6. Pig Dissection I

BI102. B.K. Penney
6. Pig Dissection I
BACKGROUND
To study the anatomy of vertebrates, we will be dissecting fetal pigs. Material for these
labs is essentially from Term 1. The context for BI02 is that I want you to understand
how a vertebrate works in order to better understand the lectures on vertebrate diversity
and human evolution.
Why do we dissect?
• to learn the ability to translate from a diagram to real structure (this means ideally
you come in with the diagram already in your head!)
• to understand complexity and interconnections among parts; models cannot do
this adequately because they are by nature simplifications of three dimensional
structures.
• to observe the variation among specimens in placement, color, etc. of structures.
Your body varies from other humans in a similar fashion.
Why pigs? Pigs look internally almost identical to a human, are readily available but are
not specifically bred or captured for dissection purposes (they are byproducts of the meat
industry that would otherwise be turned into fertilizer or disposed of some other way).
See the information on fetal pigs on the Course Links on the web page or at
http://www.goshen.edu/bio/pigbook/humanpigcomparison.html.
Diagrams are from the BIODIDAC inventory except for the abdominal cavity diagram,
which was drawn by Melissa Goczalk.
CONCEPTS
Work through the lab manual exercises in pairs or benches. Throughout,
emphasize the logical structure of how parts are connected and what they do. There are
several questions presented here that take this approach, like tracing blood flow through a
system. These will help you on the practical, as you will have a better mental map of the
various parts.
The quizzes will be fill-in-the-blank style with diagrams. The biggest trouble
some students have is that they want to memorize the diagrams- this is not the point of
this exercise! You need to appreciate variability among specimens and how things
connect in real organisms, so please ensure you use other types of observations (color,
texture, etc.) besides just location of the organ. You need to see more than one specimen
to understand this, so look around the room.
The structures you need to know are labeled on each diagram.
A.Pig Dissection I PreQuiz
Summarize functions for the following organs in 2-3 words.
Major organs are diagrammed in Fig. 20.10 (p.420) of your textbook. All are
listed in the index of your book to help you look up their functions. (There are a
LOT more for next week, so you might want to start on some of them now!)
epiglottis:
esophagus:
stomach:
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liver:
gall bladder:
pancreas:
duodenum:
small intestine:
spleen:
large intestine:
rectum:
B. External anatomy and sex of the fetal pig
GOALS:
o To become familiar with the external anatomy and major body regions of
the pig.
o To determine the sex of your pig, and become familiar with the external
genital structures.
3. External anatomy procedure
The pigs have been preserved in formalin, and then stored in a proprietary
preservative fluid. This fluid is not as toxic as formalin, but you still need
to wear gloves and goggles when handling the specimens.
Work in pairs, and ensure that you see pigs of both sexes!
1. Obtain a pig for yourself and your partner and place it in a
dissection pan.
2. Locate pig's right and left, anterior and posterior, dorsal and
ventral sides. Structures closer to the midline of the pig are
referred to as proximal, while those further from the midline are
referred to as distal.
3. Locate the following body regions: head, neck, trunk, tail, and
abdomen. Find where the thoracic cavity and abdominal cavity
will be.
4. Examine the head. Locate and describe the sensory receptors for
the five major senses.
5. Examine the pig's skin. An outer embryonic skin, the epitrichium,
may lie over the skin on some animals.
6. Locate the umblical cord, with two umbilical arteries, a larger
and flattened umbilical vein, and a very small and round allantoic
stalk (part of an extension of the urinary bladder of the fetus).
7. Locate the anus and mammary papillae (yes, male pigs will have
the papillae too!).
8. Determine the sex of the pig. Male: urogenital opening just
posterior to the umbilical cord, scrotal sacs pust ventral to the
anus. Female: urogenital opening ventral to the anus. It is
surrounded by folds called labia, and a genital papilla will
protrude just ventral to the opening.
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4. Diagram of external anatomy of the fetal pig: left side, male
ventral and female ventral.
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5. Describe circulation through the umbilical cord
Which vessels are carrying blood toward the heart of the fetus? Is this
blood oxygenated or deoxygenated? Explain.
6. To what human structures are the genital structures of the pig
comparable?
Mouth cavity
GOALS:
1) To see structures buried in the back of the mouth region, including where the
respiratory and digestive tracts cross.
1. Mouth cavity dissection procedure
1. Place the pig in the pan, ventral side up. It may make the following
dissections easier if you tie the pig in place first. Take a 2ft piece
of string, and tie a loop in one end. Slip this over one foot of the
pig, pull the string around the back side of the dissection tray, then
tie the other end onto the opposite foot. Repeat for the other pair of
legs.
2. To open the mouth fully enough to see all of the structures in this
cavity, you need to cut (with a scalpel) from each corner of the
mouth back to the jawbone. Ensure you are going straight back
from the corner of the mouth so you do not cut into any of the soft
tissues in the mouth.
3. Once you are back to the jawbone on both sides, you will have to
force the mouth open. Place one hand on the pig's snout, with your
thumb on the hard palate (careful of the teeth!). Place your other
hand on the lower jaw with your thumb on the tongue. Pull open
the mouth until you hear the jawbone dislocate. Yes, it will make a
cracking noise.
4. Cut each side of the mouth just a bit further until you can see a
small flap in the back of the mouth region.
5. Identify the structures in the mouth cavity seen in the figure: teeth,
hard palate, soft palate, opening to the nasal chamber, the
epiglottis (which will appear as a small, grayish-white flap
pointing upwards) and the tongue.
6. If you place your blunt probe back over the epiglottis and push it
down, you may be able to see the openings to the esophagus and
windpipe.
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2. Diagram of the mouth cavity, with openings leading to the
respiratory and digestive system.
Abdominal cavity
GOALS:
1. To open the abdominal cavity without damaging any of the underlying organs
2. To identify the major structures in the abdominal cavity.
1. Abdominal cavity dissection procedure
1. Begin by feeling your pig to find the sternum (breast bone) in the
center of the rib cage. Note roughly where this bone ends
(posterior end).
2. Create an opening incision by using your scalpel to make a 1cm
long cut through the skin down to the breastbone. Ensure you do
not actually cut through this structure.
3. Continue this cut CAREFULLY to the posterior end of the
breastbone, but no further. Ideally you will see a small hole
through the skin into the abdominal cavity.
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4. Continue this cut using your scissors, as it is MUCH easier to cut
without damaging organs underneath. Push the bottom point into
the hole through the skin. Turn the points of the scissors upward to
cut, so the points do not cut into any organs.
5. Cut until about 1cm before the umbilical cord, then curve the cut
around the side of the umbilical cord and continue until the start of
the thigh. Create a similar cut from just before the umbilical cord
around to the other thigh, so the umbilical cord is isolated on a flap
of skin connected at the posterior side of the pig.
6. This flap will still be connected by the umbilical vein. Lift the
overlying skin so you can see this connection between the
umbilical cord and the liver. Once you have seen this, you can cut
the umbilical vein and fold the flap of skin to the posterior of the
animal.
7. To finish opening the abdominal cavity, create a pair of cuts
through the skin at the posterior margin of the cavity. Then create a
pair of cuts at the anterior side of the cavity, following the rib cage
toward the sides of the animal.
8. Many specimens will contain a lot of fluid in the abdominal cavity,
as well as free latex or coagulated blood. Rinse your specimen
under the sink to remove this.
9. If you see any free green material, you have cut through the
intestine and the intestinal contents are leaking out. Handle the pig
gently to prevent more of the contents from escaping.
2. Identification of abdominal organs and structures
1. The liver will be the most obvious organ, at the anterior end of the
cavity. It is large, firm and typically dark brown colored (although
it may be light or green in some specimens). Its function is to
detoxify chemicals in the blood, to secrete bile and other chemicals
for digestion and to process some nutrients.
2. Immediately anterior to the liver, forming the anterior wall of the
cavity, is the diaphragm, a muscular sheet that runs side to side
underneath the rib cage. When it contracts, air is pulled into the
lungs.
3. The remaining organs are posterior or underneath the liver, and are
bound by the peritoneum, a smooth and shiny membrane. This
sheet holds the organs in place and carries blood vessels. It is
equivalent to the coelomic lining we discussed in other phyla.
4. Lift up the liver with your blunt probe, and this organ and others
will be positioned as in the diagram.
5. The gall bladder is immediately underneath the liver, normally
embedded in its underside. It is often a greenish color. Its purpose
is to store bile produced by the liver for digestion.
6. On the left side of the animal (its left, not yours) under the liver is
the stomach, a tan, floppy, baglike structure. Its purpose is to
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break down chunks of food using hydrochloric acid and
contractions.
7. Attached to the stomach on the left side is a long, firm, feathershaped organ, the spleen, which cleans the blood by recycling old
blood cells and removing antibody-coated bacteria. It is essentially
a link between the circulatory system and the lymphatic system
that prevents infection. Note that this cleaning role is different than
the "filtration" the kidneys do as part of removing nitrogenous
wastes.
8. The rest of the abdominal cavity will be filled with intestines. The
narrower diameter portion on the right of the animal is the small
intestine, which continues digesting material and absorbs
nutrients. The parts of the small intestine differ slightly along its
length: the duodenum and the jejuno-ileum.
9. The larger diameter portion on the left of the animal is the large
intestine (also called the colon), and is bound into a spiral shape.
This is one of the few differences between human and pig
anatomy, in that the human large intestine forms a loop all around
the outer portion of the abdominal cavity. The large intestine
absorbs water and other materials.
10. At the junction of the small and large intestines is the cecum, a
large baglike structure equivalent to our appendix. It aids in longterm digestion of tough plant material. Depending on your pig, the
cecum may be on top and easy to see, or it may be buried within
the coils of intestine.
11. Use your blunt probe to push the liver and stomach to the anterior
so you can see to the bottom (dorsal side) of the abdominal cavity.
At the base of the stomach, you should see a flimsy, translucentwhite organ (think scrambled eggs in a wet plastic bag). This is the
pancreas, which produces enzymes for digestion and hormones,
including insulin.
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3. Diagram of cuts to make to open abdominal cavity
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4. Diagram of organs in the abdominal cavity
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