Dissection of the Rat

Introduction
In this laboratory exercise, the anatomy of the rat will be examined in some detail. You
may recall that in your first year biology course you dissected a grass frog and a fetal pig.
You may recognize and remember structures that you learned during that dissection. In this
class, a much more detailed look at mammalian anatomy will be conducted. You will get to
know and love your preserved rat over the course of this dissection.

The classification of the Rat ( Rattus norvegicus)

Kingdom Animalia
..Phylum Chordata
...Subphylum Vertebrata
....Class Mammalia
.....Order Rodentia
......Family Muridae
.......Genus Rattus
........Species norvegicus

The lab books and 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. You are expected to have exhausted all possibilities in attempting to located
structures before asking for assistance. 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. Remember, this is a learning
experience, it is quite permissible to discuss and observe other students' specimens.
Compare you dissection with others, for animals often differ, be sure to look at animals of
the opposite sex, you will be responsible for both sexes on the lab practical.

The specimen you will receive is a preserved double-injected specimen. Double injected
refers to the arteries being filled with a red latex, and the veins being filled with blue latex.
You will notice various incisions on the external surface of the rat where the latex was
injected.

The rat is a vertebrate, which means that many aspects of its structural organization are
common with all other vertebrates, including man. The similarity of structures among
related organisms shows evidence of common ancestry. In a way, studying the rat is like
studying a human. As the leading theme of this lab, ask yourself: for every structure
observed in the rat, there is an equivalent structure in your own body - what is the structure
and where is it located.

As the second leading theme, pay particular attention to the relationships among organs and
groups of organs. Structural parts are not "just there" in random locations. Their specific
layout within the body contributes to making certain functions possible. Therefore, for
every structure seen, you should determine the following:

 What organ system it belongs to

 How it is connected with other components
 Its general function
 Its specific function (if applicable)

Dissection
Dissecting tools will be used to open the body cavity of the rat and observe the structures.
Keep in mind that dissecting does not mean "to cut up"; in fact, it means "to expose to
view". Careful dissecting techniques will be needed to observe all the structures and their
connections to other structures. You will not need to use a scalpel. Contrary to popular
belief, a scalpel is not the best tool for dissection. Scissors serve better because the point of
the scissors can be pointed upwards to prevent damaging organs underneath. Always raise
structures to be cut with your forceps before cutting, so that you can see exactly what is
underneath and where the incision should be made. Never cut more than is absolutely
necessary to expose a part.

Grading
Your grade on this laboratory will be assessed according to the following criteria

 Class Participation (serious approach, proper cleanup and lab safety)

 Lab Checklist or Class Checklist for instructors to check your progress during the
lab
 Quizzes and homework assignments
 Lab Practical Exam (at the end of lab)

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 | 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

Rat External Anatomy

Procedure: Obtained your rat. Rinse it off with water and place it in your dissecting pan
to observe the general characteristics. Make sure you know each of the highlighted words.
The rat's body is divided into six anatomical regions:

cranial region - head

cervical region - neck
pectoral region - area where front legs attach
thoracic region - chest area
abdomen - belly
pelvic region - area where the back legs attach

1. Note the hairy coat that covers the rat and the sensory hairs (whiskers) located on the
rat's face, called vibrissae.
2. The mouth has a large cleft in the upper lip which exposes large front incisors. Rats are
gnawing mammals, and these incisors will continue to grow for as long as the rat lives.
3. Note the eyes with the large pupil and the nictitating membrane found at the inside
corner of the eye. This membrane can be drawn across the eye for protection. The eyelids
are similar to those found in humans.
4. The ears are composed of the external part, called the pinna, and the auditory meatus,
the ear canal.
5. Locate the teats on the ventral surface of the rat. Check a rat of another sex and
determine whether both sexes have teats.
6. Examine the tail, the tails of rats do not have hair. Though some rodents, like gerbils,
have hair on their tails.
7. Locate the anus, which is ventral to the base of the tale.
8. On female rats, just posterior to the last pair of teats, you will find the urinary aperture
and behind that the vaginal orifice which is in a small depression called the vulva.
9. On males, you will find a large pair of of scrotal sacs which contain testes. Just anterior
to the scrotal sacs is the prepuce, which is a bulge of skin surrounding the penis. The end
of the penis has a urogenital orifice, where both urine and sperm exit.
 The Muscular and Skeletal System
of the Rat
Procedure: Skinning the Rat

You will carefully remove the skin of the rat to expose

the muscles below. This task is best accomplished with
scissors and forceps where the skin is gently lifted and
snipped away from the muscles. You can start at the
incision point where the latex was injected and continue
toward the tail. Use the lines on the diagram to cut a
similar pattern, avoiding the genital area. Gently peel the
skin from the muscles, using scissors and a probe to tease
away muscles that stick to the skin.

Muscles are attached to bones by connective tissue called

tendons that adhere to spines, knobs, and ridges on
bones. You will need to refer to the rat skeleton to
determine where the muscles are attached to bones. The end attached to the bone that does
not move during contraction is called the origin. The end of the muscle that attaches to the
bone that does move is called the insertion. The movement caused by the contraction of the
muscle is called the action. Muscles can be easily identified from one another by their
shape and overlap.

Identify the following muscles:

1. Biceps brachii - located on the anterior surface of the humerus.

2. Triceps brachii - located on the sides and back of the upper arm.
3. Spinotrapezius - located across the dorsal thoracic region of the rat.
4. Latissimus dorsi - located posterior (and partially covered) by the spinotrapezius.
5. Biceps femoris - located on the side of the thigh, in two bundles
6. Tibialis Anterior - located on the front of the leg.
7. Gastrocnemius - located on lower leg, bulk of the calf muscle. Attaches to heel by the
Achilles Tendon.
8. External Oblique - located on the sides of the abdomen.
9. Gluteus Maximus - located on the lower back and rear.
10. Pectoralis Major/Minor - located in chest

Pin the muscles listed above on a skinned rat.

Procedure:

Carefully tease away the biceps femoris and gastrocnemius to expose the 3 leg bones:
Tibia, Fibula, and Femur and the small patella (kneecap). You can also see the ligaments
around the knee that attach the bones of the lower leg to the femur and the achilles tendon
which attaches the the gastrocnemius to the ankle.

Note that the joint of the hip is called a ball and socket
Homework: Coloring: Comparing a Human to a Rat Skeleton

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. They are not always visible in the rat.

3. 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.
Procedure: Pin the structures of the head and neck - salivary glands, trachea, and lymph
nodes (if visible)
 The Thoracic Organs

Procedure: Cut through the abdominal wall of the rat following

the incision marks in the picture. Be careful not to cut to deeply
and keep the tip of your scissors pointed upwards. Do not
damage the underlying structures. Once you have opened the
body cavity, you will need to rinse it in the sink.

1. Locate the diaphragm, which is a layer of muscle that

separates the thoracic from the abdominal cavity.
2. The heart is centrally located in the thoracic cavity. The two
dark colored chambers at the top are the atria (single: atrium),
and the bottom chambers are the ventricles. The heart is covered
by a thin membrane called the pericardium. (We will come
back to the heart later.)
3. Locate the thymus gland, which lies directly over the upper
part of the heart. The thymus functions in the development of the immune system and is
much larger in young rats than it is in older rats.
4. The lungs are spongy organs that lie on either side of the heart and should take up most
of the thoracic cavity.

The Abdominal Organs

1. The coelom is the body cavity within which the
viscera (internal organs) are located. The cavity is
covered by a membrane called the peritoneum, which
is very thin and web-like, you may need to use forceps
to remove some of this membrane to see the organs
clearly.

2. Locate the liver, which is a dark colored organ

suspended just under the diaphragm. The liver has
many functions, one of which is to produce bile, which
aids in digesting fat. The liver also transforms wastes
into less harmful substances. Rats do not have a gall
bladder, which is used for storing bile in other animals.
There are four parts to the liver:

median or cystic lobe - located at the top, there is an

obvious central cleft
left lateral lobe - large and partially covered by the stomach
right lateral lobe - partially divided into an anterior and posterior lobule, hidden from view
by the median lobe
caudate lobe - small and folds around the esophagus and the stomach, seen most easily
when stomach is raised

3. The esophagus pierces the diaphragm at a spot called the hiatus and moves food from
the mouth to the stomach. It is easiest to locate where it enters the stomach.

4. Locate the stomach on the left side just under the diaphragm. The functions of the
stomach include food storage, physical breakdown of food, and the digestion of protein.

The outer margin of the curved stomach is called the greater curvature, the inner margin
is called the lesser curvature. You can make a slit in the stomach and see what is inside it.
Most of the contents should be partly digested rat food.

At each end of the stomach (on the inside) is muscular valve. The opening between the
esophagus and the stomach is called the cardiac sphincter. The opening between the
stomach and the intestine is called the pyloric sphincter.

5. The spleen is about the same color

as the liver and is attached to the
greater curvature of the stomach. It is
associated with the circulatory system
and functions in the destruction of
blood cells and blood storage. A
person can live without a spleen, but
they're more likely to get sick as it
helps the immune system function.

6. The pancreas is a brownish,

flattened gland found in the tissue
between the stomach and small
intestine. The pancreas produces
digestive enzymes that are sent to the
intestine via small ducts (the pancreatic
duct). The pancreas also secretes
insulin, which is important in the
regulation of glucose metabolism.

7. The small intestine is a slender

coiled tube that receives partially
digested food from the stomach (via the pyloric sphincter). The coils of the small intestine
are held together by a membrand called the mesentery.

The small intestine has three sections: duodenum, jejunum and ileum, (Listed in order
from the stomach to the large intestine.) The duodenum is recognizable as the first stretch
of the intestine leading from the stomach, it is mostly straight. The jejunum and ileum are
both curly parts of the intestine, with the ileum being the last section before the small
intestine becomes the large intestine.
8. Locate the colon, which is the large greenish tube that extends from the small intestine
and leads to the anus. The colon is also known as the large intestine. Food entering the
colon from the small intestine is controlled by the ileocecal valve. The colon is where the
finals stages of digestion and water absorption occurs and it contains a variety of bacteria to
aid in digestion. The colon consists of five sections:

cecum - large sac where the small and large intestine meet (the ileocecal valve regulates
passage of materials)
ascending colon – food travels upward.
transverse colon – a short section that is parallel to the diaphragm
descending colon – the section of the large intestine that travels back down toward the
rectum.
rectum - the short, terminal section of the colon that leads to the anus. The rectum
temporarily stores feces before they are expelled from the body.

Procedure: Pin the organs of the digestive cavity.

Test Your Knowledge

1. Lies under the stomach and secretes insulin ____________________________
2. The section of large intestine between the ascending and descending colon:
______________________________
3. Connects the mouth to the stomach: ____________________________________
4. Thin membrane that covers the heart: ______________________________________
5. Muscle that separates the abdominal cavity from the thoracic cavity:
___________________________________
6. Destroys old blood cells and lies within the folds of the small intestine:
___________________________________________
7. The lobe of the liver that has an obvious central cleft:
_______________________________________
8. Another name for the large intestine:
_______________________________________________
9. Organs of the respiratory system that lie on either side of the heart:
____________________________________
10. Large organ of the thoracic cavity that lies just under (posterior) to the diaphragm:
_______________________________
11. The last section of the colon, storage of feces:
___________________________________
12. The pouch of the colon that is found just where the small intestine joins it:
______________________________________
13. Valve that regulates the passage of food from the stomach to the small intestine:
__________________________________
14. Thin membrane that covers the organs of the abdominal cavity:
______________________________________________
15. The first section of the small intestine:
_______________________________________________
16. The section of large intestine that is parallel to the diaphragm:
______________________________________________
17. Structure related to the immune system, lies at the top of the heart:
_____________________________________________
18. Valve the regulates passage of materials from the small to the large intestine:
_____________________________________
19. The opening in the diaphragm where the esophagus passes through:
___________________________________________
20. Section of small intestine that comes after the duodenum:
_______________________________________________

Rat - Circulatory System

The general structure of the circulatory system of the rat is almost identical to that of
humans. Pulmonary circulation carries blood through the lungs for oxygenation and then
back to the heart. Systemic circulation moves blood through the body after it has left the
heart. You will begin your dissection at the heart. It is important that you do not cut the
vessels as you carefully
remove any muscles and
surrounding tissue to expose
them.

You may not be able to

locate all these structures
due to the placement of the
heart and vessels, but you
should be able to find a
some of them on the rat and
label the diagram to the
right. The image shows a
human heart, but a rat's heart
has the same structures.

Trace the Flow of Blood

Inside the Heart

1. Blood from the posterior

portion of the body enters the right atrium of the heart through the inferior vena cava and
the superior vena cava. Label these on the diagram.
3. Blood flows from the right atrium to the right ventricle via the tricuspid valve. Label
each on the diagram.
4. Blood is then pumped through the pulmonary semilunar valve and into the pulmonary
trunk where blood travels to the lungs. Label each.
5. Blood then flows through the pulmonary arteries to the lungs where it is oxygenated and
then returns from the lungs to enter the left atrium via four pulmonary veins. Only one of
these is visible on the diagram, a tiny vessel on the right side.
5. Blood goes from the left atrium to the left ventricle via the biscupid (or mitral) valve.
Label each.

Blood leaves the left ventricle of the heart through the aortic semilunar valve and enters the
aorta. The aorta has a visible arch with vessels that lead to the head before the artery
descends into the rat's thoracic cavity. Find the aorta on the rat and label the aorta on the
diagram.

The aorta has four general areas. Locate each of these on your rat.

ascending aorta - the upper part of the vessel that starts at the atrium
aortic arch - the place where the aorta bends to the left.
descending aorta - after the bend, the aorta can be traced toward the diaphragm
abdominal aorta - the aorta passes through the diaphragm and supplies blood to the lower
extremities

Trace the Branches of the Aortic Arch

1. Coronary arteries are located on top of the heart and supply the heart itself with blood.

2. The first visible branch from the aorta is the brachiocephalic artery, it divides into the
right common carotid artery, which supplies the right side of the neck, and the right
subclavian artery, which supplies the right shoulder and arms. Locate the carotid arteries
on your rat, they will be
obvious arteries that travel
up the side of the next.
3. At the most anterior part
of the bend in the aortic arch
is the left common carotid
artery, which supplies blood
up the left side of the neck. If
you are careful you can
follow the common carotid
to where it branches into the
internal and eternal carotid.

4. Immediately to the left of

the left common carotid
artery is the left subclavian
artery, which supplies blood
to the left shoulder and arm.
The sublclavian artery
becomes the axillary artery
as it enters the forearm.
 Procedure: Carefully tease away the muscles and tissue so that the subclavian, the axillary
and the right common carotid can be seen.

Trace The Branches of the Abdominal Aorta

Many of the arteries that branch from the aorta in this part of the rat are small and fragile.
You may not be able to find all of them, but with careful dissection a few can be exposed.
They are often named for the organ or structure the vessel supplies blood to. Find at least
three of the vessels listed for your checkpoint. If you cannot find the vessel, do not check
the box.

1. The first arterial branch from the abdominal aorta (below the diaphragm) is the celiac
artery which branches to arteries that supply the stomach (gastric artery), liver (hepatic
artery), spleen and pancreas (splenic artery) .
2. The second artery arising from the abdominal artery is the superior mesenteric artery,
which is larger than the celiac, and delivers blood directly to the small intestine.
3. The renal arteries are short and lead directly to the kidneys. These are probably the
easiest to locate.
4. Just posterior to the renal arteries are the genital arteries, which lead to the testes or the
ovaries.
5. Farther along the abdominal aorta, you can find the iliolumbar arteries which lead to
the dorsal muscles of the back.
6. Next, the inferior mesenteric artery leads to the intestinal mesenteries.
7. The abdominal aorta gives rise to the caudal artery, which goes on into the tail.
8. The abdominal aorta finally divides to form the iliac arteries, which deliver blood to the
pelvis and hind legs.
9. The iliac arteries lead to
the femoral artery in the leg.

Procedure:Attempt to locate
the vessels above, find at
least three of them.

Trace the Systemic

Veins

1. The left and right

superior vena cava conduct
blood from the upper part of
the body into the right atrium.
Trace these veins from the
atrium until you find the
small internal jugular vein and continues as the subclavian vein.
2. The subclavian vein divies into the external jugular vein and the axillary vein.
3. The inferior vena cava carries blood from the lower part of the body to the right atrium.
The hepatic vein drains the liver and enters the inferior vena cava near the diaphragm.
4. Renal veins drain the kidneys.
5. Genital veins lead from the gonads and enter the inferior vena cava.
6. The iliac and femoral veins drain the legs.
7. The caudal vein drains the tail.

Procedure: Expose the inferior vena cava and the area where it branches into the
femoral and caudal veins.

Urogenital System
The excretory and reprodutive systems of vertebrates are closely integrated and are usually
studied together as the urogenital system. However, they do have different functions: the
excretory system removes wastes and the reproductive system produces gametes (sperm &
eggs) and provides an environment for the developing embryo.

Excretory Organs
1. The primary organs of the excretory system are the kidneys. Locate these large bean
shaped structures located toward the back of the abdominal cavity on either side of the
spine. Renal arteries and veins supply the kidneys with blood.

2. Locate the delicate ureters that attach to the kidney and lead to the bladder. Wiggle the
kidneys to help locate these tiny tubes.

3. Procedure: Remove a single kidney (without damaging the other organs) and dissect it
by cutting it longitudinally. Locate the cortex (the outer area) and the medulla (the inner
area).

4. The urethra carries urine from the bladder to the urethral orifice (this orifice is found
in different areas depending on whether you have a male or female rat).

5. The small yellowish glands embedded in the fat atop the kidneys are the adrenal glands.

**You are responsible for knowing the structures of both sexes. Locate the structures in
your own rat and then observe the structures of the opposite sex from another group's rat.

The Reproductive Organs of the Male Rat

1. The major reproductive organs of the male rat are the testes (singular: testis) which are
located in the scrotal sac. Cut through the sac carefully to reveal the testis. On the surface
of the testis is a coiled tube called the epididymus, which collects and stores sperm cells.
The tubular vas deferens moves sperm from the epididymus to the urethra, which carries
sperm though the penis and out the body.

2. The lumpy brown glands located to the left and right of the urinary bladder are the
seminal vesicles. The gland below the bladder is the prostate gland and it is partially
wrapped around the penis. The seminal vesicles and the prostate gland secrete materials
that form the seminal fluid (semen).

The Reproductive Organs of the Female Rat

1. The short gray tube lying dorsal to the urinary bladder is the vagina. The vagina divides
into two uterine horns that extend toward the kidneys. This duplex uterus is common in
some animals and will accomodate multiple embryos (a litter). In contrast, a simple uterus,
like the kind found in humans has a single chamber for the development of a single
embryo.

2. At the tips of the uterine horns are small lumpy glands called ovaries, which are
connected to the uterine horns via oviducts. Oviducts are extremely tiny and may be
difficult to find without a dissecting scope.
Procedure: Pin the organs of the urogenital system.