Rat Dissection Procedure

Rat Dissection Procedure Experimental Question: HOW IS THE BODY OF A MAMMAL ORGANIZED? Objectives 1. Study the gross anatomy of a mammal that shares a common evolutionary ancestor with humans. 2. Identify and compare important external structures of the rat to human features. 3. Identify organs and organ systems associated with the major body systems of a mammal. 1. Your Specimen Clean up • If you have not finished with your specimen: Wrap your rat in wet paper towels and put it into the Ziploc bag you were given. Make sure your group is identified on the baggie. Put the rat in the plastic box in the front of the room. • When you have finished all your observations: Put your rat and all biological trash in the Ziploc bag you were given. This should be put into the black trash bag or box in the front of the room. 2. Your Tools and Workspace • • • • • Keep hands away from scalpel blade! Clean your scalpel with hot soapy water, wipe your scalpel on a paper towel and place it in the box that came with it. Clean all dissecting tools with hot soapy water and rinse in the pan of bleach water provided. Dry all equipment thoroughly so tools do not rust. Return tools in the box your group was given. Clean your work area thoroughly with disinfectant. 3. Your Self • • • If you used a dissecting apron, wipe it with disinfectant and hang to dry Wash your hands thoroughly with warm, soapy water and dry with paper towels. Use hand sanitizer. PART 1. EXTERNAL OBSERVATIONS 1. Preliminary measurements → Record the weight of your specimen. → Record the length (tip of nose to base of tail) • Observe body divisions and compare to proportions of the human head, torso and extremities (arms & legs). Locate six anatomical regions of the rat • Cranial region – head • Cervical region – neck • Pectoral – area where front legs attach • Thoracic region - chest area • Abdomen – belly, ventral side • Pelvic region - area where the back legs attach 2. Observe external features • Note the hairy coat that covers the rat and the sensory hairs (whiskers) located on the rat's face, called vibrissae. • Note the eyes with the large pupil and the nictitating membrane found at the inside corner of the eye. This membrane can be drawn across the eye for protection. • The ears are composed of the external part, called the pinna, and the auditory meatus, the ear canal. • Locate the teats (nipples) on the ventral surface of the rat. Check other groups’ animals and determine whether both sexes have teats. • Examine the tail. The tails of rats do not have hair, although some rodents, like gerbils, have hair on their tails. • Locate the anus, which is ventral to the base of the tale. Female or Male? On female rats, just posterior to the last pair of teats, you will find the urinary aperture and behind that the vaginal orifice which is in a small depression called the vulva. → Record the sex of your rat. On males, you will find a large pair of scrotal sacs which contain testes. Just anterior to the scrotal sacs is the prepuce, which is a bulge of skin surrounding the penis. The end of the penis has a urogenital orifice, where both urine and sperm are released. 3. Observe Oral Cavity • The mouth is the first part of the digestive system. Large front incisors can be seen in the large cleft in the upper lip. In rodents, these incisors continue to grow throughout the animals’ lifetime; they are continually worn down by gnawing. • Use bone scissors to cut through joint between upper and lower jaws to open the mouth. • Use a blunt probe to explore the oral cavity, locating the hard palate (the part supported by bone) and the soft palate (immediately posterior to the hard palate). The oral cavity leads into the throat (pharynx). The pharynx is a passageway used by both the digestive and respiratory systems. PART 2. OBSERVE MUSCLE 4. Secure the specimen. • Place the specimen ventral side up in the dissecting tray. • They are slightly pliable so using your hands spread the arms and legs so that the rat is balanced on its back 5. Make incision to remove skin 1. • Use forceps to lift the skin covering the ribs and use scissors to carefully make a small clip in the skin . Continue cutting to extend the slit the entire length of the rat’s body. → Do not cut too deeply, or you will cut the muscles just under the skin. 2. Use a blunt probe to loosen the skin from the underlying muscle. 3. Fold the skin laterally as you expose muscles of the thorax, abdomen and legs. Skinned Rat Rat dissection 6. Examine muscles of the abdomen and limbs. • Muscles are wrapped in sheets of filmy white connective tissue and packaged into bundles of fibers that contract together. • Observe the direction of the muscle fibers in the abdomen of the rat. Note that there are 3 layers of abdominal muscles. The fibers for each muscle layer run in different directions to allow the torso to bend and twist. Major muscles of the legs and thorax • Muscles are attached to bones by tough connective tissue called tendons. Each muscle is attached to two (or more) bones. One end of the muscle, the origin, remains stationary when the muscle is contracting. The other end, the insertion, is pulled toward the origin when the muscle contracts. The movement caused by the contraction of the muscle is called its action. Muscles can be identified by their shape, size, location, action or size. Record which of these muscles you could identify on your rat: • Pectoralis Major/Minor - located in chest area. Action: draws outstretched arm toward the midline of the body • Biceps brachii - located on the anterior surface of the upper arm. Action: bends the arm at the elbow • Biceps femoris - located on the back of the thigh, in two bundles. Action: bends knee • Rectus abdominus - located between the ribs and hip/pelvic bones. Action: bends torso. 7. Expose the bones of the leg. • Use the dissecting needle to carefully separate the muscles of the leg from the three leg bones: the large femur (thigh), tibia (shin bone) and fibula (small bone of the lower leg). You may also be able to see the tiny patella (kneecap) embedded in the patellar tendon. • Observe the ligaments that connect the bones of the lower leg to the thigh, and the thick Achilles tendon that attaches the gastrocnemius muscle to the lower leg. (Do you know how this tendon got its name?) • Carefully use a scalpel to cut or scrape muscle away from the hip joint to examine the ball and socket joint of the hip. Observe how the large head of the femur fits into the hip socket. PART 3. DISSECTION OF THE ABDOMINAL CAVITY PROCEED WITH CARE: • Make sure you look at several slides as you identify structures in the abdomen. Different structures are identified in the images. • These published images are MUCH cleaner than your rat will be; your rat will have body fluids, processed food in the intestines, cecum and large intestine. Rinse the abdomen to remove excess debris. • Work carefully and identify stomach and pancreas before you move things around very much – it is easier to find these organs if the specimen is relatively intact. • The large, greenish, squishy organ is probably the cecum NOT the stomach. Ask for help identifying organs earlier in the dissection rather than later! 8. Expose the abdomen • Use scissors to cut down the midline of the abdominal cavity, being careful not to cut the organs under the muscle layer. Continue to cut away muscle to expose the abdomen fully. • DO NOT CUT THE DIAPHRAGM • Lift off the greater omentum, an apron-like membrane that protects the organs of the abdomen. Fat stored in the omentum produces a “beer belly” in humans. A. Heart B. Lungs C. Diaphragm D. Liver E. Cecum F. Stomach G. Small intestine H. Rectum Pancreas Alert! • To locate the pancreas, first find the stomach. At the caudal end of the stomach you will see where it joins the first section of the small intestine, the duodenum. • The pancreas is connected to the duodenum and often it slips behind the stomach in the specimen. Sometimes, it may be hard to distinguish from fatty tissue. 9. Identify structures in the abdomen: a. Liver – Large, dark colored organ, caudal to the diaphragm. The liver has many functions, including aiding in digestion, storing glycogen, and detoxifying chemicals, including alcohol. Locate the 4 lobes of the liver. (Note: the rat does not have a gall bladder.) The liver can regenerate, making it possible for humans to donate a portion of the liver for transplant recipients (remember Prometheus?) b. Stomach – A C-shaped “bag” caudal to diaphragm; food enters the stomach from the esophagus. The stomach stores and digests food over a period of several hours. c. Pancreas – Lift the stomach to view this lumpylooking gland near the juncture of the small intestine and stomach. The pancreas is an important producer of digestive enzymes and chemicals, as well as insulin for regulating blood sugar. Note: this organ is sometimes hard to find. Look for the pancreas before you poke around in the abdomen. Close view of pancreas This is beautiful! Abdomen – another view 1. Liver 2. Stomach 3. Pancreas 4. Duodenum 5. Small Intestines 6. Cecum 7. Urinary Bladder 9. Identify structures in the abdomen: Small intestine has 3 sections: 1. Duodenum 2. Jejunum 3. Ileum Mesentery holds small intestine in place. Small intestine is attached to the outside of the mesentery. Large cecum is the first section of the large intestine (colon). 9. Identify structures in the abdomen: d. Spleen – Dark organ, caudal to diaphragm, attached to the stomach. The spleen’s function is to destroy damaged red blood cells and help the immune system. e. Small intestine – this long tube begins at the base of the stomach and is coiled to fill most of the abdominal cavity. It joins the large intestine at the pouch-like cecum. f. Mesentery – Lacy membrane that holds the coils of the small intestine in place. Note the great number of blood vessels in the mesentery. (See next slide) Close view of small intestine and mesentery. Small intestine is held to the outer edge of the mesentery. Click on picture for another view of mesentery. (Fun fact: the small intestine of pigs is used as casing for sausage! ) 9. Identify structures in the abdomen: e. Cecum – large pouch that is the first section of the large intestine. The cecum contains many bacteria to break down the cellulose in plant material; it is much larger in herbivores than in carnivores. f. Large intestine (colon) – Large diameter tube that frames the small intestine; leads from the cecum to the rectum. Undigestable waste from food is compacted in the large intestine. g. Rectum – Terminal part of the large intestine; leads to anus. h. Anus – Opening of digestive tract to exterior of body for solid waste removal. Stop here for instructor check. 10. Measure abdominal organs • After identifying the organs of the abdomen in place, carefully remove the liver, weigh it and find the volume of this organ. Make sure to get all four lobes of the liver. • → Record the weight and volume of the liver. Calculate the liver as % of body weight of the rat. • Remove the small intestine by cutting it at the proximal and distal ends and lifting it out of the abdominal cavity. One member of the group can use scissors to carefully trim away the mesentery that holds the intestine in place. Stretch out the small intestine and measure. • → Record the length of the uncoiled small intestine. 11. Locate deeper structures of abdominopelvic cavity. • After removing the liver, stomach and intestines, locate structures on the posterior (dorsal) wall of the abdominal cavity. • Kidneys – Paired, bean-shaped organs attached to posterior body wall. • Adrenal glands – small glands on superior margin of each kidney; part of the endocrine system. • Carefully use forceps to pull off any membrane covering the kidney. Locate the ureter, which is a tube running from the kidney to the bladder. • Urinary bladder – a sac in the pelvis that serves as a holding tank for urine. 1. 2. 3. 4. 5. 6. 7. Stomach Kidney Ovary Oviduct Uterine Horn Large Intestine Urinary Bladder 12. Observe important blood vessels. • Look along the midline of the body cavity between the kidneys for these two major blood vessels: • Inferior vena cava - This large vein returns blood to the heart from the lower regions of the body. • Descending aorta - The largest artery in the body, carries blood away from the heart down the midline of the body. It will be found deep to the inferior vena cava. • NOTE: Arteries ALWAYS move blood away from the heart; veins always move blood toward the heart. 13. DISSECTION OF REPRODUCTIVE ORGANS Observe Male reproductive organs. • Make a shallow incision into the scrotum. Loosen and lift out the oval testis. The coiled tube on the surface of the testis is the epididymus, which stores sperm cells. The tubular vas deferens moves sperm from the epididymus to the urethra. In the male, the urethra runs through the penis and carries both urine and sperm. Identify the penis, extending from the bladder to the ventral body wall. Observe Female reproductive organs. • Female reproductive organs. The short gray tube lying dorsal to the bladder is the vagina. It divides into two uterine horns that extend toward the kidneys. The structure of the rat uterus allows the female to accommodate multiple embryos. (The uterus of a human female is a singlechambered organ.) At the end of each uterine horn is a small oval ovary. A. Ovary B. Fallopian tube C. Uterine horns D. Uterus E. Fat F. Liver G. Large intestine H. Small intestine PART 4. DISSECTION OF THE THORAX 14. Expose structures of the thorax. • Beginning at the base of the neck, use scissors to cut through the bones that encase the thoracic cavity, as shown. You will be creating a “flap” that can be pulled down to expose the thorax without going into the abdomen. Cut across the top of the chest, then down both sides, cutting through the ribs. • Stop when you reach the thin muscle that separates the thorax from the abdomen (diaphragm). • Carefully cut across the lower part of the thorax – above the diaphragm - creating a window into the thoracic cavity. • This allows you to view structures in the thorax without interference from the abdominal organs. 15. IDENTIFY STRUCTURES OF THORAX, NECK & ORAL CAVITY 15. Identify structures in the thorax: 9. Lung 27. Heart a. Heart – located in the thoracic cavity medial to the lungs. Find the 2 small receiving chambers on the upper portion of the heart (atria), large left ventricle, and smaller right ventricle. b. Lungs – Right and left lungs are located lateral to the heart. Each lung has several lobes, and a spongy texture. The lungs take up much of the thoracic cavity. c. Diaphragm – thin muscle that separates thorax from abdomen. Stop here for instructor check. 16. Identify structures in the neck: Carefully use a scalpel to cut blood vessels and remove the heart to get better access to structures listed below. Weigh the heart and measure heart volume. Look for the large blood vessels that enter and exit the heart. Record the weight and volume of the heart. d. Trachea – Runs medially down the throat; the trachea (windpipe) is made of tough cartilage rings so it does not collapse with breathing. You may have to extend your incision to see the trachea. e. Bronchi - Follow the trachea into the thoracic cavity. Note that it divides into two branches; these are the bronchi. Follow the bronchi to see them enter the lungs. f. Esophagus – Use scissors to cut away a section of trachea to see the esophagus, which is found deep to the trachea. Pass a blunt probe through this muscular tube 5. Esophagus from the pharynx to the stomach. 9. Lung 22. Trachea 27. Heart Stop here for instructor check. 1. Your Specimen Clean up • If you have not finished with your specimen: Wrap your rat in wet paper towels and put it into the Ziploc bag you were given. Make sure your group is identified on the baggie. Put the rat in the plastic box in the front of the room. • When you have finished all your observations: Put your rat and all biological trash in the Ziploc bag you were given. This should be put into the black trash bag or box in the front of the room. 2. Your Tools and Workspace • • • • • Keep hands away from scalpel blade! Clean your scalpel with hot soapy water, wipe your scalpel on a paper towel and place it in the box that came with it. Clean all dissecting tools with hot soapy water and rinse in the pan of bleach water provided. Dry all equipment thoroughly so tools do not rust. Return tools in the box your group was given. Clean your work area thoroughly with disinfectant. 3. Your Self • • • If you used a dissecting apron, wipe it with disinfectant and hang to dry Wash your hands thoroughly with warm, soapy water and dry with paper towels. Use hand sanitizer.