Name.
Date.
59
Internal Anatomy of the Frog
BACKGROUND
For many decades the .study of the internal anatomy of the frog has been part of the
laboratory program in biology courses. From their study of the frog, students gain much
information about the internal organization of vertebrate bodies in general. The structure and
arrangement of organs and organ systems in the frog are ~ery similar to those of the human
body. When you have completed this laboratory study, you will know more about the organs
and organ systems of your own body.
OBJECTIVES
In this activity you will:
1. Observe the internal anatomy of the frog.
2. Study the frog as a representative
.
of the phylum Chordata, subphylum
Vertebrata.
MATERIALS
preserved
dissecting
scissors
dissecting
frog (Rana, sp)
tray with wax bed
needles
large dissecting
forceps
scalpel
hand lens
pins
PROCEDURES AND OBSERVATIONSi
Part I. Dissection of the Frog
a. Laythe frog in the dissecting tray, ventr~1side up,
FIGURE1
anterior end away from you. Look at Figure 1 to
see where the incisions should be made. Eachdissection line in Figure 1 has.a dot where the cut
shquld begin and an arrowhead indicating the direction of the cut. Pinch a bit of the skin on the
frog to feel its thickness. Note that the skin isfree
from the tissues underneath it.
b. Pinch up a bit of skinat the dot for line 1. Snipa V
with the scissors. Cut through the skin only along
line 1 shown in Figure 1, all the way to the chin.
Then cut the skin along lines 2, 3, 4, and 5: This
creates flaps of loose skin.
The skin of the frog is very important as a respiratory surface, especially in the winter wben
the frog hibernates in mud at the bottom of a
pond or stream.
.
c. Lay back the skin flaps and examine the underside
of the skin. Note the many find blood vessels.
Ttt~n
'Ii.' cut off the skin flaps.
d. Ex1tnine the muscles of the ventral side of the
frog. They are .relatedto the head, thorax, and
abdomen. Picture the movement 0.1the frog body
as each muscle contracts.
~57
e. Lookat Figure 1 again~Then cut the muscle along
line 1 just to the level of the forelimbs. Make a
shallow cut so as not to damage the organs underneath. At the level of the forelimbs you have
to cut through bone. The heart is directly beneath
the bone, so work carefully. Slidethe lower blade
of the scissors just under the bone and forward
toward the chin, until the scissors are opened
quite wide. Then bring the scissor handles to.
FIGURE2
. .~\
;..
gether in one firm cut throughthe bone.
f. Continue to cut along line 1 all the way to the jaw.
bone. Cut through muscle oitly-do not destroy
the floor of the mouth. Then make cuts along
lines 2, 3, 4, and 5 through the muscles of the
body wall.
g. Pin the specimen to the wax in the tray. Take hold
of the two forelimbs and stretch them widely
apart so that the chest cavity opens. See Figure 2.
Place a large dissecting pin in each limb at an
angle of about 45". Push the pins firmly into the
wax. Then pull the muscle flaps created by cuts 2
to 5 away from the body cavity;.Place two large
dissecting pins in each flap, at the same angle as
before. Now the body cavity will be held open as
you ,work.
h. If the frog is a female, two large masses of black
eggs may be hiding the internal organs. The eggs
are in thin ovary tissue so that you can see the
separate eggs. After examining the egg masses, if
you have a female, let other groups observe them.
Then remove the egg masses by lifting them carefully and snipping th.e two ovaries free from the
bodywith scissors.
.
The frog has a three-chambered heart. The two
atria are the receiving chambers: one receives
oxygenated blood from the lungs and one re.,
ceives deoxygenated blood from the body. The
ventricle, the pumping chamber, pumps the
blood to the body.
.
i. Locate the heart. It is in a triangl,llarpericardial
sac in the center of the body between the
forelimbS. Open the pericardial sac. Note the
pointed ventricle. Anterior to the ventricle are
the two atria.
The liver is the largest organ in the frog body.
(It is also the largest gland in the human bod~.)
Attached to the underside of the liver is the
gallbladder, which stores bile. A bile duct connects t~e gallbladder to the intestine.
j. Locate the large liver. It is reddish-brown and haS
three lobes that overlie the abdominal organs and
extend behind the heart. Carefully lift up the
lobes of the liver and look on the underside
where they are connected to each other. Identify
258
the gallbladder, which is a small, round, greenyellow sac attached to the liver. Then trim away
most of the liver, leaving some tissue attached to
the intestine.
k. Findthe anterior end of the alimentary canal, the
esophagus, posterior to the head. Trace the
esophagus as it leads into the stomach, which is a
curved, white sac. The stomach curves slightly to
the frog's right side. The posterior end of the
stomach leads into the small intestine, a small,.
coiled tube. Note that the liver is attached near
where the stomach and small intestine join.
I. Find the pancreas. It is a feather-shaped organ
.located just dorsal to the stomach and is attached
to the bile duct by a .smalltube.
.
m. Trace the small intestine as it twists and turns
toward the posterior of the frog. Do not tear any
tissue. Find where the small intestine joins the
large intestine, a shorter tube. The large intestine
leads deep into the posterior abdominal cavity.
Just anterior to the anus, between the hindlimbs,
the large intestine is joined by the urinary bladder, it two-lobed, thin-walled sac that may be
lyingon top or to one side of the large intestine.
n. Removethe alimentary canal from the abdomen.
Hold the stomach as you snip the esophagus with
the scissors. Gently free the.tube from surrounding tissues in order to keep the organs intact.
Then cut the posterior end of the canal as close to
the anus as possible.
.
Name,
Date.
The, small intestine is the principal organ of
digestion and absorption of nutrients in the frog.
As in most vertebrate digestive systems, it is held
in place by amesentery and supplied by blood
vessels in the mesentery. The mesentery is a
thin, fan-shaped membrane.
prompts the captor to release the animal-the
sired res.ponse!
r. 'locate the reddish-brown kidneys, deep in the
posterior abdominal cavity on both sides of the
backbone. Trace the path of urine from the kid,
o. Examinethe freed alimentary canal. Note the size
of the stomach and the length of the small intestine. The small intestine is twisted into a helical
form by the mesentery. Spread the coils of the
small intestine a'bit to see the shape of the mesen,tery and also to see the blood vessels in it.
p. Also attached to the mesentery is the small,
spherical, dark-red spleen. Identify it. Normallyit
is centered in the abdominal cavity between the
stomach and the large intestine.
'
The frog stores extra food at times of heavy
feeding (in the summer) in the form offat bodies.
When food is scarce, or during hibernation, the
fat bodies provide energy for life processes. They
are necesSary for the production of eggs and
sperm in the spring breeding season.
q. Observe the yellow, finger-shaped fat bodies. If
they are large, remove them. Then, at the anterior'
end of the abdominil.1 cavity, find the two darkred lungs. They m~y be deflated, long and thin, or
inflated, like long, oval balloons. Note that the
lungs-connect anteriorly with the larynx and glottis.
The frog excretes one-third of its weight in
urine each day because water is constantly entering the body through the skin. Urine is produced
in the kidneys and passes through tubes to the
cloaca, the posterior part of the large intestine. It
can be released from the body through the
cloaca, or it can be stored in the urinary bladder
for a time. When frogs and toads are captured,
they promptly
empty their urinary bladder
through the cloaca onto the captor. This often
(
de-
neysthroughtubesto the cloaca.
s. On the ventral surface of each kidney, find a
light-colored mass of tissue running almost the
length of the kidney. These are the adrenal
giands.
'
'
The male' reproductive organs in the frog are
.
the testes. In the frog and other amphibians,
s'perm travel to the cloaca through the same tubes
as the urine.
,
-
t. If your frog is a male, find the, testes on the ventral side and anterior end of the kidneys. They are
yellowish, oval bodies connected to the kidneys
by tubes.
In the female frog, th~ eggs produced by the
ovaries are swept by cilia into the funnel-shaped
openings of the oviducts near the tips of the
lungs. The eggs pass through the oviducts in
single file, all the way to the cloaca where they
are. released. As they pass through, they are
coated with a jellylike mateJjal that swells as the
eggs enter the water. Sperm from a male are released at the same time as the eggs. Fertilization
is external.
u. If your frog is a female, find the coiled, white
oviducts. Then observe a specimen ofthe oppo-
site sex to see the reproductive organs.
'
v.' Examine the backbone, or vertebral column. Each
bone is a vertebra. The vertebral column encases
the spinal cord.
'
w. Note the stringlike pairs of spinal nerves emerging from between the vertebrae. There 'are 10
pairs of spinal'nerves. Try to find the large sciatic
nerves leading to the muscles of the thighs.
Part II. Further Investigation in the Frog
a. With the scissors, cut the skin of the frog all the
way around the top of one thigh where the leg
joins the body. See Figure 3a. .
b. Fold the skin downward, inside out, toward the
knee, as in Figure 3b. Then grasp the cuff of skin
and pull firmly and steadily until the skin comes
off over the knee. Be careful-if you pull too hard
the skin might break. Also, if you hold the leg too
firmly, the bone may break. A steady, firm pull is
best.
c. The skin is difficult to detach at the ankle. Note
its strength and its attachments. Continue to pull
the skin until it comes off over the toes.
FIGURE3
.
d. Examinethe muscles of the thigh. Note that each
muscle is separate. Pull on each muscle to find
out what movement it produces when it con-
tracts.
- cuff
of skin folded down
.
The calf muscle. of the leg is called the gastrocnemius. It has the same name and function as
th~ calf muscle in the human leg. As hi all muscles that move the skeleton, each end of the gastrocnemius muscle is attached to a bone by
means of a tendon.
b
thigh muscle
e. Examine.the gastrocnemius. Find the upper tendon of the muscle and trace it to its attachment
Ofla bone.
1. To which bone is the upper end of the gastrqc;:nemiusattached?
.
f.
Cut the gastrocnemiusfree of its tendon near its
upper attachment. Pull upward on the muscle to
see how it moves the foot when it contracts.
g. Then pull down on the gastrocnemius, toward the
foot and way from the skeleton. Firmlyand steadily pull the lower tendon, the Achilles tendon,
free from the ankle bone and remove the entire
tendon from the foot.
.
2. How is the
tendon attached to the foot?
(
h. Examine the foot and toe bones and the joints
between them.
The lens of the eye in a living frog is crystalline
and transparent. The lens does not change shape
to focus as it does in the human eye. Frogs are.
nearsighted on land and farsighted under water.
260
i. With the scissors, carefully dissect the eyeball.
Removethe spherical lens. .Placethe lens on your
palm and hold it up to the light. Note that it
gathers light, even in its preserved condition.
j. With the tips of two dissecting needles, tease
apart the lens to see its structure.
k. Place the.frog dorsal side up and remove the skin
from the skull with scissors or a scalpel..
I. With a scalpel, shave or whittle the bone of the
skull between the eyes. Use very shallow strokes,
peeling off the bone in layers or shavings. When
the bone becomes thin, peel it off very carefully
using forceps. Be sure not to dig the points of the
forceps into the brain tissue. See Figure 4.
The cerebral hemispheres of the brain control
voluntary motion and conscious activities of the
frog.
m. Identify the rounded cerebral hemispheres between the eyes.
Name.
Date.
FIGURE4
n. Continue to shave bone toward the anterior of
the frog's head. Exposethe olfactory lobes.
The opti(: lobes control the activities of 'the
eyes.
olfactory lobe
cerebral hemisphere
o. Continue to shave bone, working posteriorly
from the cerebral hemispheres. Locate the optic
lobes, relatively large, hollow, rounded masses of
midbrain tissue.
optic lobe
1i'he cerebellum coordinates the muscular activities and controls the balance of the frog.
cerebellum
p. Posterior to the optic lobes, find the cerebellum,
a very short section of the brain.
The medulla is the control center in the brain
for breathing, swan owing, "digestion, and reflexes.
medulla
.
q. Trace the cerebellum as it leads into the medulla.
spinal cord
The olfactory lobes are centers for sensing
odors. Nerves lead to these lobes from the nos.
trils.
CONCLUSIONS
r. Then locate the spinal cord as it begins at the
posterior base of the medulla. Dissect a vertebra.
or two in order to see the spinal cord encased
within the vertebral column.
AND APPLICATIONS
1. Explain the three ways that respiration
occurs in the frog.
2. One atrium of the frog heart receives blood from the body, and the other receives blood from the
lungs. Both atria deliver blood into the ventricle, which pumps it to the body. Is the blood pumped to
the body oxygenated, deoxygenated; or both?
3. List the parts of the frog alimentary canal, including the glands attached to it, beginning with the
mouth and ending with the anus.
(
261
4. How does pancreatic juice get into the alimentary canal in the frog?
5. Describe the structure and function of the mesentery associated with the small intestine of the fro