rat dissection

					                  OH RATS! 
       •   Learn terminology necessary to examine animal anatomy
       •   Examine major anatomical structures of mammals using a rat model
       •   Compare structures of rat anatomy to human anatomy
       •   Understand functions of important mammalian structures and systems
       •   Review functional processes in mammalian systems

The rat is the most widely used animal in experimental studies. These include areas of disease
pathology, biological assay, and nutritional as well as behavioral research.

Both the rat and man belong to the class of vertebrates known as Mammalia. Mammals are the most
highly developed animal forms. While our major dissection subject is the rat, we will be making
reference to man through the lab guide.

Mammals range in size from minute shrews (weighing 2 grams) to giant blue whales (weighing
115,000kg). Besides mammals, the vertebrates (backboned animals) include: fish, amphibians,

There are several mammalian characteristics that set them apart from other classes of vertebrates.
Mammals are covered with fur or hair, and have mammary glands, which are milk-producing glands in
the female to nurse the young.

Live birth is a characteristic of mammals, although there are egg-laying exceptions such as the duck-
billed platypus, and echidna. Marsupials are mammals that give birth to live young born at such an
early stage of development they require further maturation in a pouch where they attach to nipples of
the mammary glands.

Rats, human, and many other species of mammal are true placental mammals grouped into the subclass
Eutheria. The embryos are retained in the uterus and are nourished by the placenta until they emerge
highly developed. Most mammals can walk and even run within a few days of birth. Man is the
mammal that remains helpless for the longest period of time following birth.

Generally speaking, orders of mammals are recognized rather easily by their external appearance.
Some of the external features separating mammals into orders are number of digits on the feet, method
of locomotion, and the characteristics of teeth.
Classification of the rat:
       Kingdom – Animalia
       Phylum –             Chordata
       Sub-Phylum –                Vertebrata
       Class –                              Mammalia
       Order –                                     Rodentia
       Family –                                           Muridae
       Genus –                                                     Rattus
       Species –                                                          norvegicus
       Variety –    albinos (white lab rat – developed as a result of repeated selections during the
                    past 90 years) Albinos differ from the wild rat in being less aggressive, more
                    readily handled without struggling. The lab rats fail to achieve the size or
                    weight of its wild cousin, having smaller brains, spinal cords, and adrenal

Breeding and Lifespan:
The female rat can begin breeding at 3 months of age. The average litter consists of 8 young and a
new litter can be produced every 21-25 days. This time frame is known as the gestation period during
which the embryo develops in the uterus. The newborn young are pink skinned, hairless, and helpless.
They are blind, deaf, have no teeth, and are unable to walk. They are suckled for about 3 weeks. The
life span of a lab rat is approximately 36 months with the females generally outliving the males. At
full maturity the rat weighs approximately 300 grams.

Structure and Function:
Note that your primary interest is in learning about your own structure and function through the
exploration of the rat. Most organs and tissues of the rat are structurally and functionally similar to
those of human. Even the names are quite similar – most often being identical.

Anatomical Terminology:

Directions of Positions used for the rat
Anterior / Cranial - toward the head                Proximal –             nearer     the    point    of
Posterior / Caudal - toward the tail                                       attachment to the trunk
Superior -             toward the head                                     (referencing appendages)
Inferior -             toward the tail              Distal -               farther away from the point
Dorsal -               toward the backbone                                 of attachment to the trunk
Ventral -              toward the belly                                    (referencing appendages)
Lateral -              toward the side              Supine -               ventral side up
Medial -               toward the midline           Prone -                dorsal side up
Superficial -          more external                Plantar -              The sole of the hind paw
Deep -                 more internal                Palmer -               The palms of the forepaw

Planes or Sections through the body:
       Transverse (cross section) - perpendicular to the long axis (separates cranial & caudal)
       Sagital -                     a longitudinal section separating the body into left and right sides
       Frontal -                     a longitudinal section dividing the specimen into dorsal and
                                     ventral sections
General Information:
All drawing should be completed with a pencil on a clean sheet of white paper without lines. You are
responsible for bringing a pencil, ruler, and the drawing packet to class each day.
Paper Preparation:
Prepare 5 pieces of paper for the drawings. Measure down from the top of the paper 3cm and make a
line parallel to the top. Measure 5cm in from the right side of the paper. Draw a line parallel to the
right side. In the top margins write the following titles (one per paper): Drawing I: External Anatomy
of the Rat, Drawing II: Abdominal Anatomy of the rat, Drawing III: Digestive System (removed),
Drawing IV: Urogenital System, Drawing V: Thoracic Cavity. In the upper right hand corner of each
paper write your name, period, and date.
The actual drawings:
Please read the entire section in your lab manual before proceeding with the drawing. Take time to
notice and draw the details of what you observe. All terms that are underlined in your lab manual must
be labeled. Indicate labels with dashed lines, drawn with a ruler. Label lines should never cross each
other. As the dashed lines approach the right margin they should be parallel to the top and bottom.
The dashed lines should immediately stop at the right margin and not cross it. Labels should be neatly
written, spelled correctly, and be located in the margin.
Each drawing represents approximately one hour worth of lab time. Attendance is pertinent to your
success in this dissection unit. Your drawings will greatly assist your memory recall while studying.
However, drawings alone will not be enough to do well on the exam. You must learn the anatomy of
the rat as you proceed through the manual. Work closely with your partner and utilize them for mini-
quizzes to ensure learning.

Rat Exam:
A practical exam will follow the rat dissection. The practical is set up as a series of stations. You will
have approximately 60 seconds at each station. You should be familiar with the introductory
information as well as all rat structures (and their functions) as detailed in the lab manual.

General Dissection Tips:
       •   Dissection is a refined method of seeking, exposing, identifying, and studying the structure
           of a specimen.
       •   Observe the dissected rats of other students in the class. Not all rats are created equal!
       •   If your rat is male you must become familiar with the anatomy of the female and visa versa.
       •   It is suggested but not necessary to wear gloves.

Receiving your rat:
You will be receiving your rat in a dissection pan. The rat will be lying in the supine position. Obtain
identification (a toe-tag) and a string for your specimen. Write your name, your partners name, period
and instructors name on the tag. Tie the toe-tag to the left hind ankle of the rat.

Clean up procedures:
One partner will place the rat in the rat bucket, then rinse and stack the dissection tray (at right angles
to each other) next to the bucket. If there are any separated tissues, they are to be disposed of in the
waste bucket. The other partner will use the table cleaning rags to wipe down your lab table. After all
rats and trays are put away, and your table is cleaned, you may wash your hands.
                     DRAWING I: External Anatomy of the Rat
Examine the specimen you received. The rat should be lying in the dissection tray in a supine position.
The body of the rat can be divided into four readily identifiable areas: The head (cranial), neck
(cervical), trunk (thoracic, abdominal, and sacral), and the tail (caudal). Any structures that are not
visible should be listed in the bottom right corner of your paper. You may draw a schematic of these
structures to assist you in studying.

The Head (Cranial) Region:
The external nares (nostrils) are found at the edge of a hairless zone at the most anterior point of the rat’s

The upper lip is split in two medially, with the cleft extending to the nostrils. This cleft is called the
philtrum (median cleft). The lower lip is not divided.
The rat has two prominent incisors and six molars (three on each side) in both the upper and lower jaw
separated by the diastema – a gap in the tooth row. The incisors grow throughout the rat’s life. The front
of the rats incisors are coated with enamel (the hard substance which surrounds your teeth) and the backs
have no enamel. There is a canal through the middle of the rat’s incisors called the pulp cavity (label in
parenthesis next to incisors). The incisors are constantly worn down and honed into sharp structures by
gnawing on hard substances. Behind the incisors, is the diastema, where the upper lips fold inward to meet
a fold of skin from the cheeks. The diastema separates the gnawing from the chewing areas of the mouth.

Draw a 3x illustration of the diastema in a circle to the side of your rat. Label the diastema, pulp cavity and
incisors directly on the 3x scale drawing (you do not need to bring the labels to the right margin).

An upper and lower eyelid protects the eyes. Spread the upper and lower eyelids. In the inner (anterior)
corner of the eye, locate a third lid-like transparent structure called the plica. The plica is a nictitating
membrane that the live animal can spread rapidly across the cornea, independent of the other eyelids,
cleansing it and keeping it moist.

Located laterally on the rat’s head are the cartilaginous folds of tissue called pinnae. Each Pinna directs
and funnels sound waves into the external auditory meatus (or ear canal) of the rat.

Vibrissae are long whiskers, or stiff hairs primarily extending from the mystacial pads located on either
side of the philtrum. Additional vibrissae can be found extending from anterior portions of the rat’s head;
it’s nose, lower jaws, cheeks, chin, and the area over the eyes. The vibrissae are extremely sensitive
sensory hairs. Each hair has a coil of sensory nerve endings at its base (within the follicle).

The Trunk Region:
The trunk of the rat is divided into the thorax located cranially and the abdomen located caudally.
Internally, a dome shaped muscle called the diaphragm, separates the two regions. The diaphragm lines the
caudal border of the rib cage. Press the ventral surface of the rat and note that the thorax is firmer than the
abdomen due to the thorax being surrounded by ribs. (Label the thorax and abdomen using brackets.)
Two pairs of appendages originate from the trunk: the forelimbs and the hind limbs. The fore limbs are
used for grasping and holding. The hind limbs are used for running, climbing, jumping, and support. Each
of the four limbs is pentadactyle, having five digits. The first digit (thumb) is very reduced in the forelimb.
The claw (nail) of the first digit is also unique in that it is flattened – not sharp and pointed. Located on the
palmer and plantar surfaces of the rat’s feet are digital pads and foot pads. These are swollen thickenings.
The digital pads are located at the tips and bases of the digits, while the six foot pads are located on the
plantar and palmer surfaces (the soles) of each foot.
Humans are biped because we walk on two appendages while rats are quadruped because they utilize four
appendages to walk. In contrast to man who uses plantigrade locomotion, walking on the soles of his feet,
a rat uses digitigrade locomotion, walking on the toes with the soles elevated.

Draw a 3x scale drawing of the right fore foot and the right hind foot of the rat. Complete your drawings in
two separate circles to the side of your rat. Draw only the palmer and plantar (sole) surfaces. Be sure to
extend the digits to show a clear view of the palmer and plantar surfaces. Indicate the number and
orientation of digits, footpads, and claws. Label them directly on the 3x scale drawing (you do not need to
bring the labels to the right margin).

The Sacral Region:
Observing the external genitalia and mammary papillae can readily differentiate male and female rats.
(Draw your rat’s genitalia; obtain a rat of the other sex –draw & label the sacral region of this rat on a separate paper.)

The female rat has three separate apertures. The most cranial of the three is the urinary aperture. Also
visible here is a short projecting organ called the clitoris (homologous with the male penis).

The vaginal aperture is the female rat’s genital opening that leads to the vagina and uteri. The term vulva
refers to the female external genitalia, which includes the labia majora, labia minora, clitoris, and the
vestibule of the vagina. The hymen closes the vulva until the animal is about three months old. The
vaginal aperture is directly caudal to the urinary aperture and clitoris.

The most caudal aperture is the female rat’s anus. The anus is located at the base of the tail. It is the
opening for the rectum allowing for the discharge of solid wastes.

Mammary papillae, commonly called nipples or teats, are more readily visible in pregnant or lactating
females. In immature females they are smaller and often hidden by fur. Generally there are twelve nipples,
six thoracic, two abdominal, and four in the sacral region. They are located along two parallel latero-
ventral lines called the milk lines. Draw all and arrange them as located on your female specimen.

The male rat has two separate apertures. The urogenital aperture is the opening of the penis. It is located
mid-ventrally in the caudal abdominal region. As the name indicates, both urinary wastes and the product
of the male genitals are discharged through this opening to the outside. This is in contrast to the female rat
where the urinary and genital apertures are separate.

The scrotum is a double pouch (two scrotal sacs) that is located caudally and laterally to the urogenital
opening. They contain testes, the male gonads, and thus appear swollen in mature males.

The second aperture of the male rat is the anus. As in female rats, it is the most caudal aperture and serves
to discharge wastes from the rectum. This opening is often hidden by the overhanging scrotum and can be
readily seen by lifting the scrotum.

The Tail:
The tail of the albino Norwegian rat is about three-fourths the length of the head and trunk combined. The
black rat’s tail is even longer. The tail is a balancing organ that gives stability in climbing, swimming, etc.
The surface of the tail is covered by rows of overlapping reptile-like scales. Three short bristles project
from under the edge of each scale.
Obtain a magnification lens and draw a 3x scale drawing of a section of the tail in a circle to the side of the
drawing of the whole specimen. Label the scales and bristles directly on your 3x scale drawing.
                      Drawing II: Abdominal Cavity of the Rat
Drawings of superficial viscera should be drawn with solid lines. Underlying, deep, viscera should
be drawn using dotted lines.

When making incisions, it is important to first pinch a piece of skin with your fingers or forceps and
make a hole. Then use your scissors to lift up while you cut. If you do not do this, you will potentially
cut through the abdominal muscles and into the digestive tract, spilling feces inside your rat.

   1. Make a mid-ventral incision in the skin from the external genitalia to approximately 2cm below
      the jaw. BE CAREFUL TO CUT THE SKIN ONLY, note the delicate underlying muscle
   2. Lift the skin and separate it from the underlying muscles using forceps. Note the two are held
      together by a white fibrous connective tissue called superficial fascia.
   3. Make four lateral cuts through the skin from the ends of the previous incision and extend the
      cuts laterally until the skin lies to the side.
   4. Observe the linea alba, a white line of connective tissue along the mid-ventral surface of the
      external obliques.
   5. Pinch the muscle in the posterior medial region of the ventral abdominal wall and make a hole.
   6. Make a mid-ventral incision in the ventral abdominal wall. Extend the cut anteriorly only as
      far as the thorax (breast bone). Be very careful to cut superficially so as not to cut the intestines
      or other abdominal structures.
   7. Make four lateral cuts through the ventral abdominal wall, from the ends of your mid-ventral
   8. Fold the skin and ventral abdominal wall to the side exposing the abdominal cavity.
   9. Rinse out the abdominal cavity by holding your rat low in the sink, moving the viscera (guts)
      gently to the side and allowing the water to wash over it.

Digestive System:
The liver is a dark brown organ with four lobes located under the dome of the diaphragm. The liver
dominates the cranial abdominal cavity. Ducts from the various lobes of the liver unite to form a tube
called the bile duct, which carries bile into the duodenum. Bile functions to emulsify fat.

The esophagus passes through the diaphragm to enter the abdominal cavity. It can be seen against the
left dorsal body wall as a tube exiting through the liver and entering the stomach (an enlarged pouched

The stomach is a large muscular pouch that lies caudal to the liver on the left side of the rat. It extends
toward the midline. Find where the esophagus joins and becomes continuous with the stomach.

Small intestine exits the stomach and is up to six times the length of the rat’s body. The coils of the
intestine are held in place by a fine peritoneal membrane called mesentery.

The small intestine opens into the large intestine at the caecum. The caecum is a blind-ended sac-like
tube that is quite long in rodents and herbivores. It is a fermentation tank, and an area of absorption.
The ceacum also serves to digest the cellulose of wood and plants. The caecum is shaped similarly to a
large “c”.
Diameter, not length gives the large intestine its name. It can be divided into three parts, the first of
which is the caecum identified previously. The colon is the major portion of the large intestine. It
ascends cranially to the level of the stomach, crosses transversely toward the rat’s left and descends
caudally near the dorsal wall. The final segment of the large intestine is the rectum. It is straight and
thick, often beaded in appearance due to the rat’s fecal pellets (waste materials) found within. The
orifice by which the fecal pellets leave the body is the anus.

Lift the first portion of the small intestine exiting the stomach, the duodenum. Spread the folds to
observe the pancreas. The pancreas is a glandular structure lighter in color than the duodenum. Parts
of the pancreas can be seen along the dorsal body wall extending to the left of the duodenum. The
pancreas secretes the hormones insulin and glucogen directly into the blood stream. Additionally, it
secretes pancreatic amylase, lipase, trypsinogen, and nucleases into the small intestine for chemical

Other abdominal viscera:
The spleen is not an organ of the digestive system, but of the circulatory system. Spleen cells destroy
worn red blood cells and platelets through a process known as phagocytosis. The spleen also acts as a
reservoir for blood storage and a site for manufacturing white blood cells. It is a dark-colored
elongated organ located on the greater curvature of the stomach.

There are several other organs located in the abdominal cavity – the kidneys, urinary bladder and
reproductive structures.
                       Drawing III: Digestive System (removed)
Carefully observe the mesentery located in the coils of the intestine. It is visible as a fine, thin membrane.
Note its shiny transparent appearance. It is responsible for the coiling of the intestines. Additionally, the
mesentery is interlaced with blood vessels, lymphatic vessels, adipose tissue, and lymph nodes. Draw a 3x
scale drawing, in a circle to the side of your drawing, of a portion of the small intestine and mesentery and
label both.

Cut the esophagus cranial to the stomach and caudal to the liver. It is ok if a piece of liver breaks off while
cutting the esophagus. Simply place it in the rat morgue. Begin at the stomach and gently remove the
digestive system from the abdominal cavity. Cut through the caudal region of the rectum being careful not
to cut any surrounding structures. You should elongate the intestines by cautiously tearing the mesentery.
When the system is removed place it in your dissection pan, elongating it with the stomach toward the top
and the rectum toward the bottom. Orient the stomach as it was found in the rat (with the intestine exiting
to what would be the rat’s right). Draw the removed system down the left side of your paper.

The esophagus passes caudally through the diaphragm to enter the abdominal cavity. At the most caudal
end of the esophagus is the entrance of the stomach. The fundus is the portion of the stomach cranial to the
gastroesophageal junction located to the left of the cardiac region. The area of the stomach between the
fundus and the greater curvature of the stomach is the body.

Partially digested food passes through the pyloric sphincter of the stomach into the duodenum. The
duodenum is the first and smallest segment of the small intestine. The structure is shaped like a “U” and
the pancreas lines the duodenal mesentery. Pancreatic ducts enter the duodenum bringing bile and
pancreatic juice. The jejunum is the section of the small intestine following the duodenum. Cut open the
jejunum, wash out its contents in the sink, and observe and feel the texture of the inner surface. Finally, the
small intestine ends in a segment known as the ileum. There is no visible difference between the jejunum
and the ileum, although there are histological (tissue) differences. The two segments are equal in size and
make up a vast portion of the small intestine. Use brackets to label the three sections of the small intestine
and indicate their respective lengths. (Determine their lengths with a ruler.) All three segments of the
small intestine contain villi (finger like projections) to increase surface area for food absorption. Remove a
segment of the small intestine and view the villi.

The large intestine begins at the caecum as described in section II. The major portion of the large intestine
is the colon. There are ascending, transverse and descending portions of the colon. The final segment of
the large intestine is the straight, thick rectum. Fecal pellets (if seen) are often enclosed in the rectum.
Remove a segment of the large intestine and make a longitudinal cut. Does it have villi?

Read thoroughly before drawing a 3x scale drawing of the detached and dissected stomach. Draw the
stomach in a circle to the side of the detached digestive system. Cut the small intestine proximal to the
stomach. Use your scissors to poke a hole through the fundus of the stomach. Cut along the greater
curvature of the stomach. Take the stomach to the sink and flush with water to remove the rat nip. The
mucosa and submucosa of the internal stomach wall are thrown into prominent longitudinal folds, called
rugae. As the stomach fills, the rugae flatten out, allowing the stomach to fill. In a full stomach the rugae
will have almost disappeared. Continue cutting, making a longitudinal section through the esophagus and
small intestine in the lesser curvature. Orient the stomach as it was found in the abdominal cavity. The
esophagus connects with the trachea in the thorax of the rat. Find where the esophagus joins with the
stomach (esophageal sphincter) in the lesser curvature. This is known as the cardiac region. The final
portion of the stomach nearest the pyloric sphincter is the pyloric region (the J of the stomach). The pyloric
sphincter is the circular muscular valve that separates the stomach from the small intestine.
                              Drawing IV: Urogenital System
The urinary and genital systems have unique and distinct functions. The urinary system removes
nitrogenous and other wastes to maintain the body’s water balance. The genital system functions in the
reproduction of the species. These systems share some common structures and are generally considered
together as the urogenital system.

Remove the liver from your rat. Use care and remove the fat found in the lower pelvic area. Be careful not
to injure or destroy delicate urinary or genital ducts or organs.

The Urinary System:
The kidneys are large, reddish-brown, bean-shaped structures found on either side of the vertebral column.
(They are located near the lumbar region of the vertebral column.) Renal is a term used for all structures
pertaining to the kidney.

Gently clear the adipose capsule (adipose tissue dominated by fat cells) away from kidneys to expose the
renal arteries, renal veins, and delicate ureters. The renal artery and vein carry blood toward and away
from the kidneys respectively.

Adrenal glands are small, round nodules of globular tissue, usually embedded in fat. They lie cranial to the
medial border of the kidney. (They are not actually part of the urinary system. However, their physical
location and blood supply are closely associated with the kidneys.)

Ureters are very fine muscular tubes that transport urine from the renal pelvis of the kidney to the urinary
bladder. CAUTION: the ureters are very narrow and easily torn. The urinary bladder is a structure that is
usually contracted into a small pear-shaped organ. It is located mid-dorsally. In males, genital glands and
ducts are found in close association with the urinary bladder. The urinary bladder temporarily stores urine
that has come down from the kidney through the ureters. Urine exits the urinary bladder and is taken to the
exterior by the urethra.

Read the following thoroughly and then draw a 3x scale cross-section drawing of the removed kidney in a
circle to the side of your paper. Remove the kidney from the rat by separating the renal fascia from the
body wall. Make a longitudinal cut through the kidney and view the frontal section. The renal cortex is the
reddish-brown outer layer of the kidney in contact with the renal capsule. The medullary region is deep to
the cortex. View the renal pelvis, which is the funnel-shaped expanded portion where the ureter, renal
artery & renal vein enter the kidney. Renal pyramids are triangular regions striated in appearance. The
base of each pyramid faces toward the cortex. Renal columns are areas of tissue, which segregate the renal
pyramids. You may be able to view these with a dissecting scope.

Female - Genital System:
Continue the dissection with your rat. Then you must draw the genital structures of the opposite sex rat on
the back of drawing IV. Include the kidneys and ureters in both drawings. You are responsible for being
able to identify the structures and discuss the functions of the reproductive organs of both sexes.

Ovaries are the female gonads. They produce the female gametes (ova) and female sex hormones. They
are located caudal and lateral to the kidneys, usually embedded in fat. Gently tease away some of the fat.
The ovaries are small round structures. Use the hand lens to see blister-like bulges upon the ovaries. These
bulges may be developing follicles that contain egg cells (ova).
Fallopian tubes (oviducts) will be found near the ovaries. They are fine-coiled tubes extending caudally
beyond the ovaries. You may use a hand lens to view them better. Trace the fallopian tubes caudally to
where they joint the much wider uterine horns (cornua). Each horn extends caudally and medially to form
the shape of a “V”. The rat embryos develop in the two horns of the uterus. This is contrary to the single
medially located uterus found in humans. Each uterine horn is attached to the dorsal body wall by a
membrane known as the broad ligament. If your rat is pregnant you are welcome to cut laterally along the
uterine horn to observe the developing fetuses.

The body of the uterus refers to the mid-dorsal structure formed by the union of the two uterine horns. The
portion of the uterus that projects into the vagina is known as the cervix. The vagina leads from the
openings of the uteri and the cervix to the exterior of the body. The body of the uterus ends in an exterior
opening is known as the vaginal aperture.

The urethra is part of the urinary system, which lies ventrally atop the vagina and terminates at the urinary
aperture. A small papilla, known as the clitoris, projects from the ventral side of the external urinary
aperture (anterior to the vaginal aperture). It is homologous to the penis in males and has associated paired
preputial glands similar to those in males.

Male - Genital System:
Locate the scrotum composed of two scrotal sacs. Carefully make a mid-ventral incision from the most
caudal region of the scrotal sac. Also, cut cranially into the urogenital aperture. This will expose the penis
and associated structures. Take care not damage any internal structures.

Testes are the male gonads that produce sperm as well as sex hormones. Locate them in the scrotum. They
are enclosed in a tough membrane. Cut the membrane open. The oval-shaped, light brown structures are
the testes.

Partially surrounding the testes (caudally) is the semi lunar-shaped epididymus, composed of convoluted
tubules. The epididymis serves as a maturation and storage area for sperm. The tube that transports sperm
and seminal fluid from the testes is the vas deferens. The vas deferens originates at the tail end of the
epididymis posterior to the testes, and joins the urethra from the urinary bladder.

Seminal vesicles are found within the abdominal cavity, near the juncture of the vas deferens and urethra.
The seminal vesicles are two leaf-shaped glands. Their lateral edges are scalloped. The seminal vesicles
add their secretions to the seminal fluid. Coagulating glands are found on the medial borders of the seminal
vesicles, and may be separated from them. They serve to coagulate the seminal fluid and are necessary for

The urethra emerges from the urinary bladder and joins the vas deferens. They continue as a single, unified
tube. The merged urethra, urogenital canal, carries sperm and seminal emissions in addition to urine.

Follow the urethra, or urogenital canal, caudally to the beginning of the penis. The prostate gland is a
small, muscular, rounded organ that surrounds the urethra as it leaves the urinary bladder. The prostatic
fluid produced by the gland is a milky, slightly acidic secretion that contributes about one third of the
volume of semen. Seminalplasmin is an antibiotic that the prostate fluid contains which prevents urinary
tract infections. The urogenital canal runs through the penis and terminates at the urogenital aperture.
                                Drawing V: Thoracic Cavity
Make an incision lateral to the mid-ventral line into the skin and musculature of the thoracic region
toward the rat’s chin. Lift the skin and muscle as you cut so that you do not damage superficial
structures. Observe the ribs, sternum and diaphragm. Next, make an incision approximately 5mm
lateral to the sternum. Be careful not to injure the lungs. Cut parallel to the sternum, through the ribs,
up to the chin. Fold back the muscles and ribs to expose the thoracic cavity.

The diaphragm is the muscular wall separating the thoracic and abdominal cavities used in respiration.
When contracted the diaphragm moves downward causing the lungs to fill with air. When relaxed the
diaphragm moves upward causing the lungs to deflate. Observe the ribs. Inspiratory volume can be
increased when the intercostal muscles contract, raising the rib cage.

The larynx is an enlarged area cranial to the trachea. Caudally, the larynx branches into the trachea
and the esophagus. The esophagus carries food to the stomach, and the trachea branches into bronchi,
which carry air to the lungs. The trachea (or windpipe) is a tube that extends along the mid-ventral
portion of the neck into the thoracic cavity. The trachea is kept open by cartilage rings. The rings give
support, shape, and work to maintain an open air passage.

The thyroid gland is composed of two relatively small lobes, which lie on either side of the base of the
larynx. The thyroid hormones regulate the body’s rate of metabolism. In growing mammals, thyroid
hormones are essential to the normal development of the skeletal, muscular, and nervous systems.

The thymus gland is relatively large in younger animals and becomes smaller as the animal matures.
In young animals it occupies a large portion of space ventral to the trachea, and cranial to the heart.
The thymus produces several hormones, collectively known as the thymosins. Thymosins play a key
role in the lymphatic system in development and maintenance of normal immunological defenses.

Examine the lungs on either side of the heart. The right lung has 4 lobes and the left lung has 1 lobe.
In humans, the right lung has 3 lobes and the left lung has 2. The reason for this is to provide room for
the heart, which lays slightly to the left.

The heart is located slightly left of medial in the thoracic cavity. The auricles (atria) are ear-like sacs
that receive blood from the entire body. The right auricle (atria) receives deoxygenated blood from the
superior and inferior vena cavas. The inferior vena cava brings oxygen poor blood from the caudal
portion of the body. The superior vena cava brings oxygen poor blood from the cranial portion of the
body (above the diaphragm). The left auricle (atria) receives oxygen rich blood from the lungs by way
of four small pulmonary veins. They are usually not injected and are difficult to find because they lie
deep. The pulmonary veins are the only veins carrying oxygen rich blood.

The ventricles are much larger and easier to see. They are the posterior portions of the heart. They
exert the force to pump blood throughout the body. The right ventricle pumps deoxygenated blood to
the lungs through the pulmonary artery (the only artery to carry oxygen poor blood). The left ventricle
pumps blood out the aorta to the entire body. The aorta exits the left ventricle passes anteriorly, curves
to the left (aortic arch), and continues dorsally, as the descending aorta, in a posterior direction along
the left side of the vertebral column. Within the thorax, the descending aorta is known as the thoracic
aorta. Below the diaphragm, the descending aorta is known as the abdominal aorta. Draw a detailed
picture of the heart including the chambers and associated vessels. Use your checklist as a guide.

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