Rat Dissection
Phylum Chordata, Class Mammalia, Rattus norvegicus
Rats share many features will other tetrapods, including the presence of two
pairs of limbs. The body of tetrapods usually is divided in three main parts: head,
trunk, and tail. Two of the most distinctive features of mammals are the
presence of hair and mammary glands. You will be dissecting both male and
female rats, but they can be easily distinguished. Males have a scrotum that
contains the testes in the ventral-posterior portion of the body. In the female,
there are two distinct openings: the urethral opening is more ventral and the
vaginal opening is more dorsal. The mammary glands may be quite visible in
some females. In both sexes, the anus is located at the base of the tail.
Before getting a rat, both you and your lab partner should first put on latex
gloves, and assemble your dissecting tools and tray. Select one rat from the
packages, and place it on your dissecting tray. First, examine the external
morphology of your rat. Is it a male or female?
Look at is head, and locate the long whiskers (or vibrissae) on the sides of the
face. These have a sensory function, and, among other things, allow rats to
immediately detect where the sides of their burrow are. Note the eyes, and
whether or not they have moveable eyelids.
Lay the animal on its back, as you will cut it open on the ventral (belly) side. You
should do this in two steps: first, cut the skin, and then afterwards cut the body
wall. The opening up should be done in a dry dissecting dish (it is easier to
separate the skin from the body wall). Grasp the skin and the abdominal wall
muscles with forceps and pull them up and away from the underlying organs.
Using a pair of scissors, make an incision just to the right of the midline of the
animal. Begin at the posterior end of the abdomen and proceed toward the neck
region. Note that it will be difficult to cut through the ribs: care is necessary to
avoid damaging the organs. Make lateral incisions at the anterior and posterior
ends of the midline incision to further expose the organs.
The diaphragm divides the body in two cavities. In the anterior, thoracic cavity,
you will find the lungs and the heart. In the posterior, abdominal cavity, you will
find most of the digestive system (the liver, the stomach, the intestine, and the
colon), the excretory system, and the reproductive system.
Now that your rat is cut open, cover the rat completely with water. It will be
necessary to pull and pin organs to the side, in order to uncover each organ.
Use the attached diagrams to aid in your dissection and locating the
organs.
Start with the digestive organs. Pin out the different organs to the right and left
(you can cut the mesentery in order to separate the organs). Remove the fat
body if necessary. Turn the liver lobes forward and hold them in place with pins
against, but not through the lobes.
The digestive system of the rat starts with the mouth, equipped with teeth and a
tongue. Food passes from the mouth into the stomach by way of the
esophagus. On the left side of the body, near the stomach (below the
diaphragm), you should see a long, narrow, ductless, deep-red organ, the
spleen, involved in the destruction of old blood cells. From the stomach, the food
moves into the small intestine, where most of the digestion and absorption
occurs. There are two large digestive glands, the liver and the pancreas,
connected to the digestive system by ducts. The liver plays several critical roles
in metabolism, detoxification, glycogen storage, as well as blood protein
synthesis. The pancreas also opens into the duodenum, and functions in
secreting digestive enzymes. The pancreas is tightly embedded within the
mesentery, along with other organs that connects to the duodenum. The small
intestine is followed by the large intestine, or colon, where water and salts are
reabsorbed and where solid wastes are formed. The large intestine contains a
large sac, the caecum.
Now examine the thoracic cavity. Locate the heart. The circulatory system
includes the heart, and a complex network of veins and arteries. The rat’s heart
has two upper chambers, the right atrium (or auricle) and the left atrium (or
auricle), and two lower ventricles. Entering the right atrium are three main blood
vessels (venae cavae) which bring deoxygenated blood back to the heart from all
regions of the body. The right and left superior venae cavae carry blood from the
right and left side of the head and neck. The inferior vena cava may be located
by lifting the heart and carefully separating the lobes of the lung. It carries blood
from the lower part of the body. The inferior vena cava is a large vein running
from the diaphragm to the right atrium.
Respiration occurs in the two lungs, located on the left and right sides. The
lungs are not perfectly symmetrical: the left lung has one lobe whereas the right
lung has four lobes. Breathing is actively aided by the ribs, the diaphragm, and
the chest muscles. The trachea carries the air from the mouth or the nostrils up
to the lungs.
Now make a sketch of your dissected rat, and label the stomach, intestines,
liver, pancreas, heart, and two lungs. Label what sex it is. Keep this
diagram to turn in with your lab report.
Leave the accompanying labeled diagrams on the next pages next to your
dissection for your classmates to observe.
Female Rat Dissection Diagrams
