Frog Dissection


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Purpose

In this lab, you will dissect a frog in order to learn more about its internal and external anatomy.

Background


As members of the class Amphibia, frogs have structural adaptations that allow them to live some of their adult lives on land, but they must return to water to reproduce. Typically, eggs are laid and fertilized in water.

On the outside of the frog’s head are two external nares or nostrils; two tympani, or eardrums; and two eyes, each of which has three lids. The third lid, called the nictitating membrane, is transparent. Inside the mouth are two internal nares, or openings into the nostrils; two vomerine teeth in the middle of the roof of the mouth; and two maxillary teeth at the sides of the mouth. Also inside the mouth behind the tongue is the pharynx, or throat. In the pharynx, there are several openings: one into the esophagus, the tube into which food is swallowed; one into the glottis, through which air enters the larynx, or voice box; and two into the Eustachian tubes, which connect the pharynx to the ear.

The digestive system consists of the organs of the digestive tract, or food tube, and the digestive glands. From the esophagus, swallowed food moves into the stomach and then into the small intestine. Bile is a digestive juice made by the liver and stored in the gallbladder. Bile flows into a tube called the common bile duct, into which pancreatic juice, a digestive juice from the pancreas, also flows. The contents of the common bile duct flow into the small intestine, where most of the digestion and absorption of food into the bloodstream takes place. Indigestible materials pass through the large intestine and then into the cloaca, the common exit chamber of the digestive, excretory, and reproductive systems.

The respiratory system consists of the nostrils and the larynx, which opens into two lungs, hollow sacs with thin walls. The walls of the lungs are filled with capillaries, which are microscopic blood vessels through which materials pass into and out of the blood. The circulatory system consists of the heart, blood vessels, and blood. The heart has two receiving chambers, or atria, and one sending chamber, or ventricle. Blood is carried to the heart in vessels called veins. Veins from different parts of the body enter the right and left atria. Blood from both atria goes into the ventricle and then is pumped into the arteries, which are blood vessels that carry blood away from the heart.

The urinary system consists of the frog’s kidneys, ureters, bladder, and cloaca. The kidneys are organs that excrete urine. Connected to each kidney is a ureter, a tube through which urine passes into the urinary bladder, a sac that stores urine until it passes out of the body through the cloaca. The organs of the male reproductive system are the testes, sperm ducts, and cloaca. Those of the female system are the ovaries, oviducts, uteri, and cloaca. The testes produce sperm, or male sex cells, which move through sperm ducts, tubes that carry sperm into the cloaca, from which the sperm move outside the body. The ovaries produce eggs, or female sex cells, which move through oviducts into the uteri, then through the cloaca outside the body.

The central nervous system of the frog consists of the brain, which is enclosed in the skull, and the spinal cord, which is enclosed in the backbone. Nerves branch out from the spinal cord. The frog’s skeletal and muscular systems consist of its framework of bones and joints, to which nearly all the voluntary muscles of the body are attached. Voluntary muscles, which are those over which the frog has control, occur in pairs of flexors and extensors. When a flexor of a leg or other body part contracts, that part is bent. When the extensor of that body part contracts, the part straightens.

External Procedure


1. Place a frog on a dissection tray. To determine the frog’s sex, look at the hand digits, or fingers, on its forelegs. A male frog usually has thick pads on its "thumbs," which is one external difference between the sexes, as shown in the diagram below. Male frogs are also usually smaller than female frogs. Observe several frogs to see the difference between males and females. (Answer Question 1 on Lab Writeup)

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2. Locate and identify the external features of the head. Find the mouth, external nares, tympanum, eyes, and nictitating membranes. (Answer Question 2 on Lab Writeup)
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3. Looking first at the posterior appendages (the hind legs), identify the thigh, shank (calf), foot and toes. (Answer Question 3 on Lab Writeup)


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4. Turn the frog on its back and pin down the legs. Cut the hinges of the mouth and open it wide. Use the diagram below to locate and identify the structures inside the mouth. Use a probe to help find each part, and point out each part to Mrs. Nash when you have discovered it: the vomerine teeth, the maxillary teeth, the internal nares, the tongue, and the eustachian tubes. (Question #5 on writeup is pointing out structures to Mrs. Nash)
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Internal Procedure



1. For the internal procedure, use this link to guide you through the dissection and to help you identify the organs. Clicking next will lead you through each of the steps for your dissection. Please use the audio and video help when you need it. Anytime you get an opportunity to "try it" go ahead and do so.


2. Before you move on, make sure you can identify and show Mrs. Nash the following organs as well as their functions: heart, lungs, liver, gall bladder, stomach, large intestine, small intestine, spleen, fat bodies, kidneys (female-ovaries and oviducts) male (testes). (She will initial this on your writeup, and you also need to label these organs on your writeup). (NOTE - on the diagram, the letters N, G, I, and B all refer to parts of the heart - the structures to choose from are artery, ventricle, left atrium, and right atrium - Letter E is the lower part of the stomach, known as the duodenum). (Question 7 and 8 on the writeup)

3. Removal of the Stomach: Cut the stomach out of the frog and open it up You may find what remains of the frog's last meal in there .Look at the texture of the stomach on the inside. (Answer Question 9 on Lab Writeup)

4. Measuring the Small intestine: Remove the small intestine from the body cavity and carefully separate the mesentery from it. Stretch the small intestine out and measure it. Now measure your frog.(Answer Question 10 on Lab Writeup)


5. Cutting about .5 inches behind the head, cut forward to attempt to find the frog brain. (Answer the rest of the questions on your writeup)
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