Download The Internal Structure of a Rat_Rat dissection

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
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
Description of contents if present
Table 2: Description of the contents of different sections of the alimentary canal of a rat
Section of alimentary canal
Stomach
Small intestine
caecum
Colon & rectum
Description of contents if present
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.)
Question 1
a) What type/s of teeth do rats have?
b)
Relate the type of teeth a rat has to the diet of a rat
The dissection
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Secure the rat to the dissecting board
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.
VCE Biology Unit 1
Figure 1
anus
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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.
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
VCE Biology Unit 1
Figure 3
Question 2
Question 2
a)
Estimate the proportion of the abdominal cavity occupied by the liver
b)
Suggest a reason for its relatively large size.
Question 3
People sometimes refer to the abdominal cavity. How much empty space is there in the abdomen?
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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.
b)
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The stomach has several muscular layers. How does this aspect of its structure suit it to its function?
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?
VCE Biology Unit 1
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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
Suggest a function for the pyloric sphincter.
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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?
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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?
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Search the mesentery of the small intestine for large blood vessels.
Question 9
While it is difficult to see this, the second half of the small intestine has a greater supply of blood vessels than
the first half. Provide an explanation for the distribution of blood vessels noted.
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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.
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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. Compare this with what gut contents
were like before they entered the large intestine.
Question 11
From your observations suggest what must happen to the gut contents as they pass along the large intestine.
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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.
VCE Biology Unit 1
Measure the lengths of each of the following sections of the alimentary canal and record these results in
table 1.
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stomach
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small intestine
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large intestine: caecum
: colon and rectum
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The caecum contains cellulose- digesting microorganisms.
Question 12
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?
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In the koala the caecum is exceptionally long; its
length is approximately 2 m.
Question 14
Suggest a reason why the caecum of the koala is so
large.
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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 15
Describe any difference between the appearance of food in each section of the alimentary tract. Account for
the differences noted.
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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 16
a) How does the thickness of the stomach wall compare with that of the small intestine?
b)
Suggest a reason for the difference noted.
VCE Biology Unit 1