Dissection of the Rat
The diagrams available to you are supplemental. You are expected to follow the
directions in this lab. You will be held responsible for being able to locate all the
structures.
Using the available material, instructions and diagrams, most students will be able to
locate many structures for themselves. If after an earnest effort, you cannot find a
structure, ask for assistance. This is a learning process, it is permissible to discuss and
observe other students' specimens.
Compare you dissection with others. Animals often differ, be sure to look at animals of
the opposite sex, you will be responsible for both sexes on the lab practical.
For the exam you’ll need to know…
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What organ system it belongs to
How it is connected with other components
Its function
Glossary of terms
Dorsal: toward the back
Ventral: toward the belly
Lateral: toward the sides
Median: near the middle
Anterior: toward the head
Posterior: toward the hind end (tail)
Superficial: on or near the surface
Deep: some distance below the surface
Sagittal: relating to the midplane with bisects the left and right sides
Transverse: relating to the plane separating anterior and posterior
Horizontal: relating to the plane separating dorsal and ventral
Proximal: near to the point of reference
Distal: far from the point of reference
Caudal: toward the tail end
Pectoral: relating to the chest and shoulder region
Pelvic: relating to the hip region
Dermal - relating to the skin
Longitudinal – lengthwise
Right & Left - refers to the specimen's right and left, not yours
Abdominal Cavity - related to the area below(posterior) the ribcage
Thoracic Cavity - related to the area above(anterior) the ribcage
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 located ventral to the anus.
Part II – Internal Anatomy
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.
3. Make sure to skin you rat completely as shown in the picture below. Expose the muscles of the forelegs
and hind legs as seen above.
Note that the muscle and skin are stuck together. Use the dull “Mall Probe” to tease apart the two types of
tissues to ease your incisions. You may need to be a little creative with your cuts to fully expose the internal
organs. Pin the skin and muscle pieces out of the way.
Rat Anatomy - Head, Thoracic, and
Abdominal Organs
Organs of the Head and Neck
1. Locate the salivary glands, which on the sides of the neck,
between muscles. Carefully remove the skin of the neck and
face to reveal these glands. Salivary glands are soft spongy
tissue that secrete saliva and amylase (an enzyme that helps
break down food). There are three salivary glands - the
sublingual, submaxillary, and parotid.
2. Find the lymph glands which lie anterior to the salivary glands. Lymph glands are circular and are pressed against the
jaw muscles.
3. After you have located the submaxillary glands, remove them to find the underlying structures.
4. The thyroid gland is a gray or brown swelling on either side of the trachea. To locate the trachea you will
need to carefully remove the sternohyoid muscles of the neck. The trachea is identifiable by its ringed cartilage
which provides support. The esophagus lies underneath the trachea, though it is easier to locate in the
abdominal cavity where it enters the stomach.
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?
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.
START HERE ON THE 2nd LAB DATE
Urogenital System
1. Beneath (dorsal to) the digestive organs which you removed, you should find the two bean shape marble
sized kidneys. Medial (toward the middle) on the dorsal body wall are the two main posterior blood vessels:
the abdominal (or dorsal) aorta, an artery, and the inferior (towards the base) vena cava, a vein. Both send
branches to the kidneys.
2. On the kidney’s anterior edge are the adrenal glands which release many hormones. The kidneys are
connected posteriorly to the urinary bladder by ureters. Urine exits the body via the urethra. These tubes may
be hard to find.
3. Determine (if you haven’t already) if you have a male or female rat and go to the appropriate section below.
Females
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.
Males
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.
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
pericardium.
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.
6. In some rats there may be a thymus gland covering the anterior half of the heart, but in older rats this gland
may not be visible.
7. Behind the heart is a stiff whitish tube, the trachea, or wind pipe, which splits into tubes, the bronchii, that
run to the lobes of the lungs. The rings around the trachea are cartilage, and reinforce the tubes in a way similar
to the way that vacuum cleaner hoses are reinforced. The trachea and bronchii bring air into the lungs.