LAB

A Cross-Species Comparison

of Organ Systems

Paul Nylund • 10BB • Polinsky • 3.18.12

Purpose

There are many differences between the organ systems of different animal

species. In this lab, the bodies of a frog and a fetal pig were dissected to have

their organs observed and compared with each other and the human body. The

lab also allows the observer to learn about the internal organs in general with

hands-on experience. I hypothesized that the frog and the pig would have

relatively similar organ structures with the exception of the intestines and

presence of the umbilical cord.

Materials

• Safety Goggles or Mask, Surgical Gloves, and a Lab Apron

• Forceps

• Preserved Frog or Fetal Pig

• 6 to 10 Dissecting Pins

• Dissecting Tray and Paper Towels

• Plastic Storage Bag and Twist Tie

• Scissors

• Dissecting Needle or Probe

• Camera

• Metric Ruler

• A Lot of String

Procedure: Frog Dissection

1. Put on safety goggles or mask, gloves, and a lab apron.

2. Place a frog on the dissection tray. Examine the fingers on its forelegs to

determine its sex. If it has thick pads on its thumbs, it is male. Male frogs

also tend to be smaller than females.

3. Identify the mouth, external nares, tympani, eyes, and nictitating

membranes on the head.

4. Turn the frog on its back and pin down the legs. Cut the hinges of the

mouth and open it wide. Use a probe to find and identify the vomerine

teeth, maxillary teeth, internal nares, tongue, openings to the Eustachian

tubes, esophagus, pharynx, and the glottis.

5. Look for the opening to the frog’s cloaca between the hind legs. Use the

forceps to lift the skin and use scissors to cut along the center of the

body from the cloaca to the lip. Turn back the skin, cut toward the side of

each leg, and pin the skin flat. The cloaca is the structure that receives

eggs, sperm, urine, and wastes from the body.

6. Lift and cut through the muscles and breast bone to open up the body

cavity. If the frog is female, the abdominal cavity may be filled with dark-

colored eggs. If this is the case with your frog, remove the eggs from the

frog.

7. Use the provided diagrams to identify the organs of the digestive system.

Take photos of the organs and label them.

Photos: Frog Dissection

1

2

1. Fat Bodies

Spaghetti shaped structures that have a bright orange or yellow color, if you

have a particularly fat frog, these fat bodies may need to be removed to see

the other structures. Usually they are located just on the inside of the

abdominal wall.

2. Peritoneum

A spider web like membrane that covers many of the organs, you may have

to carefully pick it off to get a clear view.

3. Liver

The largest structure of the body cavity. This brown colored organ is

composed of three parts, or lobes. The liver consists of a right lobe, left

anterior lobe, and left posterior lobe. The liver is not primarily an organ of

digestion; it does secrete a digestive juice called bile. Bile is needed for the

proper digestion of fats.

4. Heart

At the top of the liver, the heart is a triangular structure. The left and right

atrium can be found at the top of the heart. A single ventricle located at the

bottom of the heart. The large vessel extending out from the heart is the

conus arteriosis.

5. Lungs

Locate the lungs by looking underneath and behind the heart and liver. They

are two spongy organs.

6. Gall Bladder

Lift the lobes of the liver. There will be a small green sac under the liver. This

is the gall bladder, which stores bile.

7. Stomach

Curving from underneath the liver is the stomach. The stomach is the first

major site of chemical digestion. Frogs swallow their meals whole. Follow

the stomach to where it turns into the small intestine. The pyloric sphincter

valve regulates the exit of digested food from the stomach to the small

intestine.

8. Small Intestine

Leading from the stomach. The first straight portion of the small intestine is

called the duodenum. The curled portion is the ileum. A membrane called

the mesentery holds the ileum together. Note the blood vessels running

through the mesentery, they will carry absorbed nutrients away from the

intestine. Absorption of digested nutrients occurs in the small intestine.

9. Large Intestine

As you follow the small intestine down, it will widen into the large intestine.

The large intestine is also known as the cloaca in the frog. The cloaca is the

last stop before wastes, sperm, or urine exit the frog's body.

10. Spleen

Return to the folds of the mesentery. This dark red spherical object serves as

a holding area for blood.

11. Esophagus

Return to the stomach and follow it upward, where it gets smaller is the

beginning of the esophagus. The esophagus is the tube that leads from the

frog’s mouth to the stomach. Open the frog’s mouth and find the

esophagus, poke your probe into it and see where it leads.

Procedure: Fetal Pig Dissection

Part 1: External Anatomy

1. Obtain a fetal pig and rise off the excess preservative by holding it under

a running faucet. Lay the pig on its side in the dissecting pan and locate

the dorsal, ventral, and lateral surfaces. Also locate the anterior and

posterior ends.

2. Measure the pig’s length from snout to base of tail to determine its

approximate age. The pig we received measured 35cm, longer than the

average newborn pig. We then assumed that the pig was going to be born

soon after it was killed and preserved.

3. Examine the pig’s head. Locate the eyelids and the external ears or

pinnae. Find the external nostrils.

4. Study the pig’s appendages and examine the pig’s toes. Count and record

the number of toes and type of hoof the pig has. The pig had 4 toes.

5. Locate the umbilical cord. With scissors, cut across the cord about 1cm

from the body. Examine the 3 openings in the umbilical cord.

6. Lift the pig’s tail to find the anus. Study the ventral surface of the pig

and note the tiny bumps called mammary papillary. In females, these

structures connect to the mammary glands.

7. Determine the sex of the pig by looking for the urogenital opening. On

females, this opening is located near the anus. On males, the opening is

located near the umbilical cord.

8. If the pig is female, you should also note that the urogenital papilla is

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

9. Observe the eyes of the pig. Carefully remove the eyelid so that you can

view the eye underneath. The eyes were well developed and the pigs were

most likely born with their eyes shut.

10. Carefully lay the pig on one side in the 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.

11. With scissors, make a 3cm incision in each corner of the pig’s mouth.

Spread the jaws open and examine the tongue. Observe the palate on the

roof of the mouth. I could feel my own hard and soft palates with my

tongue.

12. Note the taste buds, or 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. Locate the canine and incisor teeth. The fetal pig had

teeth. Humans are not born with teeth.

13. Locate the epiglottis, a cone-shaped structure at the back of the mouth.

A flap of skin helps to close this opening when a pig swallows. Find the

opening to the esophagus.

14. Clean up the materials. Tie the legs of the pig together using the string

(front two and back two), and label the pig with your name(s) using a

piece of tape that is attached to on of the legs.

Part 2: The Incision

1. Place the fetal pig ventral side up in the dissecting tray.

2. Tie a string securely around a front limb. Run the string under the tray,

pull it tight, and tie it to the other front limb. Repeat this procedure with

the hind limbs to hold the legs apart so you can examine internal

structures.

3. Study the diagram below. The dashed lines numbered 1-5 show the first

set of incisions that you will make. To find the exact location for the

incision marked 2, press along the thorax with your fingers to find the

lower edge of the ribs. This is where you will make incision 2.

4. With scissors, make the incisions in order, beginning with 1. Be sure to

keep the tips of your scissors pointed upward because a deep cut will

destroy the organs below. Also, remember to cut away from yourself_

5. After you have made your incisions through the body wall, you will see the

peritoneum, a thin layer of tissue that lines the body cavity. Cut through

the peritoneum along the incision lines.

6. Spread the flaps of the body wall apart. Cut the umbilical vein, which

extends through the liver. Once the vein is cut, carefully pull the flap of

skin, including the end of the umbilical cord between the hind legs. You

are now able to see the organs of the abdominal cavity.

7. If time remains continue with part B, the digestive tract. Otherwise, clean

up and return your materials and pig as you did on day 1.

Part B: Digestive System

1. Locate the diaphragm, a sheet of muscle that separates the abdominal

cavity from the thoracic cavity. Find the most obvious structure in the

abdominal cavity, the brownish-colored liver. There were 4 lobes.

2. Find the tube-like esophagus, which joins the mouth and the stomach.

Food moves down the esophagus by muscular contractions after being

softened by saliva in the mouth. Follow the esophagus and locate the

soft, sac-like stomach beneath the liver.

3. Using scissors, cut along the outer curve of the stomach. Open the

stomach and note the texture of its inner walls. These ridges inside the

stomach are called rugae and increase the area for the release of

digestive enzymes. The stomach may not be empty because fetal pigs

swallow amniotic fluid.

4. The pig has a digestive system, which is classified as monogastric or

nonruminant. Humans also have this type of digestive system. They have

one stomach (mono=one, gastric=stomach). Locate the entrance to the

stomach or esophageal area, the cardiac region, which is largest, and the

pyloric region where the stomach narrows to join to the small intestine.

5. At the end of the stomach, there is a sphincter, or ring-shaped muscle to

control food leaving the stomach and entering the duodenum. Locate

the cardiac sphincter at the junction of the stomach and esophagus, and

the pyloric sphincter at the junction of the stomach and small intestine.

Fetal pigs receive their nourishment from their mother through the

umbilical cord.

6. Identify the first part of the small intestine, the U-shaped duodenum,

which connects to the lower end of the stomach. Pancreatic juice, made

by the pancreas, and bile, made by the liver and stored in the gall

bladder, are add to food here to continue digestion.

7. Study the rest of the small intestine. Notice that it is a coiled, narrow

tube, held together by tissue called mesentery. The soupy, partly

digested food that enters the small intestine from the stomach is called

chyme.

8. Carefully cut through the mesentery and uncoil the small intestine. Note

and record its length in centimeters. The mid-section is called the

jejunum, while the last section is called the ileum. The small intestine

measured 250cm long.

9. With scissors, remove a 3-cm piece of the lower small intestine. Cut it

open and rinse it out.

10. Observe the inner surface of the small intestine. Run your finger along it

and note its texture. Using a magnifying glass, examine the villi, the tiny

projections that line the small intestine and increase the surface area for

absorption.

11. Follow the small intestine until it reaches the wider, looped large

intestine. Cut the mesentery and unwind the large intestine (CAREFUL!)

or colon. Measure and record its length. The large intestine measured

20cm long.

12. At the junction of the large and small intestine, locate a blind pouch

called the caecum. The caecum has no known function in the pig. Notice

that the large intestine leads into the rectum, a tube that runs

posteriorly along the dorsal body wall. The rectum carries wastes to the

opening called the anus where they are eliminated.

13. Locate the thin, white pancreas beneath the stomach and duodenum.

Pancreatic juice flows through pancreatic ducts to the duodenum.

14. Between the lobes of the liver, find the small, greenish-brown gall

bladder. Locate the hepatic duct, which carries bile from the liver to the

gall bladder.

15. Find the spleen, a long, reddish-brown organ wrapped around the

stomach. The spleen filters out old red blood cells and produces new

ones for the fetus.

16. On the diagram (crude drawing), label the pig's body organs.

17. Clean up your materials and work area. Wrap the pig in damp paper

towels and put it in a plastic bag. Obtain a piece of masking tape and

label your bag with your names. Return your lab equipment and pig to

the supply cart and then thoroughly wash your hands with soap.

Photos: Fetal Pig Dissection

1. Liver and Gall Bladder (above the heart)

8. Spleen (next to the liver)

3. Heart (between lungs and liver)

4. Stomach

(Connected to Esophagus

and small intestines by sphincters)

5. Small

Intestines

(connect stomach

and large intestine)

6. Large Intestines

(Connect small intestine and rectum)

7. Lungs (lie over the heart)

8

Conclusion

This lab test made it clear that while the digestive systems in mammals

and amphibians share many of the same traits, there are differences that may

affect the eating habits of each branch. In this conclusion, the features of the

human organs will also be compared to the structures of the frog and pig. My

hypothesis was in fact correct, as the pig and frog did have different intestine

lengths, and the frog was missing an umbilical cord (a little obvious). The two

major systems that were looked at closely are the circulatory system, which

works to deliver supplies throughout an organism’s body, and the digestive

system, which digests food to extract nutrients and excrete waste.

The esophagus of the frog was much shorter than that of the fetal pig.

Pigs have a much longer esophagus proportional to their body, similar to

humans. A longer esophagus will give the animal’s salivary enzymes a longer

period of time to digest food. The frog had a shorter esophagus, most likely

because it contained more enzymes in the stomach to digest the food further

along in the digestive process.

The pig contained fewer enzymes, and because they are quite similar to

humans, one can hypothesize that humans also have fewer enzymes than a

frog. There are some ideas that may help to further explain this. A frog tends to

swallow its food whole (such as insects), while a pig takes more time to grind its

food down with its teeth. A frog would need more enzymes in its stomach to

make up for the lack of physical digestion in its mouth. Animals with more

advanced diets need longer digestive systems, because they don’t have enough

enzymes to break the food down quickly enough. The process to extract the

nutrients and minerals is also much more advanced. A longer esophagus will

contribute to a longer digestive system, while a short one contributes to a

digestive system that does not require a very long digestive tract.

The structures of the stomachs of the pig and frog were very different

from each other. The frog’s stomach was much more of a channel as part of the

digestive tract, while the pig’s stomach’s structure was much more sack-like.

Frogs don’t need to contain their food in their stomach for very long, because

their multitude of enzymes will digest it more rapidly. Pigs’ stomachs need to

contain the food for a longer period of time to allow for the enzymes to digest

it successfully. Many other mammals, including humans have the same stomach

structures.

A pig’s pancreas produces digestive enzymes for the small intestines and

insulin, that controls the blood sugar level. This is similar to human beings. Too

much or to little insulin can lead to diabetes. A frog’s pancreas also produces

digestive enzymes. The frog’s pancreas is relatively larger than the pig’s when

compared to the size of the small intestines. This may be because it has to

produce more enzymes as described in the paragraph above.

The frog had a single slit between its legs called the cloaca. All waste and

reproductive products are excreted through this opening. The pig has two

openings, however: one for feces (the anus) and another for urinary and

reproductive functions. The male genitals are located below the umbilical cord,

and the female reproductive organs are located next to the anus of the pig.

Once again, humans share more similarities with pigs, but the structures are still

very different. Human females have a separate urinary tract, and the

reproductive systems of males are located well beneath the naval cavity.

The lungs of the pig were very large in relationship to those of the frog.

The pig requires more oxygen (hence the larger lungs) to support its larger,

more intricate circulatory system. Frogs have smaller lungs, because they use

their skin for gas exchange, as well. Humans’ lungs are similar to pigs’ lungs,

because their circulatory systems are very similar.

The mammalian heart is much more advanced than the frog’s. It has a

total of four chambers, wheras the frog’s has only three chambers. A mammal’s

heart separates oxygenated blood from deoxygenated blood to accomplish

higher efficiency when delivering oxygen to a larger body. A frog’s heart

partially mixes oxygenated blood with deoxygenated blood, because it is less

important for the blood to be completely oxygenated, as it takes a much

shorter time for the blood to be reoxygenated in the frog’s smaller body.

Comparatively, the sizes of the livers of the pig and the frog were very

similar. The functions of the liver are the very same: to produce bile, store

glycogen, and to detoxify poisons. Because of the similarities between the livers

of frogs and pigs, it is very likely that the human liver has similar functions and a

similar relative size.

The pig and frog gall bladders look very different from each other. The

pig’s gall bladder hugs the underside of the liver and is colored the same brown

tint, and the frog’s gall bladder is small and green, attached to the back surface

of the liver. The function of the gall bladder is to store bile that has been

produced by the liver and to deliver the bile to the small intestines. There is not

a clear reason why there is so much color variation between the two gall

bladders, so it would be difficult to hypothesize what a human’s gall bladder

might look like (the fact was later researched to find that the human gall

bladder is usually green). There was most likely some discoloration of the pig’s

gall bladder, as well, according to external sources. During our lab, we could not

locate the gall bladder in our pig. We listed this as an experimental error.

The small intestine of the fetal pig was, comparatively, much longer than

that of the frog. It measured to be 250cm long, while the frog’s small intestines

measured only 5cm long. This relates back to the findings surrounding the

relationship between the total length of the digestive tract and the diet of the

animals (discussed in paragraphs 2 and 3 of the conclusion). If one considers the

length of the pig (because humans are also mammals), he or she can calculate

his or her intestine size. It was calculated that, a person measuring 170cm tall

would have small intestines measuring approximately 1214cm long, or enough

to make 81 hot dogs.

The function of the large intestine is to store waste before it is excreted

from the body. The large intestine of the pig was discovered to be much shorter

than the small intestine, measuring at only 20cm long. Interestingly though, the

frog’s large intestine matched the length of its small intestine at 5cm long.

Because the large intestine has no digestive function, the sizes of the large

intestines in the pig and frog are comparable. The same rule can be applied to

humans.

By performing these dissections, one is able to conclude that there are

clear differences between humans, pigs, and frogs. Most of them are due to the

fact that frogs are amphibians and have followed a very different evolutionary

path from mammals such as humans and pigs, but many similarities can be

drawn between the branches. Many of the organs are very similar to each other

and provide similar functions in three very different bodies in a facinating

display of the deeply connected biological world we live in. One also knows, as a

result of this lab, how the different digestive systems have adapted to the diet

and surrounding climate of the host animals.

This turned out to be a very successful lab, with a very minimal amount

of experimental errors. It was a good demonstration of what occurs in the

natural world all the time and was able to open up one’s perspective to observe

living creatures in a more intellectual fashion. If I could change one thing,

however, I would have the option to dissect an animal that did not have all of its

blood drained from its body (which tends to make it look lifeless and

uninviting).

Sources

Urinary and Reproductive Systems http://goo.gl/A1ATd
A Frog’s Lungs and How They Work http://goo.gl/UUVU6
Normal Heart Rate of Frogs http://goo.gl/fhy0R
Circulatory System of a Pig http://goo.gl/LbIS1
Primary Functions of the Liver http://goo.gl/98WDt

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