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Dissection II
Note: The organism you are cutting up today was once alive and vibrant, a living being. Dissection of a
complex organism is not a right, but a privilege. Please be aware that this privilege is one that many students
do not have.
Rodents (from the Latin rodere, “to gnaw”) are the largest order of mammals, comprising about 40% of the
total mammalian species. All mammals, including rats and humans, share some common characteristics:
Body covered with hair, but reduced in some
Circulatory system with a four-chambered heart
Homoeothermic (warm blooded)
Internal fertilization
Young nourished by milk from mammary glands
For many reasons rats are a much maligned group of animals. They carry and spread disease (the Bubonic
Plague or “Black Death” of the middle 14th century was spread by rats), and their toughness, intelligence and
tenacity, especially in large numbers, make them extremely difficult to deal with.
Part I – External Anatomy Briefly study the external anatomy. Locate the ears, whiskers, nostrils, and anus,
which is beneath the tail. What is/are the function(s) of the rat’s tail? Notice the claws and sharp teeth
(incisors). Determine whether your rat is a male or female. Male rats are larger, and will have testicles locate
ventral to the anus.
Before proceeding, record the following information, to be used in your lab report:
Based on your examination of the rat’s external anatomy, identify three features that you could use to
recognize a mammal. Is this a placental, marsupial, or monotreme mammal? How can you tell?
What can you tell about this animal’s lifestyle? Where does it live? What sort of things does it eat?
How is it adapted for its niche in the environment?
Name some related animals – ones that are likely to be in the same family.
Part II – Internal Anatomy Incisions
1. To make the first cut (lengthwise) pinch some skin together on the abdomen and use the scalpel (JUST
THIS ONE TIME) to open the flesh.
2. Use the scissors to continue the cut as shown. Just as in the frog, there are two layers you need to get
through, skin and muscle.
Digestive System
Warning: The Rat’s body has TWO cavities (the Thoracic and Gastrovascular), separated by the diaphragm
muscle. Be careful not to damage the diaphragm or the organs of each cavity. You may have to flush out your
rat’s abdomen under flowing water in the sink to remove the fluid in the gastrovascular cavity.
The abdominal organs may still be covered with a membrane, the peritoneum, (peritoneal membrane) but this
usually comes off with the overlying layers. If necessary drain the body cavity of excess liquids. Another
membrane, the mesentery, surrounds and supports most of the digestive system and its related vasculature
(blood vessels), and in human males is a primary storage site for fat, causing "beer bellies" in some men.
1. Locate the large, multi-lobed liver. How many lobes does the rat liver have? (How many lobes did the
fog liver have?) Don't count the long, narrow spleen, which is lateral to the stomach on the left side.
(The spleen is not a digestive organ, but rather a major storage site for oxygen-carrying red blood cells,
and immune-system white blood cells.) Also locate the diaphragm muscle, which is immediately
anterior to the liver. Notice how the liver is fed by blood vessels that pass through the diaphragm. What
other tubes pass through the diaphragm?
The diaphragm muscle in humans
2. Locate the stomach, which is in a similar location on the left as it is the frog. The pylorus is a muscle in the
stomach found at the border of the stomach and the esophagus, which is continuous with the rat’s mouth. Cut
out the stomach neatly and inspect its inner surface. Locate the pyloric sphincter, a muscular valve, at the
posterior end of the stomach. This valve mediates passage of food into the small intestine.
3. The stomach joins the small intestine via the duodenum. Notice that the small intestine is actually quite
long, and is coiled many times. What is the name of the tissue that keeps all the coils and the associated blood
vessels in place? The small intestine is continuous with the large intestine. Why is the large intestine called
“large” if the small intestine is so much longer?
4. Locate the pancreas within the mesentery. The pancreas controls blood sugar levels with hormone
commands to the liver. Once you feel confident with the organs of the digestive system remove only the
stomach and intestines CAREFULLY.
If you have not located each of the organs above, do not continue to the next section!
Before continuing, also note for your report:
Why is the large intestine called “large” if the small intestine is so much longer?
Based on what you have seen so far, what adaptations does this animal have to its lifestyle? Think of
teeth and intestines, in particular.
Do the best that you can to identify the sexual organs.
In females, the ovaries, which produce egg cells and are small and somewhat peanut-shaped, located just
posterior to the kidneys. The oviducts (AKA Cornuras, AKA Uterine horns) lead from the ovaries to the Yshaped uterus, which connects by way of the vagina to the urogenital opening.
In males, testes, which produce sperm and male hormones, start out up inside the body cavity during fetal
development, then migrate out through two canals into the scrotum. Attached to each testis is an epididymis,
which stores sperm and leads to the vas deferens, a tube that attaches to the urethra, which carries sperm out of
the body through the penis.
If you have not located each of the organs above, do not continue to the next section!
Before proceeding, note the following:
Which structures in the male and female do you think are homologous? In other words, which structures
develop from the same precursor in the embryo?
What do you think the kidneys do?
How would the kidneys need to be modified if the rat lived in water instead of on land?
The Thoracic Cavity – The Heart and Lungs
1. The organs of the thoracic cavity are covered by various thin membranes. These lubricated surfaces allow
the two sets of organs to move vigorously without wearing each other away. The heart is covered by the
2. Locate the diaphragm, which is the posterior wall of the thoracic cavity. As shown in pictures on earlier
pages, the contraction of this muscle increases the space of the chest cavity; the lungs fill with air to fill that
extra space.
3. Locate the lungs and count each of their lobes. Does one lung have more lobes than the other? Remove the
tissue around the heart and identify the following chambers.
Right atrium – receives blood from the superior and inferior vena cavas, the major veins bringing
deoxygenated (though nutrient rich) blood from the body. (FIND THEM!)
Left atrium – receives freshly oxygenated blood from the lungs via the pulmonary vein(s)
Left Ventricle – sends oxygenated blood out the ascending (going up) and then descending (going
down) aorta to the body
5. Note the network of blood vessels across the surface of the heart - these coronary arteries branch off the
aorta to feed the heart muscle, which has huge oxygen requirements! If these small vessels clog up, the parts of
the heart they would ordinarily be feeding seize up, causing a heart attack.
Lab Report:
In your lab report, type out the answers to the questions/observations (STOP SIGNS) you noted during the
dissection. You do not need to do additional research to answer these questions – they should be based solely
on your observations during dissection and your intuition and prior knowledge. Then answer the following
questions, which you might need your book and the following website:
1. Describe several key distinctive features of mammals. (Look at the WHAT MAKES A MAMMAL
section of the website)
2. Explain how the three major types of mammals (placental, marsupial, and monotreme) differ in terms of
reproduction and anatomy (read section 19.8 in the textbook)
3. How are mammals similar to other land vertebrates?
4. Describe how mammals have changed over time. (Look at the HOW DID MAMMALS EVOLVE
section of the website)
(DRAG THE SLIDER ON THE TIMELINE to see the information change)
Read each time period on the timeline and then in your own words, summarize the overall evolution of
5. On the same website, look at the 65 MYA time period. Under the “Mammal Spotlight” pane click
ZALAMBDOLESTES. Compare this ancient mammal to the rat. What features do they have in
common? How are they different?