30.2: Exercise
Dissection is a powerful tool that provides us a profound understanding of our own anatomy and physiology as living, breathing creatures and also helps us to develop a stronger understanding of evolutionary relationships between taxonomic groups. What sort of dissection experiences do you already have? As you begin this sequence of dissections, please keep in mind several things. Dissection should be done thoughtfully and respectfully. It is important to take your time carefully and to think about what you are doing. This care will help you to preserve structures for the next several weeks and will make review easier. Also, merely identifying structures is insufficient to develop fully an appreciation for how these structures work. You must think about the function of these structures. How do they work?! Also, please do these dissections respectfully. These organisms were euthanized so that we might have a wonderful opportunity to learn something greater about the living world. Please keep that in mind as you are working.
Squid Dissection
Procedure:
External Anatomy
- Place the squid in the dissection pan with the mantle (major body part) facing away from you and the tentacles and arms towards you.
- Turn the squid so that the siphon faces you. It is located between the eyes. By expelling water through the siphon the squid can effectively move through the water.
- Notice the chromatophores on the mantle. They allow the squid to change color and blend into the environment.
- At the pointed tip of the mantle there are two fins that help stabilize and propel the squid
- Notice the eyes on either side-they are well developed and allow the squid to have excellent vision
- Distinguish between the tentacles and the arms. The tentacles are longer and are used to pass food to the arms.
- Count the number of arms. How many are there?
- Notice the suction cups on both the tentacles and the arms. How does the distribution of the suction cups differ between these two structures?
- Pull back the arms and locate the beak or mouth in the middle
Draw the external anatomy of your squid. Label the listed structures on your drawing and give functions.
- Tentacles
- Arms
- Suction cups
- Beak
- Eyes
- Mantle
- Fins
- Siphon
Dissection
- Ensure the siphon is facing up. Grab the mantle with your hand and squeeze until you "pop the collar". There should now be space under the mantle. Using scissors, cut the mantle up the middle from the funnel to the tip at the top of the squid. Be careful to lift up with the scissors while cutting to avoid cutting the internal organs. The mantle should now fall open. Pin the mantle sides to your dissecting tray using dissecting pins.
-
Spread the mantle open and try to identify the following internal structures
- Feathery gills
- Heart, located at the base of each gill. Squid actually have three hearts!
- Liver, probably yellowish in color and long in shape running down the middle
- Ink sack, which looks like a small silver fish. If you find it, cut it out at both ends and you can extract some of the ink and try to write with it!
- The pen, which is all that remains of the shell. To try and find the pen, lift the head of the squid and place it down over the organs. You should notice a pointy area along the midline of the body, the tip of the pen. If you grasp the tip and pull the pen will release from the mantle. It resembles a transparent feather.
Internal Anatomy
Draw the internal anatomy of your squid. Label the listed structures on your drawing and give functions.
- Esophagus
- Stomach
- Caecum
- Ink sac
- Gills
- Hearts
- Pen
- Brain
-
Eyes: cut open an eye, taking the lens out.
- What shape is the lens?
- How is this lens similar to and different from the lens in the eye of a terrestrial mammal?
- Gonads: is your squid male or female? How can you tell?
- Nidamental gland (if female)
Frog Dissection
Procedure:
External Anatomy
-
Place the frog in the dissection pan legs down.
-
Identify the eyes, covered by a
nicitating membrane
, the
external nares
(nostrils), and the
tympanum
located behind each eye.
- What is the function of the tympanum?
- Examine the front and back limbs. How many phalanges are on the hindfeet? The forefeet? Which pair of limbs is the longest? How does this assist the frog in its movement?
-
Identify the eyes, covered by a
nicitating membrane
, the
external nares
(nostrils), and the
tympanum
located behind each eye.
-
Mouth
-
Turn the frog over and open the mouth as wide as you can. You can cut the hinges of the jaw if necessary. Identify the following structures:
- Two vomerine teeth located in the middle of the roof of the mouth
- Maxillary teeth (smaller) located on the sides of the upper jaw
- Tongue
- Pharynx (located behind the tongue)
- Esophagus , the opening leading to the stomach
- Glottis , slit where air passes through to enter the trachea , which leads to the lungs
- Eustacian tubes (2) openings that lead to the ears. They are located in the angle of the jaw.
-
Turn the frog over and open the mouth as wide as you can. You can cut the hinges of the jaw if necessary. Identify the following structures:
Draw the external anatomy of your frog. Label the listed structures on your drawing and give functions.
- Eyes
- Nares
- Tympanum
- Mouth with opening to esophagus and trachea
- Skin - Describe the skin. What do you notice about it? Why is the color and texture the way that it is?
Dissection
-
Body Dissection
- Place the frog belly side up in the dissecting tray. You can pin down the limbs if necessary.
- Lift up the skin with forceps midway between the hind legs of the frog. Use scissors to cut the skin along the midline of the frog starting between the hind legs and ending at the neck. Be careful not too cut too deeply.
- Cut the skin horizontally above the hind legs and below the front legs creating skin flaps.
- Pick up a skin flap with forceps and use a scalpel to separate the skin from the muscle below.
- Pin the skin flaps to the dissection tray.
- Repeat the same procedure to cut through the muscles. Create one long incision along the midline of the frog from between the hind legs to the neck. Be careful not to cut too deeply and damage the internal organs. When you reach the area just below the front legs of the frog, turn your scissors sideways to cut through the chest bones and avoid damaging the heart and lungs. Then make horizontal incisions above the rear legs and between the front legs. Use forceps and a scalpel to separate the muscle from the tissue below. Then pin the muscle to the dissection tra
Internal Anatomy
- The most prominent organ is the liver , dark brown in color, and taking up most of the abdominal cavity
- Identify the lungs , two small pouches on opposite sides of the frog midline. They may be partially hidden by the liver.
- Lift up the liver and underneath locate the gallbladder.
-
Identify the
heart
covered by the protective
pericardium
. Frogs have a three chambered heart with two atria and one ventricle. Try to locate these different areas of the frog heart.
- How is it a disadvantage to have a 3 chambered heart?
- The stomach is a j-shaped organ located underneath the left lobe of the liver. It connects to the esophagus bringing food from the mouth and the small intestine used for nutrient absorption.
- The small intestine connect to the large intestine which carry any undigested material to the cloaca . Frogs have one opening to the outside environment and the cloaca receives materials from the intestine, the urinary system and the reproductive system.
- Find the pancreas , a yellow ribbon located between the stomach and the small intestine.
- Locate the spleen , shaped similarly to a pea and located near the stomach.
- You will be able to see the yellow, finger like, fat bodies , which the frog uses to store fat.
- The kidneys of the frog are long and narrow and located along the back body wall.
- Try to find the mesonephric ducts , thin white tubes that carry urine from the kidney to the cloaca.
- If your frog is female, the abdominal cavity will be filled with black and white eggs. The eggs are stored in the ovaries.
-
If you have a male frog, locate the
testes
. The testes are shaped like a bean and located at the top of the kidneys. They are yellow/tan in color.
- Do you have a male or female frog?
Draw the internal anatomy of your frog. Label the listed structures on your drawing and give functions.
- Liver
- Lungs
- Gallbladder
-
Heart
- Once you have completed your dissection, take the heart out and cut it open. How many chambers does it have?
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Pancreas
- Spleen
- Fat bodies
- Kidneys
- Ovaries or testes
Pigeon Dissection
Procedure:
External Anatomy
- Place the pigeon in the dissection pan with ventral side up.
-
Note the major anatomy of the bird
- Head with beak and large eyes
- Wings
- Trunk
- Legs
- Feel the ventral side of the bird and find the keeled sternum . This bone is flat in humans, but has a keel on it in birds for muscle attachment
- On the legs, note the angle of the joints. Where is the femur located? Where are the knee and ankle joints? Compare bird legs with joints to that of a mammal.
What major anatomical modifications do birds have for flight?
Draw the external anatomy of your pigeon. Label the listed structures on your drawing and give functions.
- Eyes
- Nares
- Beak
- Wings
- Sternum
- Legs
- Feet with claws
- Cloaca
Dissection
- With the ventral side of your bird up, first use a scalpel to cut the skin from neck to vent and peel the skin back. Note how thin the skin is, and the large pectoralis muscle underneath.
- Feel down the sternum until you find the end, where the belly is soft.
- Using a scalpel or scissors cut the skin underneath the sternum from one side to the other, exposing the organs underneath.
- Cut up each side of the bird, until you come to a shoulder joint.
- Using a scissors, cut through the bone at each shoulder joint.
- You should now be able to lift the sternum with muscle attached up towards the head of the bird, and get a good view of all the organs inside.
- Note that birds do not have a separate thoracic and abdominal cavity, as we will see in mammals.
Internal Anatomy
Draw the internal anatomy of your pigeon. Label the listed structures on your drawing and give functions.
- Esophagus
- Proventriculus
- Gizzard
- Intestine
- Pancreas
- Spleen
- Heart
-
Lungs
- How do birds ventilate their lungs?
- Gonads: is your bird male or female? How can you tell?
Rat Dissection
Procedure:
External Anatomy
- Based on the external anatomy is your rat male or female? How can you tell?
- Can you locate any hair on your rat, a common characteristic of mammals? Where is there the most hair?
Draw the external anatomy of your rat. Label the listed structures on your drawing and give functions.
- Mouth
- Nose
- Eyes
- Body
- Feet with claws
- Tail
The Incision
Place your rat in the dissecting pan ventral side up. Use string to tie the legs behind the back of the pan. Use scissors to cut through the skin and muscles according to the diagram.
Your rat may be filled with water and preservative, drain over the sink if necessary.
Neck Region:
Using the diagram to the right begin your dissection in the neck region. Try to cut as little as possible. Once you open the body cavity, you will generally be able to separate the different organs by simply pulling them apart with your fingers, forceps, or a probe. The more you cut things up, the harder it will be to figure out what you’re looking at. Cut midline on the ventral surface of the neck to expose the underlying muscles. Carefully separate the muscles to observe the underlying structures. Locate and understand the functions of the following structures:
- Larynx : an enlarged structure on the trachea. If you cut it open, you can see the vocal cords inside.
- Thymus Gland : an endocrine (hormone-secreting) gland that helps regulate the immune system. It’s a large, spongy structure covering the ventral surface of the trachea, extending up along either side, and also resides over the heart. It is easy to cut so be extra careful.
- Thyroid Gland : another endocrine gland; it’s a small bilobed (two parts) structure just posterior to the larynx. The thyroid secretes hormones that help regulate metabolism.
- Trachea : the airway; it's reinforced with rings of cartilage so it does not collapse.
- Esophagus : carries food from mouth to stomach; soft and muscular so it can move a food bolus by peristalsis. It is located dorsal to the trachea (but appears behind it because the specimen is upside down).
Thoracic Cavity:
Vertebrates have true coeloms (a body cavity). In mammals, the coelom is divided into two main cavities: the thoracic cavity, which contains the lungs, and the abdominal cavity, which contains the digestive system. The thoracic cavity and the abdominal cavity are separated by the diaphragm. Note the many membranes lining the coelom and holding the organs in place. Look for these structures in the thoracic cavity:
- Lungs : they have several lobes. Note how spongy the tissue is.
- Heart : muscular and easy to find. The heart is surrounded by a pericardial sac. Note the aorta, where high-pressure blood leaves the heart on its way to the systemic circulation. You may also see the right and left carotid arteries, which supply blood to the head. For now, don’t spend too much time on the various lobes of the heart and the many blood vessels. Come back to these later.
- Diaphragm : a sheet of muscle and connective tissue that helps in breathing and divides the two cavities described here.
Abdominal Cavity (Digestion & Absorption):
Locate and understand the functions of the following structures:
- Liver : very large and dark. It has several lobes. You’ll need to lift it out of the way to see the organs beneath. The liver produces bile that is stored in the gall bladder. The gall bladder is a small organ attached to the underside of the liver; it's usually greenish due to the bile. It connects to the small intestine by the bile duct.
- Stomach : a muscular, sac-like organ that sits posterior to and to the left of the liver. Here is where the gastric juices released by glands continue enzymatic digestion started in the mouth. In particular, proteins are hydrolyzed via pepsin. At each end of the stomach are valves that regulate food entering and leaving the stomach. At the esophagus is the cardiac sphincter valve, and at the duodenum is the pyloric sphincter valve. View the inside of the stomach by slicing it open lengthwise.
- Small & Large Intestine : tube-like structures that continue the movement of food (now called chyme). The small intestine is first. The initial part of the small intestine (duodenum) is responsible for the final steps of enzymatic digestion and then eventually absorption of the degraded molecules. The large intestine primarily functions to compact the remaining waste material by absorbing water invested in the digestion and lubrication process. Also absorption of vitamins completes here.
- Rectum : the final portion of the large intestine where wastes are stored before elimination through the anus.
- Mesenteries : thin, transparent sheets of connective tissue containing blood vessels connecting the intestine and other organs.
- Pancreas : white and looks a little bit like cauliflower and located along the underside of the stomach, a pancreatic duct leads to the duodenum – first part of the small intestine. The pancreas also makes insulin, which is necessary for the proper uptake of sugars from the blood. It secretes digestive enzymes and buffers as well that contribute to the digestion of material within the small intestine.
- Spleen : The spleen is a flat organ located near the stomach. It performs several functions related to producing and maturing new blood cells and eliminating old ones. Blood passes through open sinuses in the spleen, rather than being confined to narrow blood vessels.
QUESTION
- What do the hard and soft palate separate?
Cut into the neck region. Make sure you can locate the following structures:
- Trachea
- Thymus
- Thyroid
- Esophagus
QUESTION
- Is the trachea in front of or behind the esophagus?
Cut into the thoracic cavity beneath the rib cage. Make sure you can locate the following structures:
- Heart
- Lungs
- Bronchi
- Diaphragm
QUESTIONS
- How many chambers does the rat heart have?
- How does the size of the rat lungs compare to the size of the frog lungs you dissected previously?
- What role does the diaphragm play in respiration?
- What cavity contains the lungs?
- What cavity contains the heart?
Focus next on the abdominal cavity. First look at the digestive system organs. Make sure you can locate the following organs:
- Stomach
- Spleen
- Liver
- Gallbladder
- Small intestine
- Large intestine
- Pancreas
QUESTIONS
- What is the function of the liver?
- What is the function of the gallbladder?
- What type of digestion occurs in the stomach?
- Name the three sections of the small intestines in order.
- Name one process that occurs in the large intestine.
- Which digestive organs located in the abdominal cavity are considered to be accessory organs?
Also in the abdominal cavity you will find the excretory system organs. Make sure you can locate:
- Kidneys
- Bladder
Finally in the abdominal cavity are the reproductive organs. If you have a female rat look at another group’s male rat and vice versa. You should be able to find:
- Testes (male)
- Uterus with horn (female)
- Ovary (female)
The arteries and veins are challenging to identify, especially if the pig is not injected with dye. Arteries carry blood away from the heart. Veins return blood to the heart. Try to identify the following:
- Aorta
- Pulmonary artery
- Coronary arteries
- Jugular vein
- Carotid artery
- Renal artery
- Renal vein
LICENSES AND ATTRIBUTIONS
CC LICENSED CONTENT, SHARED PREVIOUSLY
- Biology 102 Labs. Authored by : Lynette Hauser. Provided by : Tidewater Community College. Located at : [www.tcc.edu] . License : CC BY-NC-SA: Attribution-NonCommercial-ShareAlike