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59

Internal Anatomy of the Frog

BACKGROUND For many decades the .study of the internal anatomy of the frog has been part of the laboratory program in biology courses. From their study of the frog, students gain much information about the internal organization of vertebrate bodies in general. The structure and arrangement of organs and organ systems in the frog are ~ery similar to those of the human body. When you have completed this laboratory study, you will know more about the organs and organ systems of your own body. OBJECTIVES In this activity you will:

1. Observe the internal anatomy of the frog. 2. Study the frog as a representative

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of the phylum Chordata, subphylum

Vertebrata.

MATERIALS

preserved dissecting scissors dissecting

frog (Rana, sp) tray with wax bed needles

large dissecting forceps scalpel hand lens

pins

PROCEDURES AND OBSERVATIONSi

Part I. Dissection of the Frog a. Laythe frog in the dissecting tray, ventr~1side up, FIGURE1 anterior end away from you. Look at Figure 1 to see where the incisions should be made. Eachdissection line in Figure 1 has.a dot where the cut shquld begin and an arrowhead indicating the direction of the cut. Pinch a bit of the skin on the frog to feel its thickness. Note that the skin isfree from the tissues underneath it. b. Pinch up a bit of skinat the dot for line 1. Snipa V with the scissors. Cut through the skin only along line 1 shown in Figure 1, all the way to the chin. Then cut the skin along lines 2, 3, 4, and 5: This creates flaps of loose skin. The skin of the frog is very important as a respiratory surface, especially in the winter wben the frog hibernates in mud at the bottom of a pond or stream.

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c. Lay back the skin flaps and examine the underside of the skin. Note the many find blood vessels. Ttt~n 'Ii.' cut off the skin flaps.

d. Ex1tnine the muscles of the ventral side of the frog. They are .relatedto the head, thorax, and abdomen. Picture the movement 0.1the frog body as each muscle contracts. ~57

e. Lookat Figure 1 again~Then cut the muscle along line 1 just to the level of the forelimbs. Make a shallow cut so as not to damage the organs underneath. At the level of the forelimbs you have to cut through bone. The heart is directly beneath the bone, so work carefully. Slidethe lower blade of the scissors just under the bone and forward toward the chin, until the scissors are opened quite wide. Then bring the scissor handles to.

FIGURE2

. .~\ ;..

gether in one firm cut throughthe bone.

f. Continue to cut along line 1 all the way to the jaw. bone. Cut through muscle oitly-do not destroy the floor of the mouth. Then make cuts along lines 2, 3, 4, and 5 through the muscles of the body wall. g. Pin the specimen to the wax in the tray. Take hold of the two forelimbs and stretch them widely apart so that the chest cavity opens. See Figure 2. Place a large dissecting pin in each limb at an angle of about 45". Push the pins firmly into the wax. Then pull the muscle flaps created by cuts 2 to 5 away from the body cavity;.Place two large dissecting pins in each flap, at the same angle as before. Now the body cavity will be held open as you ,work. h. If the frog is a female, two large masses of black eggs may be hiding the internal organs. The eggs are in thin ovary tissue so that you can see the separate eggs. After examining the egg masses, if you have a female, let other groups observe them. Then remove the egg masses by lifting them carefully and snipping th.e two ovaries free from the

bodywith scissors.

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The frog has a three-chambered heart. The two atria are the receiving chambers: one receives oxygenated blood from the lungs and one re., ceives deoxygenated blood from the body. The ventricle, the pumping chamber, pumps the blood to the body.

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i. Locate the heart. It is in a triangl,llarpericardial sac in the center of the body between the forelimbS. Open the pericardial sac. Note the pointed ventricle. Anterior to the ventricle are the two atria. The liver is the largest organ in the frog body. (It is also the largest gland in the human bod~.) Attached to the underside of the liver is the gallbladder, which stores bile. A bile duct connects t~e gallbladder to the intestine.

j. Locate the large liver. It is reddish-brown and haS three lobes that overlie the abdominal organs and extend behind the heart. Carefully lift up the lobes of the liver and look on the underside where they are connected to each other. Identify 258

the gallbladder, which is a small, round, greenyellow sac attached to the liver. Then trim away most of the liver, leaving some tissue attached to the intestine. k. Findthe anterior end of the alimentary canal, the esophagus, posterior to the head. Trace the esophagus as it leads into the stomach, which is a curved, white sac. The stomach curves slightly to the frog's right side. The posterior end of the stomach leads into the small intestine, a small,. coiled tube. Note that the liver is attached near where the stomach and small intestine join. I. Find the pancreas. It is a feather-shaped organ .located just dorsal to the stomach and is attached to the bile duct by a .smalltube. . m. Trace the small intestine as it twists and turns toward the posterior of the frog. Do not tear any tissue. Find where the small intestine joins the large intestine, a shorter tube. The large intestine leads deep into the posterior abdominal cavity. Just anterior to the anus, between the hindlimbs, the large intestine is joined by the urinary bladder, it two-lobed, thin-walled sac that may be lyingon top or to one side of the large intestine. n. Removethe alimentary canal from the abdomen. Hold the stomach as you snip the esophagus with the scissors. Gently free the.tube from surrounding tissues in order to keep the organs intact. Then cut the posterior end of the canal as close to the anus as possible.

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The, small intestine is the principal organ of digestion and absorption of nutrients in the frog. As in most vertebrate digestive systems, it is held in place by amesentery and supplied by blood vessels in the mesentery. The mesentery is a thin, fan-shaped membrane.

prompts the captor to release the animal-the sired res.ponse!

r. 'locate the reddish-brown kidneys, deep in the posterior abdominal cavity on both sides of the backbone. Trace the path of urine from the kid,

o. Examinethe freed alimentary canal. Note the size of the stomach and the length of the small intestine. The small intestine is twisted into a helical form by the mesentery. Spread the coils of the small intestine a'bit to see the shape of the mesen,tery and also to see the blood vessels in it. p. Also attached to the mesentery is the small, spherical, dark-red spleen. Identify it. Normallyit is centered in the abdominal cavity between the stomach and the large intestine. '

The frog stores extra food at times of heavy feeding (in the summer) in the form offat bodies. When food is scarce, or during hibernation, the fat bodies provide energy for life processes. They are necesSary for the production of eggs and sperm in the spring breeding season. q. Observe the yellow, finger-shaped fat bodies. If they are large, remove them. Then, at the anterior' end of the abdominil.1 cavity, find the two darkred lungs. They m~y be deflated, long and thin, or inflated, like long, oval balloons. Note that the lungs-connect anteriorly with the larynx and glottis. The frog excretes one-third of its weight in urine each day because water is constantly entering the body through the skin. Urine is produced in the kidneys and passes through tubes to the cloaca, the posterior part of the large intestine. It can be released from the body through the cloaca, or it can be stored in the urinary bladder for a time. When frogs and toads are captured, they promptly empty their urinary bladder through the cloaca onto the captor. This often

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de-

neysthroughtubesto the cloaca.

s. On the ventral surface of each kidney, find a light-colored mass of tissue running almost the length of the kidney. These are the adrenal

giands.

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The male' reproductive organs in the frog are .

the testes. In the frog and other amphibians, s'perm travel to the cloaca through the same tubes as the urine. ,

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t. If your frog is a male, find the, testes on the ventral side and anterior end of the kidneys. They are yellowish, oval bodies connected to the kidneys by tubes. In the female frog, th~ eggs produced by the ovaries are swept by cilia into the funnel-shaped openings of the oviducts near the tips of the lungs. The eggs pass through the oviducts in single file, all the way to the cloaca where they are. released. As they pass through, they are coated with a jellylike mateJjal that swells as the eggs enter the water. Sperm from a male are released at the same time as the eggs. Fertilization is external. u. If your frog is a female, find the coiled, white oviducts. Then observe a specimen ofthe oppo-

site sex to see the reproductive organs.

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v.' Examine the backbone, or vertebral column. Each bone is a vertebra. The vertebral column encases

the spinal cord.

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w. Note the stringlike pairs of spinal nerves emerging from between the vertebrae. There 'are 10 pairs of spinal'nerves. Try to find the large sciatic nerves leading to the muscles of the thighs.

Part II. Further Investigation in the Frog a. With the scissors, cut the skin of the frog all the way around the top of one thigh where the leg joins the body. See Figure 3a. . b. Fold the skin downward, inside out, toward the knee, as in Figure 3b. Then grasp the cuff of skin and pull firmly and steadily until the skin comes off over the knee. Be careful-if you pull too hard the skin might break. Also, if you hold the leg too firmly, the bone may break. A steady, firm pull is best. c. The skin is difficult to detach at the ankle. Note its strength and its attachments. Continue to pull the skin until it comes off over the toes.

FIGURE3

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d. Examinethe muscles of the thigh. Note that each muscle is separate. Pull on each muscle to find out what movement it produces when it con-

tracts.

- cuff

of skin folded down

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The calf muscle. of the leg is called the gastrocnemius. It has the same name and function as th~ calf muscle in the human leg. As hi all muscles that move the skeleton, each end of the gastrocnemius muscle is attached to a bone by means of a tendon.

b thigh muscle

e. Examine.the gastrocnemius. Find the upper tendon of the muscle and trace it to its attachment Ofla bone. 1. To which bone is the upper end of the gastrqc;:nemiusattached?

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f.

Cut the gastrocnemiusfree of its tendon near its upper attachment. Pull upward on the muscle to see how it moves the foot when it contracts.

g. Then pull down on the gastrocnemius, toward the foot and way from the skeleton. Firmlyand steadily pull the lower tendon, the Achilles tendon, free from the ankle bone and remove the entire tendon from the foot. .

2. How is the

tendon attached to the foot? (

h. Examine the foot and toe bones and the joints between them. The lens of the eye in a living frog is crystalline and transparent. The lens does not change shape to focus as it does in the human eye. Frogs are. nearsighted on land and farsighted under water. 260

i. With the scissors, carefully dissect the eyeball. Removethe spherical lens. .Placethe lens on your palm and hold it up to the light. Note that it gathers light, even in its preserved condition. j. With the tips of two dissecting needles, tease apart the lens to see its structure. k. Place the.frog dorsal side up and remove the skin from the skull with scissors or a scalpel.. I. With a scalpel, shave or whittle the bone of the skull between the eyes. Use very shallow strokes, peeling off the bone in layers or shavings. When the bone becomes thin, peel it off very carefully using forceps. Be sure not to dig the points of the forceps into the brain tissue. See Figure 4. The cerebral hemispheres of the brain control voluntary motion and conscious activities of the frog. m. Identify the rounded cerebral hemispheres between the eyes.

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Date. FIGURE4

n. Continue to shave bone toward the anterior of the frog's head. Exposethe olfactory lobes. The opti(: lobes control the activities of 'the eyes.

olfactory lobe

cerebral hemisphere

o. Continue to shave bone, working posteriorly from the cerebral hemispheres. Locate the optic lobes, relatively large, hollow, rounded masses of midbrain tissue.

optic lobe

1i'he cerebellum coordinates the muscular activities and controls the balance of the frog.

cerebellum

p. Posterior to the optic lobes, find the cerebellum, a very short section of the brain. The medulla is the control center in the brain for breathing, swan owing, "digestion, and reflexes.

medulla

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q. Trace the cerebellum as it leads into the medulla.

spinal cord

The olfactory lobes are centers for sensing odors. Nerves lead to these lobes from the nos. trils.

CONCLUSIONS

r. Then locate the spinal cord as it begins at the posterior base of the medulla. Dissect a vertebra. or two in order to see the spinal cord encased within the vertebral column.

AND APPLICATIONS

1. Explain the three ways that respiration

occurs in the frog.

2. One atrium of the frog heart receives blood from the body, and the other receives blood from the lungs. Both atria deliver blood into the ventricle, which pumps it to the body. Is the blood pumped to the body oxygenated, deoxygenated; or both?

3. List the parts of the frog alimentary canal, including the glands attached to it, beginning with the mouth and ending with the anus. (

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4. How does pancreatic juice get into the alimentary canal in the frog?

5. Describe the structure and function of the mesentery associated with the small intestine of the fro