NOTE BEFORE YOU BEGIN: You will be working with both eyes and orbits today. On one side you will open the roof of the orbit, explore the orbital contents from above, and explore the eyelids. On the other side you will remove the eyelid and fat and explore the eye from the front and then remove that eye from the orbit.
1. SUPERIOR APPROACH TO THE ORBIT
Re-examine the contents of the cavernous sinus.
ON ONE SIDE, use a hand saw to make two vertical cuts (as shown by the dotted lines in the figure below) through the frontal bone on the medial and lateral portions of the orbit.
Use a small chisel to carefully chip through the thin orbital roof in the anterior cranial fossa.
Flip the piece of the frontal bone forward to remove.
Continue to chip away the thin bone of the orbital roof until the orbital contents are exposed.
Once the frontal bone has been cut, you can use hemostats or forceps to grip the thin bone of the orbital roof and gently pull it away to reveal the structures within the orbit.
Once you have removed the orbital roof, you will see the periorbita: the periosteal sac that envelops the contents of the orbit.
Carefully cut through the periorbita and immediately beneath find two branches of the ophthalmic nerve (CN V1):
The large frontal nerve, which further breaks up into supratrochlear and supraorbital branches as it travels forward.
The small lacrimal nerve running laterally towards the lacrimal gland.
2. NERVES OF THE EYE
Trace these frontal and lacrimal nerves as far forward as you can.
In addition to these branches of V1, locate the small trochlear nerve (CN IV) at the back of the orbit (medial to the ophthalmic).
Trace the trochlear nerve anteromedially to the superior oblique muscle.
The ophthalmic division (CN V1) of the trigeminal nerve (CN V) enters the orbit as a cluster of three branches: A frontal branch: to the supraorbital skin splitting into the supratrochlear and supraorbital nerves. A lacrimal nerve: sensory fibers to the lacrimal gland. A nasociliary nerve: general sensory nerve for the eyeball, the medial wall of the orbit, and parts of the inside and outside of the nose. The ciliary nerves (branches off the nasociliary nerve) carry sympathetic fibers to the muscle that dilates the pupil. These fibers originated in the sympathetic trunk and travel to the orbit via the internal carotid artery and its branches, and join the ophthalmic nerve in the cavernous sinus.
The ophthalmic division (CN V1) is purely sensory. However, in addition to its own the sensory fibers, the lacrimal nerve serves as a pathway for parasympathetic motor fibers that originate with the facial nerve (CN VII) and then by the maxillary division (CN V2) before joining the lacrimal branch to innervate the lacrimal gland.
Underneath the frontal nerve, identify the levator palpebrae superioris muscle.
Below this muscle, identify the superior rectus muscle.
Carefully dissect the connective tissue and fat between the levator palpebrae superioris/superior rectus and the superior oblique muscles until you expose the branches of the ophthalmic artery and the nasociliary nerve.
g. On the lateral wall of the orbit, find the lateral rectus muscle.
h. Pull the lateral rectus away from the bony wall to expose its nerve, the abducent (CN VI).
i. Relocate the lacrimal nerve running out to the lacrimal gland between the levator palpebrae superioris and the superior rectus muscles, as well as the lateral rectus muscle.
j. Identify its accompanying artery -- the lacrimal artery (a branch of the ophthalmic artery)
k. Transect the levator palpebrae and the superior rectus muscles.
l. Carefully reflect them posteriorly.
m. Locate the nerve fibers entering their lower surface from the superior branch of the oculomotor nerve (CN III).
n. Make an attempt to find the tiny ciliary ganglion below the superior branch of the oculomotor and the nasociliary nerve.
NOTE: the ciliary ganglion is very difficult to find. It is towards the back of the orbit.
o. You may find a very small branch from the nasociliary nerve entering the ganglion.
3. OPTHALMIC ARTERY
The ophthalmic artery, arising from the internal carotid artery, enters the orbit with the optic nerve by passing through the optic canal. Its branches cover the same territory as the branches of the ophthalmic nerve. The chief differences are that:
The artery enters the orbit below the superior rectus muscle.
Its frontal and nasal branches share a common trunk.
4. EXPLORE EYELID (on the same side as orbital roof dissection)
The upper and lower eyelids contain a plaque of fibrous tissue called the tarsus (tarsal plate). This fibrous tissue serves as a skeleton for the lids. The tarsal plate is covered with skin in front, and with a delicate mucous membrane (the conjunctiva) behind. A conjunctival sac, formed by the mucous membrane, reflects off both the upper and the lower eyelids and onto the outer surface of the eyeball (the sclera).
Turn the lower lid down, and examine the conjunctiva.
Explore the lower portion of the conjunctival sac with your finger.
Feel the reflection of the conjunctival sac from the lid onto the eyeball.
NOTE: Many eyeballs are dehydrated and some are covered with a protective shield. Remove that shield when present. Ask your instructor to help you rehydrate the eye.
d. Remove the thin skin of the eyelids carefully.
e. Note the fibers of the orbicularis oculi muscle between the skin and the connective tissue covering the tarsal plate .
NOTE: the orbicularis oculi muscle is very thin in the eyelid. You may accidentally remove the fibers when you carefully peel back the skin.
f. Make an incision along the eyelid and then a sagittal incision to look for slips of the levator palpebrae superioris muscle.
NOTE: these slips are the smooth muscle portion of lev. palp. superioris, which insert directly into the tarsal plate.
The muscles that open the eyelids attach to the tarsus. The levator palpebrae superioris is a striated voluntary muscle. On the deep surface of its aponeurosis are smooth muscle fibers that attach to the superior tarsus. These constitute the tarsal muscle (or Müller’s muscle), which are innervated by postganglionic sympathetic fibers.
5. ANTERIOR APPROACH TO THE ORBIT (on the other side)- LACRIMAL APPARATUS AND MUSCLES OF THE EYE
ON THE OTHER SIDE, remove both eyelids and the orbital septum.
expose the lacrimal gland in the upper lateral corner of the orbit.
Tears are secreted into the upper lateral corner of the conjunctival sac by the lacrimal gland. The tears then flow medially across the conjunctiva, and are collected by tiny lacrimal canaliculi in the medial corner of the eye. The lacrimal canaliculi lead into the lacrimal sac and the nasolacrimal duct, and ultimately empty into the nasal cavity.
Note that there is a passage for tears from the orbit to the nose that begins at the lacrimal cannaliculi, passes into the lacrimal foramen, and flows into the nasal cavity. You can ask your instructor for a thin wire and try to explore this passage if you want to. This should be done gently.
Clear away the periorbital fat and examine the attachments of the six extrinsic muscles:
Note that on the medial-superior wall of the orbit, there is a cartilaginous and ligamentous trochlea that acts as a pulley for the superior oblique tendon.
Clean away the obscuring fat to expose the trochlea on the medial wall of the orbit.
Identify the inferior oblique muscle. Look for its origin from the medial floor of the orbit.
On the anterior cheek beneath the bony orbit, locate the infraorbital foramen.
Examine the fibers of the infraorbital nerve, a branch of V2, spreading out to innervate the skin.
The optic axes of the eye are parallel; that is, both eyes ordinarily point in the same direction. However, the axes of the orbital cones diverge, because the orbits are wider in the front than in the back. Therefore, the rectus muscles of the eyeball, which originate from the margin of the optic foramen, travel obliquely sideways to their attachments. This means that both the superior and inferior recti direct the eye medially.
6. REMOVAL OF THE EYEBALL
On the eye you examined from the front, use a probe to pick up the tendon of the medial rectus muscle.
Transect the tendon with scissors (see figure below).
NOTE: when transecting these muscles, leave enough of each muscle on the eye so that you can orient the eye correctly once it is removed from the orbit.
Use forceps to grasp the lateral rectus muscle.
Pull anteriorly to adduct the eyeball (turn it medially).
Insert the scissors into the orbit on the lateral side of the eyeball and cut the optic nerve.
Pull the eyeball farther anteriorly and transect the superior and inferior oblique tendons near the surface of the eyeball.
Remove the eyeball from the orbit.
Use forceps to pick out lobules of fat from the orbit and study the orbit as viewed anteriorly.
Find the nerve to the inferior oblique muscle, and follow it posteriorly to the inferior division of the oculomotor nerve (CN III).
Trace the four rectus muscles to their attachments on the common tendinous ring (also known as the annulus of Zinn).
Identify the structures that pass through the ring:
The ophthalmic artery which gives a branch bundled with the optic nerve, the central artery of the retina.
The optic nerve (CN II).
Superior and inferior divisions of the oculomotor nerve (CN III)
Abducent nerve (CN VI)
Nasociliary nerve (branch of CN V1)
7. INTERNAL STRUCTURE OF THE EYE
Use a SHARP scalpel to transect the eye, making a transverse cut through the optic nerve.
Identify the cornea.
Behind that the cornea, identify the aqueous humor, the iris and the pupil, the lens, and the large vitreous body.
Locate the optic disc.
Laterally to the optic disk, locate the fovea (which may only be visible as a dark patch- it is unlikely that you will be able to see the pit-like depression of this structure).
Note how easily the retina becomes detached from the choroid, which contains the blood vessels.
Note that all the above are delicate structures and may be damaged in the process of embalming and dissection. Do your best but don’t worry if you can’t see all the structures listed.
Histology of the eye (note: SUPPLEMENTARY MATERIAL!!!)
Orbital cellulitis is a bacterial infection of the muscles and fat of the orbit. Its symptoms include swelling of the eyelids, pain with eye movements, fever, protrusion of the eye, discharge and general discomfort. The infection usually begins in the ethmoid air cells and spreads laterally into the orbit. There is only a thin layer between the ethmoid air cells and the orbit, and there are nerves and blood vessels that perforate this layer to enter into the air cells. This is the route through which infection can travel. Thus, inflammation is commonly seen in the medial area of the orbit. Subperiosteal abscesses are commonly found in this location as a result of the infection. If left untreated, this condition can spread to the apex of the orbit, which can cause blindness through interference with the optic nerve or its blood supply. The infection can also drain into the cavernous sinus, resulting in cavernous sinus thrombosis.
Lab 21 - Dissection of the Ear (OPTIONAL)
Suggested readings from Gray's Anatomy for Students, 2nd ed.
Ch. 8, p. 812-818; 830-855; 873-877
Suggested readings from Langman's Medical Embryology, 11th ed.
Ch. 16: 265-287
Note that the following dissection is optional and challenging. But also very interesting and useful.
Examine the auricle of the cadaver and identify the:
Helix – the rim of the auricle
Antihelix – the curved prominence anterior to the helix
Concha – the deepest part of the auricle
Lobule of the auricle
Note that the auricular cartilage gives the auricle its shape. There is no cartilage in the lobule. The external acoustic meatus begins at the deepest part of the concha and ends at the tympanic membrane (a distance of about 2.5 cm in adults). The outer one-third of the external acoustic meatus is cartilaginous and the inner two-thirds is bony. Note that the external acoustic meatus is S-shaped; first curving posterosuperiorly and then anteroinferiorly. The external acoustic meatus is straightened for examination by pulling the auricle upward, outward, and backward.
2. INNER EAR
The tympanic cavity is an air-filled space within the temporal bone. It is separated from the external acoustic meatus by the tympanic membrane and from the middle cranial fossa by the tegmen tympani.
The tympanic cavity will be approached by removing the tegmen tympani from the floor of the middle cranial fossa.
Peel any remaining dura mater away from the superior surface of the temporal bone, starting at the petrous part and pulling anteriorly.
Look for the greater petrosal nerve in its groove.
NOTE: the greater petrosal nerve is quite small and a very difficult nerve to find.
Note that the greater petrosal nerve lies between the dura mater and the bone.
In the posterior cranial fossa, identify the facial nerve (CN VII) and the vestibulocochlear nerve (CN VIII) as they enter the internal acoustic meatus.
Follow the facial and vestibulocochlear nerves laterally as they pass through the internal acoustic meatus.
Use a chisel to shave off the roof of the internal acoustic meatus, remaining superior to the nerves.
HINT: Your goal is to shave off the temporal bone until you get something like that shown in the figure below.
The facial nerve (CN VII) courses laterally entering the ‘Z’ shaped facial canal through the internal acoustic meatus. The facial canal is 3cm long and completely contained within the wall of the temporal bone, it is the longest ossified canal in the human body. At its initial bend, the facial nerve forms the geniculate ganglion, which is primarily a collection of sensory neuron cell bodies. At the geniculate ganglion, the facial nerve gives off the greater petrosal nerve which synapses in the pterygopalatine ganglion and supplies parasympathetic fibers to the lacrimal gland. The facial nerve continues posteriolaterally from the geniculate ganglion in the wall of the middle ear cavity before exiting the middle ear through the stylomastoid foramen and going on to innervate the muscles of facial expression. While in the middle ear, the facial nerve gives off two important branches. First, a small nerve to innervate the stapedius muscle whose role is to stabilize the stapes and help control mechanical sound wave amplitudes. The second branch is the chorda tympani which weaves through the malleous and incus before exiting the middle ear through the petrotympanic fissure. After exiting the middle ear, the chorda tympani joins the lingual nerve (branch of mandibular division of trigeminal CN V.3) through which parasympathetic fibers course to synapse in the mandibular ganglion and then innervate the submandibular and sublingual salivary glands. Afferent taste fibers travel through the chorda tympani and lingual nerve providing taste sensation for the anterior two-thirds of the tongue.
Look for the modiolus of the cochlea, this may not be possible, depending on the plane of your cut.
The semicircular canals lie posterior to the internal acoustic meatus.
Find the semicircular canals, a series of tiny holes in the bone.
To open the tympanic cavity, continue shaving off the bone postero-lateral to the geniculate ganglion to remove the tegmen tympani (the roof of the middle ear cavity).
Observe the auditory ossicles within the tympanic cavity.
Note that the malleus is attached to the tympanic membrane
Note that the incus occupies an intermediate position
Note that the stapes is the most medial of the auditory ossicles.
NOTE: The malleus and incus are easily seen from the superior view. The stapes is located more inferiorly, making observation more difficult.
Looking down from above, identify the tympanic membrane on the lateral wall of the tympanic cavity.
You may also be able to see the tendon of the tensor tympani muscle, a thin strand of tissue that spans from the medial wall of the tympanic cavity to the handle of the malleus and the chorda tympani nerve. This nerve passes between the malleus and the incus.
FUNCTIONAL ANATOMY: The features of the walls of the tympanic cavity can be summarized as follows:
Lateral – the tympanic membrane, malleus, and chorda tympani nerve (passing between the malleus and incus)
Posterior – the aditus (L. aditus, inlet or access), an opening into the mastoid air cells
Medial – the promontory and oval window (fenestra vestibuli) containing the base (footplate) of the stapes
Anterior – the opening of the pharyngotympanic (auditory) tube
Superior – tegmen tympani
Inferior – the floor of the tympanic cavity, which is closely related to the jugular fossa and the jugular bulb
Note that the tympanic cavity and its associated recesses and air cells are covered with mucous membrane.
The glossopharyngeal nerve (CN IX) innervates the mucous membrane of the tympanic cavity. It forms the tympanic plexus under the mucosa that covers the promontory.
Histology of the Ear (G 7.78a, 7.78b) (note: SUPPLEMENTARY material!!)