Week 14 LabThe Special Senses
LEARNING OUTCOMES:
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To identify landmark anatomical structures on a dissected cow eye specimen, human eye model, or
appropriate diagrams, and to relate their structures to their functions.
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❍
To perform simple diagnostic eye tests and relate their findings to possible corrective procedures.
To identify landmark anatomical structures on human ear models, or appropriate diagrams, and to relate
their structures to their functions.
BEFORE WE BEGIN:
When it comes to dissections, there are three types of people: cutters (people who don’t mind cutting things),
touchers (people who don’t like to cut, but will touch afterwards), and lookers (who won’t touch anything).
❍ Every group needs to have at least one cutter. Otherwise you’ll just sit there and stare at the brain.
❍ Only cutters and touchers needs gloves.
❍ Lookers are good for turning pages and taking notes.
ACTIVITY 1: Cow Eye Dissection
The anatomy of the cow eye and sheep eye is similar to that of the human eye. Be careful while dissecting
the eyeball because the sclera is fibrous and difficult to cut. Use small strokes with the scalpel, and cut
away from your fingers. Do not allow your laboratory partner to hold the eyeball while you dissect.
You must practice the highest level of laboratory safety while handling and dissecting the eyeball. Keep the
following guidelines in mind during the dissection.
1. Wear gloves and safety glasses to protect yourself from the fixatives used to preserve the specimen.
2. Be extremely careful when using a scalpel or other sharp instrument. Always direct cutting and scissor
motions away from you to prevent an accident if the instrument slips on moist tissue.
3. Before cutting a given tissue, make sure it is free from underlying and/or adjacent tissues so that they are
not accidentally severed.
4. Never discard tissue in the sink or trash. Your instructor will inform you of the proper disposal
procedure.
In Lab:
1. Examine the external features of the cow or sheep eye. Depending on how the eye was removed from
the animal, your specimen may have, around the eyeball, adipose tissue, portions of the extraocular
muscles, and the palpebrae. If so, note the amount of adipose tissue, which cushions the eyeball. If your
specimen lacks these structures, observe them in the figure above.
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Identify the optic nerve (cranial nerve II) exiting the eyeball at the posterior wall.
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Examine the remnants of the extraocular muscles and, if present, the palpebrae and eyelashes.
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Locate the corneal limbus, where the white sclera and the cornea join. The cornea, which is
normally transparent, will be opaque if the eye has been preserved.
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2. Holding the eyeball securely, use scissors to remove any adipose tissue and extraocular muscles from
the surface, taking care not to remove the optic nerve.
3. Hold the eyeball securely in the dissection pan, and with a sharp scalpel make an incision about 0.6 cm
(0.25 in) back from the cornea. Use numerous small, downward strokes over the same area to penetrate
the sclera.
4. Insert the sharp blade of the scissors into the incision, and cut around the circumference of the eyeball,
being sure to maintain the 0.6-cm distance back from the cornea.
5. Carefully separate the anterior and posterior cavities of the eyeball. The vitreous body should stay with
the posterior cavity. Examine the anterior portion of the eyeball.
•
Place a blunt probe between the lens and the ciliary processes, and carefully lift the lens up a little.
The halo of delicate transparent filaments between the lens and the ciliary processes is formed by
the suspensory ligaments. Notice the ciliary body, where the suspensory ligaments originate, and the
heavily pigmented iris with the pupil in its center.
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Remove the vitreous body from the posterior cavity, set it on a piece of paper with text (preferably
from the recycling bin), and notice how it causes refraction (bending) of light rays.
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The retina is the tan membrane that is easily separated from the heavily pigmented choroid of the
vascular tunic.
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Examine the optic disc, where the retina attaches to the posterior of the eyeball.
Week 14 Lab: The Special Senses
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•
The choroid has a greenish-blue membrane, the
tapetum lucidum (ta-PE-tum, a carpet; LU-sid-um,
clear), which improves night vision in many animals,
including sheep and cows. When headlights shine in a
cow’s eyes at night, this membrane reflects the light and
makes the eyes glow. Humans do not have this
membrane, and our night vision is not as good as that of
animals that have the membrane.
6. Never discard tissue in the sink or trash. Your instructor will inform you of the proper disposal
procedure.
7. We have a variety of eye models in the lab, the nicest being the one that has the eyeball sitting in a little
socket. On the eye model, you should be able to find:
❍ sclera
❍ extraocular (a.k.a. extrinsic) eye muscles
❍ optic nerve (CN II)
❍ cornea
❍ lens
❍ choroid
❍ iris
❍ pupil
❍ ciliary body
❍ suspensory ligaments
❍ retina
❍ optic disc
ACTIVITY 2: Eye Tests
In these short activities, you will take some simple diagnostic eye tests that address various aspects of eye
physiology.
In Lab:
Blind-Spot Mapping:
The optic disc, or blind spot, is an area of the retina lacking photoreceptors. Normally you do not see your
blind spot because the visual fields of your two eyes overlap and “fill in” the information missing from the
blind spot.
Week 14 Lab: The Special Senses
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1. If you wear glasses, try the mapping procedures both with and without your glasses. Hold the figure
below about two inches from your face with the cross in front of your right eye. Close your left eye, and
stare at the cross with your right eye.
2. Slowly move the page away from your face. The dot disappears when its image falls on your blind spot.
3. If you have difficulty mapping your blind spot, remember not to move your eyes as the page moves.
Visual Acuity:
Sharpness of vision, or visual acuity, is tested with a Snellen eye chart, which consists of black letters of
various sizes printed on white cardboard. A person with a visual acuity of 20/20 is considered to have
normal, or emmetropic (em-e-TRO-pik), vision. If your visual acuity is 20/30, for example, you can see at 20
feet what an emmetropic eye can see at 30 feet; 20/30 vision is not as sharp as 20/20 vision.
An eye that focuses an image in front of the retina is myopic (mi-O-pik), or nearsighted, and can clearly see
close objects but not distant ones. An eye that focuses an image behind the retina is hyperopic (hi-per-Opik), or farsighted, and can only see distant objects clearly. Corrective lenses are used to adjust for both
conditions.
1. If you wear glasses or contact lenses, remove them before performing the vision test.
2. Stand at the 20-foot mark in front of the Snellen eye chart on the wall, and cover your left eye with
either a cupped hand or an index card. While your partner stands next to the eye chart, read a line
where you can easily make out all the letters. Continue to view progressively smaller letters until your
partner announces that you have not read the letters correctly. Record the visual-acuity value of this line
on your lab report.
3. Repeat this process with your right eye. Record your data on the lab report.
4. Now, using both eyes, read the smallest line you can see clearly. Record your data on the lab report.
5. If you wear glasses or contact lenses, repeat the test while wearing your corrective lenses. Record your
data on the lab report.
Week 14 Lab: The Special Senses
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ACTIVITY 3: The Ear — Gross Anatomy
The anatomy of the ear is designed to transmit the energy of sound waves into the mechanical energy of
vibrating ear structures and finally into action potentials that are then carried to the brain via the
vestibulocochlear nerve.
In Lab:
1. We have two types of ear models: a straightforward anatomical model and a “functional” ear model that
you can play with to demonstrate how the pressure waves of perilymph generate what we perceive as
sound.
2. On the anatomical ear model (we have two copies of this), you should be able to find:
•
external (outer) ear:
❍ auricle
❍ external acoustic meatus
•
middle ear:
❍ tympanic membrane
❍ malleus
❍ incus
❍ stapes
anatomical ear model
❍ auditory tube (a.k.a. pharyngotympanic or Eustachian
tube)
•
internal (inner) ear:
❍ vestibule
❍ semicircular canals
❍ cochlea
❍ vestibulocochlear nerve (CN VIII)
3. The functional ear model is much lighter on the actual anatomy, but you should be able to find:
❍ tympanic membrane
❍ malleus
❍ incus
❍ stapes
❍ oval window (under the stapes)
Note: the ossicles (malleus, incus, and stapes) are much nicer
on this model, mainly because they are larger.
functional ear model
Week 14 Lab: The Special Senses
5
Pig Heart Dissection
Name:
Date:
Introduction
Mammals have four-chambered hearts and double circulation. The heart of a bird or
mammal has two atria and two completely separated ventricles. The double-loop
circulation is similar to amphibians and reptiles, but the oxygen-rich blood is completely
separated from oxygen-poor blood. The left side of the heart handles only oxygenated
blood, and the right side receives and pumps only deoxygenated blood. With no mixing of
the two kinds of blood, and with a double circulation that restores pressure after blood has
passed through the lung capillaries, delivery of oxygen to all parts of the body for cellular
respiration is enhanced.
Objective
Using a pig heart, students will observe the major chambers, valves, and vessels of the
heart and be able to describe the circulation of blood through the heart to the lungs and
back and out to the rest of the body. (The pig heart is used because it is very similar to the
human heart in structure, size, & function.)
Materials: Dissecting pan, dissecting kit, safety glasses, lab apron, pig heart, & gloves
Procedure – External Structure
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Examine the heart and locate the thin membrane or pericardium that still covers
the heart. The pericardium or pericardial sac, is a double-layered closed sac that
surrounds the heart and anchors it.
After examining the pericardium, carefully remove this tissue. Located below the
pericardium is the muscle of your heart called the myocardium. Most of the
myocardium is located in the lower two chambers of the heart called ventricles.
Locate the tip of the heart or the apex. Only the left ventricle extends all the way to
the apex.
Place the heart in the dissecting pan so that the front or ventral side is towards you
(the major blood vessels are on the top and the apex is down). The front of the
heart is recognized by a groove that extends from the right side of the broad end of
the heart diagonally to a point above & to your left of the apex.
Front or Ventral Side of the Heart
1. Locate the following chambers of the heart from this surface:
▪ Left atria – upper chamber to your right
▪ Left ventricle – lower chamber to your right
▪ Right atria – upper chamber to your left
▪ Right ventricle – lower chamber to your left
2. While the heart is still in this position in the dissecting
pan, locate these blood vessels at the broad end of the
heart:
▪ Coronary artery – this blood vessel lies in the groove
on the front of the heart & it branches over the front
& the back side of the heart to supply fresh blood
with oxygen & nutrients to the heart muscle itself.
▪ Pulmonary artery – this blood vessel branches &
carries blood to the lungs to receive oxygen & can be found curving out of the right
ventricle (upper chamber to your left)
▪ Aorta – major vessel located near the right atria & just behind the pulmonary arteries to
the lungs. Locate the curved part of this vessel known as the aorticarch. Branching
from the aortic arch is a large artery that supplies blood to the upper body.
▪ Pulmonary veins – these vessels return oxygenated blood from the right & left lungs to
the left atrium (upper chamber on your right)
▪ Inferior & Superior Vena Cava – these two blood vessels are located on your left of the
heart and connect to the right atrium (upper chamber on your left). Deoxygenated
blood enters the body through these vessels into the right receiving chamber. Use your
probe to feel down into the right atrium. These vessels do not contain valves to control
blood flow.
Procedure – Internal Anatomy
1.
2.
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5.
Use scissors to cut through the side of the pulmonary artery and continue cutting down
into the wall of the right ventricle. Be careful to just cut deep enough to go through the
wall of the heart chamber. (Your cutting line should be above & parallel to the groove of
the coronary artery.)
With your fingers, push open the heart at the cut to examine the internal structure. If
there is dried blood inside the chambers, rinse out the heart.
Locate the right atrium. Notice the thinner muscular wall of this receiving chamber.
Find where the inferior & superior vena cava enter this chamber & notice the lack of
valves.
Locate the valve that between the right atrium and right ventricle. This is called the
tricuspid valve. This valve allows blood flow from the right atrium into the right ventricle
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during diastole (period when the heart is relaxed). When the heart begins to contract
(systole phase), ventricular pressure increases until it is greater than the pressure in the
atrium causing the tricuspid to snap closed.
Use your fingers to feel the thickness of the right ventricle and its smooth lining. Also
note the network of irregular muscular cords on the inner wall of this chamber.
Find the septum on the right side of the right ventricle. This thick muscular wall
separates the right & left pumping ventricles from each other.
Inside the right ventricle, locate the pulmonary artery that carries blood away from this
chamber. Find the one-way valve called the pulmonary semilunar valve that controls
blood flow away from the right ventricle at the entrance to this blood vessel.
Using your scissors, continue to cut open the heart. Start a cut on the outside of the left
atrium downward into the left ventricle cutting toward the apex to the septum at the
center groove. Push open the heart at this cut with your fingers & rinse out any dried
blood with water.
Examine the left atrium. Find the openings of the pulmonary veins form the lungs.
Observe the one-way, semi-lunar valves at the entrance to these veins.
Inside this chamber, look for the valve that controls blood flow between the upper left
atrium and lower left ventricle. This valve is called the bicuspid or mitral valve. This
valve consists of two leaflets & blood flows from the left atrium into the left ventricle
during diastole.
Examine the left ventricle. Notice the thickness of the ventricular wall. This heart
chamber is responsible for pumping blood throughout the body.
Using your scissors, cut across the left ventricle toward the aorta & continue cutting to
expose the valve.
Count the three flaps or leaflets on this valve leading from the left ventricle into the aorta
and note their half-moon shape. This is called the aortic semilunar valve.
Using scissors, cut through the aorta and examine the inside. Find the hole or coronary
sinus in the wall of this major artery. This leads into the coronary artery which carries
blood to and nourishes the heart muscle itself.
When you have finished dissecting the heart, dispose of the heart as your teacher
advises and clean, dry, and return all dissecting equipment to the lab cart. Wash
your hands thoroughly with soap.
Lab Questions
1. Why are pig hearts used to study the anatomy of the human heart?
2. How can you tell which side of the heart is the ventral surface?
3. How many chambers are found in the heart? What other group of organisms would have this
same number of chambers?
5. Which chambers are the pumping chambers of the heart?
6. Which chambers are the receiving chambers of the heart?
7. How do the walls of the atria compare with the walls of the ventricles and why are they different?
8. What is the purpose of heart valves?
9. Name & compare the heart valves found between the upper & lower chambers of the right and
left sides of the heart.
10. Vessels that carry blood away from the heart are called __________, while __________ carry
blood toward the heart.
11. Which artery is the largest and why?
12. What is the purpose of the coronary artery and what results if there is blockage in this vessel?
13. Use the diagram of the heart below to trace blood flow through the heart:
AGREE/DISAGREE/NOT SURE
NAME:
DATE:
Next to each statement, put a(n) A (agree), D (disagree) or NS (not sure). Be sure to write
a brief explanation to defend your answer.
1. The heart is made of 5 chambers that pumps blood through the body.
Explanation:
2. The left side of the heart receives oxygenated blood while the right side receives
deoxygenated blood.
Explanation:
3. The aorta sends oxygenated blood out to the whole body.
Explanation:
4. The heart is composted of atria and ventricles with the atria located on the bottom
and ventricles on the top of the heart.
Explanation:
5. As blood begins to circulate, it leaves the heart from the left ventricle and goes into
the aorta.
Explanation:
6. The pulmonary artery takes blood to the lungs to pick up oxygen.
Explanation:
7. It is not possible to get a heart transplant.
Explanation:
8. When we inhale air, fresh oxygen goes into our blood.
Explanation:
9. Blood can travel back and forth through 2-way valves located in the heart.
Explanation:
10. The heart is a double pump with two pumping chambers- the left and right
ventricles.
Explanation:
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