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AP1 Lab 11 – Ear and Eye, Dissection of Eye
THE EAR – FOR HEARING AND BALANCE Figs. 15.24-15.31
The senses of both hearing and equilibrium are located in the ear.
The ear is divided into three anatomical divisions. Identify the following on the models.
1) EXTERNAL EAR - consists of the auricle (a.k.a. pinna) which is the outwardly visible
structure surrounding the entrance to the ear canal and the ear canal itself more properly called
the external auditory canal (a.k.a. external acoustic meatus). At the end of the external
acoustic meatus is the tympanic membrane (ear drum).
The auricle or pinna collects sound waves and directs them into the external acoustic
meatus (ear canal) in order to strike the tympanic membrane or tympanum (eardrum) and
cause it to vibrate.
Lining the walls of the external acoustic meatus are ceruminous glands secreting a waxy
secretion known as cerumen (a.k.a. ear wax). What is the function of cerumen?
2) MIDDLE EAR
The middle ear begins at the tympanum and is an air filled chamber located within the
temporal bone. It contains the three small bones called auditory ossicles, which amplify and
transmit the vibrations of the tympanum inward to the inner ear. Individually these 3 bones are
malleus (hammer), incus (anvil), and stapes (stirrup) [pronounced “stay-peez”]
There is also a flexible, pliable, mucous membrane lined tube that connects the middle ear
cavity to the nasopharynx (upper throat) [pronounced “Nay-zo-fair-inks”]. This tube is known
by several names: auditory tube, pharyngotympanic tube [“Far-in-jo-tim-panic”], and
formerly the eustachian tube. Describe the function of this tube.
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The normal position of this tube when an adult is upright is for it to run obliquely downward
thus providing natural ‘drainage’ of any liquids that might enter the middle ear cavity. Can you
see how an infection of the throat could travel to the middle ear? In small children this is even
more likely to happen because this tube doesn’t yet tilt downward and liquids in the throat can
easily travel toward the middle ear. Resulting middle ear infections are usually called “Otitis
media”. This term translates literally as “inflamed middle ear.”
Identify the tensor tympani muscle attached to the malleus. This muscle and a much smaller
muscle called the stapedius muscle [not visible on our models] are involved in a reflex called
the attenuation reflex.
**Confirm all of above and previous page with your instructor.**
OYO: Look up this attenuation reflex online and explain.
Other online OYOs:
What is an otoscope?
What is a myringotomy and why is it done?
What are ventilation tubes and why would they be installed?
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3. INTERNAL OR INNER EAR
This contains the sensory organs for both hearing and equilibrium. (Table 15.2) The inner ear
can be described as a membranous labyrinth – a series of fluid filled tubes and sacs. This
membranous labyrinth is suspended in a bony labyrinth (of nearly the same shape) by a
fluid called perilymph. Imagine your hand fitting in a stiff glove filled with water. The glove is
the bony labyrinth and your hand and fingers make up the membranous labyrinth. Your hand
never really touches the glove because of the water. The perilymph fluid is very similar to
CSF and is continuous with it. Leakage of this fluid from your patient’s ear canal is indicative
of a basilar skull fracture. You would typically only see this in traumatic head injuries.
The 2 parts of the membranous labyrinth are the snail shaped Cochlea (it contains the
receptor structures for hearing) and the tubular vestibular complex (consisting of a chamber
called the vestibule and 3 semicircular canals.) The vestibule and semicircular canals
contain the sensory receptors for equilibrium. All parts of the membranous labyrinth are filled
with a fluid called endolymph.
When the 3 auditory ossicles vibrate in response to sound waves the endolymph vibrates in
the cochlea stimulating receptors that send impulses that the brain interprets as sounds.
Positions and movements of the head and body cause movements of the endolymph in the
vestibule and the 3 semicircular canals. Sensors here send nerve impulses that your brain
interprets as body position or movement so you know when you’re leaning forward or back or
to the side or turning, spinning, etc. Name and identify on the model the cranial nerve that is
100% sensory for hearing and balance ___________________________
**Confirm your identifications with your instructor.
OYO: Go online to eHow.com and learn about motion sickness and explain several ways
motion sickness (“antimotion”) drugs work?
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OYO: Dysfunctions of Hearing and Balance
1. Otitis Media – Inflammation and infection of the middle ear. Typically, acute otitis media
follows a cold: after a few days of a stuffy nose the ear becomes involved and can cause severe
pain. The pain will usually settle within a day or two but can last over a week. Sometimes the ear
drum ruptures discharging pus from the ear, but usually the ruptured drum will heal rapidly.
At an anatomic level, the typical progression of acute otitis media occurs as follows: the tissues
surrounding the Eustachian tube swell due to an upper respiratory infection, allergies, or
dysfunction of the tubes. The Eustachian tube remains blocked most of the time. The air present in
the middle ear is slowly absorbed into the surrounding tissues. A strong negative pressure creates
a vacuum in the middle ear. The vacuum reaches a point where fluid from the surrounding tissues
accumulates in the middle ear.
2. Hyperacusis – Extreme sensitivity to noise – can be caused by exposure to loud noise, or
paralysis of an attenuating muscle, the stapedius. Paralysis of the stapedius allows wider
oscillation of the stapes, resulting in heightened reaction of the auditory ossicles to sound vibration.
3. Presbycusis – age related hearing loss. Tends to start with higher frequencies, moving down
into the voice range. Typical upper limit of hearing at age 50 is about 11,000 Hz.
4. NIHL – Noise induced hearing loss – There are two basic types of NIHL: NIHL caused by
acoustic trauma and gradual developing NIHL. Acoustic trauma can be due to explosions, gunfire,
etc. Gradual NIHL is repeated exposure to loud sounds over a period of time. This would include
musical concerts, workplace noise, and IPod’s. Over 85 dB (A) of noise for 8 hours per day is the
OSHA limit. NIHL usually occurs first at high frequencies (3000- 8000 Hz) and then spreads down
to low frequencies. See the Web of Life website www.brazosport.edu/weboflife for a “virtual
exhibit.”
5. Tinnitus – is the perception of sound in the human ear in the absence of corresponding
external sound. Pulsatile tinnitus is usually objective in nature, resulting from altered blood flow or
increased blood turbulence near the ear. Subjective tinnitus may be due to excessive exposure to
loud noise, or ototoxic medications.
6. Meniere’s Syndrome - Disorder of all three parts of the labyrinth
 Symptoms - episodes of vertigo (sensation of a spinning motion), fluctuating hearing loss,
tinnitus and sometimes feeling of fullness or pressure in ear and often accompanied by
nausea.
 Normally affects one ear only
 Treated by antimotion drugs, low-salt diet and diuretics
Creative uses of deafness:
“The Mosquito” – high frequency sounds designed to prevent loitering of younger people
“Teen Buzz” or “Mosquito ringtone” – A modulated 17 kHz sound that most people over
20 years of age cannot hear.
Factoid:
Human communication is in the range of 200 and 8,000 hertz. Teenagers can hear frequencies as
high as 20,000 hertz. Therefore, teenagers are not human.
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THE EYE Figs. 15.1-15.9
Identify these on diagrams, models, and your lab partner.
SUPERFICIAL ANATOMY
EXTRINSIC EYE MUSCLES: These muscles control directional movements of the eye.
Name the 3 cranial nerves that innervate these muscles: __________________________,
______________________________, & ____________________________
LACRIMAL GLAND: (pronounced LAC-cri-mal) located in superior, lateral border of eye socket.
The models with eyelids have this structure… other models without eyelids don’t.
Secretes tears to lubricate surface of eye. Also secretes tears associated with crying.
Tears contain an antibacterial enzyme called LYSOZYME.
LACRIMAL PUNCTA: holes on the medial edges of all four eyelids for drainage of tears. View
these on your lab partner. Tears are absorbed here, not produced here. Explain why
your nose ‘runs’ when you cry.
CONJUNCTIVA: (pronounced “kon-JUNK-ti-va”) the delicate, almost invisible, mucous
membrane covering the visible parts of sclera and underside of eyelids. Lubricates and
reduces friction between eyelid and cornea when blinking.
 Have your lab partner pull down their lower eyelids to see this pinkish membrane.
SCLERA: The tough, white, outermost layer of the eyeball wall. Maintains shape of the globe.
Look at your lab partner’s eyes. The ‘whites’ of the eyes are the sclera. “Don’t shoot till
you see the scleras of their eyes.”
CORNEA: The clear, “bubble” on the front of the eye... Is actually an extension of the sclera.
 Performs the initial refraction (bending) of light as it enters the eye. The lens makes
final adjustments in refraction for focusing.
 Is the most easily transplanted body part because there is no direct blood flow to the
cornea so rejection is less likely. Look at your lab partner’s eyes from the side to see
the cornea.
OPTIC NERVE: Function? ______________________________________________________
And it is cranial nerve #? ___
INTERNAL ANATOMY
IRIS: the “colored” part of your eye. Consists of pigments and two layers of muscle that adjust
the diameter of the pupil to control the amount of light entering the eye. Constricts pupils
in bright light. Dilates pupils in dim light to allow in more light.
These are smooth muscles under involuntary control of the ANS. Which division of the
ANS would dilate the eye? _______________ which would constrict? ______________
PUPIL: the hole at the center of the iris that allows light to enter and strike the retina. The
diameter of the pupil is controlled by contraction & relaxation of the iris.
LENS: a bi-convex disc directly behind the pupil. Performs accommodation (focusing). Its
curvature is adjusted by ciliary muscle contraction to refract (bend) light for focusing of the
image on the retina.
**Confirm identifications with instructor
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CILIARY MUSCLE/BODY: The ring of muscle around the lens adjusting its shape for focusing. It
attaches to the edges of the lens. [On the models, it appears as white stripes on the
inside of the anterior portion of the globe.]
RETINA: innermost layer of the eye wall containing photoreceptors and axons leading out to
optic nerve. (Appears light brown or reddish-tan on models.) Covers only the posterior
2/3 to ¾ of globe of eye. Is where the energy of light is converted into electrical nerve
impulses by PHOTORECEPTORS called RODS and CONES.
CHOROID LAYER: middle of the 3 layers of the eye wall. (On models: appears very dark brown
to black.) Is highly vascular and darkly pigmented. Nourishes the retina and assists with
absorption of light to prevent its scattering within the eye.
ANTERIOR SEGMENT (CAVITY): space between the lens and cornea filled with AQUEOUS HUMOR.
“Humors” is an old, generic name for body fluids. Subdivided into anterior and posterior
chambers by the iris.
AQUEOUS HUMOR: a thin, watery fluid in the anterior segment. Is continuously replenished as
older fluid drains out through CANALS OF SCHLEMM.
POSTERIOR SEGMENT (CAVITY): space between the lens and retina filled with vitreous humor.
VITREOUS HUMOR: a thick, viscous fluid which helps hold the delicate retina against the
choroid. Is not replaced. If you lose it, it’s gone. On the models it is represented by the
clear plastic globe.
The eyes contain 99% of all the sensory receptors in the body. They are called
photoreceptors.
What are the two types of photoreceptors found in the retina? __________ & __________
Which is the more abundant type of photoreceptor? ______________
Which type is more sensitive to light? They only need a little bit of light to work?_________
Cones use a chemical photopigment called photopsins. Rods use a chemical
photopigment called rhodopsin. Both receptor types are activated when their
photopigment is “broken down” by light. Online OYO: 1) The last time you stepped into a
dark theater it took several minutes before you could see well. Explain.
2) When you come out of the theater into bright sunlight your eyes hurt for a minute or two.
Explain.
OPTIC DISC: that point on the retina where all the nerve fibers (axons) from the photoreceptors
converge and exit to become the optic nerve. Is sometimes called the "BLIND SPOT"
because there are no photoreceptors here.
FOVEA CENTRALIS: an area lateral to each optic disc. Cones are concentrated here providing
you the sharpest visual acuity.
**Confirm identifications with instructor.
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OYO: Disorders of the Eye
DETACHED RETINA: It’s usually not the retina separating from the choroid. Instead there are
actually two layers of the retina. When they separate it leads to death of photoreceptors and
neurons because of separation from blood flow from the underlying choroid layer.
GLAUCOMA:
excess pressure in the anterior segment created when aqueous humor does not drain
properly. Pressure is transmitted through the vitreous humor to the retina where blood flow
can be cut off causing permanent damage to the optic nerve.
CONJUNCTIVITIS: inflammation of the conjunctiva… many
“PINK EYE” is conjunctivitis due to bacterial infection
STY:
possible causes.
and is very contagious.
inflammation of a sebaceous gland at the base of an eyelash.
CATARACT:
a gradual “clouding” of the lens likely due to excess UV exposure. When it interferes
significantly with vision the lens is replaced with an artificial one.
ASTIGMATISM:
a defect or abnormal curvature in the cornea. Parts of an image are distorted.
MYOPIA (NEAR SIGHTEDNESS):
Close images are seen clearly but distant images are focused in front
of the retina and therefore out of focus. Is the result of the globe of the eye being too elongated
or the cornea being curved too sharply. Usually correctable with glasses, contacts, or laser
surgery.
HYPEROPIA (FAR SIGHTEDNESS):
opposite of above.
COLOR BLINDNESS:
lack of ability to distinguish colors accurately. Due to absence of certain cones
or lack of the appropriate photopigment. More common in men.
MACULAR DEGENERATION: Breakdown of cells in the central portion of the retina resulting in loss of
central vision but not peripheral. Cause is usually unknown and there is no known way to prevent
or correct the problem. Most often appears in persons over age 50.
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DISSECTION OF THE EYE
Images from Cat Dissection Manual p. 165 and the “Photographs” manual p. 95 will be
helpful.
ID the following superficial structures:
Cornea - is not clear on these specimens because the Na+ pumps are no longer working (after
death) and water has accumulated and light rays are scattered rather than passing straight
through.
Conjunctiva – appears a dull tan / gray color
Extrinsic Eye Muscles – remnants may or may not be present on your specimen amongst a
lot of connective tissue
Optic Nerve – you will find the stump of the nerve attached to the back side of the eyeball.
Now bisect the eyeball along the coronal/frontal plane separating it into anterior and posterior
halves.
This will be difficult as the sclera is amazingly tough. Please be careful with the sharp tools.
Pierce it with the point of a knife and then use long smooth strokes to complete the cut. Cut all
the way through the eye separating it completely into anterior and posterior halves.
ON THE ANTERIOR PROTION:
Lens and vitreous humor.
Often these will fall out and may resemble raw egg white with a hard, whitish yolk.
If they are still in place use your finger to scoop the lens and jelly-like vitreous humor away
from where the lens is attached to the ciliary muscle.
Use small scissors to cut around the perimeter of the cornea (not the white sclera) to
completely remove it. Identify:
Iris - will not be clearly visible through the cloudy cornea but is recognizable once the
cornea is removed. Try to see fibers that radiate outward from the pupil as well as fibers
that run in a circular around the pupil.
Pupil - duh
Ciliary Muscle – Look from the posterior side of the anterior half of the eye and see the dark
muscle fibers radiating from where the lens was. Bits of this muscle are likely still attached to
the edges of the lens.
ON THE POSTERIOR PORTION:
Sclera
Choroid Layer (black and blue in color)
Notice that part of the choroid layer is a shiny blue color. This reflects light rather than
absorbs it enabling this animal to see better at night than you and I. The retina actually
gets stimulated by light passing in and on its way back out. The choroid layer in humans is
solid black so light is not reflected back out. Therefore we see poorly at night.
Retina
If this very delicate layer hasn’t already detached it will be tan in color. Use your probe to
move it around and discover it is attached at only one spot… the ____________________.
Optic Disc – the retina will appear to converge and be attached here.
**Confirm identifications with instructor.
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