Objectives
• Identify the
organs that make up the internal systems of the frog.
Materials
• preserved frog
• dissecting
tray and paper towels
• dissecting
kit
Purpose
In this lab, you will dissect
a frog in order to observe the external and internal structures
of frog anatomy.
Background
As members of the class Amphibia,
frogs may live some of their adult lives on land,
but they must return to water to reproduce. Eggs are laid
and fertilized in water.
External Anatomy:
On the outside of the frog’s head are two external
nares, or nostrils; two tympani, or eardrums;
and two eyes, each of which has three lids. The third
lid, called the nictitating membrane, is transparent.
Mouth Dissection:
Inside the mouth are two internal nares, or openings
into the nostrils; two vomerine teeth in the middle
of the roof of the mouth; and two maxillary teeth at
the sides of the mouth. Also inside the mouth behind the
tongue is the pharynx, or throat. In the pharynx,
there are several openings: one into the esophagus,
the tube into which food is swallowed; one into the glottis,
through which air enters the larynx, or voice
box; and two into the Eustachian tubes, which connect
the pharynx to the ear.
Digestive System:
The digestive system consists of the organs of the digestive
tract, or food tube, and the digestive glands. From the
esophagus, swallowed food moves into the stomach and
then into the small intestine. Bile is a digestive
juice made by the liver and stored in the gallbladder.
Bile flows into a tube called the common bile duct,
into which pancreatic juice, a digestive juice
from the pancreas, also flows. The contents of
the common bile duct flow into the small intestine, where
most of the digestion and absorption of food into the
bloodstream takes place. Indigestible materials pass through
the large intestine and then into the cloaca,
the common exit chamber of the digestive, excretory,
and reproductive systems.
Respiratory
System: The respiratory system consists of
the nostrils and the larynx, which opens into two lungs,
hollow sacs with thin walls. The walls of the lungs
are filled with capillaries, which are microscopic
blood vessels through which materials pass into and out
of the blood.
Circulatory
System: The circulatory system consists of
the heart, blood vessels, and blood. The heart has two
receiving chambers, or atria, and one sending chamber,
or ventricle. Blood is carried to the heart in
vessels called veins. Veins from different parts of the
body enter the right and left atria. Blood from both atria
goes into the ventricle and then is pumped into the arteries,
which are blood vessels that carry blood away from
the heart.
Urinary
System: The urinary system consists of the
frog’s kidneys, ureters, bladder, and cloaca. The kidneys
are organs that excrete urine. Connected to each kidney
is a ureter, a tube through which urine passes
into the urinary bladder, a sac that stores urine
until it passes out of the body through the cloaca.
Reproductive
System: The organs of the male reproductive
system are the testes, sperm ducts, and cloaca. Those
of the female system are the ovaries, oviducts, uteri,
and cloaca. The testes produce sperm, or
male sex cells, which move through sperm ducts, tubes
that carry sperm into the cloaca, from which the sperm
move outside the body. The ovaries produce eggs,
or female sex cells, which move through oviducts
into the uteri, then through the cloaca outside
the body.
Nervous
System: The central nervous system of the frog
consists of the brain, which is enclosed in the
skull, and the spinal cord, which is enclosed in the backbone.
Nerves branch out from the spinal cord.
Skeletal
System: The frog’s skeletal and muscular systems
consist of its framework of bones and joints, to which
nearly all the voluntary muscles of the body are
attached.
Muscular
System: Voluntary muscles, which are those
over which the frog has control, occur in pairs of flexors
and extensors. When a flexor of a leg or other
body part contracts, that part is bent. When the
extensor of that body part contracts, the part
straightens.
Procedure
1. Obtain all necessary
equipment form the lab table.
2. Place a frog on
a dissection tray. To determine the frog’s sex,
look at the hand digits, or fingers, on its forelegs.
A male frog usually has thick pads on its "thumbs," which
is one external difference between the sexes, as shown
in the diagram below. Male frogs are also usually smaller
than female frogs. Observe several frogs to see the difference
between males and females.
3. Use the diagram
below to locate and identify the external features of
the head. Find the mouth, external nares, tympani,
eyes, and nictitating membranes.
4. Turn the frog
on its back and pin down the legs. Cut the hinges of the
mouth and open it wide. Use the diagram below to locate
and identify the structures inside the mouth. Use a probe
to help find each part: the vomerine teeth, the
maxillary teeth, the internal nares, the
tongue, the openings to the Eustachian tubes,
the esophagus, the pharynx, and the slit-like
glottis.
5. Look for the opening
to the frog’s cloaca, located between the hind legs. Use
forceps to lift the skin and use scissors to cut along
the center of the body from the cloaca to the lip. Turn
back the skin, cut toward the side at each leg, and pin
the skin flat. The diagram above shows how to make these
cuts.
6. Lift and cut through
the muscles and breast bone to open up the body cavity.
If your frog is a female, the abdominal cavity may be
filled with dark-colored eggs. If so, remove the eggs
on one side so you can see the organs underlying them.
7. Use the diagram
below to locate and identify the organs of the digestive
system: esophagus, stomach, small intestine, large
intestine, cloaca, liver, gallbladder, and pancreas.
8. Again refer to
the diagram below to identify the parts of the circulatory
and respiratory systems that are in the chest cavity.
Find the left atrium, right atrium, and ventricle
of the heart. Find an artery attached to the heart
and another artery near the backbone. Find a vein
near one of the shoulders. Find the two lungs.
9. Use a probe and
scissors to lift and remove the intestines and
liver. Use the diagram on the next page to identify
the parts of the urinary and reproductive systems. Remove
the peritoneal membrane, which is connective
tissue that lies on top of the red kidneys. Observe the
yellow fatbodies that are attached to the kidneys.
Find the ureters; the urinary bladder; the testes
and sperm ducts in the male; and the ovaries,
oviducts, and uteri in the female.
10. 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. Note the size and
texture of the leg muscles.
11. Dispose of your
materials according to the directions from your teacher.
12. Clean up your
work area and wash your hands before leaving the lab.
