Download Frog Dissection - Seattle Girls` School 6th Grade

Frog Dissection
A. External Anatomy of the Frog
Before dissecting your frog, observe it.
o Examine its front and hind legs.
o How are the toes connected?
o What does its skin feel like?
o How is the head attached?
o Note similarities and differences with the
homologous structures on the human body.
1. Determine the frog’s sex. The male frog has thick pads on its “thumbs” and tends to be
smaller. What do you think the pads are used for?
2. Use the diagram above to locate and identify the external features of the head.
3. Look at its eyes and tympanum (organ for hearing). How is the placement of these
structures adapted for swimming?
B. Interior Anatomy of the Frog
1. Put your frog on its dorsal side (ventral side
facing up).
2. Using your forceps to hold up the skin as you are
working, make a shallow cut with your scissors
from the cloaca (opening around the urogenital
tract) to the lip. As you cut, pull the skin up and
away from the muscles to be sure that you are
cutting only the skin.
3. Then cut toward the side of each leg and pin the
skin flat. (Look at the diagram to the right.)
4. Carefully cut around the cloaca, down one of the
hind legs, and around the ankles.
5. Gently pull the skin away from the leg muscles
and notice that it does not stick tightly to the
muscles themselves. It is attached to connective tissue called septa. Also notice that
there are no fat deposits between skin and muscles (where humans do have a fatty layer).
Muscular System
1. Compare the muscles visible on your frog to the diagrams provided. Try to identify a
few individual muscles in your frog.
2. With your forceps, move the back legs and try to identify which muscles straighten the
legs and which bend the legs.
3. See how the leg muscles are attached at the top and bottom of the muscles. Notice to
which bones the muscles attach and look for the tendons (connective tissue attaching
muscles to bones).
Internal Organs
1. Using the scalpel (or scissors), make a shallow cut a little to one side of the center line.
Lift up with forceps as you cut through the muscles from the jaw down the entire length
of the frog’s abdomen to the cloaca. DO NOT CUT TOO DEEPLY. It is easy to injure
the internal organs.
2. Make cuts at right angles to your first cut above the hind legs and just above the front
legs. Open the body cavity carefully. You may have to gently cut the connective tissue
that holds the central vein to the muscles so that the vein stays with the internal organs.
Use four dissecting pins to hold the flaps of the muscle open. If your frog is female, you
will have to move the eggs (the dark mass which fills most of her abdominal cavity) to
one side to see the organs underlying them.
Digestive System
3. Pry open the mouth. If necessary, cut the hinges of
the mouth. Notice the teeth (maxillary and
vomerine); run your fingers along the top of the
mouth to feel the maxillary teeth. A frog does not
chew its food. What do the positions of its teeth
suggest about how the frog uses them and how it
feeds?
4. Notice the tongue and where the tongue is attached
(it is not like yours).
5. Identify other structures in the mouth as shown in
the diagram to the right.
6. Back in the body cavity, use your forceps and
dissecting needle to gently push the heart and liver
out of the way so you can locate the esophagus,
stomach, gall bladder, pancreas, small intestine,
large intestine, and cloaca (opening
for both the digestive and
reproductive systems). Use the
diagram to help you identify these
organs.
7. Notice that the small intestine is
attached by connective tissue called
mesentery. The mesentery is richly
supplied with blood vessels. What is
its function?
8. Using your scalpel and forceps,
carefully remove the liver, stomach,
and small intestine. Approximately
how long do you think the frog’s
small intestine is?
9. Cut open the stomach and observe
any contents.
Respiratory and Circulatory Systems
10. Identify the lungs and heart. The small lungs lie on either side of the heart. Notice that
the frog, unlike the human, does not have a
diaphragm.
11. Remove (very carefully) the pericardium
(layer of tissue) surrounding the heart. Refer
to the diagram to identify the following: left
atrium, right atrium, and ventricle of the heart.
Note that the frog heart has three chambers
compared to the four chambers found in
mammalian hearts. Locate the arteries and
veins that lead towards and away from the
heart.
Urinary and Reproductive Systems
12. Use the diagram on the right to
identify the parts of the urinary and
reproductive systems.
13. Remove the peritoneal membrane,
which is connective tissue that lies
on top of the red kidneys. Observe
the yellow fat bodies that are
attached to the kidneys.
14. Find the long twisting ureters that
drain each kidney. The ureters lead
to the urinary bladder, located
posteriorly, which empties into the cloaca.
15. Find the ovaries, oviducts, and uteri (if your frog is female) or testis and sperm ducts (if
your frog is male).
Nervous System (if time permits)
16. Remove the kidneys and look for threadlike spinal
nerves that extend from the spinal cord. Dissect a
thigh, and trace one nerve into a leg muscle.
17. Remove the pins that hold down your frog and turn it
onto its ventral side. Be sure to make very shallow
cuts so that you do not damage delicate structures.
Remove the skin on the head. Scrape away or pick
off the top of the skull, the cranium, until the brain
lies exposed. Identify the olfactory bulb, cerebral
hemispheres, optic lobe, cerebellum, and medulla.
Look for large nerve bundles (ganglia) leading from
the brain. Lift up the brain and look for the pituitary
glad, located beneath the cerebellum at the base of
the optical nerve.