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Transcript for:
Fetal Pig Dissection Guide

Title: Fetal Pig Dissection (GUIDE)

URL Source: file://pdf.07b4c380db68501984b59f00a32cedb9/

Markdown Content:

Fetal Pig

Dissection

Manual Introduction

Fetal pigs are readily available, since farmers fi nd it pro fi table to breed female pigs

which they plan to sell. Thus, pig fetuses are byproducts of the slaughterhouses. The

period of gestation is 112 to 115 days, and there are, on the average, about seven to eight

off spring in a li er. At birth the pigs vary from 12 to 14 inches in length. The

approximate age of the fetus can be determined by measuring the length of the body

from the tip of the snout to the rump (not including the tail). The following are

approximate body length to age relationships:

Gestation Table

Length of Specimen Approximate Age in Days from Fertilization 4 cm 56 days 20 cm 75 days 25 cm 100 days 30 cm 105 days 35 cm 111 days 40 cm 115 days (full term)

As a laboratory animal the fetal pig has several advantages. It is relatively

inexpensive so that usually a maximum of two students can be assigned to an animal.

Since they are small, they do not require much storage space. The animals are mammals

and, therefore, their structures are like those of humans. In addition to relatively mature

organs, there are also fetal structures present that are directly comparable to those of

human beings. These include the umbilical cord and the circulatory structures which

are specialized for fetal circulation.

As the fetal pig is dissected and studied, the structures identi fi ed should be

compared with those of the human. Dissection is not merely cu ing the animal, but a

systematic technique of bringing into view structures which, in their normal position,

cannot readily be seen. Follow instructions exactly. Do not cut or remove any structure

unless directed to do so. Always separate structures carefully, especially blood vessels,

by moving connective tissues out of the way. It is best to use the dull probe for this task.

You may fi nd that the substances used to preserve the specimens are irritating to

your skin. If so, wear thin vinyl or plastic gloves. Remove as much of the preservative

from your specimen as possible by frequently washing it with tap water. Keep your

fi ngers away from your eyes during dissection.

At the conclusion of each laboratory period, clean up the working area thoroughly.

Put the pig in the container provided by your teacher. Do not leave any solid material

in the sink. Clean and dry the laboratory table and the dissection tools that were

assigned to you. The terms right and left always refer to the pigs right and left. In a quadruped,

anterior or cranial refers to the head end; posterior or caudal to the tail end; dorsal or

superior to the back; ventral or inferior to the belly. Lateral refers to the side, medial to

the position of a structure nearer the midline of the body.

Day 1: External Anatomy Examination

  1. Each group should obtain a dissecting tray, fetal pig, dissecting equipment (scalpel,

scissors, probe, forceps), and personal equipment (goggles and gloves).

  1. Cut the fetal pig bag near the top and carefully drain all the preservative fl uid into

the sink. If any splashes on the counter, rinse o ff thoroughly. To make the room

smell as best as possible, run the water for a few seconds to wash all the fl uid down

the drain, making sure to rinse the whole bo om of the sink o ff .

Image 1: Fetal Pig

  1. Examine the pig for body hair, although this is usually not visible currently. Is body

hair present?

a. Look under the chin for some longer hairs.

  1. Note the epitrichium , the layer of embryonic skin that is visibly peeling. This is lost

as the hair develops.

  1. On the head locate the following structures:

a. The mouth , bounded by upper and lower jaws and soft lips, is sometimes

partially open, revealing a soft tongue . The front end of the head is prolonged

into a snout . The snout is used for rooting around in the soil for roots, insects, and other materials used by the pig for food. Do you have a snout?

b. Observe the two nostrils (external nares) at the end of the snout.

c. The eyes (usually closed) are covered by upper and lower eyelids fringed with

eyelashes. Use a probe and pull the upper eyelids apart. The nictitating

membrane should be visible in the medial corner of the eye. This transparent

membrane, which is referred to as a third eyelid, can move across the eyeball

with the eye open, thus providing protection. Check your partners eye for this

structure. Is it present?

d. Observe the eyes of the pig, carefully remove the eyelid so that you can view

the eye underneath. Does it seem well developed? Do you think pigs are born

with their eyes open or shut?

e. The opening into the ear is called the external acoustic (auditory) meatus and

the fl aened fl ap of skin is called the pinna , or auricle . The pinna and the

external acoustic meatus make up the external ear in the pig as well as in the

human.

  1. Carefully lay the pig on one side in your dissecting pan and cut away the skin from

the side of the face and upper neck to expose the masseter muscle that works the

jaw, lymph nodes , and salivary glands . The salivary glands kind of look like

chewing gum and are often lost if you cut too deeply.

Image 2: Lateral View of Fetal Pig Head

  1. Open mouth: Open the pig's mouth and locate the hard and soft palate on the roof

of the mouth. Can you feel your own hard and soft palates with your tongue?

  1. Note the taste buds (also known as sensory papillae ) on the side of the tongue.

Locate the esophagus at the back of the mouth. Feel the edge of the mouth for teeth.

Does the fetal pig have teeth? Are humans born with teeth?

  1. Locate the epiglo is , a cone-shaped structure at the back of the mouth; a fl ap of skin helps to close this opening when pigs swallow. The pharynx is the cavity in the back

of the mouth - it is the junction for food (esophagus) and air (trachea).

  1. Using your scissors, cut the corners of the jaw so that the mouth will remain open.

  2. Carefully examine the external features of your pig beginning with the head. Pay

aention to the amount and color of hair, birthmarks, and other unique markings.

  1. The trunk can be divided approximately into two general regions, consisting of an

anterior thorax and a posterior portion, the abdomen .

Image 5: Urogenital View of Fetal Pig

  1. Determine the sex of your pig by looking for the urogenital opening . In females,

this opening is located near the anus . In males, the opening is located near the

umbilical cord .

a. If your pig is female, you should also note that urogenital papilla is

present near the genital opening. Males do not have urogenital papilla.

b. Both males and females have rows of nipples , and the umbilical cord will

be present in both.

  1. Determine the gender of your pig by using the fi gures above. If your pig is male,

you will see the urogenital opening posterior to the umbilical cord. It serves as a

passageway for urine and semen. Depending on the age you may or may not see

scrotal sacs. The penis is not visible, but you can feel it by pressing the skin between

the urogenital opening and the scrotal sacs. The males mammary papillae will

never develop but in females they will develop into teats during pregnancy. If your

pig is female, you will see the genital papillae under the tail. This releases metabolic

wastes and is the opening to the reproductive system. Be sure to observe another

groups pig of the opposite gender. 15. Note that there are only four toes or digits on each limb as compared to fi ve in

humans.

  1. Examine the legs and note that they have the same general structure as that of

humans and other animals, although they are somewhat modi fi ed.

  1. Use a piece of string to measure the length of your pig. Stretch the string along its

back from the base of its tail to the tip of its nose. Make your measurement in

centimeters. How many centimeters is it? Use the data from the Gestation Table to

determine the age of your pig in terms of gestation. How many days was the pig in

gestation?

Day 2: Thoracic Cavity

  1. Put paper towels under the dissecting tray. Place the pig in the dissecting tray on its

back. Remember: when observing structures from the ventral side, left and right will be

reversed.

  1. For a be er view of the abdomen, you can pull the legs further apart by tying string

around each leg and twisting the string around the spools in each corner of the tray

(Image 8). You can also pull the string underneath the dissecting pan and tie it to the

other leg. Dont tie this too tight as you may want to adjust the ties as you open the

chest cavity.

Image 8: Tying the Fetal Pig

  1. With a scalpel , make a Y-shaped incision (Image 9). Use the fi gure below to guide you

as to how to make the cuts and where to cut according to the gender of your pig.

The arms of the Y start from the top of each shoulder anterior to the front legs and

come down to the sternum which is directly over the heart between the front legs.

The incision should be just deep enough to cut through the muscular chest wall.

Cut away the tissue from the underside of the fl ap of skin formed by the arms of

the Y. Continue to cut the tissue as you pull the fl ap back toward the nose until the

protruding larynx is exposed. Image 9: Incision Guidelines

The organ systems that we will be exploring in depth during this dissection will be:

The Respiratory System

The Circulatory System

The Digestive System

The Excretory System

The Reproductive System

Prior to exploring each system in the fetal pig, you may virtually dissect at the following

website: h p://www.whitman.edu/biology/vpd/main.html

Examination Procedures

  1. Make a median longitudinal incision through the muscles in the neck in order to

expose the larynx and trachea . A empt to not sever the blood vessels or nerves

located on either side of the trachea.

a. The trachea contains rings of cartilage in its walls. Determine whether these

rings are complete on the dorsal surface of the trachea.

  1. Remove muscular tissue from the larynx. Make a longitudinal incision through the

ventral wall of the larynx and locate the vocal folds , which are two small, shelf-like

membranes. These are poorly developed in the fetal pig. Image 10: Fetal Pig Internal Organs

  1. Free the trachea, laterally, from the preceding blood vessels and nerves. Lying along

the trachea, and a ached to it, are the two slender inferior laryngeal nerves. These

nerves which are essential for speech in humans originate from the vagus nerve and,

although they are small and delicate, are easily seen against the trachea on either

side.

  1. Locate the esophagus , the muscular tube dorsal to the trachea.

  2. Examine the interior of the thoracic cavity.

a. Note that the thoracic cavity is divided into two lateral pleural cavities . which

contain the lungs. The pericardial sac , which contains the heart. is in the space

(mediastinum ) between the lungs.

b. The pleura is a double layered membrane which lines the thorax. That portion

of the pleura lining the thoracic wall is called the parietal pleura ; that which

covers and adheres to the lungs is called the visceral pleura .

c. The pericardium , the membrane surrounding the heart, is also composed of two layers: the outer parietal layer and the inner visceral a ached to the heart.

Much of the parietal pleura forming the medial walls of the pleural cavities is

tightly bound to the parietal pericardium.

  1. Remove thymus tissue in the thoracic cavity in order to study the lungs. a. Note that

the lung is a ached to other structures in the thorax only by the root. The root of

the lung is formed by the bronchus , pulmonary artery and vein, bronchial arteries

and veins, nerves, lymphatic vessels, and bronchial lymph nodes, all encircled by

pleura.

Image 11. Super fi cial View of the Thoracic Cavity with the Neck

Dissected

b. Determine the number of lobes in each lung. Each lung is divided into three

major lobes: apical , cardiac , and diaphragmatic . The right lung has an

intermediate lobe beneath the apex of the heart.

c. Cut o ff a small section of the left lung and note the density of the lung. The

lungs have not yet fi lled with air, since they are nonfunctional before birth. Image 12. Dissection of the Organs of the Thoracic Cavity of the Fetal Pig

Day 3: Abdominopelvic Cavity

Examination Procedures

  1. Expose the organs in the abdominal cavity by making the incisions through the body

wall as shown in Figure 13. First trace each incision by making a shallow cut with a

scalpel through the

skin, then continue the cut through the rest of the body wall with a pair of scissors. Lift

the body wall toward you as you do this to avoid cu ing internal organs.

  1. Make a short cut that extends cranially from the umbilical cord to the posterior end

of the sternum (breastbone), which you can feel. Look into the abdominal cavity and

notice the muscular diaphragm that forms a border between the abdominal and

thoracic cavities.

  1. Make lateral incisions through the body wall just posterior to the a achment of the

diaphragm, which you can feel with your fi ngers. These cuts should follow the

aachment of the diaphragm all the way to the back muscles. You should now have

the abdominal cavity exposed. If the body cavity is fi lled with a dark fl uid, fl ush it

out with water; be careful not to damage any organs. Image 13. Super fi cial View of the Digestive Organs of the Fetal

Pig

  1. Use Images 13 and 14 to assist you in locating the following organs:

a. Locate the large, reddish-brown colored liver , posterior to the diaphragm. Note

that the superior surface of the liver is convex to match the concavity of the

diaphragm. The liver produces bile, which is emptied into the duodenum and

serves to break up fats.

b. Count the number of lobes in the liver. The pig liver is divided into fi ve lobes:

the right lateral, right central, left central, left lateral, and a small caudate lobe.

The caudate lobe is posterior to the right lateral lobe.

c. Lift the right lobe of the liver and locate the gallbladder , the small, pear

shaped sac embedded in the right central lobe. The gallbladder stores bile

produced by the liver.

d. The umbilical vein can be found entering the liver to the left of the

gallbladder. Image 14. Digestive Organs with the Liver and Spleen Pulled

Back

  1. Lift the liver to expose the stomach, the large, somewhat J-shaped organ located on

the left side of the abdominal cavity.

a. Locate the entrance of the esophagus into the stomach . The stomach is the sight

of mechanical digestion and, through actions by the enzyme pepsin, the initial

site of protein digestion.

b. Identify the following regions of the stomach: the greater curvature , the side to

which the spleen is a ached; the cardiac region where the esophagus joins the

stomach; and the pyloric region , the region opening into the duodenum.

c. Use a longitudinal incision from the cardiac region to the pyloric region to cut

open the stomach. The green debris found here and elsewhere in the digestive

tract is called meconium. It consists of a bile-stained mucus, epithelial cells

sloughed o ff from the skin and lining of the digestive tract, and amniotic fl uid

swallowed by the fetus. It is discharged in the fi rst bowel movements of the

newborn. Wash the meconium out of the stomach.

d. Observe the gastric mucosa lining the stomach and the rugae , the longitudinal

folds visible in the interior of the stomach.

e. Locate the cardiac sphincter , a circular ring of smooth muscle surrounding the

opening of the esophagus into the stomach. Note that the sphincter is tightly

closed. This sphincter allows food into the stomach and prevents food from

backing up into the esophagus.

f. Continue the same longitudinal incision through the pyloric sphincter . This sphincter valve keeps food in the stomach until it is su ffi ciently broken down to

be handled by the duodenum.

  1. Locate the small intestine beginning at the posterior end of the stomach. The small

intestine is a long, coiled tube, divided into three regions: the duodenum , the

jejunum , and the ileum .

a. The anterior curved portion of the small intestine leaving the stomach is the

duodenum . This portion is approximately 1 cm long. The common bile duct

from the liver and gallbladder can be seen entering the duodenum. Pancreatic

enzymes enter the duodenum and serve as the primary digestive agents in this

site of most chemical digestion.

b. Open the duodenum by continuing the longitudinal incision through its wall

from the pyloric sphincter, on the side away from the opening of the common

bile duct.

c. The two remaining portions of the small intestine, the jejunum and the ileum ,

are approximately equal in length and have no readily distinguishable

boundary. The jejunum is the middle portion of the small intestine, and the

ileum is the la er half that enters the large intestine. These two sections of the

intestine represent the location of the greatest amount of absorption by the

digestive tract. Cut open a section of the small intestine and observe the velvet

like texture of the interior of the small intestine. This texture is due to small

fi nger-like projections called villi that greatly increase the absorptive surface of

the small intestine.

  1. Locate the spleen, the long, dark organ to the left of the stomach. It is a ached to the

greater curvature of the stomach by means of the greater omentum , a specialized fold

of the peritoneum. The spleen functions in the destruction of worn out red blood

cells and the production of some lymphocytes.

  1. The pancreas lies in the angle between the curve of the stomach and the duodenum.

The greater part of the gland is located dorsal to the stomach. The pancreas secretes

enzymes that act upon all major categories of food (carbohydrates, lipids, proteins,

and nucleic acids); it also contains endocrine patches (the islets of Langerhans) that

produce insulin and glucagon, hormones essential for normal glucose metabolism.

The pancreas is connected to the duodenum by the pancreatic duct . This duct is

small and need not be dissected out.

  1. Locate the peritoneum , the double membrane lining the abdominal cavity.

a. The parietal layer of the peritoneum lines the body wall; the visceral

layer covers the abdominal organs.

b. Locate the mesentery , the double layer of the peritoneum extending from the

dorsal wall of the abdominal cavity to the small intestine. The mesentery

contains blood vessels, lymph vessels and nerves. Nutrients are absorbed into

the bloodstream by the mesentery and then sent to the liver. 10. Unravel and string out the small and large intestines by carefully cu ing the

mesentery that holds the intestines in a tight ball. Do not dissect out these organs

and be as careful as possible. If successful, you will fi nd a continuous tube starting

from the pyloric sphincter and ending at the rectum of the large intestine.

  1. Trace the ileum to its point of a achment with the large intestine.

a. The ileum opens into the side of the colon, forming a blind pouch, the cecum ,

at the beginning of the colon. The cecum contains bacteria that serve to

break down much of the cellulose that is present in the diet of herbivores.

b. Cut into the cecum, wash out the contents, and observe the ileocecal sphincter ,

which is found at the entrance to the small intestine and prevents material in

the colon from backing up into the small intestine.

c. The fi rst part of the large intestine in the fetal pig is called the spiral colon . It is

visible as a compact coiled mass on the left side of the abdominal cavity. This

structure is characteristic of the pig and is not found in humans.

d. The posterior portion of the large intestine is the rectum . Locate this structure

passing from the spiral colon as a straight tube into the pelvic region. The

external opening of the rectum is the anus .

Day 4: Heart, Arteries and Veins

Background

The blood vessels of a fetal mammal, such as the pig, closely resemble those of the

human adult. Modi fi cations for fetal life include a placental circulation, by way of the

umbilical cord, and two devices to bypass the lungs, since the lungs are not functional

before birth. The arteries of the fetal pig may have been injected with red latex and the

veins with blue latex.

Examination Procedure

  1. You are to follow the steps listed below and fi nd each structure that is in bold print.

Do not go to the next structure until you have successfully located the indicated bold

term on your fetal pig.

The Heart

  1. Observe the pericardium surrounding the heart. After determining the structures to

which it is a ached, remove the parietal layer of the pericardium. The visceral layer

of the pericardium forms the epicardium of the heart, the outermost layer of the

heart.

  1. Note that the apex of the heart is directed toward the left. The heart is tilted so that

the greater part of the right ventricle lies directly in front, along the ventral surface of

the heart. The left ventricle forms the apex of the heart. 3. The atria lie anterior to the ventricles. Each atrium has a conspicuous ear-like

appendage called the auricle on the ventral surface.

  1. A groove, the coronary sulcus , separates the right atrium from the right ventricle.

The anterior longitudinal sulcus is the groove that separates the right ventricle from

the left ventricle. Dorsal to this sulcus is the interventricular septum . The coronary

blood vessels are in these grooves.

The Venous System

  1. Observe the anterior vena cava , the large vessel entering the anterior part of the

right atrium. (This vein is called the superior vena cava in humans.) The anterior

vena cava drains the head, neck, and arms.

  1. Locate the posterior vena cava posterior to the heart and trace it forward to the point

where it drains into the right atrium. This large vein, called the i nferior vena cava in

humans, drains the lower portion of the body.

The Arterial System

  1. Locate the pulmonary artery on the ventral surface of the heart. Trace it down to its

origin in the right ventricle; then follow it toward the lungs, noting that it branches

into a right and left pulmonary artery.

  1. The aorta arises from the left ventricle. Locate this vessel dorsal and anterior to the

pulmonary artery. The fi rst branches of the dorsal aorta are the small, right and left

coronary arteries, which arise from the base of the aorta. The left coronary artery is

visible on the ventral surface of the heart in the anterior longitudinal sulcus; the right

coronary artery is in the coronary sulcus. The coronary arteries supply the heart

muscle with fresh, oxygenated blood and other needed nutrients. Even though blood

constantly fl ows through the inside of the heart, nutrients are not transferred to the

heart muscle at this point but rather from the hearts own arterial vessels.

Dissection of the Fetal Heart

  1. To dissect the fetal heart, make an incision through the lateral wall of each atrium or

auricle.

  1. Observe the point of entrance of the superior and inferior venae cavae into the right

atrium. Day 5: Urinary and Reproductive Systems

Urinary System Background

The organs in the urinary system of the fetal pig are very similar to those in the human.

As you dissect the organs, be prepared to trace the path of urine from its site of

production to the point at which it passes to the outside. The pig kidney will be

sectioned in order to study its internal structure, since it provides a good example of a

typical mammalian kidney.

Examination Procedure

  1. Observe the paired kidneys on the dorsal body wall of the pig.

  2. Locate the adrenal gland , a narrow band immediately above each kidney.

  3. Observe the ureter , the narrow, white convoluted tube which drains the urine from

each kidney. Trace the ureter from the hilum , the opening on the medial border of

each kidney, to the urinary bladder , freeing it from the peritoneum. The urinary bladder

is a ached to the re fl ected ventral strip of the abdominal wall.

  1. Locate the urethra, the duct which conducts urine from the posterior end of the

bladder to the outside. The remainder of the urethra will be freed when the

reproductive system is dissected.

5.Remove one kidney. Make a longitudinal section through the kidney. Observe.

Reproductive System Background

The reproductive organs of the fetal pig are similar to those of the human. Note,

however, the di ff erence in the structure of the uterus of the female. Although you will

dissect the reproductive system of only one sex, you are responsible for that of both

sexes.

Examination Procedure

Part 1. The Female Reproductive System

  1. Identify the ovaries, a pair of small light colored oval bodies located posterior to the

kidneys.

  1. The uterine tubes (Fallopian tubes) are very small, highly convoluted tubes lying on

the dorsal surface of the ovaries. The expanded end of the Fallopian tube, which partially

covers the ovary and picks up the eggs from the ovary, is called the ostium .

  1. Trace the Fallopian tube until reaching a larger tube next to each ovary. These tubes,

the uterine horns or horns of the uterus, are the beginning of the uterus. The eggs are

carried through the Fallopian tubes to the uterine horns where, if fertilized, they

develop. The fetuses tend to be equally spaced throughout the two horns.

  1. The two horns unite in the midline to form the body of the uterus which lies dorsal to

the urethra. Part 2. The Male Reproductive System

  1. Locate the scrotum , the sac visible under the skin ventral to the anus. Early in fetal

development the testes are located below the kidneys; however, they migrate

before birth through the inguinal canal into the scrotum.

  1. The vas deferens carries the sperm from the epididymis through the inguinal canal to

empty into the urethra. Trace the vas deferens through the inguinal canal to the urethra,

noting how it loops over the ureter and enters the dorsal surface of the urethra.

  1. Locate the penis , the long muscular tube lying just under the skin immediately

posterior to the umbilical cord and the urogenital opening in the midventral strip of

the abdominal wall. Remove the overlying skin so that the penis is exposed.

  1. Locate the seminal vesicles and prostate glands at the beginning of the urethra. They

can be found on the dorsal side of the urethra and ventrally to the rectum. These

glands, along with the Cowpers glands, will produce seminal fl uid that nourishes

and protects the sperm cells.

Day 6: BRAIN & SPINAL CORD

Follow the directions below and locate each of the structures.

  1. Rotate the pig so the skull is up. Using scissors cut away the skin on the head to

explore the skull. The bone is not yet completely calci fi ed, and the skull is

largely cartilaginous. Use scissors to carefully cut a circle in the top of the skull.

Remove the disk you have cut. You have revealed the two hemispheres of the

cerebrum . The cerebrum controls thinking, senses, etc.

  1. Use your scissors to expose the rest of the brain. Locate the longitudinal fi ssure

or indention that runs laterally between the right and left cerebral hemispheres.

The olfactory lobes that control smell are at the front of the cerebrum.

  1. Posterior to the cerebrum is the cerebellum. Locate the cerebellum and the

transverse fi ssure that separates it from the cerebrum. The cerebellum consists

of 2 lateral hemispheres and is involved with the control of muscles and

coordination.

  1. Carefully cut posteriorly from the skull to the vertebral column in order to

locate the hind section of the brain known as the medulla oblongata and the

spinal cord . The medulla is on the ventral side of the cerebellum, so you may

need to look under the cerebellum to fi nd it. The medulla connects the brain to

the spinal cord and controls all vital functions of the body such as heartbeat

and breathing. Try to expose where the spinal cord a aches to the medulla

oblongata.

  1. With forceps and a razor remove the muscle from the mid-dorsal line of the

back. Carefully use the razor to make a series of slices through the vertebrae to

expose the thick, whitish spinal cord.