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1 Identify the dorsal and ventral surfaces of the fetal pig

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1 Identify the dorsal and ventral surfaces of the fetal pig

    Animal Biology 74

    Body cavity Dissection

    anteriorAbdomen dissectionA. Abdominal cavity On the ventral surface Figure3anterior to the umbilical cord locate the

    sternum (breast-bone) and the rib cage.

    Locate the lower edge of the ribs. With your finger trace the edge of the rib cage (your make initialfingertips will trace in an arc across the belly slit here

    and around to the sides). With your razor thoraxblade make an initial slit in the abdominal 122cavity just below the rib cage and above the edge of rib cage3umbilical cord [figure 3(1)]. You need to cut umbilical chordabdomendeep enough to enter the abdominal cavity, 44

    but not so deep you cut into the liver, a large 55brown organ right under your incision. Once

    you have entered the cavity, switch to

    scissors and cut the abdominal cavity open by following the same arc you traced with your

    finger [figure 3 (2)]. Now go back to the

    posteriorcenter again and make two incisions around

    the umbilical cord and down to the legs [figure 3 (3) and (4)]. Now make two incisions

    [figure 3(5)] over the top of the legs and toward the back (dorsal surface).

    ; The umbilical cord should still be attached by the

    umbilical vein to the liver. You will need to cut this

    vein in order to completely open the abdominal

    anteriorThorax dissectionFigure5

    junction ofclavicles andsternum

    punctureliver33here

    2

    11

    cavity (see figure 4) umbilical vein

    umbilical chord

     B. Thoracic Cavity To keep the

    diaphragm intact, locate it again (above

    the liver) in the abdominal cavity. Intestine

    Position your scissors on the sternum

    (breast bone) a short distance above the

    diaphragm (figure 5). Puncture the

    thoracic wall [figure 5(1)] with the point

    of the scissors (you may want to use a

    posterior

    Animal Biology 75

    razor blade to make the first cut.) The thoracic wall is thicker than the abdominal wall and you will be pushing the scissors through cartilage, so a fair amount of pressure will be needed to enter the cavity. You are posterior to the lungs and heart so you will not damage these organs. Once you have entered the cavity cut an arc along the ribs (similar to the one made in the abdominal cavity) both to the left and the right side[figure 5(1)]. Now look for a hard bump between the arms where the collar bones (clavicles) and breast bone (sternum) meet. At the

    location where you entered the thoracic cavity make a second cut up the sternum to this bump [figure 5(2)]. You need to be careful to cut only the body wall, including the cartilage of the ribs and sternum. The lungs and heart will be just below where you are cutting and are attached by some of the membranous lining of the body (called the pleural membranes over the

    lungs and the pericardium around the heart). It is okay to cut the lining, but not the organs. Stop at the sternal bump and then cut laterally toward and under the arms to the back [figure 5(3)]. Spread the rib cage (you need to break the ribs) and expose the heart and lungs (figure 8).

    ; Some of the specimens may contain excess preservative fluid or coagulated blood. You may need to wash out the exposed cavities. Hold the pig under a moderate flow of water and rinse gently. You may find it necessary to move the intestine, or other organs slightly to remove all of the excess coagulated blood. Use paper towels to soak up excess water.

    ; Many of the organs in the abdominal and thoracic cavity should now be observable. It will, however, be necessary to move some organs from their natural positions in order to see those “hidden”.

    I. Abdominal Cavity Identify (figures 6 and 7) the following structures:

    1. Liver This dark brown organ dominates the abdominal body cavity. It should be

    observable at the location where you made the first incision into this cavity. There are 5

    lobes. Four are easily seen, while the fifth can only be seen by moving the intestinal

    coils to the left.

    Animal Biology 76 .

    Figure 62. Diaphragm This is a dome-shaped

    muscular wall that separates the thoracic gall bladderumbilical veincavity from the abdominal cavity. Use liver

    the liver to orient your search for the

    diaphragm. The diaphragm is just above

    the liver and is a sheet of muscle bile ductcompletely separating the body cavities. umbilicalstomachThe diaphragm arches over the top of cord

    the liver.

    3. Gall bladder Lift the right central lobe

    spleenof the liver. The gall bladder, a

    collapsed balloon-shaped reservoir, is intestineembedded in the liver’s under surface.

    umbilical4. Stomach This is a large muscular arterypouch on the left side of the pig. It is

    partly under the liver. The esophagus

    enters the stomach from the thoracic cavity. It can be observed by lifting the liver and finding the top of the stomach. The esophagus is easier to find in the thoracic cavity.

    At its lower end the stomach forms a short “U” or “L” and ends in the muscular pyloric

    sphincter. The pyloric sphincter will feel hard and forms a bump.

    5. Small intestine There are

    three regions to the small

    intestine. The upper portion is a

    short region called the

    duodenum The duodenum in

    Latin means “12 finger”, = 12

    inches today. This is about the

    length of the human duodenum.

    It is continuous with the pyloric

    sphincter. Small tubes called

    ducts open into the duodenum

    from both the gall bladder (bile

    duct) and the pancreas

    (pancreatic duct). Trace the bile

    duct from the gall bladder down

    to the duodenum. You may look

    for the pancreatic duct later.

     The remaining small

    intestine is composed of two

    functional regions; however,

     externally they are not

    distinguishable. Notice how the small intestine is coiled and held in place by a thin membranous

    material. This membrane is called the mesentery. Lift a coil of the small intestine and stretch

    Animal Biology 77

    the area out. Note the narrow vessels (blood and lymphatic) and lymph nodes interlaced in the mesentery. You will not be able to distinguish the blood vessels from the lymphatic vessels, but the lymph nodes will appear as small light brown oval-shaped structures. Stretch out a coil of the small intestine and cut the mesentery with your scissors. Continue to stretch out coils and cut the mesentery until you have completely unraveled the small intestine. Stop when you reach the large intestine (colon).

    6. Large intestine At the end of the small intestine (ileum) there is a short blind sac

    called the caecum. In humans the caecum has an appendix. The large intestine (colon)

    is shorter, darker, and thicker than the small intestine. The rectum is the uncoiled

    posterior portion of the colon. The rectum descends along the back and disappears

    under the urinary bladder. The human colon is shorter than that of the pig, and is not

    coiled.

    7. Spleen This is the reddish-brown leaf-shaped organ seen on the left side near the

    stomach. The spleen is attached by a membrane to the stomach and you have probably

    been pushing it out of the way as you traced the stomach and small intestines. 8. Pancreas Lift the stomach and spleen (and the small intestine if it is still coiled at the

    duodenum.) The pancreas is a lobular light-colored structure that has a beady

    appearance. Its main part lies in the loop of the duodenum, but it extends to the left

    toward the spleen. The pancreatic duct enters the duodenum from the pancreas, but it is

    small and difficult to find, so do not spend time looking for it.

    9. Kidney These large bean-shaped organs are embedded in the dorsal body wall, one on

    each side. Notice they are covered by a thin membrane that not only wraps them but

    covers all of the dorsal body wall. This is the peritoneum. The mesentery of the small

    intestine is also a part of the peritoneum. You cut through the ventral portion of the

    peritoneum when you entered the abdominal body cavity. To remove the peritoneum

    from the pig’s left kidney, use your forceps to break and pick up the peritoneum on the

    lateral side of the kidney and pull it slowly from the kidney and the ureter (a white wavy

    tube) that descends from the kidney. Remove the peritoneum carefully from the ureter

    as you trace it to the point where it enters the urinary bladder. Most of the female and

    some of the male reproductive structures are located at the base and a little

    anterior to the urinary bladder. Be careful not to pull or remove any tubes or

    oval-shaped structures as you remove the peritoneum. We will look at the

    reproductive system on the demonstration pigs.

    10. Urinary bladder In the fetal pig, the urinary bladder is an elongated sac that is

    attached to the inner surface of the ventral abdominal wall and the umbilical cord. The

    two vessels, one on each side of the urinary bladder, are the umbilical arteries.

    II. Thoracic Cavity: Use figures 8 and 9 to help you located the thoracic structures.

Find the following organs and glands:

    1. Thymus gland This granular tissue partially covers the heart and extends up into the

    neck. The gland is enlarged in the fetus and in young animals. It is greatly reduced in

    size as an animal matures. You will need to remove this tissue from the heart, but be

    careful not to remove the atria (upper chambers; atrium singular) of the heart at the

    Animal Biology 78

    same time. This will not be a problem if the pericardium is intact, however if it is cut, be careful. The atria are usually a darker color and not granular.

    2. Pericardium You will need to remove this sac from the heart. The phrenic nerves

    that control the contraction of the diaphragm during breathing will be present below the pericardium and will interfere with its removal. The nerves are white thread like structures that are not easily broken, but in order to completely clean away the pericardium you may have to break some of them.

    3. Heart This a conical organ located in the center of the thorax in the space between the lungs. Locate the following parts: two atria and two ventricles, coronary arteries

    and veins on the surface of the heart, pulmonary artery, aortic arch, and the ductus

    arteriosus.

    a. Pulmonary artery This is the large blood vessel located between the two atria.

    It will be the most easily seen of the vessels.

    b. The aortic arch is located behind the pulmonary artery. Place your finger on the

    pulmonary artery and the pig’s left atrium (your right side). Now press down and

    a little to your left. Remove any pericardium that is covering the aortic arch and

    pulmonary artery as you trace them lower down behind the heart. You should be

    able to see where the pulmonary artery enters the aorta (figure 9). The

    pulmonary artery transports blood to the lungs; however, in a fetus, the lungs are

    almost completely bypassed until just before birth. What you are observing is the

    bypass called the ductus arteriosus. This shunt must close at or soon after birth.

    If you roll the heart a little farther over and remove the membranes and break

    some of the nerves just below the ductus arteriosus, you can see the much smaller

    part of the pulmonary artery going downward toward the lungs.

    4. Lungs Note the spongy lungs. The right lung is the largest, and is divided into four lobes. The left lung is divided into three lobes (in humans the right lung has three and the left lung has two). The lungs are covered with membranous sacs, the pleurals.

    There are two different types of pleural linings. One lines the thoracic cavity, the other lines each lung. Remove any remaining pleural membrane from the lungs.

    5. Dorsal aorta Roll the left lung so you can see behind it. You may have to break loose more pleural sac and nerves. With the left lung rolled to the right and up, you will be able to see three tubes. Starting at the ductus arteriosus, follow the tube down the back this is the dorsal aorta. Toward the center and a little ventral is the esophagus.

    The other tube is the posterior vena cava, the large vein that brings the blood back to

    the heart from the lower body.

    Animal Biology 79

    anteriorThoracic dissection

    Figure 8

    coronary arterythymus gland

    inner right lobe lung

    left atrium

    left ventricle

    left lobe of lung

    location of diaphragm

    umbilical vein

    spleenstomach

    pancreas

    kidney

    colon

    ureter

    rectum

    ovary

    intestine

    caecum

    posterior;

Animal Biology 80

Animal Biology 81

    Reproductive systems:

     Find the following structures:

    A. Female: (Fig. page 81)

    1. Ovaries These are oval or bean-shaped structures at the tips of a Y-shaped pair of tubes

    coming up from behind (dorsal to) the urinary bladder.

    2. Oviducts These are very tiny tubes that are located on the backside of the ovaries. These

    are called the fallopian tubes in humans.

    3. Horns of the Uterus These are the two larger tubes of the Y. The region where the horns

    of the uterus join and disappear behind the urinary bladder is the body of the uterus. In humans

    the uterus is not split into horns, and the oviducts would look much like the horns of the uterus.

    The split uterus is an adaptation for multiple births.

    4. Body of Uterus The region just below where the two horns join together.

    5. Cervix Located at the end of the uterus, this is a hard and slightly enlarged region. 6. Vagina This starts just below the cervix and is the tube down to the connection to the

    urethra.

    7. Urethra This is part of the excretory system. The urethra is the tube that leads to the

    outside of the body from the bladder. Notice how the vagina and urethra join in the pig as a

    urogenital sinus. The vagina and the urethra remain separated in humans.

    B. Male reproductive system: (Fig. Page 81)

1. Urogenital opening Just below the umbilical cord (Figure 2, page X-2)

    2. Penis The tube that has been separated from the ventral body wall and muscle. The penis

    continues as a U-shaped tube to just above the anal opening.

    3. Urethra It continues with the penis and begins at the enlarged area on the U-shaped tube

    and ends at the bladder.

    4. Scrotal sacs There are two scrotal sacs. Each contains a testis.

    5. Testes The oval shaped structures inside the scrotal sacs. One testis has been exposed by

    opening one sac. When the fetal pigs are very young, the testes are in the body cavity. They

    will descend into the scrotal sac through the inguinal canal. Some of your pigs may have the

    testes either still in the body cavity, or just beginning to descend into the scrotum. The opening

    into the inguinal canal is often associated with a hernia in man. The intestine may push down

    into the scrotum through the canal.

    6. Epididymus These are the small coiled tubular structures on the testes.

    7. Vas deferens These are the tubes that lead from each testis out of the scrotal sac and into

    the urethra. The two vas deferens can be seen at the testis or where they from a Y and enter

    the urethra (dorsal side at the posterior end of the bladder).

    8. Glands that produce the seminal fluid

    a. Bulbourethral glands (Cowper’s) — These are located just anterior to where the penis and

    urethra join. They are two large pill-shaped glands on each side of the urethra.

    b. Seminal vesicles These are located on either side of the vas deferens as it becomes a

    single tube and enters the urethra.

    Animal Biology 82

    c. Prostate gland A tiny bump (in an adult human, the prostate is about the size of a

    walnut) just below where the vas deferens becomes a single tube. The vas deferens

    appears to pass through the prostate as it goes toward the urethra. The seminal vesicles

    are on both sides of the prostate (figure 12).

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