Laboratory Rat Dissection 141u5a

Laboratory Rat Dissection OBJECTIVES: 1. To name mammalian organ systems, and state the function of each 2. To identify these organs in a dissected rat 3. To relate the functions of these organs MATERIALS: Preserved rat Dissecting tray String Dissection scissors dissecting needle Scalpel

Probe Metric tape measure Plastic bag Masking tape String

BACKGROUND INFORMATION: Mammals are endothermic vertebrates that usually have hair or fur, bear live young, and nourish their young with milk produced in mammary glands. The placental mammals are the largest group of mammals. Placental mammals develop completely inside the mother’s uterus before they are born. During this time of development, these animals are connected by the umbilical cord to the placenta, a capillary-rich organ that attaches to the wall of the uterus. There, nutrients and oxygen from the mother’s body are exchanged with wastes and carbon dioxide from the developing fetus. In this investigation, you will be studying the rat. An in-depth examination of this organism will help you to better understand the features and body systems of all placental mammals, including humans. PRE-LAB: 1. Obtain a rat and lay it on its side in a dissecting tray, Observe the major body areas of the rat: head, m\neck, trunk (which includes the chest or thoracic region, and the abdominal region), and a tail. 2. On the diagram of the external features of the rat on your answer sheet, label the anterior and posterior ends of the rat, the dorsal, ventral, and lateral surfaces, the head, neck, trunk, and tail. 3. Use the tape measure to measure the length of the rat from the tip of the nose to the base of the tail. Record the measurement on you answer sheet. 4. Examine the rat’s head. Notice the cleft in the center if the lip. It is called the philtrum. On either side of the philtrum are mystacial pads which contain long sensory hairs called vibrissae. Locate the rostrum or snout, eyelids, and the external ears pr pinnae. Find the external nostrils or nares. On the diagram of the rat on your answer sheet, label the rostrum, mystacial pads, eyelids, and nares. 5. Study the rat’s appendages. The forefoot is called the manus, and the hind foot is called the pes. Notes the claws. Label the manus and pes on your answer sheet. 6. Lift the rat’s tail to find the anus. Study the ventral surface of the rat. Notice the small bumps called mammary papillae. These are present in both sexes. In the female, these structures connect to the mammary glands which produce milk to nourish the young. In the rat, there are usually six pairs of mammary glands. 7. Determine the sex of your rat by observing the ventral body surface beneath the tail. If a sac like scrotum and a single body opening are visible, the animal is a male. If three body openings are present, it is a female. Refer to diagram 1 for help. 8. Answer questions for this section on your answer sheet. 9. CLEAN UP YOUR MATERIALS. Place the rat in a plastic bag. Label a piece of masking tape with your name and class period. Place the masking tape on your bag, and close the bag with a rubber band. 10. Wash all dissecting equipment and the dissecting tray. Line up all equipment in the pan. Wash your table with soap and water. Be sure to have your area checked by your teacher before you leave the room. INVESTIGATION PROCEDURE: Part A: The Incision 1. Place a rat ventral side up in your 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 the internal structures.

3. Study the diagrams 2-7. Diagram 4 shows the first incision you will make. Follow the dashed lines on diagram 2 to make your next incisions. To find the exact location for the second incision, press along the thorax with your fingers and find the lower edge of the ribs. This is where you will make incision 2. 4. With scissors, make the incisions in order. Be sure to keep the tips of your scissors pointed upwards. Lift the skin away from the body as you cut. CAUTION: A DEEP CUT WILL DESTROY THE ORGANS BELOW!! 5. Follow the original incision lines and cut through the muscle tissue (body wall). Cut away any remaining connective tissue that holds the skin to the internal organs. 6. Cut through the sternum to reveal organs underneath. Rinse and drain the body fluids in the sink. 7. 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. Spread the flaps of the body wall apart. You should be able to see the organs of the abdominal cavity. Part B: Digestive System 1. Refer to diagram 8. 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 brownishcolored liver. 2. Find the tube-like esophagus which s the mouth and the stomach. Follow the esophagus and locate the soft, sac-like stomach beneath the liver. Be careful not to destroy the diaphragm as you proceed to step three. 3. With scissors, cut at each sphincter to remove the stomach. Open the stomach with your scissors, scrape out the contents, and note the texture of its inner walls. Record this information on your lab answer sheet. 4. Identify the first part of the small intestine, the U-shaped duodenum, which connects with the lower end of the stomach. 5. Study the rest of the small intestine. It is held in place by a membrane called the mesentery. Notice that it contains many blood vessels and may contain large fat deposits. 6. Carefully cut through the mesentery and uncoil the small intestine. Record its length on your answer sheet. 7. With scissors, remove a 3-cm long piece of the lower small intestine. Cut it open and gently rinse it out. Handle the tissue gently to prevent destruction of the villi. 8. Observe the inner surface of the small intestine. Using a dissecting scope, examine the villi, the tiny projections that line the small intestine. Run your fingers along it and note its texture. 9. Follow the small intestine until it s the wider, looped large intestine. Unwind the large intestine and record its length. 10. 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 anus where they are eliminated. 11. Locate the thin, white pancreas beneath the stomach and the duodenum. Digestive enzymes produced by the pancreas flow into the small intestine through the pancreatic duct. Unlike most other mammals, the rat has no gall bladder. Bile es directly into the duodenum. 12. Find the spleen, a long, reddish organ wrapped around the stomach. The spleen filters out old red blood cells and is often referred to as the red blood cell graveyard. 13. On the diagram of the digestive system on your answer sheet, label the following organs: liver, small intestine, mesentery, large intestine, pancreas, and spleen. Answer the appropriate questions for this section on you answer sheet. Part C: Urogenital System 1. Once the digestive organs are removed, you may study the excretory and reproductive organs which make up the urogenital system. 2. Refer to diagram 9 to locate the large, bean-shaped kidneys lying against the dorsal wall. They filter nitrogenous wastes from the blood. Notice that they are covered by the peritoneum. Identify the adrenal glands that are located at the top of each kidney. 3. Find the ureters, tubes which extend from the kidneys to the baglike urinary bladder where urine is stored until it is released from the body. 4. Lift the urinary bladder to find the urethra, the tube which carries urine out of the body. Follow the urethra to the urethral opening on the outside of the rat’s body. Answer the appropriate questions on your lab report. 5. On your answer sheet, label the following structures: kidney, ureter, urinary bladder, and urethral opening. Color the inferior vena cava blue. Color the descending aorta red. 6. Answer the questions for this section on your answer sheet. Male Reproductive System

1. Make a shallow incision into the scrotum. Identify the oval testis. The testes produce sperm and testosterone, the male hormone 2. Remove and dissect the testis. Make a longitudinal cut to dissect the testis into halves. Observe the structure of the testis with the dissecting scope. The coiled tubules that make up the testis are the seminiferous tubules and are the site where sperm are produced. 3. Label the following structures: testis, scrotum, vas deferens, and penis. 4. Answer the questions for this section on your answer sheet. Female Reproductive System 1. Inspect the pelvic cavity to identify the Y-shaped uterus lying against the dorsal body wall and beneath the bladder. Follow one of the uterine horns superiorly to identify an ovary, a small oval structure at the end of the uterine horn. (The rat uterus is different from the uterus of human females, which is a single-chambered organ about the size and shape of a pear.) The lower, undivided part of the rat uterus s the vagina, which leads outside the body. Identify the external vaginal opening. 2. Remove the uterine horn and dissect it. If your rat was pregnant, you should be able to see the fetuses. 3. Label the following structures: ovary, uterine, uterus, vagina, vaginal opening, and urethral opening. 4. Answer the questions for this section on your answer sheet. Part D:

Respiratory System

1. Examine the diaphragm, a sheet of muscle that stretches across the abdominal cavity and separates it from the thoracic cavity. 2. Carefully separate the pericardium (the sac surrounding the heart) and the diaphragm from the body wall. 3. Study the two spongy lungs that surround the heart. 4. Locate the trachea, a large air tube which lies anterior to the lungs. The trachea is easy to identify because of its cartilaginous rings. Notice that the trachea branches into the bronchi which lead into each lung. 5. Locate the thymus gland. It is a large, irregular mass of glandular tissue over the heart. The thymus plays a role in the immune system of the body. 6. Remove the thymus gland. 7. Anterior to the trachea, find the hard, white larynx or voice box. This organ contains the vocal cords that enable the animal to produce sounds. 8. Answer the questions for this section on your lab report. Part E: Circulatory System 1. Locate the heart. The heart is protected by a thin membrane called the pericardium. Study the structure of the heart. Compare the heart to diagrams 10 and 11, and identify tits parts. Answer the appropriate questions on your lab report. 2. Examine the atria and ventricles. With your probe, gently press on an atrium and a ventricle and compare the way they feel. Record this information on your answer sheet. 3. Notice that the surface of the heart is covered with blood vessels. These vessels are part of the coronary circulation, a set of arteries and veins whose only job is to nourish the heart tissue. Answer the appropriate questions on your lab report. 4. Now lift the heart to view its dorsal surface. Observe the inferior or caudal vena cava that carries blood from the posterior part of the body and empties it into the right atrium. 5. Anterior to the heart, locate another large vein that enters the right atrium. This vein, the superior or cranial vena cava, brings blood to the right atrium from the anterior part of the body. 6. Find the pulmonary artery which leaves the right ventricle. This vessel carries blood to the lungs. They are dark in color. 7. Locate the pulmonary veins that enter the left atrium. These vessels bring blood back into the heart from the lungs. 8. Identify the aorta, a large light gray artery that transports blood from the left ventricle. Many arteries that carry blood throughout the body branch from the aorta. Note the two branches that go to the head region 9. Remove the heart by cutting through the vessels attached to it. 10. Hold the dorsal and ventral surfaces of the heart with your thumb and forefinger, and rest the ventricles on your dissecting tray. Obtain a scalpel from your teacher. Cut the heart into longitudinal halves. CAUTION: The scalpel is very sharp. Use it carefully always cut away from yourself.

11. Observe the light colored septum that completely separates the right and left sides of the heart. 12. Remove any material inside the heart and expose the walls of the atria and the ventricles. 13. Study the internal features of these chambers and note where vessels enter or leave each chamber. Locate the valves between each atrium and ventricle. These structures prevent blood from flowing backward in the heart. 14. Answer the questions for this section on your lab report. 15. Label the following structures on your answer sheet: right atrium, right ventricle, left atrium, left ventricle and aorta. 16. Once you have completed your dissection, put the rat and any body tissue in the plastic bag and close it. All bags will be thrown into the trash. Carefully wash all your dissecting tools and your dissecting tray. Line up all equipment in the pan. Wash your table with soap and water. Wash your hands thoroughly with soap and water. Be sure to have your area checked by your teacher before you leave the room.