Rat Lab Key
Examine the genital area of your rat. Do you have a male or
Rats are rodents, which have a dental formula of 1/1, 0/0,
3/2, 3/3. Verify that your speciman has this dental
Your next challenge is to remove the skin of the rat. This is
a task that must be done to expose the muscles below.
Begin by cutting the skin from the chin to the base of the
tail. Then from the center line to each ankle. Some small muscles are attached to
the skin, but very few. Run the scalpel between the skin and the muscle. Do not
spend a lot of time trying to be neat. Just get the skin off!
Your rat is "double injected". That means that the arteries have been injected
with a red plastic and the veins have been injected with blue.
Generally, a muscle is attached at each end.
The less movable attachment is called the
origin, the more movable attachment, the
insertion. The fleshy central portion of a
muscle is called the belly. The attachment
may be to a bone by means of a narrow band
of connective tissue called a tendon.
We will be concerned only with some of the more easily identified, superficial
skeletal muscles. Find the muscles labeled on the images below on your skinned rat.
At the posterior angle of the jaw, locate the large masseter muscle, which elevates
the jaw. Note the various neck muscles that elevate, depress and rotate the head.
On the chest, notice the pair of flat, triangular pectoralis muscles, one on each
side of the midline.The pectoralis major originates on the sternum and inserts on
the humerus. The anterior margin of each pectoral muscle is marked by the
disappearance beneath it of the external jugular vein, and by the lateral passage
of a superficial vein from the shoulder.
The lateral border of the pectoral muscle is somewhat disguised in the region of
the armpit by the cutaneous maximus muscle which originates in the region of the
armpit. The cutaneous maximus muscle inserts on the skin. This muscle is used for
shaking the skin.
Originating around the shoulder joint of the scapula (shoulder blade) and appearing
from beneath the distal attachment or insertion of the pectoralis muscle is the
biceps muscle. This muscle inserts on the radius just distal to the humerus. It
flexes and rotates the forearm. On the back side of the arm, antagonistic to the
biceps, is the triceps muscle. It originates from the humerus and the scapula and
inserts on the ulna. The triceps muscle extends the forearm.
Running back from the armpit region on the medial side of the humerus, and
somewhat hidden by the cutaneous maximus, is the latissimus dorsi muscle which
passes posterior and dorsal. The latissimus dorsi originates on the thoracic and
lumbar vertebrae and inserts on the shaft of the humerus. Its action is to pull the
arm backward and upward.
On the foreman (distal to the humerus), note the fleshy muscles near the elbow
which extend as long tendons over the wrist and attach to the digits. Most
muscular control of the digits comes from these muscles. Wiggle your fingers and
clench your fist and watch the play of muscles in your upper forearm. You can see
and feel the tendons on the front of your wrist and the back of your hand.
Like all tetrapods, your rat has two "cavities" in its body. The abdominal cavity,
containing the gut and its associated organs and the thorasic cavity, containing the
heart and lungs. The two cavities are separated by the diaphragm, breathing
Before you begin to open these cavities, remember this:
Your goal is to open both body cavities
without spoiling the internal organs!
The abdominal wall is very thin, but
tough. Begin by making a shallow incision
into the abdominal cavity, just anterior
to the genitals, with the scalpel. Insert
the tip of the scissors into the incision
and carefully extend the cut forward.
The abdominal cavity is not protected by
ribs. As you move forward, be ready for the resistance when your scissors meet
bone. This is a good indication you have reached the thorasic cavity. STOP!
Make a 90o cut to each side. Cut back down each side of the rat and remove the
two flaps of muscle to expose the entire abdominal cavity.
Note the large liver with its five lobes:
the right and left central lobes to the right and left of the midline
the right and left lateral lobes, lateral to the central lobes. The left lobe is
large and overlaps the stomach, while the right is a double lobe overlapping
the right kidney.
The rat has no gall bladder.
The organs of the gut are probably packed tightly in the abdomonal cavity.
Carefully stretch the intestines so the parts beneath them are exposed. Examine
the thin mesentery membrane attached to the small intestine. This membrane
holds the intestine in some type of
The stomach, which is divided into the
anterior cardiac and more posterior
pyloric portion, lies in the left side of
the abdominal cavity just below the liver.
The esophagus, which runs through the
thorax from the pharynx, enters the
cardiac portion of the stomach.
The pyloric sphincter muscle controls
movement of food from the pyloric
portion of the stomach to the small
The small intestine consists of three parts:
The duodenum is in the form of a loop with descending, transverse and
ascending limbs. The pancreas and pancreatic ducts are within the bend of
The jejunum, which is thick-walled, begins where the duodenum turns
posteriorly and terminates with the commencement of the thin-walled and
darker colored ileum.
The ileum runs to the caecum.
The anatomy of the digestive tract is variable in animals, depending upon food
habits. Carnivorous animals have a relatively short digestive tract. Herbivores, on
the other hand, which take in large quantities of difficult to digest cellulose, have
long digestive tracts, often equipped with various internal pouches or expansions
where intestinal microorganisms can aid in cellulose breakdown.
Rodents like the rat and lagomorphs like the rabbit possess a large caecum, a blind
sac which attachs to the gut at the junction of small and large intestines. The
caecum is important in cellulose digestion.
The large intestine consists of the caecum, colon, and rectum. The rectum is that
portion of the large intestine which continues through the pelvic muscles. The
alimentary canal terminates with the anus.
In the rat, as in all tetrapods, there is a sharing of the anterior structures of the
digestive and respiratory system. A special gate-like structure, the epiglottis, is
present at the entrance to the respiratory structures leading to the lungs, which
makes possible a time-sharing arrangement between the two systems. Usually much
of the oral cavity and the nasopharyngeal region perform a respiratory function,
passing air in and out of the lungs, past the raised epiglottis. When food is taken
into the mouth the oral cavity becomes a part of the digestive system. The act of
swallowing, resulting in the passage of food back through the pharynx to the
esophagus, simultaneously causes the epiglottis to close off the respiratory
system, thereby ensuring that food does not enter the passages leading to the
The thorasic cavity is opened by cutting through the center of the ribs. Carefully
extend the cut into the hollow of the throat between the bottom jaws.
The respiratory system consists of two lungs and the passages by which their
internal cavities are connected to the exterior.
The nasal cavities, which are separated from one
another by the nasal septum and from the mouth
cavity by the palate.
The pharynx is divided into the naso- pharynx
above the palate (roof of the mouth), and the oro-
pharynx behind the mouth cavity. The edge of the
soft palate acts as a valve to prevent food from
passing into the naso-pharynx and then into the nasal
cavities during swallowing.
The opening from the pharynx into the larynx, or
voice box, is called the glottis. As mentioned above,
the glottis is closed over, during swallowing of food,
with a gate-like epiglottis, to prevent the passage of
food into the larynx and lower respiratory passages.
The heart is covered by a membrane known as the pericardium. Remember that, in
vertebrates, the blood circulates in a closed system of vessels with the heart
maintaining a more or less constant flow throughout the entire system.
In general, blood leaves the heart and enters one or more large arteries which
branch repeatedly into more and more arteries of gradually decreasing size.
Collectively, these many small arteries are able to carry the same volume of blood
per unit of time as do the smaller number of large arteries (sometimes just one)
leaving the heart. Within the body tissues the small arteries, arterioles, lead on to
smaller blood passages, the capillaries, where nutrients and gases are exchanged
between the blood and the tissue. The portion of this system between the heart
and the capillaries is the arterial system. Upon leaving the capillaries, the blood
again enters small blood vessels, venules, which are part of the venous system. The
venules soon join together with others, forming larger veins that lead back towards
the heart. The venous system thus consists of that portion of the circulatory
system leading from the capillaries of the body tissues and lungs to the heart.
In the rat, as in all mammals, blood flows
through the heart twice during the course of
one complete circuit of the system, once
entering the heart after going through the
lungs, and once after being distributed to the
tissues. The four-chambered heart keeps the
blood from mixing during these two passages.
Because of this, the term double circulatory
system is used in describing the blood
vascular system of mammals.
Functionally, the rat heart may be considered to consist of two separate pumps,
made up of the right and left halves, which are located together in the same organ.
The left half receives oxygenated blood from the lungs and pumps it out to the
body tissues. The deoxygenated blood returning from the tissues passes through
the right half of the heart, which pumps it out to the lungs. Each half of the heart
consists of an atrium which receives the blood and a thick-walled, muscular
ventricle that pumps it out.
Study the heart in position to see its major features. The apex of the heart is
formed by the large, muscular, left ventricle. The right ventricle is not clearly
distinguished externally from the left, but is smaller and has thinner walls. The
pulmonary artery carrying blood to the lungs, leads anteriorly from the right
ventricle. It crosses under and behind the aorta, which arises from the large left
ventricle. The aorta passes forward then loops to the left and back. The left and
right atria lie anterior to the ventricles. They can be seen on either side of the
pulmonary artery and aorta.
Find the two superior venae cavae which drain the anterior body regions, and the
inferior vena cava which returns blood to the heart from the abdominal region. All
of these enter the right atrium. Pulmonary veins from the lungs enter the left
Make a sketch of the heart and label the chambers and major vessels.
The Circulatory System:
Locate the dorsal aorta which may be readily seen where it leaves the heart and
curves to the left and dorsally into the thoracic cavity. Trace the dorsal aorta as it
leaves the heart and locate the following branches:
The innominate artery is the first major branch of the aorta. The
innominate is short and, if followed, subdivides into the right common
carotid artery to the head, and the right subclavian artery to the right
The second branch off the aorta is the left common carotid artery. Both
the right and left common carotids divide into an internal carotid artery
and an external carotid artery which supply the brain and outer surface of
the head, respectively.
The third artery leaving the aorta is the left subclavian artery to the left
forearm. As the aorta passes back through the thorax, it gives off numerous
pairs of small, intercostal arteries to the dorsal body wall.
Continue to follow the dorsal aorta as it
passes through the diaphragm from the
thoracic cavity into the abdominal cavity.
The aorta runs alongside the inferior
vena cava. In the abdominal region, you
can find several major unpaired
branches of the aorta that supply
various parts of the digestive tract.
The most anterior (first branch in
the abdominal cavity after the
diaphragm) of these is the coeliac
artery which supplies the spleen,
pancreas and stomach.
A short distance posterior to the coeliac artery is the superior mesenteric
artery which supplies the small intestine.
The inferior mesenteric artery is the most posterior unpaired branch of
the aorta, arising from it just before it passes into the hind legs and tail
regions. The inferior mesenteric supplies the colon and rectum.
Posterior to the superior mesenteric artery, locate the pair of arteries to
the kidneys, the renal arteries.
Locate the spermatic or ovarian arteries, which usually arise from the
abdominal aorta just posterior to the kidneys. The spermatic arteries supply
the testes, while the ovarian arteries supply the ovaries.
Next, locate a pair of small iliolumbar arteries that arise from the aorta
posterior to the genital arteries and send branches to the dorsal
musculature of the body wall.
The right and left common iliac arteries arise at the posterior end of the
abdominal cavity, where they form the two major terminal branches off the
aorta supplying the legs.
A third terminal branch, which is a continuation of the aorta into the tail, is
the caudal artery.
Make a sketch showing all of these major branches of the aorta.
The venous system consists of those vessels returning blood from the capillaries to
the heart. The hepatic portal system, carrying blood from the alimentary tract to
the liver, consists of the hepatic portal vein and its tributaries from the gut.
Two main veins drain the gut, ultimately joining to form the hepatic portal
vein which enters the liver. The superior mesenteric vein arises as many
small branches from the small intestine and runs towards the liver. It is
joined by the inferior mesenteric vein from the colon.
Three smaller veins the pancreatic, duodenal and gastric veins also enter
the hepatic portal vein before it enters the liver.
Again locate the left and right superior vena cava, which enter the right atrium of
the heart. Each receives blood from the neck and head region of one side of the
body. In the neck region, locate the corresponding external jugular vein, the
largest vein coming from the neck and head region. If you did your dissection of
the thorax carefully you should be able to follow the external jugular into the
thoracic cavity. The external jugular is joined by the subclavian vein from the
forelimb, and a small internal jugular vein from deep in the neck. Once joined,
these veins form the superior vena cava, which continues to the heart.