make a coronal cut through the dura mater from the midpoint of the parasagittal cut (near the vertex) to just above the ear (Fig. 7.43). Repeat on the opposite side of the midline.
  10. The result of these cuts is a median strip of dura mater containing the superior sagittal sinus and four flaps of dura mater that are similar in position to the scalp flaps (Fig. 7.43). Fold the dural flaps inferiorly over the cut edge of the skull. Use scissors to detach any small adhesions or blood vessels that constrain the flaps.
  11. Observe the arachnoid mater (Gr. arachnoeides, like a cobweb, in reference to the spider web-like connective tissue strands in the subarachnoid space). The arachnoid mater loosely covers the brain and spans across the fissures and sulci. In the living person, the arachnoid mater is closely applied to the internal meningeal layer of the dura mater with no space between (Fig. 7.42).
  12. Observe the cerebral veins that are visible through the arachnoid mater. The cerebral veins empty into the superior sagittal sinus through short communications called bridging veins. Bridging veins may be torn in cases of head trauma.
  13. Use scissors to make a small cut (2.5 cm) through the arachnoid mater over the lateral surface of the brain. Use a probe to elevate the arachnoid mater and observe the subarachnoid space. In the living person, the subarachnoid space is a real space that contains cerebrospinal fluid.
  14. Through the opening in the arachnoid mater, observe the pia mater (L. pia mater, tender mother) on the surface of the brain. The pia mater faithfully follows the contours of the brain, passing into all sulci and fissures. The pia mater cannot be removed from the surface of the brain.

IN THE CLINIC: When the middle meningeal artery is torn in a head injury, blood accumulates between the skull and the dura mater (epidural hematoma).


IN THE CLINIC: As a complication of head injury, bridging veins may bleed into the potential space between the dura mater and the arachnoid mater. When this happens, the blood accumulates between the dura mater and arachnoid mater (a "subdural space" is created), and this condition is called a subdural hematoma.


Dissection Review



BRAIN REMOVAL
Dissection Overview
The internal meningeal layer of the dura mater forms inwardly projecting folds (dural infoldings) that serve as incomplete partitions of the cranial cavity. Three of these folds (cerebral falx, cerebellar tentorium, and cerebellar falx) extend inward between parts of the brain. These infoldings must first be cut before the brain can be removed.

The order of dissection will be as follows: The brain will be removed intact, along with the arachnoid mater and pia mater. The dura mater will be left in the cranial cavity, where the dural infoldings will be studied.

  1. Use a skull to identify the following features (Fig 7.44):
    • Crista galli
    • Cribriform plate
    • Anterior clinoid process
    • Posterior clinoid process
    • Superior border of the petrous part of the temporal bone
    • Internal acoustic meatus
    • Jugular foramen
    • Hypoglossal canal
    • Foramen magnum
    • Groove for the sigmoid sinus
    • Groove for the transverse sinus
  2. Place the cadaver in the supine position. Place 2 to 3 blocks under the scapulae to prop up the body to an angle of about 30 to 45 degrees.
  3. In the midline, use your fingers to gently retract one cerebral hemisphere 1 or 2 cm laterally and observe the cerebral falx (L. falx, sickle) between the cerebral hemispheres (Fig. 7.45). The cerebral falx is attached to the crista galli at its anterior end and the cerebellar tentorium at its posterior end.
  4. Use your hand to gently lift the frontal lobes (anterior part of the brain) and use scissors to cut the cerebral falx where it is attached to the crista galli.
  5. Use scissors to cut the cerebral veins where they enter the superior sagittal sinus (Fig. 7.43) so that the veins will remain on the surface of the brain. Grasp the cerebral falx near the crista galli and pull it superiorly and posteriorly from between the cerebral hemispheres. At its posterior end, the cerebral falx will remain attached to the cerebellar tentorium. Do not detach the cerebral falx from the cerebellar tentorium.
  6. On the right side, gently lift the occipital lobe (posterior part of brain) and observe the cerebellar tentorium. Beginning anteriorly, use a scalpel to cut the cerebellar tentorium from the petrous part of the temporal bone as close to bone as possible. Sever the cerebellar tentorium from the posterior clinoid process and then from the superior border of the petrous part of the temporal bone (Fig 7.44). The cut should continue to the posterolateral end of the superior border of the petrous part of the temporal bone, near the groove for the sigmoid sinus. Repeat the cut on the left side of the cadaver.
  7. Pull the cerebral falx and cerebellar tentorium posteriorly from between the cerebral hemispheres and cerebellum. Do not cut and detach the cerebral falx and cerebellar tentorium from its remaining attachments to the cranium. Simply pull these structures out the back of the head. This procedure will tear the great cerebral vein (Fig 7.46).
  8. With the dural infoldings reflected, the brain may be gently moved to expose the cranial nerves and vessels that are located on its inferior surface.
  9. Use your fingers to gently elevate the frontal lobes. Use a probe to lift the olfactory bulb from the cribriform plate on each side of the crista galli (Fig 7.50).
  10. Use a scalpel to cut the following structures bilaterally: optic nerve (Cranial Nerve II), internal carotid artery, and oculomotor nerve (Cranial Nerve III) (Fig 7.50). Cut the stalk of the pituitary gland in the midline.
  11. Raise the brain slightly higher and cut the following structures bilaterally: trochlear nerve (Cranial Nerve IV), trigeminal nerve (Cranial Nerve V), and the abducens nerve (Cranial Nerve VI) (Fig 7.50).
  12. Elevate the cerebrum and brainstem still further and cut the following structures bilaterally: facial (Cranial Nerve VII) and vestibulocochlear nerves (Cranial Nerve VIII) near the internal acoustic meatus; glossopharyngeal (Cranial Nerve IX), vagus (Cranial Nerve X), and accessory nerves (Cranial Nerve XI) near the jugular foramen; and hypoglossal nerve (Cranial Nerve XII) near the hypoglossal canal (Fig 7.50).
  13. Support the brain with the palm of one hand under the frontal lobes and your fingers extending down the ventral surface of the brainstem. Insert the tip of your middle finger into the cut that was made across the cervical spinal cord to support the brainstem and cerebellum. Roll the brain, brainstem, and cerebellum posteriorly and out of the cranial cavity in one piece.
  14. After you have removed the brain bring it along with an evaluation form to a faculty member for evaluation. The faculty will evaluate the brain for the intactness of the cranial nerves and vasculature. High scoring dissecting teams will be invited to a celebratory dinner at Dr. Giffin's house.


THE DURAL VENOUS SINUSES

Dissection Overview - Dural Infoldings and Dural Venous Sinuses
The two layers of the dura mater separate in several locations to form dural venous sinuses. The dural venous sinuses collect venous drainage from the brain and conduct it out of the cranial cavity. The order of dissection will be as follows: The dura mater will be repositioned to recreate its three-dimensional morphology during life. The infoldings of the dura mater and the associated dural venous sinuses will be identified.
  1. Return the dura mater to its correct anatomical position.
  2. On the right side of the head, open the two flaps of dura mater and identify the cerebral falx (falx cerebri) (Fig 7.46). In the living person, the cerebral falx lies between the cerebral hemispheres. The cerebral falx is attached to the crista galli, the calvaria on both sides of the groove for the superior sagittal sinus, and the cerebellar tentorium (tentorium cerebelli).
  3. Identify the cerebellar tentorium (tentorium cerebelli; L. tentorium, tent) (Fig. 7.45). The cerebellar tentorium is attached to the clinoid processes of the sphenoid bone, the superior border of the petrous portion of the temporal bone, and the occipital bone on both sides of the groove for the transverse sinus. The opening in the cerebellar tentorium is called the tentorial notch (tentorial incisure), and the brainstem passes through it. In the living person, the cerebellar tentorium is between the cerebral hemispheres and the cerebellum.
  4. Identify the cerebellar falx (falx cerebelli), which is located inferior to the cerebellar tentorium in the midline (Fig 7.46). Note that the cerebellar falx is attached to the inner surface of the occipital bone and that it is located between the cerebellar hemispheres.
  5. Review the position of the superior sagittal sinus (Fig 7.46). Note that the superior sagittal sinus begins near the crista galli and ends near the cerebellar tentorium by draining into the confluence of sinuses.
  6. Identify the inferior sagittal sinus, which is in the inferior margin of the cerebral falx (Fig 7.46). The inferior sagittal sinus begins anteriorly and ends near the cerebellar tentorium by draining into the straight sinus. Note that the inferior sagittal sinus is much smaller in diameter than the superior sagittal sinus.
  7. The straight sinus is located in the line of junction of the cerebral falx and the cerebellar tentorium. At its anterior end, the straight sinus receives the inferior sagittal sinus and the great cerebral vein. The straight sinus drains into the confluence of sinuses.
  8. Review the position of the transverse sinuses (right and left). Each transverse sinus carries venous blood from the confluence of sinuses to the sigmoid sinus. Use a scalpel to open the lumen of the transverse sinus.
  9. Identify the sigmoid sinus. The sigmoid sinus begins at the lateral end of the transverse sinus and ends at the jugular foramen. Use a scalpel to open the lumen of the sigmoid sinus and trace it to the jugular foramen. The internal jugular vein is formed at the external surface of the jugular foramen.
  10. Observe the floor of the cranial cavity. Note that the dura mater covers all of the bones and provides openings through which the cranial nerves pass. There are small dural venous sinuses located between the layers of the dura mater in the floor of the cranial cavity (Netter, Plate 98).

Dissection Review



THE CRANIAL FOSSAE
Learning Objectives - The Cranial Fossae:
Upon completion of this session, the student will be able to:
  1. Describe the general organization of the cranial fossae.
  2. Describe the osteology of the anterior, middle , and posterior cranial fossae.
  3. List the foramina and contents of each of the cranial fossae.
  4. Demonstrate the anatomy of the cavernous sinus

Dissection Overview - The Cranial Fossae
The order of dissection will be as follows: The bones of the floor of the cranial cavity will be studied and the boundaries of the cranial fossae will be identified. The vessels and the nerves of each cranial fossa will be studied. Because the floor of the cranial cavity is covered by dura mater, the dissection is much easier if a skull is held beside the cadaver specimen during dissection to permit direct observation of the foramina.

Use the skull to identify:
Identify the foramen lacerum, which is formed by portions of the greater wing of the sphenoid bone and the temporal bone. The anterior cranial fossa is separated from the middle cranial fossa by the right and left lesser wings of the sphenoid bone and the sphenoidal limbus (a variably prominent ridge on the body of the sphenoid bone forming the posterior border of the jugum and the anterior border of the prechiasmatic sulcus). The middle cranial fossa is separated from the posterior cranial fossa by the superior border of the petrous part of the right and left temporal bones and the dorsum sellae. The cerebellar tentorium is attached to the superior border of the petrous part of the temporal bone and it forms the roof of the posterior cranial fossa.

Dissection Instructions - Anterior Cranial Fossa:
  1. On the right side of the cadaver only, use a probe to loosen the dura mater along the cut edge of the frontal bone. Grasp the dura mater with your fingers and pull it posteriorly as far as the lesser wing of the sphenoid bone. Use scissors to detach the dura mater along the lesser wing of the sphenoid bone and along the midline and place it in the tissue container.
  2. Note that the sphenoparietal venous sinus is located along the lesser wings of the sphenoid bone and that its lumen may now be visible where you detached the dura mater.
  3. Identify the three bones that participate in the formation of the anterior cranial fossa: sphenoid bone, ethmoid bone, and orbital part of the frontal bone (Fig 7.49). Note that the orbital part of the frontal bone forms the roof of the orbit.
  4. Before the brain was removed, the cerebral falx was attached to the crista galli and the frontal lobe of the brain rested on the orbital part of the frontal bone. The olfactory bulb rested on the cribriform plate and the fibers of the olfactory nerve (Cranial Nerve I) passed through the openings of the cribriform plate to enter the nasal cavity (Fig 7.50).

Dissection Instructions - Middle Cranial Fossa:

Recall that the middle cranial fossa contains the temporal lobe of the brain.
  1. Observe the dura mater that covers the floor of the middle cranial fossa. The dura mater hides all of the openings in the skull and the nerves and vessels that pass through them (Fig 7.50).
  2. Identify the middle meningeal artery that can be seen through the dura mater (Fig 7.50). It appears as a dark line extending laterally from the deepest point of the middle cranial fossa. The middle meningeal artery enters the middle cranial fossa by passing through the foramen spinosum.
  3. ON THE RIGHT SIDE OF THE CADAVER ONLY, grasp the dura mater along the lesser wing of the sphenoid bone and peel it posteriorly as far as the superior border of the petrous part of the temporal bone. Note that the middle meningeal artery adheres to the external surface of the dura mater. Use a probe to tease the proximal part of middle meningeal artery away from the dura mater and leave it in the skull.
  4. ON THE RIGHT SIDE OF THE CADAVER ONLY, use scissors to detach the dura mater along the superior border of the petrous part of the temporal bone and place it in the tissue container. Do not cut the cranial nerves that cross the anterior end of the superior border of the petrous part of the temporal bone (oculomotor, trigeminal, trochlear, and abducens). Note that the lumen of the superior petrosal sinus can be seen along the line of the cut (Fig 7.50).
  5. Observe that the floor of the middle cranial fossa is formed by two bones: sphenoid and temporal (Fig 7.49).
  6. Identify the optic nerve (Cranial Nerve II) (Fig 7.50). The optic nerve passes through the optic canal to enter the orbit. The optic nerve is surrounded by a sleeve of dura mater as it exits the middle cranial fossa.
  7. Use a probe to identify the superior orbital fissure that is located inferior to the lesser wing of the sphenoid bone (Fig 7.49). Three cranial nerves and part of a fourth exit the middle cranial fossa by passing through the superior orbital fissure:
    • Oculomotor nerve (Cranial Nerve III) - passes over the superior border of the petrous part of the temporal bone and passes anteriorly within the lateral wall of the cavernous sinus.
    • Trochlear nerve (Cranial Nerve IV) - courses anteriorly within the lateral wall of the cavernous sinus immediately inferior to the oculomotor nerve (Fig 7.50). The trochlear nerve is a very small nerve that enters the dura mater at the anterior end of the tentorial notch. It may have been cut during brain removal but should be intact farther anteriorly.
    • Ophthalmic division of the trigeminal nerve (Cranial Nerve V1) - arises from the trigeminal ganglion and passes anteriorly within the lateral wall of the cavernous sinus inferior to the trochlear nerve (Fig 7.50).
    • Abducens nerve (Cranial Nerve VI) - enters the dura mater over the clivus of the occipital bone (Fig 7.50). The abducens nerve passes anteriorly within the cavernous sinus in close relationship to the lateral surface of the internal carotid artery.
  8. Use a probe to clean the nerves that pass through the superior orbital fissure. Note that three of these nerves are located within the lateral wall of the cavernous sinus (Cranial Nerves III, IV, V1) and one is within the cavernous sinus (Cranial Nerve VI) (Fig 7.51).
  9. Identify the trigeminal nerve (Cranial Nerve V) (Fig 7.50). It is the largest cranial nerve and is easily found where it crosses the superior border of the petrous part of the temporal bone.
  10. Follow the trigeminal nerve anteriorly and identify the trigeminal ganglion. Use a probe to define the three divisions (nerves) that arise from the anterior border of the trigeminal ganglion (ophthalmic [V1], maxillary [V2], and mandibular [V3]). Note that these three divisions are named according to their region of distribution and are numbered from superior to inferior as they arise from the trigeminal ganglion.
  11. Identify the maxillary division of the trigeminal nerve (Cranial Nerve V2) (Fig 7.50), and follow it anteriorly to the foramen rotundum (Fig 7.49), where it exits the middle cranial fossa. The maxillary division courses within the lateral wall of the cavernous sinus just inferior to the ophthalmic division of the trigeminal nerve (Cranial Nerve V1) (Fig 7.51).
  12. Identify the mandibular division of the trigeminal nerve (Cranial Nerve V3) (Fig 7.50) and follow it inferiorly to the foramen ovale (Fig 7.49), which is where it exits the middle cranial fossa.
  13. Return to the area of the cavernous sinus and use a probe to retract the cranial nerves. Identify the internal carotid artery (Fig 7.50). The internal carotid artery enters the cranial cavity by passing through the carotid canal. It makes an S-shaped bend in the cavernous sinus and emerges near the optic nerve. Cranial nerves III, IV, V1, V2, and VI cross the lateral side of the internal carotid artery. Among this group of nerves, the abducens nerve (Cranial Nerve VI) is most closely related to the internal carotid artery (Fig 7.51).
  14. Identify the region of the hypophyseal fossa (Fig 7.49). The hypophyseal fossa is covered by the sellar diaphragm (diaphragma sellae), which is a dural infolding (Fig 7.51). The stalk of the pituitary gland passes through an opening in the sellar diaphragm. The pituitary gland is still contained within the hypophyseal fossa.
  15. Anterior and posterior to the stalk of the pituitary gland are two small dural venous sinuses called the anterior and posterior intercavernous sinuses (Fig 7.50). The intercavernous sinuses connect the right and left cavernous sinuses across the midline. Do not attempt to dissect the intercavernous sinuses.
  16. Use an atlas illustration to identify all of the veins and venous sinuses that drain into or out of the cavernous sinus.

IN THE CLINIC: Cavernous Sinus
In fractures of the base of the skull, the internal carotid artery may rupture within the cavernous sinus. The release of arterial blood into the cavernous sinus creates an abnormal reflux of blood from the cavernous sinus into the ophthalmic veins. As a result, the orbit is engorged and the eyeball is protruded and is pulsating in synchrony with the radial pulse (pulsating exophthalmos).


Dissection Instructions - Posterior Cranial Fossa:
  1. Recall that the posterior cranial fossa contains the cerebellum and the brainstem. At the foramen magnum, the brainstem becomes continuous with the cervical spinal cord. The features of the posterior cranial fossa will be studied with the dura mater intact.
  2. Identify the facial nerve (Cranial Nerve VII) and the vestibulocochlear nerve (Cranial Nerve VIII) where they enter the internal acoustic meatus (Fig 7.50). Do not follow them into the bone at this time.
  3. The jugular foramen is inferior to the internal acoustic meatus (Fig 7.49). Identify the rootlets of the glossopharyngeal nerve (Cranial Nerve IX), the vagus nerve (Cranial Nerve X), and the accessory nerve (Cranial Nerve XI) where they enter the jugular foramen. Because cranial nerves IX and X are formed by rootlets, it is difficult to distinguish one nerve from the other as they enter the jugular foramen. However, the accessory nerve can be positively identified because it enters the posterior cranial fossa through the foramen magnum and crosses the inner surface of the occipital bone (Fig 7.50).
  4. Identify the hypoglossal nerve (Cranial Nerve XII) where it enters the hypoglossal canal (Fig 7.50).
  5. Review the course of the transverse sinus and sigmoid sinus. Observe that the sigmoid sinus ends at the jugular foramen posterior to the exit point of cranial nerves IX, X, and XI.
  6. On the left (undissected) side of the cranial cavity, identify all 12 cranial nerves in order from anterior to posterior (Fig 7.50).


Dissection Review
  • Review the bones that form the floor of the cranial cavity.
  • Read an account of the dural venous sinuses and review them in the cadaver.
  • Summarize the cranial nerves, review the course of each cranial nerve, and name the opening through which each passes to exit the cranial cavity. In the skull, review the openings (foramina and fissures) through which the cranial nerves pass.
  • If the brain is still available, hold it beside the cranial cavity so that you can see its ventral surface. Review the severed cranial nerves and vessels on the brain (Fig 7.48) and correlate with the severed ends remaining in the cadaver. If the brain is not available, compare an atlas figure of the ventral surface of the brain.



THE ORBIT AND ITS CONTENTS
Learning Objectives:
Upon completion of this session, the student will be able to:
  1. Identify the prominent bony features of the orbit with included foramina and fissures.
  2. Describe the components of the eyelids with associated muscles, tarsal glands, connective tissue fascia and conjunctiva.
  3. Identify the extraocular muscles, their function and innervation.
  4. Identify all sensory, motor and autonomic nerves of the orbit and trace their routes to and within the orbit.
  5. Identify branches of ophthalmic arteries and veins.

Dissection Overview
The orbit contains the eyeball and extraocular muscles. The eyeball is about 2.5 cm in diameter and occupies the anterior half of the orbit. The posterior half of the orbit contains fat, extraocular muscles, branches of cranial nerves, and blood vessels. Some vessels and nerves pass through the orbit to reach the scalp and face.

The order of dissection will be as follows: The bones of the orbit will be studied. The floor of the anterior cranial fossa will be removed and the orbit will be dissected from a superior approach. Cranial nerves III, IV, V1, and VI will be followed through the superior orbital fissure into the orbit and the extraocular muscles will be identified. On the left side only, the anatomy of the eyelid will be studied. The attachments of the extraocular muscles will be studied.

Osteology of the Orbit
The bones of the orbit form a four-sided pyramid. The base of the pyramid is the orbital margin and the apex of the pyramid is the optic canal. Viewed from above, the medial walls of the two orbits are parallel to each other and about 2.5 cm apart. The lateral walls of the orbits form a right angle to each other. Refer to a skull and identify the bones that participate in the formation of the walls of the orbit (Fig 7.52):
On the medial wall of the orbit, identify the anterior and posterior ethmoidal foramina. These are typically small and absolute identification may not be possible.

Identify the inferior orbital fissure, which is a gap between the maxilla and the greater wing of the sphenoid bone. Note that the lateral wall of the orbit is stout and strong but the part of the ethmoid bone that forms the medial wall is paper thin and for this reason it is called the lamina papyracea. Examine a coronal section through the orbit and note the following relationships (Fig 7.53):

Surface Anatomy of the Eyeball, Eyelids, and Lacrimal Apparatus
(do this some other time when you are not in the Gross Anatomy lab)
Use a mirror or recruit the assistance of your lab partner to inspect the living eye. Identify:
In the medial angle of the eye, observe:
Evert the lower lid slightly and observe:
Use an illustration to study the following features and relate them to the living eye:
Dissection Instructions - Eyelid and Lacrimal Apparatus
  1. Dissect the eyelid and lacrimal gland only in the left eye.
  2. Review the attachments of the orbicularis oculi muscle. Use a probe to raise the lateral part of the orbital portion of the orbicularis oculi muscle and reflect the muscle medially.
  3. Raise the thin palpebral portion of the orbicularis oculi muscle off the underlying tarsal plate and reflect the muscle medially.
  4. The orbital septum is a sheet of connective tissue that is attached to the periosteum at the margin of the orbit and to the tarsal plates (Fig 7.54 and Fig 7.55). The orbital septum separates the superficial fascia of the face from the contents of the orbit.
  5. Identify the tarsal plates, which give shape to the eyelids (Fig 7.54). Tarsal glands are embedded in the posterior surface of each tarsal plate. Tarsal glands drain by small orifices that are located posterior to the eyelashes. Tarsal glands secrete an oily substance onto the margin of the eyelid that prevents the overflow of lacrimal fluid (tears).
  6. The lacrimal gland occupies the lacrimal fossa in the frontal bone (Fig 7.54). To find the lacrimal gland, use a scalpel to cut through the orbital septum adjacent to the orbital margin in the superolateral quadrant of the left orbit. Pass a probe through the incision and free the lacrimal gland from the lacrimal fossa. Note that the lacrimal gland drains into the superior conjunctival fornix by 6 to 10 short ducts (Fig 7.56).
  7. Use a skull to identify the lacrimal groove at the medial side of the orbital margin (Fig 7.52).
  8. Two lacrimal canaliculi drain lacrimal fluid from the medial angle of the eye into the lacrimal sac. The nasolacrimal duct extends inferiorly from the lacrimal sac and enters the inferior meatus of the nasal cavity (Fig 7.56).
  9. Lacrimal fluid flows from the lacrimal gland across the eyeball to the medial angle of the eye.
    • During crying, excess lacrimal fluid cannot be emptied through the lacrimal canaliculi and tears overflow the lower eyelids. Increased drainage of tears into the nasal cavity results in sniffling, which is characteristic of crying.
IN THE CLINIC: Tarsal Glands
If the duct of a tarsal gland becomes obstructed, a chalazion (cyst) will develop. A chalazion will be located between the tarsal plate and the conjunctiva. In contrast, a hordeolum (stye) is the inflammation of a sebaceous gland associated with the follicle of an eyelash.


Dissection Instructions - Orbit
The first 12 steps of this dissection are to be done bilaterally. The deep orbit dissction is to be done on the right side only.
  1. Dissect the orbits from the superior approach. Wear eye protection for all steps that require the use of bone cutters.
  2. In the floor of the anterior cranial fossa, tap the orbital part of the frontal bone with the handle of a chisel until the bone cracks. Use bone cutters to pick out the bone fragments and enlarge the opening in the roof of the orbit. Remove the roof of the orbit as far anteriorly as the supraorbital margin.
  3. The frontal bone contains the frontal sinus that may extend into the roof of the orbit. Medially, the ethmoidal air cells may extend into the roof of the orbit. If either situation occurs in your cadaver, you must remove the mucous membrane that lines the sinus and remove a second layer of thin bone to open the orbit.
  4. Identify the membrane just inferior to the roof of the orbit. This is the periorbita, which lines the bones of the orbit.
  5. Push a probe posteriorly between the roof of the orbit and the periorbita. The probe should pass inferior to the lesser wing of the sphenoid bone, through the superior orbital fissure, and into the middle cranial fossa. Use the probe to break the lesser wing of the sphenoid bone.
  6. Use bone cutters to remove the fragments of the lesser wing of the sphenoid bone. Chip away the roof of the optic canal and remove the anterior clinoid process (Fig 7.57).
  7. Examine the periorbita and note that the frontal nerve may be visible through it. Use scissors to incise the periorbita from the apex of the orbit to the midpoint of the supraorbital margin. Use forceps to lift the periorbita off deeper structures and make a transverse incision through the periorbita, close to the supraorbital margin. Use a probe to tease open the flaps of periorbita and use scissors to remove them.
  8. The use of a fine probe and fine forceps is recommended from this point onward in the dissection.
  9. Three nerves enter the apex of the orbit by passing superior to the extraocular muscles:
    • Frontal nerve (a branch of Cranial Nerve V1) - courses from the apex of the orbit toward the supraorbital margin (Fig 7.57). Trace the frontal nerve anteriorly and observe that it divides into the supratrochlear nerve and the supraorbital nerve.
    • Lacrimal nerve (a branch of Cranial Nerve V1) - passes through the superior orbital fissure lateral to the frontal nerve and courses along the lateral wall of the orbit. The lacrimal nerve is much smaller than the frontal nerve (Fig 7.57). Follow the lacrimal nerve anterolaterally toward the lacrimal gland.
    • Trochlear nerve - passes through the superior orbital fissure medial to the frontal nerve (Fig 7.57). Follow the trochlear nerve to the superior border of the superior oblique muscle, which it innervates. The trochlear nerve usually enters the superior border of the superior oblique muscle in its posterior one-third.
  10. While preserving the nerves, use forceps to pick out lobules of fat and expose the superior surface of the levator palpebrae superioris muscle (Fig 7.55 and Fig 7.57). The levator palpebrae superioris muscle attaches to the upper eyelid, which it elevates.
  11. On the medial side of the orbit, identify the superior oblique muscle and trace it anteriorly (Fig 7.58). Observe that the tendon of the superior oblique muscle passes through the trochlea (L. trochlea, pulley), bends at an acute angle, and attaches to the posterolateral portion of the eyeball.
  12. On the lateral side of the orbit, identify the lateral rectus muscle (Fig 7.58). The lateral rectus muscle arises by two heads from the common tendinous ring. The common tendinous ring surrounds the optic canal and part of the superior orbital fissure, and it is the posterior attachment of the four rectus muscles. The optic nerve (Cranial Nerve II), nasociliary nerve, oculomotor nerve (Cranial Nerve III), and abducens nerve (Cranial Nerve VI) pass through the common tendinous ring.

    13. PERFORM THE FOLLOWING STEPS ON THE RIGHT SIDE OF THE CADAVER ONLY

  13. Transect the levator palpebrae superioris muscle as far anteriorly as possible and reflect it posteriorly.
  14. Identify the superior rectus muscle that lies immediately inferior to the levator palpebrae superioris muscle (Fig 7.55 and Fig 7.57). Clean the superior rectus muscle and observe that it is attached to the eyeball by a thin, broad tendon.
  15. Transect the superior rectus muscle close to the eyeball and reflect it posteriorly (Fig 7.58). Note that a branch of the superior branch of the oculomotor nerve (Cranial Nerve III) reaches the inferior surface of the superior rectus muscle. A branch of the superior branch of the oculomotor nerve passes around the medial side of the superior rectus muscle to innervate the levator palpebrae superioris muscle.
  16. Use scissors to cut the common tendinous ring between the attachments of the superior rectus and lateral rectus muscles. All structures passing through the common tendinous ring are now exposed.
  17. Identify the abducens nerve (Cranial Nerve VI). The abducens nerve passes between the two heads of the lateral rectus muscle, turns laterally, and enters the medial surface of the lateral rectus muscle. Find the abducens nerve on the medial surface of the lateral rectus muscle near the apex of the orbit (Fig 7.58).
  18. Identify the nasociliary nerve, which is a branch of Cranial Nerve V1 (Fig 7.58). Trace the nasociliary nerve through the orbit and note that it is much smaller than the frontal nerve. The nasociliary nerve crosses superior to the optic nerve and gives off several long ciliary nerves to the posterior part of the eyeball (note that the long ciliary nerves are very delicate; care should be exercised when looking for them).
  19. Follow the nasociliary nerve toward the medial wall of the orbit. Use forceps to pick out the fat that fills the intervals between muscles, nerves, and vessels.
  20. Identify the anterior ethmoidal nerve, which is a small branch of the nasociliary nerve that passes through the anterior ethmoidal foramen. The anterior ethmoidal nerve supplies part of the mucous membrane in the nasal cavity. Its terminal branch is the external nasal nerve that innervates the skin at the tip of the nose.
  21. In the middle cranial fossa, identify the oculomotor nerve within the lateral wall of the cavernous sinus. Follow the oculomotor nerve to the superior orbital fissure where it branches into two divisions:
    • Superior branch - innervates the levator palpebrae superioris and the superior rectus muscles
    • Inferior branch - innervates the medial rectus, inferior rectus, and inferior oblique muscles
  22. The ciliary ganglion is a parasympathetic ganglion located between the optic nerve and the lateral rectus muscle. It is approximately 2 mm in diameter and is located about 1 cm anterior to the apex of the orbit (Fig 7.58). Short ciliary nerves connect the ciliary ganglion to the posterior surface of the eyeball. Study the autonomic function of the ciliary ganglion. You are not required to dissect the ciliary ganglion. If you have time, you may want to make an attempt at finding the ciliary ganglion and the short ciliary nerves. If you do so, you will undoubtedly impress your instructors.
  23. Identify the optic nerve (Cranial Nerve II) (Fig 7.58). The optic "nerve" is actually a brain tract and it is surrounded by the three meningeal layers: dura mater, arachnoid mater, and pia mater.
  24. Identify the ophthalmic artery where it branches from the internal carotid artery (Fig 7.50). In its course through the orbit, note that the ophthalmic artery usually crosses superior to the optic nerve and reaches the medial wall of the orbit. The ophthalmic artery gives rise to posterior ciliary arteries that supply the eyeball. Do not look for the posterior ciliary arteries.
  25. Identify branches of the ophthalmic artery: the supraorbital artery exits the supraorbital foramen in the company of the supraorbital nerve to supply the forehead and scalp; the lacrimal artery passes along the superior border of the lateral rectus muscle to supply the lacrimal gland, conjunctiva, and eyelids.
  26. Use an atlas illustration to study the course of the superior ophthalmic vein in the orbit. At the medial angle of the eye, the superior ophthalmic vein anastomoses with the angular vein (a tributary of the facial vein). The superior ophthalmic vein pursues the same course as the ophthalmic artery, and receives tributaries corresponding to the branches of that vessel. Forming a short single trunk, it passes between the two heads of the lateral rectus muscle and through the medial part of the superior orbital fissure, and ends in the cavernous sinus.
  27. The medial rectus muscle can be easily seen along the medial wall of the orbit. The inferior rectus and inferior oblique muscles are not easily seen from the superior approach. Carefully push aside the dissected structures to catch a glimpse of these muscles.
IN THE CLINIC: Ophthalmic Veins

Anastomoses between the angular vein and the superior and inferior ophthalmic veins are of clinical importance. Infections of the upper lip, cheeks, and forehead may spread through the facial and angular veins into the ophthalmic veins and then into the cavernous sinus. Thrombosis of the cavernous sinus may result, leading to involvement of the abducens nerve and dysfunction of the lateral rectus muscle.


Dissection Review