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Frog Dissection

Objectives & Purpose: To study the internal and external anatomy of a frog.
Describe the appearance of various organs found in the frog.
Name the organs that make up various systems of the frog.
• safety goggles, gloves, and a lab apron
• forceps
• preserved frog
• dissecting pins (6–10)
• dissecting tray and paper towels
• plastic storage bag and twist tie
• scissors
• marking pen
• dissecting needle

Kingdom - Animalia Phylum - Chordata Class - Amphibia Order - Ranidae
Genus - Rana Species - pipiens
As members of the class Amphibia, frogs may live some of their adult lives on land, but they must return to water to reproduce. Eggs are laid and fertilized in water.
External Anatomy:
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. 

Frog Dissection (answer questions on separate page)

1. Put on safety goggles, gloves, and a lab apron.      
2. 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.
Is your frog male or female?
( answer this on handout page)

3. Use the diagram below to locate and identify the external features of the head. Find the mouth, external nares, tympani, eyes, and nictitating membranes. You can also use the dissection drawings. Be sure to label the diagram handout.


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: the vomerine teeth, the maxillary teeth, the internal nares, the tongue, the openings to the Eustachian tubes, the esophagus, the pharynx, and the slit-like glottis.  

(Be sure to label the diagram handout)
5. Look for the opening to the frog’s cloaca, located between the hind legs. Use forceps to lift the skin and use scissors to cut along the center of the body from the cloaca to the lip. Turn back the skin, cut toward the side at each leg, and pin the skin flat. The diagram above shows how to make these cuts.
Examine the lower side of the skin and note the pattern of the blood vessels. Study the muscle layers exposed by the removal of the skin.Now, remove the muscle layers in the chest region in same manner in which you removed the skin.In the region of the thorax, it will be necessary to cut through the bones of the arms and the shoulders.Remove the muscle layers with your forceps.

6. Lift and cut through the muscles and breast bone to open up the body cavity. If your specimen happens to be a female, the body cavity will probably be nearly filled with eggs. In this case, it will be necessary to remove the eggs with your forceps before proceeding further. Note that the eggs are loose in the body cavity. You will find also, two long coiled , white oviducts leading from the anterior end of the body cavity to the posterior end. These are the tubes through which the eggs pass and should not be confused with other tubular structures to be referred to later.. If so, remove the eggs on one side so you can see the organs underlying them.
7. Use the diagram above to locate and identify the organs of the digestive system: esophagus, stomach, small intestine, large intestine, cloaca, liver, gallbladder, and pancreas.
Find the 3-lobed liver high in the body cavity.Spread the lobes of the liver apart with your probe and find the bright, green gall bladder.Raise the liver on the left side and notice the position of the stomach.Follow the intestine from the lower end of the stomach to the large intestine. Examine the mesentery and the manner in which it holds the intestine in place. Find the spleen on the mesentery. The pancreas is attached to the lower part of the stomach.
Next, remove the liver, using care not to damage other organs. Cut through the gullet at its upper end.Raise the stomach with your forceps and carefully loosen the intestine as far as possible.The cloaca lies below the large intestine between the hind legs.Lay the alimentary canal in your dissecting pan. Pin the stomach at the top and the large intestine at the bottom. Find the muscular valve, the pylorus, at the lower end of the stomach, where it joins the intestine.
8. Again refer to the diagram to identify the parts of the circulatory and respiratory systems that are in the chest cavity. Find the left atrium, right atrium, and ventricle of the heart. Find an artery attached to the heart and another artery near the backbone. Find a vein near one of the shoulders. Find the two lungs.
Examine the heart,which lies in a thin sac, the pericardium.Using your probe,find the great veins and arteries leading to and from the heart. Examine the lungs and notice the pulmonary veins and arteries connecting the heart and lungs.See if you can find the trachea leading from the mouth to the lungs.Remove the heart, severing the necessary blood vessels. Examine the front side of the heart with our hand lens.The large vessel arising from the ventricle, extending over the front surface of the hart,and forming a Y at the top of the heart is the conus arteriosus. This great compound vessel sends one branch to the head, a second branch to the body, and a third branch to the lungs and skin. Locate the right auricle, the left auricle, and the ventricle in the front of the heart. Examine the back side of the heart with your hand lens. Three branches of the vena cava unite in a thin-walled sac, the sinus venosus, on the to of the back wall of the heart. The sinus venosus opens into the right auricle. Pulmonary veins from the lungs join the left auricle. Examine the dark, red, lobed kidneys lying along the back of either side of the spine. Find a small,twisting tube,the ureter leading from each kidney to the cloaca for storage. Do not confuse the ureters with the oviducts in a female.If your frog is an immature female, you will find two lobed, grayish ovaries lying close to the kidneys.The testes of the male are in a corresponding position. Find the yellow, finger-like fat bodies attached above the kidneys.
9. Use a probe and scissors to lift and remove the intestines and liver. Use the diagram on the next page to identify the parts of the urinary and reproductive systems. Remove the peritoneal membrane, which is connective tissue that lies on top of the red kidneys. Observe the yellow fat bodies that are attached to the kidneys. Find the ureters; the urinary bladder; the testes and sperm ducts in the male; and the ovaries, oviducts, and uteri in the female.  


10. If bullfrogs are used for this work, they have been specially prepared in order to facilitate the removal of the bony skull. With the scissors, cut the skin along the mid-dorsal line, turning back the flaps. Notice the large lymph spaces just under the skin on the dorsal side.Small nerves and blood vessels may be seen passing to the skin. With the scalpel, cut through the skull a short distance anterior to the eyes, taking care not to cut deep enough to injure the brain.Then with scissors, scalpel, and forceps, work until the brain and spinal cord are completely exposed.
In identifying these structures,constantly refer to the accompanying plate.The parts are described below starting with the most anterior and naming them in order passing posteriorly.
Olfactory lobes - two lobes that are fused together; they give rise to the olfactory nerves which pick up stimuli from the external nares.
Cerebral hemispheres - two large hemispheres which continue forward as the olfactory lobes, sometimes called the telencephalon.
Midbrain -a diamond-shaped portion posterior to and partly enclosed by the cerebrum, sometimes called the diencephalon. This region bears the epiphysis on the dorsal side (you probably have torn it off during your dissection)and the hypophysis (Pituitary gland) on the ventral side. The hypophysis is an endocrine gland.
Optic lobes - outgrowths of a region of the brain known as the mesencephalon. They are large, pigmented, spherical bodies.
Cerebellum - a narrow band passing transversely between the optic lobes and the medulla, sometimes spoken of as metencephalon.
Medulla oblongota - the posterior part of the brain and continuous with the cord. The anterior end of the medulla contains the fourth ventricle. this region of the brain is sometimes referred to as the myelencephalon.
11. It is advisable that every student work to see the cranial nerves. You may not find all of them but do the best you can.They are on the attached diagrams as they occur in the animal, starting from the anterior end and working posteriorly. Numbers accompany the nerves.

SENSE ORGANS: 12. If you have not already done so, cut open the nasal capsule located under the external nares.This is the end organ for the olfactory nerves.Not the great surface areas as a result of the convoluted lining of the capsule.Why is such a large surface necessary? Is the sense of smell acute in the frog? How do you know? How could you find out?
13. Study one of the eyes in situ(in place).Note the muscles that turn the eyeball in its socket.There are 6 such muscles. The four rectus muscles (external, internal, superior, and inferior) arise from the posterior portion of the eye socket (orbit). Note their insertions on the eyeball. In what directions can they move the eye? The two oblique muscles (superior and inferior) arise from the anterior portion of the eye socket. Note their insertions on the eyeball. How do they move the eye? By the combined action of these six muscles, the frog is able to move its eyes in all directions, much the same as we can. Remove the eye and examine it. Cut it open and note any structures with which you are already familiar.
14. You have already seen the typanic membrane located externally. Cut through it and note the small bone, the columella touching the drum.Sound vibrations are transmitted from the drum through this structure to the inner ear where they set up impulses that are carried through the auditory nerve and recorded in the brain.You may be able to dissect out the otic capsule which contains not the auditory apparatus,but also the semi-circular canals which are organs of equilibrium.An elaborate extension of the auditory apparatus is the endolymphatic organ which probably is important in the behavior of the frog.An elongate sac connects with the brain (region of the cerebellum) and extends down the spinal cord as far as the filum terminale.At intervals, it gives off side pouches (paraganglionic gland) which extend out from the vertebrate. The sac is filled with a milky fluid and at intervals (both in the cerebellar region and long the column) contains masses of small crystals of a calcium compound (aragonite) which gives the appearance of lumps of calcium carbonate.

: 15. During this work, refer to the accompanying chart of the nervous system in ventral view. Study the spinal cord and note that it narrows down to a thread-like body, filum terminale at the posterior end.Now turn the frog over and pin it out. Remove all of the viscera, taking care not to injure the nerves.There are ten spinal nerves given off in pairs from the spinal cord. Locate these nerves.
The 1st spinal nerve, (small) sends fibers to the tongue. It represents the 12th cranial nerve (hypoglossal) of the higher vertebrates. A few fibers pass back and unite with the large 2nd spinal nerve. The 3rd also joins the 2nd, the three nerves forming the brachial plexus. From the brachial plexus, the brachial nerve is formed which goes to the forelimb. The 4th, 5th, and 6th nerves are small and pass obliquely at successively greater angles to the muscles of the back. The 7th, 8th, and 9th are larger and run backward nearly parallel with each other. Follow them closely and note that they finally ute to for the sciatic plexus. At the point where the 7th joins the plexus, it gives off a branch to adjacent muscles. Trace the plexus backward until the large sciatic nerve arises from it. Variations in the form of the plexus will be found. The 10th spinal nerve is small and emerges through a foramen (opening) in the urostyle and runs backward close to the mid-dorsal line. The pair of nerve cords belonging to the sympathetic system should be located. They extend along the dorsal wall of the body cavity. Ten enlargements may be found in each cord. These indicate the points where branches from the spinal nerves unite with this system. What is the function of this system of nerves?


16: Remove the skin from an uninjured hind leg.The flesh is now seen to be made up of a considerable number of muscles,which may be readily separated from each other.These muscles fall into two categories:the flexors, which bend a joint; and the extensors, which straighten it.Each muscle has a thickened middle portion and tapers toward the ends. Muscles are attached to bones by tendons, which are continuous with the connective tissue sheath that surrounds and binds together the numerous muscle scales making up each muscle. Study the gastrocnemius muscle, the large muscle in the calf of the leg. To what bones is it attached? The large tendon at its posterior end is the Achilles tendon. Compare this with the same tendon in man and other animals. Determine the relation of the gastrocnemius to movements of the knee and ankle by moving the joints. Look on the front leg for muscles that oppose the action of the gastrocnemius in the ankle region. Observe the two sets of muscles in the thigh. To what bones are they attached? What muscles are brought into play when the frog suddenly straightens the leg, as in jumping and swimming? Remove the muscles of the thigh, noticing their attachments and their mechanical possibilities.
17. Make a diagrammatic diagram showing the relation of the gastrocnemius muscle to the bones of the leg.Indicate in the diagram or in writing,the motions produced by its contraction.

18. Compare the skeleton of the frog with that of man and other vertebrates.
How do the several parts correspond? How do they differ?
The bones of the skeleton may be classified as follows:
1. The Axial Skeleton: skull, vertebral column, ribs and sternum
The skull, or skeleton of the head, includes the cranium, or brain case, the bones of the face and jaws, and the hyoid apparatus, or skeleton of the tongue
The vertebral column is composed of nine separate vertebrae and the elongate urostyle. the most anterior vertebra is the cervical vertebrae which articulates with the posterior end of the skull and lacks the transverse processes seen in other vertebrae. The most posterior vertebra is the sacral vertebra, to which the pelvic girdle is articulated. The urostyle lies posterior to the sacral vertebrae. Typically, the ribs articulate with the vertebrae, as can be seen from other skeletons which may be available for observation. In the frog, the ribs fuse with the ends of the transverse processes of the vertebrae during development. The sternum is that part of the skeleton found in the ventral body wall. In the frog, it is divided into the omosternum, which lies anterior to the ventral parts of thepectoral girdle, and the sternum proper, which lies posterior to the girdle. Both these parts have cartilaginous extensions.
2. The Appendicular Skeleton: pectoral and pelvic girdles, and the bones of the appendages.
The pectoral or shoulder girdle consists ventrally of two pairs of bones which join a cartilaginous plate in the mid-line. The anterior of these two is the clavicle, the posterior , the coracoid. Laterally, the girdle extends dorsally as the bony scapula and cartilaginous suprascapula.In some frogs, a thin bone, the cleithrum,is closely applied to the suprascapula. At the base of the scapula is a depression, the glenoid fossa, with which the humerus of the forelimb articulates.
The pelvic or hip girdle is a V-shaped bone formed on each side by the fusion of three elements: the ilium, the ischium, and the pubis. The elongate portions which articulates with the 9th vertebra on each side are the bony iliac processes. The bony ischiac portion lies posterior to the cartilaginous pubic region. A depression, where the 3 parts meet on each side, forms the acetabulum with which the head of the femur of the hind leg articulates.
The skeletons of the fore and hind limbs are articulated with the pectoral and pelvic girdles, respectively. The identity of the fundamental arrangement of the bones in the two pairs of limbs is shown by the following tabulations,in which the bones are listed in order from the proximal to the distal end of the limb - that is, beginning with the bones that articulates with the girdle.
Bones of the Fore Limb
Humerus (upper bone)
Radio-ulna (fore arm)
Carpals (wrist, small bones)
Metacarpals (hand)
Phalanges (fingers
Bones of the Hind Limb
Femur (thigh)
Tibiofibula (shin)
Tarsals (ankle, 2 large bones plus small bones)
Metatarsals (foot)
Phalanges (toes)

Make a sketch of the skeleton of the frog from the dorsal view, showing as many of the bones as possible. Be careful to label all the bones shown.