VCE Biology Unit 1
THE DIGESTIVE SYSTEM OF A RAT
Purpose:
To examine the internal structure of a rat, in
particular the organs of the digestive system, in
order to:
 Determine the sequence in which the "food"
passes through each part of the alimentary
canal.
 Related structure to function where inference
is possible.
 Identify main ways in which the rat's digestive system is similar to and different from our
own.
Materials and equipment
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Rat
Dissecting board
Dissecting kit (scalpel, probe,
fine forceps, blunt forceps)
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Dissecting pins
String
Paper towel
Newpaper
Gloves
If you are unable or unwilling to
complete the rat dissection please
complete the questions by using
the virtual dissection at
:http://www.biologycorner.com/wor
ksheets/rat_dissection06.html
And
the slides on Biology sites
Resource folder on on Quia
Results: As you following the procedure outlines in the section below, fill in results
tables as required.
Table 1: Lengths of the different sections of the alimentary canal of a rat
Section of alimentary canal
oesophagus
Stomach
Small intestine
caecum
colon
Length (cm)
Table 2: Description of the contents of different sections of the alimentary canal of a
rat
Section of alimentary canal
Description of contents if
present
Stomach
Small intestine
caecum
Colon & rectum
Procedure and Discussion Questions:
Part A:
The mouth: Examine the structure and arrangement of the rat's teeth. To do this successfully
you may need to cut down both sides of the mouth with scissors. This will mean cutting
through bone so you will need to apply a little pressure to cut through. Open the mouth up
wide.
Note the type of teeth present and examine the surface of the molars. Examine the structure
and arrangement of teeth in a human. (ask your partner nicely to look at their teeth.)
VCE Biology Unit 1
Question 1
a) What type/s of teeth do rats have?
There is a pair of incisors and 3 pairs of molars in each jaw.
(dental formula 1 0 0 3 )
1003
b) Suggest how each tooth type might help a rat to eat or digest the type of food it
consumes as a part of its diet.
While rats eat just about anything much of their diet is grains and tougher plant matter
l
Incisors used to gnaw through roots and tough plant matter
Molars use to grind up grain and cereals
The dissection
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Secure the rat to the dissecting board
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Beginning the dissection: Use forceps to lift the skin of the lower ventral surface of the
rat and make a single cut with the scissors held horizontally. Place the rounded end of
the scissors forward into the cut and push forward as demonstrated by your teacher.
Hold the edge of the skin with forceps and use a scalpel to cut the connective tissue so
that the skin can be pulled back. Secure the skin with pins. See figure 1.
Figure 1
anus
VCE Biology Unit 1
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Opening up the abdominal cavity: Pick up the abdominal body wall (which is largely
muscle) in the mid ventral line and cut an opening in it with scissors held horizontally.
Place the rounded end of the scissors forward into the cut and push forward as
demonstrated by your teacher. Cut along the bottom (posterior) of the rib cage and pin
out the two flaps of abdominal tissue.
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Identify the liver and the intestine. Be careful not to damage either. Use the pictures
below (Figure 2 and Figure 3) to help you identify structures throughout the rest of the
practical activity.
Figure 2
Figure 3
VCE Biology Unit 1
Question 2
The liver occupies a large proportion of the upper abdomen. It is an extremely important
organ with many diverse functions. What role does the liver play in digestion?
Produces bile; which emulsifies lipids
Question 3
People sometimes refer to the abdominal cavity. How much empty space is there in the
abdomen?
Almost none. It is a filled up cavity.
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With your fingers, carefully pull the lobes of the liver to your left to allow you to identify
and examine the stomach. Feel it with your gloved fingers and note its consistency.
Question 4
a) Describe in words or a drawing the shape of the stomach.
A “J” shaped sack
b) The stomach has several muscular layers. How does this aspect of its structure suit it
to its function?
The contraction and relaxation of the stomach muscles churns the content of the
stomach. While this will incidentally help to break up food a bit (this isn’t really an
important function of the stomach) the main reason for the churing is to mix the food
with the gastric juice to optimize contact between acid, enzymes and food particles
and thus facilitating faster chemical digestion of protein.
The stomach is also used to store food, and several musclular layers make the
stomach a tough and secure storage container.
Just behind the stomach you should be able to locate the spleen. It is a long red organ quite
similar in colour and texture to the liver.
Using a probe or forceps carefully separate the liver and stomach and look for the
oesophagus, a fairly narrow tube which enter the stomach near its middle.
Feel the stomach and compare its diameter with that of the oesophagus.
Question 5
Through which would you expect food to move faster; the stomach or oesophagus? Why?
Oesophagus: no processing occurs here so it is moved quickly along. In the
stomach gastric juice is released, mixed with food and chemical digestion of
protein is initiated. Food needs to be in a soup like consistency before it is released
VCE Biology Unit 1
and this takes time. Thus food moves more slowly through the stomach than
through the oesophagus.
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Examine the region where the stomach joins the small intestine. You will see a
constriction and a thickened ring of muscle, the pyloric sphincter. This relaxes and
widens periodically, then closes again.
Question 6
What is the function of the pyloric sphincter.
To regulate the release of chyme from the stomach into the intestine(a few cubic
cms of chyme at a time)
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Trace the alimentary tract along its length from the stomach to the posterior, shifting the
abdominal organs from side to side to do so. Notice that all of the abdominal organs have
transparent connective tissue - the mesentery- associated with them. Observe the
mesentery.
Question 7
What appears to be the function of the mesentery?
To secure the intestines in place
To hold the blood vessels leading to and from the intestine
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Examine the intestine. The first part of the small intestine (immediately after the pyloric
sphincter) is called the duodenum.
Scattered through the mesentery in the first loop of the duodenum is another organ, the
pancreas. Try to identify the pancreas and any duct leading from it.(ducts will be
extremely difficult to identify.
Question 8
What function might any ducts from the pancreas have?
To transport pancreatic juice(containing enzymes and bicarbonate) to the
duodenum.
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Search the mesentery of the small intestine for large blood vessels.
Question 9
Would you expect to find a denser supply of blood vessels leading to the tissue of the first
part(duodenum) or the last part(jejunum and ileum) of the small intestine? Explain your
reasoning.
Likely to be a richer blood supply to jejunum and ileum as most absorption occurs in
these regions and many nutrients are absorbed into the blood stream.
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Veins lead from the small intestine into the liver.
Question 10
List the substances you would expect to find in the blood in these vessels.
VCE Biology Unit 1
Veins leading from the intestine to the liver will carry blood containing;
monosaccharides like glucose(product of carbohydrate digestion), amino
acids(product of protein digestion) and water.
Also other material not necessarily related to digestion like hormones, oxygen carbon
dioxide, nitrogen, drugs etc etc
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Follow the small intestine. After a time it widens abruptly to become the large intestine.
At its junction with the small intestine there is a bag like organ called the caecum. The
large intestine is comprised of the caecum, colon and the rectum. The colon has three
main sections (ascending, transverse and descending colon).
The rectum follows the colon and terminates at a ring of muscle know as the anus.
Identify these structures.
Notice the shape and consistency of the faeces in the rectum if present.
Carefully free the alimentary tract from its mesentery by pulling it apart gently and freeing
it where necessary with the scalpel. Completely remove the alimentary tract by cutting
through the oesophagus and the rectum (close to the anus).
Stretch the alimentary tract on the bench and measure the length of the distinguishable
sections.
Measure the lengths of each of the following sections of the alimentary canal and
record these results in table 1.
 stomach
 small intestine
 large intestine: caecum
: colon and rectum
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The caecum contains cellulose- digesting microorganisms.
Question 11
Humans do not have a distinct caecum like the rat’s. (though the blind end of the colon is
the caecum its structure is not distinct and it certainly isn’t large). Why does the rat have
a large distinct caecum while, humans do not?
While most rats will eat just about anything (meat, plants etc) their diet is rich in grain
and plan materials. To get sufficient nutrition from this materials microbial
fermentation is needed. In comparison humans have a more varied diet and get
sufficient energy from the food in their diet to not need to digest cellulose. Human
ancestors did not evolved structures for significant microbial fermentation.
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Using scissors cut open parts of the oesophagus, stomach, small intestine and large
intestine. Observe and describe the contents of each section. Record your results in
Table 2
Question 12
a) Describe any difference between the appearance of the material found in the
stomach and the small intestine. Provide a reason for the difference.
The material in the stomach looked like crushed up food. It was light in colour,
grainy in texture and quite dry in consistency compared to the content of the
small intestine which was darker, more watery and less grainy.
b)
(i) How does the consistency of the material in the large intestine change along its
length? The material becomes less fluid and more solid along its length.
VCE Biology Unit 1
c)
(ii)Provide a reason for the change noted.
Absorption of water from the material in the large intestine makes the material become
increasingly more solid and compact along its length
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Rinse a section of the stomach and small intestine under water to clean them. With
gloved fingers feel the difference in thickness of the walls.
Question 13
a) How does the thickness of the stomach wall compare with that of the small intestine?
The stomach wall is thicker than the wall of the small intestine
b) Suggest a reason for the difference noted.
The stomach muscles need to be stronger to move and churn a large volume of food
while the muscle in the wall of the intestine only needs to be strong enough to move
small volumes of food by peristalsis.