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RAT/FROG DISSECTION
INTRODUCTION
In this lab we will examine the anatomy of a rat or a frog. By dissecting the specimen, we may look at the
internal organs that we have studied in close 3D detail! The internal anatomies of a rat and frog are very similar
in construction to a humans’.
OBJECTIVES
•
•
Students will be able to gain dexterity with instruments used in the dissection procedure
Students will be able to compare and/or contrast the anatomy of the frog/rat with that one of the human
body
•
Students will be able to put anatomical vocabulary into practice
MATERIALS
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Frog/Rat
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Scissors Forceps
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Dissection Pan
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Probe
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Gloves
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Dissection Pins
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Razors
PROCEDURES - FROG DISSECTION
Pre-Dissection
• Obtain your materials and your frog to be placed on the dissection tray
Frog Dissection - External Anatomy
1. Place your frog with its ventral side on the tray and its dorsal side up
2. A male frog usually has thick pads on its "thumbs," which is one external difference between the sexes,
as shown in diagram 1-2. Male frogs are also usually smaller than female frogs. Comparing to the
diagram, determine the gender of your frog.
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3. Using diagram 3 to locate and identify the external features of the head: mouth, external nares,
tympani, eyes, and nictitating membranes. The eyes have a non-moveable upper and lower lid and a
nictitating membrane, which serves to moisten the eye. The tympanum is located behind each eye.
4. When examining the external nares, insert a probe into the
external nares and note down where your probe exits.
5. Turn the frog on its dorsal side and pin down the legs.
6. Cut the hinges of the mouth and open it wide. Use diagram
4 to locate and identify the structures inside the mouth. Use
a probe to help find: 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.
Frog Dissection - Dissection
7.
First Skin Incision - use the forceps to lift the skin midway between the rear
legs of the frog. Use the razor to make a cut along the center/midline of the
frog, therefore, cutting it equally.
8.
Continue the Skin Incision - use the scissors to cut all the way up the frog's
body to the neck. Be very careful not to cut too deeply.
9.
Leg Incisions - use the scissors to make horizontal incisions above the rear
legs and at the bottom of the front legs.
10.
Separate the Skin & Muscle - Once you have finished the incisions between
the front and rear legs of the frog you need to separate the skin flaps from the
muscle below. To do this: pick up the flap of skin with the forceps, and use the
razor to help separate the skin from the muscle below. Pin down the skin.
11.
First Muscle Incisions - Now that the skin has been removed, begin the
abdominal muscle incision by using the forceps to lift the muscle midway
between the rear legs of the frog. Next use the razor to start the incision in the
direction of the chin.
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12. Continue the Muscle Incisions - Using the scissors, carefully continue the incision up the midline of
the frog, but do not cut too deeply as to damage the organs.
13. Turn Scissor Blades - This is very important. When you reach a point just below the front legs, turn the
scissors blades sideways to cut through the bones in the chest. This should prevent damage to the heart
or other internal organs. When your scissors reach a point just below the frog's neck you have cut far
enough.
14. Second Muscle Incisions - Next, using the scissors, make horizontal incisions through the muscle
between both the front legs and above the back legs. To finish opening up the frog's body cavity
therefore exposing the abdominal region, use the forceps to hold the muscle flaps while separating the
muscle from the tissues below with a razor. Pin down the flaps.
15. Removal of Fat Bodies - If there are yellow bodies in your frog, carefully remove them.
Frog Dissection - Respiratory System
16. Lungs - Identify the lungs, two small sacs on either side of the
midline and partially hidden under the liver. Trace the path of air
from the external nares to the lungs. The lungs are shallow and do
not supply enough oxygen to support the frog without help of the
skin and mouth lining.
17. Liver - Locate the liver, the large, prominent, dark-brown organ in
mid ventral portion of trunk. Under the liver, find the gallbladder.
Frog Dissection - Circulatory System
18. Heart - Lift the liver gently. Identify the heart, covered by a membranous covering (the pericardium).
With forceps, lift the covering, and gently slit it open. Amphibian hearts have 3 chambers. The heart
consists of a single, thick-walled ventricle and two (right and left) anterior, thin-walled atria.
Frog Dissection - Digestive System
19.
Esophagus - Identify the esophagus, a very short connection between the
mouth and the stomach. Lift the left liver lobe, and identify the stomach,
which is whitish and J-shaped. The stomach connects with the esophagus
anteriorly and with the small intestine posteriorly.
20. Cloaca - Find the small intestine and the large intestine, which enters the
cloaca. The cloaca lies beneath the pubic bone and is a general receptacle
for the intestine, the reproductive system, and the urinary system. It opens
to the outside by way of the anus. Trace the path of food in the digestive
tract from the mouth to the cloaca.
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21. Stomach - Slit open the side of the stomach, and notice its ribbed internal surface. Try to identify its
contents. As you lift the small intestine you will see the pancreas, a thin, yellowish ribbon, between the
small intestine and the stomach. Locate the spleen, a small pea-shaped body near the stomach. It
produces new blood cells and disposes of old ones.
Frog Dissection - Excretory System
22. Kidney and bladder - Identify the kidneys, which are long narrow organs lying against the dorsal wall.
The bladder is a thin sac attached to the cloaca, it may be difficult to locate. Identify the urinary bladder,
attached to the ventral wall of the cloaca. In frogs, urine backs up into the bladder from the cloaca.
Frog Dissection - Reproductive System
23. Males - Locate the testes in the male frog. They are yellow or tan-colored, bean-shaped organs near the
anterior end of each kidney. Several small ducts, the vasa efferential, carry sperm into the kidney ducts
that also carry urine from the kidneys. Fat bodies, which store fat, are attached to the testes.
24. Females - Locate the ovaries in the female frog. They are attached to the dorsal body wall. Fat bodies
are attached to the ovaries. Highly coiled oviducts lead to the cloaca. The ostium (opening) of the
oviducts is dorsal to the liver.
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PROCEDURES - RAT DISSECTION
Pre-Dissection
• Obtain your materials and your rat to be placed on the dissection tray
Rat Dissection - External Anatomy
1. Place your frog with its ventral side on the tray and its dorsal side up
2. The rat’s body is divided into 6 anatomical regions: the cranial region, cervical region, pectoral region,
thoracic region, abdomen, and the pelvic region.
3. Note the hairy coat that covers the rat and the sensory hairs (whiskers) located on the rat's face, called
vibrisse. The mouth of the rat has a large cleft in the upper lip which exposes large front incisors. Rats
are gnawing mammals, and these incisors will continue to grow for as long as the rat lives.
4. Note the eyes with the large pupil. The eyelids are similar to those found in humans.
5. Locate the teats on the ventral surface of the rat.
Check a rat of another sex and determine
whether both sexes have teats. On female rats,
just posterior to the last pair of teats, you will
find the urinary organ. On males, you will find a
large pair of scrotal sacs which contain testes.
Just anterior to the scrotal sacs is the prepuce,
which is a bulge of skin surrounding the penis.
6. Examine the tail. Though some rodents, like
gerbils, have hair on their tails, rats do not.
Locate the anus, which is ventral to the base of
the tale.
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Rat Dissection - Skinning the Rat
7. Skinning the Rat - You will carefully remove the skin of the rat to expose the muscles
below. This task is best accomplished with scissors, forceps and the probe. Use the
lines on the diagram to cut a similar pattern, avoiding the genital area. Gently peel the
skin from the muscles, using the probe to tease away muscles that stick to the skin.
8. Refer to Frog Dissection - For the dissection part, refer back to Frog dissection and
use the same dissection procedure. (Use frog dissection procedure 7-14)
Rat Dissection - Thoracic Region
9. Diaphragm - Locate the diaphram, which is a thin layer of muscle that separates the thoracic cavity
from the abdominal cavity.
10. Heart - The heart is centrally located in the thoracic cavity. The two dark colored chambers at the top
are the atria (single: atrium), and the bottom chambers are the ventricles. The heart is covered by a thin
membrane called the pericardium. (We will come back to the heart later.)
11. Lungs - The bronchial tube branch from the trachea and enter the lungs on either side. The lungs are
large spongy tissue that take up a large amount of the thoracic cavity. Bronchial tubes may be difficult
to locate because they are embedded in the lungs.
Rat Dissection - Abdominal Region
12. Liver - Locate the liver, which is a dark colored organ suspended just under the diaphragm. The liver’s
functions are production of bile which aids in digesting fat, storage of glycogen and transforms wastes
into less harmful substances. Rats do not have a gall bladder which is used for storing bile in other
animals.
13. Esophagus - The esophagus pierces the diaphragm and moves food from the mouth to the stomach. It is
distinguished from the trachea by its lack of cartilage rings.
14. Stomach - Locate the stomach on the left side just under the diaphragm. The opening between the
esophagus and the stomach is called the cardiac sphincter. Slit the stomach lengthwise and notice the
ridges, called rugae.
15.
16.
Spleen - The spleen is about the same color as the liver and is attached
to the stomach. It functions in the destruction of blood cells and in
blood storage. A person can live without a spleen, but they're more
likely to get sick as it helps the immune system function.
Pancreas - The pancreas is a brownish, flattened gland found in the
tissue between the stomach and small intestine. The pancreas produces
digestive enzymes that are sent to the intestine via small ducts (the
pancreatic duct). The pancreas also secretes insulin which is important
in the regulation of glucose metabolism. Find the pancreas by looking
for a thin, almost membrane looking structure that has the consistency
of cottage cheese.
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17. Small Intestine - The small intestine is a slender coiled tube that receives partially digested food from
the stomach (via the pyloric sphincter). It consists of three sections: duodenum, ileum, and jejunum. Use
your scissors and probe to cut the connective tissue of the small intestine. If you are careful you will be
able to stretch it out and untangle it so that you can see the relative lengths of the large and the small
intestine.
18. Colon - Locate the colon, which is the large greenish tube that extends from the small intestine and
leads to the anus. The colon is also known as the large intestine. The colon is where the finals stages of
digestion and water absorption occurs and it contains a variety of bacteria to aid in digestion.
19. Cecum - Locate the ceacum - a large sac in the lower third of the abdominal cavity. It is a dead-end
pouch and is similar to the appendix in humans. It also is the point at which the small intestine becomes
the large intestine.
20. Rectum - Locate the rectum - the short, terminal section of the colon between the descending colon and
the anus. The rectum temporarily stores feces before they are expelled from the body.
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DISCUSSION - FROG DISSECTION QUESTIONS
1. Write the taxonomy classification of the frog, include the kingdom, phylum, class, subclass, and order.
(2 mark)
2. Identify the following structures, their locations and functions. (Copy this table on your report). (10
marks)
Structure
Function
Location
Vomerine teeth
Eustachian tubes
Nictitating Membrane
Tympanic Membrane
Esophagus
Glottis
Left Atrium
Right Atrium
Ventricle
Vasa Efferential
3. The ventricle has thicker, more muscular walls than the atria. Relate this difference in wall structure to
the functions of the 2 types of heart chamber. (2 marks)
4. List the organs of digestion in order from the esophagus to cloaca (in the form of a mind map). (3 marks)
5. Why is the inside of the stomach folded? (1 marks)
6. How do the following adaptations help it survive in the wild? (9 marks)
Adaptation
Eye position
External nares (nostrils)
Vocal sacs
Nicitating membrane
Tympanum
Maxillary and Vomerine teeth
Vestigial Thumb
Eustachian tube
Tongue
How it Aids in Survival
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7. A mature female may have its body cavity full of black and white eggs. The black portion of the egg
contains the living embryo. The white portion is yolk, containing stored food. When placed in water the
eggs will float with the light side down. What is the advantage of this? (3 marks)
8. If your frog is an immature female, you will find 2 grayish or pinkish ovaries in either side of the
abdominal cavity. The testes of the male are in the corresponding positions. Attached to these organs are
yellow fat bodies. What is the function of fat bodies? (1 mark)
9. During one mating of frogs, the female lays some 2,000 to 3,000 eggs in water as the male sheds
millions of sperm over them. How do these large numbers relate to the frog’s fitness for life in water? (2
marks)
10. What do the large optic lobes of a frog’s brain suggest? What does the large olfactory lobe of a frog’s
brain suggest? (2 marks)
DISCUSSION - RAT DISSECTION QUESTIONS
1. Write the taxonomy classification of the rat, include the kingdom, phylum, class, subclass, and order. (2
mark)
2. Identify the following structures, their locations and functions. (Copy this table on your report). (10
marks)
Structure
Function
Location
Vibrisse
Teats
Scrotal Sacs
Pericardium
Esophagus
Diaphragm
Spleen
Pancreas
Colon
Cecum
3. Describe the anatomy of the liver and the role of the liver in homeostasis. Be specific. (3 marks)
4. Compare and contrast the anatomy of the rat to that of a human. Describe at 4 similarities/differences
and include importance of structure in relation to function. (4 marks)
Similarities:
Differences:
5. Pick one organ system and illustrate the organs that make up the organ system in the rat. Link the
organs in a mind map, how each organ is related to the next. (3 marks)
6. Describe a rat’s foot structure and for what it might be useful for. (2 marks)
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7. Do rats hunt for their food? If not, how do they obtain or capture food? What sort of food might a rat be
better suited to digest than a human being is? (3 marks)
8. How does the reproductive system of a female rat allow for multiple developing fetuses? (2 marks)
9. Locate the trachea and the esophagus. Describe 3 ways in which you could tell them apart. (3 marks)
10. Chemical digestion occurs at three points along the digestive track. Name those three points and
describe the type of digestion. (3 marks)
Mark Break Down
Pre-Lab Diagram
Lab Identification
/5
Lab Report
/10
Total
/35
/50