FROG DISSECTION External Anatomy 1. The division of a frog s body includes the head, trunk and limbs. Examine the front and hind limbs of the frog. The hind limbs are the long, more muscular limbs of the frogs. Find the digits, which are finger like projections on the forelimbs and hind limbs. The limbs of a frog consist of an upper arm, wrist, hand and fingers (digits). On a mature male frog there is a pad located on the innermost finger of the forelimbs. This pad is used by a male to grasp a female during mating and ensures the male stays in contact with the female as her eggs are fertilized when they re released into the water. a. Why is there a difference in size proportion between the hind and fore limbs? 2. Notice the size and location of the frog s eyes. a. How does the location of the frog s eyes help improve its vision? 3. A frog doesn t chew its prey but uses its eyes to help gulp food down. Press down the eyes of the frog. a. Explain how the eyes help in gulping food. 4. Frogs have a special third eyelid covering their eye known as the nictitating membrane. This membrane covers the eye when the frog is under the water. a. How is this covering an adaptation for the frog? 5. Observe the brow spot found between and anterior to the two eyes. This organ is the remaining indication of a third or middle eye. It was a photo-sensory organ active in triggering hormone production (including reproduction) and thermoregulation. 6. There are two openings on the anterior and dorsal part of the skull. These are the external nares (nostrils) which are used to take in oxygen when a frog s mouth is full of food. 7. Locate the tympanic membrane or the eardrum located on each side of the head. These are used to detect sound waves. 8. Pry open them mouth. Use the scissors to cut the corners of the mouth where the maxilla (upper jaw) and mandible (lower jaw) join together. Examine the frog s mouth. Locate the tongue, which is a muscular, sticky flap on a living frog. A grass uses a flip and grab technique to catch its prey. a. Describe where the tongue is located in the frog s mouth. b. How is this location an adaptation for a feeding frog? 9. Feel the maxillary teeth that are found along the rim of the upper jaw. Notice that only the upper jaw has these teeth. These teeth are used to make sure captured prey does not escape, if not all the way in the mouth. Find the two vomerine teeth attached to the skull bones of the roof of the mouth. These teeth are used to position prey directly over the gullet. 10. Locate the internal nares, which are found slightly above and on either side of the vomerine teeth. These are connected to the external nares.
11. On either side of the upper jaw near the jaw hinge, locate the openings to the Eustachian tubes. The Eustachian tube openings lead to each ear and ensure equal air pressure on both sides of the tympanic membrane. 12. Find the gullet, which is the wide opening leading to the esophagus. The esophagus is the first tube of the alimentary (digestive) canal leading to the stomach. Below the gullet is the glottis, a small slit like opening leading to the trachea and the rest of the respiratory system. 13. Sound is produced only in male frogs as moving air passes between vibrating vocal cords, as well as by rapid expulsion of air from paired vocal sacs, which are located on either side of the lower jaw near the jaw hinge. Internal anatomy of a frog 14. Place the frog on its dorsal side in the dissecting pan. To hold the frog in position, place dissecting pins through the tip of the snout and each of the legs as shown in Figure 1 Figure 1 15. With the forceps, lift the skin and insert the point of the scissors to right of the midline near the pelvis. Make the first incision from the pelvis to the throat as shown by line a in Figure 1. 16. At the forelimbs and the hind limbs, make transverse cuts on the lines b, c, d, and e as shown in Figure 1. Use dissection pins to hold the flaps of skin back. 17. Locate the linea alba, or narrow white line of connective tissue between the abdominal muscles. This is what divides Mr. Hathaway s six pack. You will also see a ventral abdominal vein. 18. Make an incision to the right of the linea alba through the muscles from the pelvis to the throat as in step 1. Note: Be careful not to damage underlying organs as you cut through the muscle layers. Make transverse cuts from this incision at the same places that cuts were made in step 16. 19. Carefully cut through the sternum that lies between the forelimbs. Fold and pin back the muscles. 20. Once inside the frog, you will see the peritoneum, a thin membrane covering the body cavity. Cut or peel away this membrane, which protects and suspends internal organs. 21. If your frog is a mature female, the body cavity may be filled with black-colored eggs covering the transparent ovaries. Remove the eggs and ovaries if they are present. 22. Find the heart in the center of the chest cavity. Notice that the heart lies in a thin sac called the pericardium. Remove the pericardium to observe the three-chambered heart. The two, dark-walled chambers, called atria, receive blood. The right atrium receives deoxygenated blood from the veins of the frog s body. The left atrium receives oxygenated blood from the lungs. Each atrium empties into the ventricle, which is the lighter-colored, thick-walled part of the heart. The large vessel forming a y-shape at the anterior end of the ventricle is the conus arteriosus. Blood is pumped from the ventricle to the conus arteriosus where it will then be pumped through a system of arteries to the rest of the frog. a. Why do frogs have a three-chambered heart (a fish has two chambers and birds and mammals have four chambered hearts)?
23. Locate the two lungs. They are small, spongy brown sacs that lie to the right and left of the heart. Look for the bronchial tubes that extend from the anterior part of the lungs and join with the trachea. Insert a dropper into the glottis and pump air into the lungs. Observe what happens. 24. Locate the large reddish-brown liver in the anterior of the body cavity. The liver is used to make the digestive enzyme bile. a. How many lobes are there? 25. Lift up the liver and attached to the underside will be the small transparent gall bladder. It is usually filled with green bile that is made in the liver and stored here. The bile duct, brings bile from the gall bladder to the duodenum. 26. Curving from underneath the liver, is the large, white, tubular stomach. The stomach is the first site for chemical and mechanical digestion and narrows at the pylorus, where it connects to the small intestine. Inside the pylorus is the pyloric sphincter which is a muscle used to regulate food entering the small intestine from the stomach. 27. The duodenum is the short, uncoiled first section of the small intestine where bile enters from the gall bladder. The middle part of the small intestine is the jejunum. The last part of the small intestine is the ileum. Absorption of digested nutrients occurs in the small intestine, through small folds called villi. The ileum is held together by a membrane called the mesentery. Gently pull on the mesentery and notice all of the blood vessels. a. Why are there so many blood vessels in the mesentery? 28. Also located in the mesentery is a small, reddish-brown, round structure known as the spleen. The spleen is in the lymphatic system and is charge of filtering out dead blood cells and harmful bacteria. 29. The pancreas is a thin, yellowish gland that is difficult to find. It is located in the mesentery between the duodenum and the upper part of the stomach. The pancreas secretes digestive enzymes and can also be put in the endocrine system as it is in charge of creating insulin. Insulin is used to regulate blood sugar levels. 30. The end of the ileum widens to form the large intestine. The colon is one region of the large intestine and is responsible for absorption of water and elimination of solid waste material. 31. The large intestine terminates at the cloaca. The cloaca is a general catch-all basin and opens to the outside of the frog through the cloacal opening. This is where digestive, reproductive, and excretory products are expelled. a. Do mammals such as humans have a cloaca? b. Why or why not? 32. The glottis leads to the trachea which is known as the windpipe in higher animals. The trachea branches into bronchial tubes, which connect to each lung. 33. Push the intestines aside to see the two dark, oblong kidneys attached to the dorsal body wall. Find the, ureter, which is a tube leading out of each kidney. The ureters carry the urinary wastes from the kidney to the urinary bladder. The chemical waste is called urine and is stored in a thin-walled urinary bladder that empties into the cloaca. 34. The adrenal gland is part of the endocrine system and secretes hormones throughout the frog. They can be seen as a yellow stripe on each kidney. The fat bodies are the yellow finger-like structures near each kidney. These structures are used for food during hibernation and nourishing gametes. 35. In male frogs each yellowish, oval-shaped testis is attached by tubes to the kidney. During mating the sperm mixes with and follows the exit route of the urine. In female frogs the two ovaries appear as egg sacs. Eggs released from the ovaries enter a long, coiled oviduct at the anterior opening. Eggs pass down the creamcolored oviduct and are held in a uterus, before entering the cloaca. From the cloaca the eggs pass from the body to be fertilized outside the frog s body.
Nervous system and the brain 36. Turn over your frog so that the dorsal side once again faces up. Insert the point of your scissors through the skin at the base of the head and remove the skin from the head area. 37. Bend the frog to determine the approximate region of the neck. Insert your scissors and clip across the upper spinal cord in the region of the neck. 38. Locate the white spinal cord enclosed within the vertebrae. This structure relays information from the brain to the body and the body to the brain. 39. Use your forceps to remove the bone above the spinal cord, working forward until you have reached the nostril area. You will now be able to view the exposed brain as show below 40. Locate the olfactory lobes which are in charge of smell and the optic lobes which are in charge of vision. 41. Locate the cerebrum which is small in size in relation to a human brain. It is here that things like perception, imagination, thought, judgment, and decision occur. a. Why would there be such a proportional size difference between the cerebrum of a frog and the cerebrum of a human? 42. Locate the cerebellum which is just posterior to the optic lobes. This structure is in charge of the coordination of voluntary motor movement, balance and equilibrium and muscle tone. 43. The last brain structure to locate is the medulla oblongata which is in charge of nonvoluntary processes within an animal. Nonvoluntary processes are life functions that an animal doesn t think about to stay alive, such as breathing. a. Give two examples of a nonvoluntary process found in humans. Label the following structures on the diagram below: head region, trunk region, hind limb, forelimb, nictitating membrane, tympanic membrane, external nares and mouth.
Label the follow structures on the diagram below: maxillary teeth, vomerine teeth, internal nares, Eustachian tube, gullet, glottis, jejunum, vocal sac opening, tongue, liver, gall bladder, bile duct, stomach, pylorus, duodenum, ileum, large intestine, cloaca, mesentery, spleen, and pancreas
Label the follow structures on the female frog: fat body, urinary bladder, cloaca, ureter, kidney, ovary, oviduct Label the follow structures on the male frog: fat body, urinary bladder, cloaca, testis, ureter, kidney