duke anatomy - lab 20_ eye & ear

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8/25/2014 Duke Anatomy - Lab 20: Eye & Ear

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Lab 20 - Eye & OrbitSuggested 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. 18 (Ear): pp. 327-334; Ch. 19 (Eye): pp. 335-344

12th ed - Ch. 19 (Ear): pp. 321-328; Ch. 20 (Eye): pp. 329-338

LEARNING RESOURCES ACLAND VIDEOS - Skull ACLAND VIDEOS - Eye & Orbit

Click here to view a summary review table of eye anatomy.

Primary Lab Objectives and Goals:

1. Remove orbital roof on one side. Note the relationship between the bone and the contents immediately

beneath it.

2. Identify frontal nerve and trace branches. Locate the motor nerves to the eye muscles - trochlear,

abducent and oculomotor nerves.

3. Identify the ophthalmic artery and its branches that supply the eye and orbit.

4. Review the movements produced by the muscles of the eye.

5. Dissect eyelidsand lacrimal gland and explore the small structures ofthe eyelid.

6. Remove the eyeball on the other side and examine the extra-ocular muscles, including their origins and

attachments.

7. Transect the eyeball and identify the internal structures associated with vision: the cornea, lens, pupil,

vitreous body, retina, and the optic disc.

**The next few steps are optional but are also very interesting**

8. Study the histology of the eye (optional).

9. Dissect the outer, middle, and inner ear (optional).

10. Study the histology of the inner ear (optional).

Dissection Instructions

NOTE BEFORE YOU BEGIN:You will be working with both eyes and orbits today. On one side you will

explore the eyelids and associated lacrimal gland and lacrimal passageways. On that same side you will

open the roof of the orbit from above. 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.

TOP
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1. SUPERIOR APPROACH TO THE ORBIT

a. Continuing on the same side where the eyelid and lacrimal apparatus was dissected, re-examine the

contents of thecavernous sinus

b. Then use a 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.

c. Use a small chisel to carefully chip through the thin orbital roof in the anterior cranial fossa.

d. Flip the piece of the frontal bone forward to remove.

e. 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.

f. Once you have removed the orbital roof, you will see the periorbita: the periosteal sac that envelops the

contents of the orbit.

g. 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.
http://www.duke.edu/web/anatomy/Lab20/Lab21-ModifiedImages/figure_7.038-modified.jpg


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2. NERVES OF THE EYE

a. Trace these frontal and lacrimal nerves as far forward as you can.

b. In addition to these branches of V1, locate the small trochlear nerve (CN IV) at the back of the orbit

(medial to the ophthalmic).

c. Trace the trochlear nerve medially to the superior obliquemuscle.

Atlas Image:

FUNCTIONAL ANATOMY:

The ophthalmic division (V1) of the trigeminal nerve (CN V) is purely sensory. However, its lacrimal

branch serves as a pathway for parasympathetic motor fibers that are carried first by cranial nerve VIIand then by V2 before joining this lacrimal branch to reach the lacrimal gland.

The ophthalmic nerve (CN V1) enters the orbit as a cluster of three branches: A frontal branch: to the

supraorbital skin. A lacrimal branch:sensory fibers to the lacrimal gland. This branch also carries with

secretomotor parasympathetic fibers from the facial nerve (CN VII) via the zygomaticotemporal nerve

(branch of CN V2), which joins the lacrimal nerve near the gland. A nasociliary branch: general


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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 arteryand its branches, and join the ophthalmic nerve in the cavernous sinus.

d. Underneath the frontal nerve, identify the levator palpebrae superiorismuscle.

e. Below this muscle, identify the superior rectus muscle.

f. Carefully dissect the connective tissue and fat between the levator palpebrae superioris/superior rectus

and the superior obliquemuscles until you expose the branches of the ophthalmic arteryand thenasociliary nerve.

a. On the lateral wall of the orbit, find the lateral rectusmuscle.

b. Pull the lateral rectus away from the bony wall to expose its nerve, the abducent (CN VI).

c. 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.

d. Identify its accompanying artery -- the lacrimal artery(a branch of the ophthalmic artery)

e. Transect the levator palpebraeand the superior rectus muscles.

f. Carefully reflect them posteriorly.

g. Locate the nerve fibers entering their lower surface from the superior branch of the oculomotor nerve

(CN III).

h. Make an attempt to find the tiny ciliary ganglionbelow 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.

i. You may find a very small branch from the nasociliary nerve entering the ganglion.

Atlas Images:


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3. OPTHALMIC ARTERY

The ophthalmic arteryenters the orbit with the optic nerve by pass ing 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 rectusmuscle.

Its frontal and nasal branches share a common trunk.

4. EYELID AND LACRIMAL APPARATUS (one side)

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 (theconjunctiva)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

(thesclera).

Atlas Images:
http://www.duke.edu/web/anatomy/siteParts/GrantsDissector14e/7-55.jpg


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a. Dissect the upper and lower eyelids on ONE SIDEonly.

b. Turn the lower lid down, and examine the conjunctiva.

c. Explore the lower portion of the conjunctival sac with your finger.

d. 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 thatshield when present. Ask your instructor to help you rehydrate the eye.

Atlas Image:

e. Remove the thin skin of the eyelids carefully.

f. Note the fibers of the orbicularis oculimuscle 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.

g. Make an incision along the eyelid and then a sagittal incision to look for slips of the levator palpebrae

superiorismuscle.

NOTE:these slips are thesmooth muscle portionof lev. palp. superioris, which insert directly into

the tarsal plate.
http://www.duke.edu/web/anatomy/siteParts/GrantsDissector14e/7-54.jpg


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FUNCTIONAL ANATOMY:

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 Mllers muscle), which are innervated by

postganglionic sympathetic fibers.

5. MUSCLES OF THE EYE (anterior approach)

a. ON THE OTHER SIDE, remove both eyelids and the orbital septum.

b. expose the lacrimal gland in the upper lateral corner of the orbit.

FUNCTIONAL ANATOMY:

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 canaliculiin the

medial corner of the eye. The lacrimal canaliculi lead into the lacrimal sacand 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.

Atlas Image:


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c. Clear away the periorbital fat and examine the attachments of the six extrinsic muscles:

1. Superior oblique

2. Superior rectus

3. Lateral rectus

4. Medial rectus

5. Inferior oblique

6. Inferior rectus

Note that above the lacrimal sac, on the medial wall of the orbit, there is a cartilaginous and

ligamentous trochleathat acts as a pulley for thesuperior obliquetendon.

d. Clean away the obscuring fat to expose the trochlea on the medial wall of the orbit.e. Identify the inferior obliquemuscle. Look for its origin from the medial floor o f the orbit.

f. On the anterior cheek beneath the bony orbit, locate the infraorbital foramen.

g. Examine the fibers of the infraorbital nerve, a branch of V2, spreading out to innervate the skin.

Atlas Image:


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FUNCTIONAL ANATOMY:

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

a. On the eye you examined from the front, use a probe to pick up the tendon of the medial rectus muscle.

b. Transect the tendon w ith 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.

c. Use forceps to grasp the lateral rectus muscle.

d. Pull anteriorly to adduct the eyeball (turn it medially).

e. Insert the scissors into the orbit on the lateral side of the eyeball and cut the optic nerve.

f. Pull the eyeball farther anteriorly and transect the superior and inferior oblique tendons near the surface

of the eyeball.

g. Remove the eyeball from the orbit.


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h. Use forceps to pick out lobules of fat from the orbit and study the orbit as viewed anteriorly.

i. Find the nerve to the inferior oblique muscle, and follow it posteriorly to the inferior division of theoculomotor nerve (CN III).

j. Trace the four rectus muscles to their attachments on thecommon tendinous ring.

k. Identify the structures that pass through the ring:

The optic nerve(CN II) and the central artery of the retina.

Superior and inferior divisions of the oculomotor nerve(CN III)

Abducent nerve(CN VI)

Nasociliary nerve (branch of V1)

Atlas Images:

7. INTERNAL STRUCTURE OF THE EYE

a. Use a SHARP scalpel to transect the eye horizontally through the optic nerve.

b. Identify the cornea.

c. Behind that the cornea, identify the chamber containing theaqueous humor, the iris and the pupil, the

lens, and the large vitreous body.

d. Locate the optic disc.

e. Laterally to the optic disk, locate the fovea.

f. Note how easily the retina becomes detached from the choroid, which contains the blood vessels.


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Note that all the above are delicate structures and may be damaged in the process of embalming and

dissection. Do your best but dont worry if you cant see all the structures listed.

Atlas Image:

Histology of the eye (note: SUPPLEMENTARY MATERIAL!!!)

Click here to open an optional section covering the histology of the eye.

CLINICAL CORRELATION

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 w ith the optic nerve or its blood supply. The infection can a lso drain into the cavernous sinus,

resulting in cavernous sinus thrombosis.
http://www.duke.edu/web/anatomy/Lab20/EyeEar.html#eye


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orbital cellulitis

Lab 20 - 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 fromLangman's Medical

Embryology, 11th ed.

Ch. 16: 265-287

LEARNING RESOURCES ACLAND VIDEOS - Ear

Click here to view a summary review table of ear anatomy.

Note that the following dissection is optional and challenging. But also veryinteresting and useful.

Dissection Instructions

1.EXTERNAL EAR

a. Examine the auricleof 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 auricleTragus

Antitragus

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.

Atlas Image:
https://web.duke.edu/anatomy/Lab20/Lab20_tableEar.html


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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.

a. Pee l any remaining dura mater away from the superior surface of the temporal bone, starting at the

petrous part and pulling anteriorly.

b. Look for the greater petrosal nervein its groove.

NOTE:the greater petrosal nerve is quite small and a very difficult nerve to find.

c. Note that the greater petrosal nerve lies between the dura mater and the bone.

d. In the pos terior cranial fossa, identify the facial nerve (CN VII)and the vestibulocochlear nerve (CN

VIII)as they enter the internal acoustic meatus.

e. Follow the facial and vestibulocochlear nerves laterally as they pass through the internal acoustic

meatus.

f. Use a chisel to shave off the roof of the internal acoustic meatus, remaining superiorly to the nerves.

HINT:Your goal is to shave off the temporal bone until you get something like that shown in the figure

below.


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Atlas Image:

FUNCTIONAL ANATOMY:

The facial nerve(CN VII) courses laterally until it makes a sharp bend in the posterior direction. At

this bend, thegeniculate ganglionmay be seen along with the origin of thegreater petrosal nerve.

The geniculate ganglion contains cell bodies of sensory neurons. The greater petrosal nerve carries

preganglionic parasympathetic fibers to the pterygopalatine ganglion for innervation of the mucous

membranes of the nasal and upper oral cavities, and the lacrimal gland. The preganglionic

parasympathetic nerve fibers do not synapse in the geniculate ganglion.

Thegreater petrosal nervecourses anteromedially within the temporal bone and emerges in the

middle cranial fossa, at the hiatus for the greater petrosal nerve. It then passes inferiorly and medially

on the surface of the temporal bone, in the groove for the greater petrosal nerve. The greater petrosalnerve enters the carotid canal. On the surface of the internal carotid artery, the greater petrosal nerve

joins the deep petrosal nerve to form the nerve of the pterygoid canal. The nerve of the pterygoid canal

carries the preganglionic parasympathetic fibers of the greater petrosal nerve and postganglionic

sympathetic fibers of the deep petrosal nerve to the pterygopalatine fossa, from there they supply

structures in the midface.

The cochlealies anterior to the internal acoustic meatus in the angle formed by the facial nerve (CN

VII), the geniculate ganglion, and the greater petrosal nerve.

g. Look for the modiolus of the cochlea, this may not be possible, depending on the plane of your cut.

Atlas Images:


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Thesemicircular canalslie posterior to the internal acoustic meatus.

h. Find the semicircular canals, a series of tiny holes in the bone.

i. 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).

j. Observe the auditory ossicleswithin the tympanic cavity.

k. Note that the malleusis attached to the tympanic membrane

l. Note that the incusoccupies an intermediate position

Note that thestapesis 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.

m. Looking down from above, identify the tympanic membraneon the lateral wall of the tympanic cavity.

n. 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.

Atlas Image:
https://web.duke.edu/anatomy/siteParts/Grays2e/Grays2ed-Ch-008-f121.jpg


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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 thepromontoryand oval window(fenestra vestibuli) containing the base

(footplate) of the stapes

Anterior the opening of thepharyngotympanic (auditory) tube

Superior tegmen tympani

Inferior the floor of the tympanic cavity, which is closely related to thejugular fossaand the

jugular bulb

Note that the tympanic cavity and its associated recesses and air cells are covered with mucous

membrane.


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Theglossopharyngeal nerve (CN IX) innervates the mucous membrane of the tympanic cavity. It

forms the tympanic plexusunder the mucosa that covers the promontory.

Histology of the Ear (G 7.78a, 7.78b) (note: SUPPLEMENTARY material!!)

Click here to open an optional section covering the histology of the ear.

Click here to submit questions or comments about this site.

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duke anatomy - lab 20_ eye & ear

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