Download DISSECTION OF A RAT

7.1 DISSECTION OF A RAT
Time required: 2 periods
Name: ________________
Introduction: Dissection is a technique used to study the internal organs of a dead animal. It shows students the
internal structure of the animal, how the tissues look and feel, and how they are interrelated. All mammals
possess similar type of organs and systems whether they are small or large in size. To understand the structure
and functions of our organ systems, it is helpful to dissect smaller mammals such as rats. Since rats are
herbivorous, they have 4 long, sharp incisors for gnawing and 12 molars for chewing but have no canines and
premolars. The toothless space between the incisors and molars is called the diastema. Their digestive system is
similar to ours except that they have longer small intestine and a large caecum to digest cellulose. They have no
gall bladder. There are no differences between human and rat excretory systems. The female rats have a Vshaped uterus that can hold many fetuses where as human uterus is smaller and usually holds only one fetus at
a time.
Learning outcomes: After completing this dissection, students will become proficient in dissecting and become
familiar with the structure and functions of various organs and systems of the rat and also note the differences
between human and rat systems
Group size: 2-3 per group
Objectives: To dissect a rat and note the similarities and differences between rat’s digestive, excretory and
reproductive systems with that of human systems.
Apparatus and materials per group:
A preserved male rat or female rat
One female rat per class
Scalpel
Sharp scissors
Dissecting probe
Forceps
A long string
Plastic bag
Dissecting needle Drawing pins
Disinfectant solution
Dissecting board A small ruler
Model of human torso
Disposable gloves Old newspaper Rat dissection chart and video
Lab coat
Observing the external features
1.
2.
3.
4.
Put on gloves and a lab coat before dissecting the rat.
Get the necessary dissecting equipment, dissecting board, string, drawing pins, a ruler and a newspaper.
Lay the newspaper on top of the dissecting board.
Obtain a preserved rat specimen for your group and note whether it is a male or a female by checking for the
presence of external testes.
Note: The penis in the male rat will be withdrawn inside the body
5. Observe the external ears, eyes, mouth, whiskers, feet and tail.
6. Note the body colour and type of body covering.
7. Place the rat down on its back on the dissecting board and pin the front and hind feet to the dissecting board
or secure the rat firmly to the dissecting board with a string.
Note: Your teacher will
show you how to do this.
Dorsal side
Anterior side
posterior side
Ventral side
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Figure1: External features of a rat
http://intro.bio.umass.edu/manual/images/e/e1/ExternalLateral.jpg
Oral cavity
8. Using a scalpel, cut through the angles of the jaws so that the
lower jaw collapses, exposing the buccal cavity (oral cavity)
Teeth
Figure 2- Rat’s skull and teeth
http://www.spflrc.org/user/rats/Teeth.htm
9. Open the mouth of the rat and observe the shape, size and
number of incisors and molars. Note the empty space between
incisors and molars called
diastema. Check if the rat has canines and premolars. (Refer to
figure 4)
Nostrils
10. Find the openings of the nostrils in the roof of the mouth at the anterior end.
The Tongue
11. Locate the tongue and note whether it is attached at the front or at the back.
The Gullet and Glottis
12. Posterior in the mouth is the gullet, the opening to the oesophagus. Just ventral to the gullet is a vertical slit
called the glottis. The glottis opens into the trachea, or windpipe, which branches into 2 bronchi in the lungs.
START OF THE DISSECTION
4th incision
1st incision
2nd incision
3rd incision
Figure 3-Guide to make incisions on the
Figure 4-Internal structures of the rat
ventral side of the rat
http://www.biologycorner.com/bio3/anatomy/rat_head.html
Cutting the skin
Incision 1:
1. Grab a pinch of skin between the hind legs and make a V-shaped cut with a pair of sharp scissors. Hold the
skin with forceps and continue to cut upwards along the mid-line up to the throat. (Figure 5)
Incision 2:
2. Using scissors cut across both the front legs and back legs taking care not to cut the underlying muscles.
3. Using the scalpel and forceps, peel the skin off the rat and pin the skin to the dissecting board.
Caution- To avoid damage to the internal organs, be sure to lift the muscles with the forceps as you
cut. Point the blunt end of the scissors towards the rat .Be careful when using a scalpel
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Cutting the muscles
Incision 3:
4. Begin the vertical incision by lifting the muscle layer with forceps between the rear legs of the frog. Make a
small cut with the scissors
5. Continue the incision up the midline to a point just below the front legs.
Caution: Be careful that you don't cut too deeply. The muscle is thin. It is easy to damage the organs
underneath
Incision 4:
6. Just as you did with the skin, make a sideways incision in the muscle with the scalpel.
Make the first incision between the front legs and the next incision is just above the rear legs.
Caution: Again, be careful that you don't cut too deeply.
7. Separate the muscle flaps from the organs below. Pull back and hold the muscle flaps with the forceps.
8. Use the scalpel to separate the muscle from the underlying organs.
9. Pin the muscle flaps back far enough to allow easy access to the internal organs.
Cutting the chest bone
Incision 5:
10. Expose the chest cavity by cutting around the ribs on both sides.
11. Lift the rib cage and set it aside to expose the chest cavity.
12. Observe the lungs and the heart located in the chest cavity.
Note: The chest cavity is separated from the abdominal cavity by a thin muscular membrane called the
diaphragm.
ORGANS IN THE CHEST CAVITY
Lungs- Refer to figure 5
12. Notice the size and colour of the lungs which are at the anterior end of the chest cavity on either side of the
heart.
Trachea
13. Observe one trachea as it divides into two bronchi going into each lung.
Heart- Refer to figure 6
14. Observe the muscular heart located between the two lungs. It is covered by a thin membrane known as the
pericardium. Carefully cut through the pericardium to reveal the lower part of the heart, the ventricle, and the
upper part, which is composed of two atria. The walls of the atria can be pushed in easily. The walls of the
ventricle are sturdier.
15. Remove the lungs and the heart and place them aside on the news paper.
Figure 5- Lungs and heart of a rat
Figure 6- Heart of a rat
http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb8pg10.htm lungs
http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb8pg12.htm heart Please draw these diagrams
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ORGANS IN THE ABDOMOINAL CAVITY
Digestive system
Oesophagus
16. Using forceps pick up the oesophagus as it comes
through the diaphragm and note where it enters the
stomach. (Refer to figure 3)
Liver
17. Just above the stomach you will see the reddish
brown liver which is divided into five lobes.
Note: Bile produced by the liver is stored in the gall
bladder and helps to emulsifies fats so that they can be
digested by lipases. The rat has no gall bladder
because it does not eat fatty food.
Spleen
18. Note the spleen which is a flattened, reddish organ
lying posterior to the stomach.
Pancreas
19. A glandular organ called the pancreas is found near
the stomach.
Small intestine
Figure 7- Rat’s digestive system
Please draw a similar diagram
http://kentsimmons.uwinnipeg.ca/16cm05/16labm
an05/lb8pg3.htm
20. Carefully stretch the small intestines and examine
the thin mesentery membrane which holds the
intestine together.
21. Separate the intestines from the mesentery
membrane using your fingers and pin it to one side on
the dissecting board.
Note: The small intestine consists of three parts:
Duodenum, jejunum and ileum
Large intestine
22. Now follow the small intestine all the way along until it becomes the large intestine.
Note: The large intestine consists of: colon and rectum. The colon is the longest part of the large intestine,
and it is further subdivided into ascending, transverse and descending colon. The last part of the large
intestine is the rectum.
Caecum
23. At the junction between the small and large intestines, there is a blind pouch called the caecum which is
important in cellulose digestion.
Note: Rats are garnivores, which means they eat seeds and grains. In the caecum are bacteria that are able
to digest the cellulose in the seeds. The food sits in the caecum and essentially rots. The caecum acts like a
fermentation chamber. Humans do not have a caecum because humans are nor pure herbivores. Humans
have an appendix which is a vestigial organ.
Rectum:
24. At the end of the large intestine there is an expanded portion called the rectum, which opens to the exterior
through the anus.
25. Using a ruler Measure the length of the following organs: a) Oesophagus b) stomach d) small intestine e)
large intestine and f) caecum
26. Enter your measurements in table1.
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URINO-GENITAL SYSTEM
Urino-genital system consists of excretory system and reproductive system.
Figure 8- Male urino-genital system
www.tutorvista.com/content/biology/biology-ii…
Figure 9- Female urino-genital system
please draw similar diagrams
A. Excretory system
Kidneys
27. Wash the abdominal cavity with water and note the colour and size of the kidneys located on either side of
the backbone.
Ureter and urinary bladder
28. Trace the ureters leading from the kidney to the urinary bladder which expels urine via the urethra.
B. Reproductive system
i) Male reproductive system
Scrotum and testes
29. Examine the genital area of your rat and identify whether it is a male or a female.
Note: The male rat has two external testes located in scrotal sacs and a penis located between two back legs.
These organs are absent in the female rat.
30. Locate the male’s scrotum and cut longitudinally through the skin to locate the testes where sperms are
produced.
31. Separate the skin from the testes and notice a sac called epididymis lying around each testis. This sac stores
sperms.
32. Find the vas deferens that leads from each testis to the urethra. This duct carries sperms from the testes to the
penis.
33. To either side of urinary bladder you will find two sets of glands. The smaller, round, more posterior ones are
the prostate glands. The larger pair of glands are the seminal vesicles which produce semen.
34. Follow the urethra as it goes through the penis. Both sperms and urine are expelled from the urinogenital
orifice at the tip of the penis.
ii) Female reproductive System
Uterus
35. Observe two V shaped uteri which join and open into the vagina.
Note: Humans have one uterus because they usually produce one baby at a time.
Ovary and oviduct
36. At the anterior end of each uterus there is a short, convoluted oviduct, which opens into a small, round ovary.
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Pregnant female rat
37. If your rat is pregnant, cut longitudinally along two uteri and note the number of embryos.
38. On the exterior, just ventral to the tail, you will find the anus, the vaginal opening and the urethral opening.
39. Observe the reproductive organs of a male rat/female rat dissected by other groups and note the differences.
Cleaning up
40. After finishing the dissection, wrap the rat and the organs in the newspaper and place it in a bag provided for
disposal.
Caution: Do not discard any organs in the sink.
41. Clean your dissecting board and instruments with a detergent and pack up.
42. Dispose of your gloves in the bag provided, and wash your hands thoroughly with an antiseptic solution.
Results: Table 1-Enter your measurements of the digestive organs of the rat
Organ measured
Length (cm)
Oesophagus
Stomach
Small intestine
Large intestine
Caecum
Total length
More than a metre
2. Label the diagrams given below:
1
2
4
3
5
6
7
8
9
A. Label the organs of the digestive
system:
1. Liver
2. Stomach
3. Spleen
4. Duodenum
5. Pancreas
6. Jejunum
7. Ileum
8. caecum
9. Large intestine
10. Rectum
11. Anus
10
11
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B. Label the male urino-genital
system:
1. Kidney
2. Ureter
3. Seminal vesicle
4. Vasdeference
5. Epididymis
6. Testes
7. Scrotal sac
8. Bladder
9. Prostate gland
10. Penis
1
2
8
3
9
4
5
1
0
6
7
1
2
4
5
6
3
7
C. Label the female urino-genital
system
1. Kidney
2. Ureter
3. Bladder
4. Ovary
5. Oviduct
6. Uterus
7. Urethra
8. Vagina
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Discussion questions:
1. What covers the body of the rat? The rat’s body is covered with fur.
2. What type of teeth do rats have? Rats have incisors and molars in both upper and lower jaws. They do not
have canines and premolars. The space between the incisors and molars is known as diastema.
3. Is the rat a herbivore or a carnivore? The rat is a herbivore.
4. What separates the chest cavity from the abdominal cavity?
A muscular diaphragm separates the chest cavity from the abdominal cavity.
5. Describe the location and appearance of the lungs.
The lungs are located on either side of the heart in the chest cavity. They are pink in colour and spongy.
They are divided into 2-3 lobes and covered with a pleural membrane.
6. What is the purpose of trachea and bronchi?
The trachea leads from the mouth cavity to the lungs where it divides into two bronchi. Each bronchus
further divides into bronchioles which end in air sacs called alveoli. Exchange of oxygen and carbon
dioxide takes place by diffusion in the alveoli.
7. Describe the location and structure of rat’s heart.
The heart of a rat is located in the chest cavity between two lungs. It is a muscular organ divided into 4
chambers: the right and left atria and the right and left ventricles.
8. Describe the shape and structure of the stomach? The stomach is a J-shaped muscular organ.
9. Which is the largest organ situated in the abdominal cavity?
The liver is the largest organ in the abdominal cavity.
10. Why doesn’t the rat have a gall bladder?
Since the rat does not eat fatty foods, there is no need for a gall bladder.
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11. What differences did you notice between the small intestine and the large intestine?
The small intestine is narrow in diameter and is very long and coiled. The large intestine is wider in
diameter and is shorter in length than small intestine.
12. Where is the caecum situated? What is its purpose?
The caecum is located where small intestine joins the large intestine. It contains bacteria which help in
digesting cellulose in raw plant food.
13. Why is the caecum absent in human digestive system?
Since humans eat meat and cooked plant food there is no need for caecum.
14. Describe the shape and position of the kidneys? The kidneys are located on either side of the back bone.
They are bean shaped and dark brown in colour.
15. What are the functions of the ureter, urinary bladder and urethra?
The ureter carries urine from the kidneys to the urinary bladder where the urine is stored for a while and
then is expelled by the urethra.
16. What are the functions of the following reproductive organs in the male rat:
a)Penis- expels urine and sperm.
b)Testes- produces the sperms
c) Epidydymus-where sperms mature and are stored.
17. What are the functions of the following reproductive organs in the male rat:
a) Ovaries- produce ova.
b) Oviducts- carry ova from the ovary to the uterus.
c) Uterus- where the fetus develops.
18. What is the difference between human uterus and rat uterus?
Rats have two V-shaped uteri which join together but humans have one uterus
19. How many fetuses did you notice in the rat? 7-8 fetuses were noticed in rat’s uteri. (Answers will vary)
Conclusion: Look at the model of a human torso and describe the differences between the rat and human systems
in the table given below:
Organs and systems
Rat
Human
Outer covering
Covered with fur
Covered sparsely with hair
Teeth
Only incisors and molars are present
Incisors, canines, pre molars and molars
Has a space in between incisors and
are present
molars called diastema.
No diastema present
Digestive system
Has a large caecum
Has no caecum but has appendix
The small intestine is very long
The small intestine is shorter
Reproductive system The female rat has two uteri
Human female has one uterus
Number of fetuses
6-8 develop at a time
One fetus develops at a time
Web sites:
http://members.madasafish.com/~cj_whitehound/Rats_Nest/artwork/clipart/b+w_ship_rat.gif
www.tutorvista.com/content/biology/biology-ii...
http://www.spflrc.org/user/rats/Teeth.htm
http://www.biologycorner.com/bio3/anatomy/rat_head.html
http://www.biologycorner.com/bio3/anatomy/rat_head.html
http://www.biolsci.monash.edu.au/undergrad/erat/erat/Shared/Main/Erat1.htm Interactive site with video
http://educatus.com/main/samples/viewAll.asp?lid=801486&scid=8014860000
http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb8pg5.htm
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