Rat Dissection Protocol
You will find it helpful to understand the following terms used to describe position or direction
(see Figure 1):
Dorsal - the back or upper surface
Ventral - the belly or lower surface
Lateral - the side
Anterior - the front or head end
Posterior - the hind or tail end
Medial - toward the midline of the animal
Proximal - closer to the midline of the body
Distal - farther from the midline of the body
Superficial - near the surface
Deep - a distance below the surface
External Features
As you study the external anatomy of your specimen, identify the following features and
compare with Figure 2 and Figure 3:
1. Hair - Remember that this is a characteristic of mammals in general. The fur color is white
because an albino lacks pigment.
2. Body regions - The rat’s body consists of several regions: the head, trunk, appendages, and
tail. Compare the length of the tail and the rest of the body. The tail is used for balance and
support, particularly when the animal is sitting erect and using the forelimbs in a manner more
or less similar to the way the arms are used by humans. Notice that the appendages are
adapted for walking.
3. Vibrissae - These are groups of very long hairs found just behind the nose and above the
eyes. They are tactile organs very useful to a nocturnal animal. While rats scurry for food, often
in total darkness, these bristles enable them to maintain contact with walls and other solid
objects that will guide their search.
4. Ear - The long flexible fold of the ear is called the '''pinna'''. It is composed of cartilage
covered with skin. In the living animal, the pinna can be rotated to catch the slightest sound
from almost any direction. This is useful for an animal whose activity is mainly confined to
darkness. The sound caught by the pinna is directed to the external auditory canal. The sound
waves move through this conduit and impinge on the tympanic membrane, causing it to vibrate
in synchrony with the wave. The movements of the tympanic membrane cause the bones of the
middle ear to move, thus converting the sound wave into mechanical energy and transmitting it
to the inner ear, where it is detected by branches of the auditory nerve.
5. Eyes - Notice that the eyes are placed anteriorly in the skull. The central area through which
light enters the eye is the '''pupil'''. It is surrounded by a circular structure called the '''iris'''. In
most animals, the iris is pigmented. By its contraction, the iris can regulate the size of the pupil
and, therefore, the amount of light that enters the eye. However, the albino rat lacks pigments
except for hemoglobin, the red pigment of the blood. Because the rat has no pigment in the
iris, it cannot regulate the amount of light entering the eye. In the living animal, the eyes look
pink because the blood vessels are visible through the non-pigmented iris. This is a
characteristic of all albinos, including humans.
6. External nares - Look for this pair of openings near the tip of the snout. Air can be drawn
through these openings into the respiratory system. While passing through the nasal passages,
gaseous substances in the air can stimulate the very sensitive olfactory receptors.
7. Mouth and incisors - Notice the way in which the lower incisors shear against the upper ones.
8. External reproductive structures and openings - Determine the sex of your rat (see Figure 3).
Compare it with one of the opposite sex. In the male, the '''scrotal sac''' projects posteriorly
between the thighs beyond the base of the tail. Within this pouch lie the '''testes'''. In the midventral line, anterior to the scrotum, find a small sheath of loose skin, the prepuce. Look for the
opening in the apex of the '''prepuce'''. During sexual excitement, the penis can be protruded
through this opening. In the female, find the '''vaginal opening'''. This will be seen between the
anus, at the base of the tail, and the opening of the '''urethra''', which is more ventral. Look for
the '''mammary glands'''. There are usually six pairs–three in the thoracic region and three in
the abdominal region. They will be small and difficult to see in both immature females and in
males.
9. Opening of the digestive and excretory systems - The posterior opening of the digestive
system is the anus, found at the base of the tail. The excretory system opens via the urethra. In
the male, the urethra runs through the '''penis'''. The urethral opening of the female is anterior
to the vaginal opening and the anus.
Skeletal System
The skeletal system supports the body against gravity and provides a protective encasement for
some of the vital organs such as the brain and heart. There are two main parts to the
vertebrate skeleton: the axial skeleton, which is composed of the skull, mandible, vertebral
column, ribs, and sternum; and the appendicular skeleton, which is composed of the limbs and
the pectoral and pelvic girdles.
Axial Skeleton
Study Figure 4 and Figure 5 to compare the rat and human skeletons.
1. Skull - This is composed of several firmly connected bones. Note the large opening at the
posterior and ventral part of the skull. This is the foramen magnum, through which the spinal
cord enters the skull. The mandible is the lower jaw.
2. Vertebral column - This is essentially a beam that acts to support the head and body and to
transfer the weight to the appendages. The five regions of the column have vertebrae that are
modified to suit the various requirements of support and movement. Most mammals have a
freely movable neck with 7 '''cervical vertebrae'''. Look carefully at the first two cervical
vertebrae–the '''atlas''' and the '''axis'''. Notice how they are modified to form a sort of
universal joint for head movements. Rats and mice have 13 '''thoracic vertebrae''' to which the
ribs are attached. Notice the large transverse process of the '''lumbar vertebrae'''. There are 3
or 4 sacral vertebrae that are fused to form the sacrum. The '''sacrum''' articulates with the
pelvic girdle and transfers the weight of the body to the hind legs. Weight is transferred to the
front legs by a muscular sling extending from the distal end of the ribs to the pectoral girdle.
Rats have 26 to 30 '''caudal vertebrae''' in their tail.
Appendicular Skeleton
1. Pectoral girdle - This is the arch that supports the front limbs. Only muscles and ligaments
connect it to the axial skeleton. The largest component of the pectoral girdle is the bladeshaped '''scapula''', commonly called the shoulder blade. This articulates with the clavicles,
commonly called the collarbones.
2. Front limb - The '''humerus''' is the bone of the upper forelimb. It connects to the scapula
with a “ball and socket” type of joint. Swing your arm around in a circle. Notice the freedom of
movement this type of joint allows. The bones of the forearm are the '''radius''' and '''ulna'''.
The wrist is composed of small '''carpal bones'''. The metacarpals are the bones of the hand and
the '''phalanges''' (singular, phalanx) are the bones of the fingers.
3. Pelvic girdle - This arch supports the hind limbs. It consists of three bones: the '''ilium''',
'''ischium''', and '''pubis'''. These three bones meet and fuse at the hip socket.
4. Hind limb - The thigh bone is the '''femur'''. It connects to the pelvic girdle with a ball and
socket joint. The bones of the lower hind leg are the '''tibia''' and '''fibula'''. The '''tarsals''',
'''metatarsals''', and '''phalanges''' are the bones of the ankle, foot, and toes, respectively.
A Comparative Study
Specialized structural characteristics of animals are related to their environments. Striking
skeletal modifications can be seen among vertebrate animals and are often indications of their
mode of life or ecological niche.
Study the fish skeleton and determine how it is adapted to its way of life. The head and trunk of
a fish usually move as a unit. Are the first two vertebrae the same as in the skeletons we have
looked at that permit free movement of the head and neck? Notice that a sternum is missing.
Also, the appendicular skeleton is simple and involved with fin movement.
The snake is specialized for slithering along the ground. A snake skeleton has no pectoral or
pelvic girdles. This is an example of degenerate specialization. What has happened to the limbs
of the snake?
The skeleton of the bird provides an opportunity to study some adaptations for flight. The
bones are hollow and thin, making the skeleton lighter relative to the rest of the body weight
than it is in most mammalian skeletons. According to physical principles, a tube is more
resistant to certain types of stress than is a rod of equal weight. Most of the bony substance of
the bird's bones is at the periphery, forming a tubelike structure and providing better structural
support. Further strength is added to these bones in the form of internal struts. Similar to struts
found inside the wing of an airplane, these provide strength without adding too much
additional weight.
Compare the length of the neck and the articulation between the skull and the vertebral
column of the bird, rat, and human. The neck of the bird is long and there is a good deal of
freedom of movement between the head and neck. This movement is important because the
bill of the bird is used for such varied activities as feeding, nest building and defense.
Now compare the trunk regions. Notice that, in the bird, this region is shortened and the
vertebrae are firmly united, providing a strong fulcrum for the action of the wings. This area
also provides a strong point for the attachment of the pelvic girdle and the hind legs. This is
important because, when the bird is on the ground, its hind limbs must bear the entire weight
of the animal. What structural modification has occurred in the sternum of the bird? Notice the
large ventral keel that provides a broad area for the attachment of flight muscles.
The appendages have also been modified. The bones of the wing are homologous with those of
the arm of other vertebrates. The fifth toe of all birds, and the fourth of some, have been lost.
In most species of birds the first toe is turned back. This serves as a prop that increases the
grasping action of the foot during perching. Notice that the limb bones have become fused in
some places. This decreases the probability of dislocation and injury when the bird is landing.
The bat is a mammal adapted for flight. Notice the modified forelimbs. The fingers are greatly
elongated and joined by a membrane that extends to the sides of the body and legs, as well as
between the legs and the tail. Notice that the shoulder girdle is more well developed than the
pelvic girdle. The sternum usually has a keel for muscle attachment.
Some of the structural modifications accompanying an erect posture can be seen in a
comparison of the rat and human skeletons. The major change is in the balancing of the skull
atop the backbone, instead of slinging it in front of the backbone. Notice that the human
vertebral column has a double curvature instead of a single arch as is seen in the quadruped
rat. This keeps the head and shoulders balanced over the hips. The hipbones of the rat are long
and narrow. Compare these to the short broad bones of the human hip. This modification
found in humans helps to support the internal organs. Look now at the feet of the skeletons. All
vertebrates except humans stand either on the toes or the flattened sole. Humans stand with
the heels, toes and outer border of the foot pressed to the ground. The arch formed by the rest
of the foot provides a means of distributing the weight over a triangular area much larger than
the base of the leg bones.
Digestive System
The digestive system is a long tube inside the animal, with the mouth as the opening at the
anterior end and the anus as the opening at the posterior end. The process of digestion, the
enzymatic breakdown of complex food substances into their simpler components, occurs in the
lumen (cavity) of the digestive tube. The small molecules resulting from digestion are then
absorbed by the cells lining the gut and transferred to all the other cells of the body via the
circulatory system. Within the cells, these molecules may be burned to release energy for
cellular activity, built into the structural elements of the cell, or stored for later use. The
undigested material passes along the gastrointestinal tract and out of the anus as feces.
1. Mouth - The mouth is the most anterior part of the digestive system. Within the mouth, the
food is ground up by chewing and mixed with saliva, which contains carbohydrate-splitting
enzymes and lubricating mucus.
2. Salivary glands - Make an incision on one side of the body from the region of the shoulder to
the angle of the jaw. Continue cutting along the lower jaw to reveal the salivary glands. There
are three pairs of salivary glands. The largest lies just behind the ear and extends to the
ventrolateral surface of the neck. The other glands are more ventral and extend anteriorly
under the lower jaw (Figure 6). The saliva, as previously mentioned, contains enzymes, which
begin the digestion of carbohydrates, and mucus, which moistens food and sticks it together to
facilitate swallowing.
3. Floor of the mouth and pharynx - Locate the '''tongue'''. It plays a role in the swallowing
response. The food moves from the mouth into a chamber shared by the respiratory system
called the '''pharynx''' and on into the '''esophagus'''. The esophagus can be seen under the
trachea (a tube recognized by its cartilage rings) in the neck region.
4. Viscera - The other organs of the digestive system are located within the body cavities. All the
organs of the body cavity, particularly those of the digestive system, are called the viscera.
These organs are supported from the dorsal body wall by mesenteries. The wall of the body
cavities and the organs are lined with a thin, moist membrane, the peritoneum.
Opening the Body Cavity:
To continue your study, you must expose the viscera.

From the cut at the throat of the rat, cut down the center of the rat until you reach the genitals.

Be careful to only cut the body wall.

At the posterior end of your cut (by the genitals), cut laterally so that you are creating two flaps
of skin that will open from the center of the rat.

Feel for the bottom of the rib cage.

Make similar lateral cuts just below the rib cage.

You should now be able to open the flaps of skin.
Locate:
Liver - Locate the liver first, because it is an obvious landmark. It is the large, reddish brown
mass that lies immediately posterior to the diaphragm (the muscle dividing the thoracic and
abdominal cavities). The liver has a great number of functions. However, its role in digestion is
to produce bile, a substance that emulsifies fats (breaks them into minute droplets), making
them easier to digest. In humans, the bile is stored in the gall bladder before being released
into the small intestine. However, the rat lacks a gall bladder. Therefore, the bile is released
through a duct directly into the small intestine, where it acts.
Stomach - The food passes from the esophagus into the stomach. Locate this bean-shaped sac,
which is partially covered by the left lateral lobe of the liver. By carefully removing this lobe you
may easily observe the entire stomach and esophagus. One function of the stomach is to act as
a storage organ so that fewer and larger meals can be consumed. Within the stomach, food is
coated with mucus and digestion of proteins begins. The cells lining the stomach secrete the
protein-splitting enzyme pepsin. This enzyme is active only in an acidic environment. The cells
of the stomach lining also produce hydrochloric acid, which activates the pepsin. The muscular
walls of the stomach churn the food, mixing it with enzymes and helping to fragment it. During
this time, circular muscles, called sphincters, located at each end of the stomach, prevent the
food from escaping.
Small intestine- Examine the stomach and locate the place where it joins with the small
intestine. Without tearing the mesentery that binds the coils together, trace the small intestine
to its junction with the large intestine. Most of the digestion and the absorption of the products
of digestion take place in the small intestine. Glands in the wall of the small intestine secrete
enzymes for the breakdown of both proteins and carbohydrates. Secretions of the pancreas
enter the small intestine and contain enzymes for the breakdown of fats, carbohydrates, and
proteins. We have already noted that the digestion of fats is aided by bile. Although bile is not
an enzyme, it helps digestion by emulsifying the fats. The alkaline environment of the small
intestine inactivates the pepsin from the stomach. Enzymes from the small intestine continue
the digestion of protein. Epithelial cells lining the small intestine absorb the digested
substances and pass them on to the blood capillaries or the lymphatic system for distribution.
Pancreas - The pancreas is an irregular mass of brownish glandular tissue in the mesentery
dorsal to the stomach. We have already discussed the digestive function of this diffuse gland. It
also produces a hormone, insulin, which passes directly into the circulatory system and is not
involved with digestion.
Caecum - At the junction of the small and large intestine you will find a blind sac, the caecum or
caecum. The caecum is a place where ingested cellulose is diverted from the main track and is
digested by microbial fermentation. Rats and lagomorphs (rabbits, hares) will produce a special
feces formed from the caecum product. They will then ingest this feces again, to digest it a
second time. This behavior is called coprophagy. Not all mammals have a caecum. Humans do
have a short caecum terminating in the appendix. The appendix may serve a function in the
immune system, making antibodies. The human caecum provides space for digestion, but does
not have the microbes for cellulose fermentation.
Large intestine or colon - Running from the caecum, the colon ascends, crosses the abdominal
cavity, and descends again. The colon connects posteriorly with the poorly differentiated
'''rectum''' of the rat. The rectum connects the colon and the anus. The primary function of the
large intestine is to absorb most of the water of the digestive secretions, conserving it for use
within the body.
Spleen is not part of the digestive system. However, it is an obvious structure, and this is a
convenient time to locate it. It is a dense, red, elongate structure located on the left side of the
rat’s body. It is part of the circulatory system and plays a role in the production and destruction
of blood cells.
Review Questions
Skeletal System
Describe at least 3 differences between the rat and the human skeleton that evolved with an
upright posture.
Describe the differences in the wing structure of the bat and the bird.
Describe the differences in the articulation between the skull and the vertebral column of a rat,
bird, and human.
Describe the differences in the trunk region of a cat and a bird. What modifications for flight
can be seen in the bird?
What modifications for way of life can be seen in the in the skeleton of the snake?
What modifications for way of life can be seen in the in the skeleton of the fish?
Digestive System
Trace the path of food through the digestive system, naming each structure.
Describe the observed anatomy of each of the digestive structures you observed and relate the
anatomy of the structure (its form) to its function using (Figure 8 and Figure 9)
Respiratory, Circulatory, and Urogenital Systems
Respiratory System
Food materials are broken down into simpler subunits by the digestive system, and these
simpler substances are delivered to the cells by the circulatory system. Within body cells,
simple molecules may be broken down to release energy. Some of this energy is used to form
ATP, which can be used to supply energy for cellular activities. The process of breaking down
food molecules to form ATP is called cellular respiration. Part of this energy transaction is to
pass hydrogen ions along the electron transport chain. At the end of the chain, the hydrogen is
joined to its final acceptor, oxygen, forming water (H2O). In the absence of oxygen, the
hydrogen transfers are prevented and no ATP can be formed. This is the reason that oxygen
must be delivered to the cells. In many invertebrates, the size of the body is small enough for
oxygen to reach the cells by diffusion. In larger animals such as rats and humans, however, a
respiratory system has evolved, creating an internal surface area large enough to allow oxygen
to diffuse into and be carried to the cells by the circulatory system. The respiratory and
circulatory systems must function together.
It is now time to cut open the thoracic cavity on your specimen:

You will now make the lateral cuts at the top of the chest.

Even though you already have the cuts at the bottom of the rib cage, the body wall may not
peel back because it is attached at the bottom of the rib cage to the diaphragm.

You will have to carefully separate the body wall from the diaphragm.

It will also be necessary to break the ribs close to where they join at the sternum.

Cut the rib bones with the points of the scissors always angled slightly upwards, so that you do
not damage any internal organs.

Now that the left and right rib cages are no longer joined, carefully open them to expose the
thoracic cavity.
Trachea - To reach the lungs, air travels through the nose or mouth to the pharynx and then to
the trachea. Find the trachea in the neck region. You will notice that it has C-shaped rings of
cartilage to prevent it from collapsing as air rushes through it. Note that in the anterior region
the cartilaginous bands are replaced by a single, larger housing of cartilage. This initial portion
of the trachea is the larynx or voice box, within which lie the vocal cords. The vocal cords are
folds of epithelium that vibrate, producing sounds as air passes over them. The small brownish
glandular mass found on either side of the anterior end of the trachea is the thyroid gland. It is
not part of the respiratory system, but you may have wondered what it was. In the thoracic
cavity, the trachea branches into the right and left bronchi. The forking of the trachea occurs
immediately dorsal to the aorta and cannot be seen at this time. Each bronchus leads to a lung
where it branches further into bronchioles.
Lungs - Identify the lungs and note that there are four lobes on the right lung and only one on
the left lung. Within the lungs, the bronchioles carry the air to their endings, tiny air sacs.
Inside, these air sacs are further partitioned into chambers called alveoli. This greatly increases
the surface area available for gas exchange. In the human, the internal surface area of the lung
is equal to about half the area of a tennis court. In fact, our lungs have a greater surface area
than our skin. The alveoli are only one cell layer thick and have capillaries immediately outside
of them. The gas exchange occurs across these moist surfaces by simple diffusion. The oxygen,
being in higher concentration in the inspired air, diffuses across the alveolar and capillary walls
and is picked up by the red blood cells of the blood. The concentration of carbon dioxide is
higher in the blood that has carried it from the cells, where it was produced by the oxidation of
foodstuffs. The carbon dioxide will diffuse into the alveoli and leave the body during exhalation.
Diaphragm - Notice that the lungs are located within closed cavities, the thoracic or pleural
cavities, which are lined by membranes, called the pleura. The parietal pleura line the wall of
the cavity (body wall, diaphragm, and median septum) and visceral pleura line the lungs. Find
the diaphragm, a muscular wall separating the thoracic and abdominal cavities.
The fact that the lungs lie within closed cavities is critical to the mechanism of breathing. During
inspiration, the size of the chest cavity is increased, creating a negative pressure or vacuum that
draws air into the lungs. This action is accomplished by the contraction and flattening of the
dome-shaped diaphragm and the contraction of the muscles between the ribs. The contraction
of the rib muscles raises the ribs, thus increasing the size of the thoracic cavity. Air fills the
lungs. During expiration, the diaphragm and rib muscles relax and decrease the size of the chest
cavity, forcing the air out. The maintenance of a closed chest cavity is, therefore, essential to
the breathing mechanism. This is similar to the way a bicycle pump works. When you pull out
the handle, a piston is drawn back inside the cylinder, increasing its volume and creating a
negative pressure that draws air into the pump. Pushing in the handle moves the piston so that
the internal volume is lessened and air is forced out of the pump. Obviously, a hole in the side
of the pump would prevent it from working.
Circulatory System
The Heart
The rat heart is small so that the details of its structure are difficult to observe.
The heart is basically two pumps: one circulates blood to the lungs for oxygenation and the
other circulates blood to the body cells. Each side of the heart has an atrium (the anterior
chamber that receives blood from a vein) and a ventricle (a thicker walled chamber whose
contractions drive the blood into an artery) (Figure 7).
Orient the heart so that the thicker-walled left ventricle is on your right. This is the position of
the heart when it is exposed from the ventral side of the animal, as your rat’s heart is now.
Right side - The right side of the heart circulates blood to the lungs. The ''right atrium'' receives
blood returning from the body via two large veins called the ''superior and inferior vena cava''.
When the right atrium contracts, blood is forced into the ''right ventricle''. The contraction of
this chamber pushes the blood into the ''pulmonary artery'' and on to the lungs, where it picks
up oxygen.
Left side - The oxygen-rich blood is returned from the lung to the ''left atrium'' of the heart
through the ''pulmonary vein''. Contraction of the left atrium moves the blood into the ''left
ventricle''. The powerful contraction of the muscles of the left ventricle drives the blood out of
the heart through the ''aorta'' to the rest of the body.
Valves - The effectiveness of these contractions is increased by the presence of valves that
prevent the backflow of blood. Locate the ''semilunar valves'' between the aorta and the left
ventricle. A similar set of valves is found at the junction of the pulmonary artery and the right
ventricle. These valves prevent blood from flowing back into the ventricles from the arteries.
Between each atrium and its corresponding ventricle is a set of ''atrioventricular valves''. When
the ventricles contract, blood is forced against these valves, forcing them shut and preventing
the flow of blood back into the atria. Do you see the heartstrings? Technically called the
''chordae tendineae'', these prevent the valves from flapping back into the atria, which would
permit the backflow of blood.
Blood Vessels
The circulatory system performs the essential duties of transporting oxygen and nutrients to
metabolizing body tissue and carries off carbon dioxide and other metabolic waste for eventual
removal from the body. The rat has a closed circulatory system, which means that the blood
remains within a system of vessels through which it is pumped by the heart. A vessel that
carries blood away from the heart to a capillary bed is an ''artery''. A ''vein'' carries blood in the
reverse direction, from the capillaries back to the heart. A ''portal vein'' carries blood from one
capillary bed to another. Note that normally an artery carries oxygenated blood, and a vein
carried deoxygenated blood. However, the pulmonary artery and vein are backward in this
regard because they carry the blood between the heart and lungs.
Veins
Begin your dissection by locating the rat’s heart. The vessels of your specimen have been
injected with latex to make them easier to locate. Those with blue latex are veins. The arteries
contain red latex.
Vena cava - The vena cava returns blood from the body to the right atrium.
Pulmonary vein- This vein enters the left atrium with blood from the lungs.
Internal and external jugular veins - locate these veins in the neck region. These carry blood
from the head region back toward the heart.
Arteries
Aorta - Emerging from the left ventricle, near the midline of the heart, is the '''aortic arch'''. It
bends around dorsally, giving rise to three branches very near its origin.
The first is the ''innominate artery''. This vessel divides again very shortly to form the ''right
subclavian'' and ''right common carotid''.
The next branch of the aorta is the ''left carotid".
The third branch is the ''left subclavian''.
The aorta continues dorsally, giving rise to the other arteries of the body.
Urogenital System
The urogenital system comprises the excretory and reproductive systems. The relationship
between the systems is closer in males than in females. As you begin your study of this system
compare your findings with Figure 10 and Figure 11.
The Excretory System
The excretory system functions in removing the nitrogenous waste products of cellular
metabolism, as well as in removing a number of other materials that may be present in the
blood in excess of the body’s needs. However, it conserves materials that are not in excess. In
this way it plays a vital role both in excretion and in maintenance of a fairly constant internal
environment.
Kidneys - Find the pair of bean-shaped kidneys lying against the back muscles on the dorsal side
of the animal. These are often embedded in fat. The blood is brought into the kidney for
processing and taken out of the kidney via the vessels that can be seen near the medial
indentation. Remove some of the fat from around the kidneys and locate the adrenal glands.
These are endocrine glands, not involved in excretion and found just anterior to the kidney. The
hormone they are most famous for is adrenaline (epinephrine). This substance has a variety of
effects on a body, preparing it for emergency situations. The cortex of the adrenal glands
produces other hormones that can be categorized into three groups: (1) regulators of
carbohydrate and protein metabolism, (2) regulators of salt and water balance, and (3) sex
hormones.
Ureter - The material removed from the blood by the kidney is gathered internally in collecting
ducts that empty into the larger ureter through which the urine leaves the kidney. Locate this
duct as it passes from the indentation on the medial side of each kidney close to the blood
vessels. The path of the ureter is toward the posterior end of the animal.
Urinary bladder- This sac stores the urine prior to its passage outside the body. The organ is
usually contracted in a preserved specimen and appears as a small pear-shaped muscular sac.
Urethra - This is the tube the urine flows through as it exits the body. It can be seen if the
mesenteries holding the bladder are dissected away and the bladder is displaced dorsally. In the
male, the urethra extends through the penis. It will carry sperm from the testes and secretions
of reproductive accessory glands as well as the urine. In the female, the urethra opens to the
exterior separately from the reproductive system.
The Reproductive System
The Male
Scrotum - You have already identified the scrotum, the sac housing the testes. In the
nonreproductive season, the testes may be withdrawn into the abdominal cavity.
Testes - Cut open the scrotum to reveal the testes. Slit one testis open and notice that it
contains a mass of tubules. These are the ''seminiferous tubules'', where the sperm are
produced. In addition to sperm, the testis produces male sex hormones.
Epididymis - The sperm pass from the seminiferous tubules into this highly coiled tubule. The
epididymis covers both ends and the lateral surface of the testis.
Vas deferens - This tubule carries the sperm from the epididymis to the urethra. It passes
anteriorly and joins the urethra very close to the spot where the latter leaves the urinary
bladder.
Urethra - You have previously identified this tubule.
Accessory glands - The secretions of all the accessory glands form the seminal fluid that carries
the sperm during ejaculation, activates the sperm, provides some nutrients for them, and
contains substances that help to neutralize the acidity of the vagina. The products of the
coagulating gland probably contribute to the vaginal plug, which is formed in the female rat
after copulation. There are several accessory glands, we will only concern ourselves with
"seminal vesicles" and the "prostate gland".
a. The vesicular glands and the coagulating glands comprise the "seminal vesicles". They are
shaped like wings and are located anterior to the urinary bladder.
b. The ''prostate gland'' is large and has lobes. Look for it where the urethra and vas deferens
join. It usually surrounds the bladder.
Penis - You have already identified this structure. Its erectile tissue is composed of three
cylindrical masses of sponge-like vascular tissue. During sexual excitement, the arteries leading
to this tissue become dilated and the veins draining it become constricted. This causes the
spongy tissue to become engorged with blood, making the penis hard and erect. This rigidity
facilitates insertion into the female’s vagina. In addition, the rat has a rod of connective tissue
called a baculum or “penis bone” enhancing this rigidity.
The Female
Uterus - Locate this structure in the region posterior to the kidneys. In the rat, the uterus is
actually divided into two complete uteri, which open separately into the vagina. The two uterus
system is know as the unterine horn. Open the uteri and examine any embryos you find.
Ovaries - These are located near the anterior end of the uterus. They are often embedded in
fat, which must be carefully dissected away. The ovaries produce the ova, which, if fertilized,
will develop into embryos.
Oviducts - Each oviduct is a highly coiled tubule found on the surface of an ovary. The ova pass
from the ovary into the oviduct, which carries them to the uterus. The ovary is actually
surrounded by the funnel-shaped opening of the oviduct, so that the eggs are released directly
into this tubule.
Figure 1. Orientation terms as depicted on a rat (lateral view).
Figure 2. Lateral view of a rat, indicating major external features.
Figure 3. Ventral views of a male and female rat, noting external reproductive features.
Figure 4.
Figure 5.
Figure 6.
Figure 7. Placement of incision lines to enter the rat. The anterior-most lateral cut will be made
next
Figure 8.