Chordate II, Internal Anatomy

    Frogs serve as excellent animals for study by the general biology student, as their anatomy and physiology are similar to man's.  Frogs are equally at home on land or in water, and they require water during the early parts of their life cycle.  They hibernate during cold seasons under water or in other protected places, and they are very active in the spring and summer.

The classification of the bullfrog and man are compared thus:

    Common name:        Bullfrog                      Human
    Phylum:                    Chordata                    Chordata
    Subphylum:              Vertebrata                  Vertebrat
    Class:                       Amphibia                    Mammalia
    Order:                      Anura                         Primate
    Family:                     Ranidae                      Hominidae
    Genus:                     Rana                           Homo
    Species:                   catasbeiana                sapiens

Question 1:  Many reptiles, such as crocodiles and alligators, spend much of their lives in water.  Why then are they not classified as amphibians?

LAB OBJECTIVES:
    1.  To identify and name all structures that are to be located.
    2.  To give the function of the structures.
    3.  To note the differences and similarities between certain anatomical features of the frog and human.

In this exercise you will examine the external features, mouth cavity, skeleton, muscles, and internal structures of the bullfrog.  You will work in groups.  Each group will obtain a frog, dissecting pan, and other supplies provided.  Examine and study your frog using the instructions outlined below.

I.  External Features
    Identify the head and trunk of your frog.  The head is roughly triangular in shape, with a slit-like opening extending around its forward edge and forming the mouth.  The cloacal opening (anus and reproductive organ) is at the posterior and between the hind limbs.  The sense organs include the nostrils, eyes, and ears (tympanum).  The two openings just posterior to the mouth are the external nares (nostrils).

    The bulging eyes are equipped with a more or less non-movable upper lid, a non-movable lower lid, and a transparent fold, the nictitating membrane.  The latter is attached to the inside of the lower lids and can be moved up over the eye.  A frog can close its eyes altogether by retracting them deep into their sockets, a process which brings the upper and lower lids together. 

    Find the ear drum (tympanum), a disk-like structure just behind the eye.  The tympanum is the outer wall of the middle ear.

    The appendages are paired.  The short forelimb consists of an upper arm, a forearm, and a hand.  The hand has four digits and a vestigial thumb.  The hind limb consists of a thigh, shank, and a foot.  The foot has five digits with a small prehallux, which is a rudimentary sixth digit.  Find a vestigial thumb and prehallux on the demonstration skeleton.

Question 2:  What physicals features of the frog suggest that it is adapted to life in water?

Question 3:  What is the advantage of the coloration and the patterns of the frog's skin?

II.  The Mouth Cavity
    Open the mouth very wide, cutting the angle of the jaws if necessary.  Pin back the lower jaw.  Examine the tongue.  Note its point of attachment.  Find the small maxillary teeth in the margins of the upper jaw.  Between the internal nares are two small prominances covered with teeth.  These are the vomerine teeth.  Near the angles of the jaws are the circular openings of the eustachian tubes.  These tubes connect the tympanic cavity, just beneath the eardrum, with the pharynx (throat).  Look for the slit-like glottis.  It opens into the larynx (voice box).  Locate the opening of the esophagus.

    The internal nares may be located by pushing a probe through the external nares.  Also push your probe through the eustachian tube openings to demonstrate that they do connect to a chamber beneath the tympanum.  Refer to figure 1 to help you locate the underlined structures.

Question 4:  What feature of the frog suggests that it is adapted to a diet of insects and other small animals?

III.  Skeleton
    Refer to the demonstration frog skeletons and figures 2 and 3 to locate and learn bones your instructor emphasizes.  Using your text, compare these bones to corresponding human bones.

Question 5:  List the differences between the positions and structures of the bones in the frog as compared to man.

IV.  Muscles
    To examine the muscles of the frog, it will be necessary to remove the skin.  Note that the skin is loose and not closely connected to the underlying muscles.  This is due to lymph spaces between the skin and the muscle layer.  Place the frog on its back and using your scissors, make a cut through the skin only along the midventral body line from between the legs to the chin.  Now make a cut down the inside of the left arm to the wrist and down the inside of the left leg to the heel.  Gently remove the skin from the left side of the body leaving the skin attached on the right side.  Utilizing frog models or other sources, your instructor may emphasize important muscles.  Using your text, compare these muscles to corresponding human muscles.

Question 6:  Note some of the differences between the sizes, shapes, and locations of the human and frog muscles.

V.  Internal Structure
   
    A.  To open the body cavity, place the frog on its back (dorsal side):  locate the linea alba.  With your scissors make a longitudinal cut through the muscular layer of the abdominal wall slightly to the right of the linea alba from the pelvic girdle to the pectorial girdle.  Refer to figure 4.  The ventral abdominal vein is attached to the undersurface of this muscular layer along the linea alba.  Be careful not to cut this vein.  Lift the muscular wall, find the large ventral abdominal vein, and careful pull it away from the muscle.  Next, make two transverse cuts through the muscle: one just anterior to each hind leg and the other one just posterior to each front leg.  Continue the cuts to the back and turn back the flaps of the body wall.  The flaps may be pinned down in the dissecting pan.  Finally, lift up the pectorial girdle, carefully free the structures that are attached to it, and continue your longitudinal cut (first cut) forward through it.  This girdle is bone and will not cut as easily as the muscular wall; a strong pair of scissors will be needed. 

    You have now exposed the body cavity or coelom in which the visceral organs are located.  The coelom is completely lined by a shiny layer of epithelium called the peritoneum.  The coelom of the frog is divided into two chambers or lesser cavities:
        1.  a small pericardial cavity underneath the pectorial girdle where the heart lies; the membrane lining the pericardial
    cavity is the pericardium;
        2.  a large pleuroperitoneal cavity, containing the lungs and all other organs.

    B.  General Visceral Organs
    Identify the organs in the pleuroperitoneal cavity.  Refer to figure 5.

        1.  The large, dense, lobed organ occupying much of the anterior part of the cavity is the liver.
        2.  Lift up the right side of the liver and find the gall bladder, a small dark sac attached to the dorsal surface of the liver.
        3.  The stomach lies dorsal to the liver on the left side of the body.
        4.  One lung lies dorso-lateral to the stomach; the other lies in a comparable position on the other side of the body.
        5.  Lift the posterior end of the stomach.  The small, dense round body on the left side of the membrane (mesentery) supporting the intestine is the spleen, an organ in which blood cells are produced and stored.
        6.  A small intestine leads from the posterior end of the stomach; it is relatively small and convoluted.
        7.  The lobate whitish tissue lying in the loop between the first part of the small intestine and the stomach is the pancreas.
        8.  Follow the coils of the small intestines, and they will lead to the wider large intestine.
        9.  The yellowish mass of fingerlike lobes of fat that must be pushed aside are the fat bodies, reserved food used chiefly for hibernation.
        10.  One mass of fat bodies is attached to the anterior end of each testis or ovary.
            a.  The testes of the male are a pair of small, whitish, oval bodies, one on each side of the mesentery that supports the intestines.  (See mount of male frog.)
            b.  Ovaries occupy the same position but vary greatly in size according to the reproductive state of the female.
            (See mount of female frog.)
        11.  An elongated dark kidney lies against the back on each side of the back bone and dorsal to each gonad (testes or ovary).
        12.  In females a long coiled white tube, the oviduct, will be found lateral to each kidney.  Males may have a smaller vestigial oviduct.
        13.  The urinary bladder is a sac located just anterior to the hind legs and ventral to the large intestine.  When distended, it is a large bilobed organ; when contracted, it is much smaller.

Question 6:  What is an organ?

Question 7:  What is the relationship between organs and systems?

Question 8:  What similarities are there between these organs of the frogs and those of humans?  Refer to the text.

Question 9:  Define:
                    visceral-

                    mesentery-

                    thoracic cavity-

VI.  The Digestive System
    All animals must take into their bodies a variety of materials to sustain metabolism.  These include food, vitamins, mineral ions, water and oxygen.  Most of the water enters through the skin of the frog; the uptake of ions and gas exchange are also skin functions.  Oxygen and carbon dioxide enter and leave through the lungs and skin.  Other materials enter through the digestive tract.  Roughage and other undigested material, bacteria, and bile pigments derived from the breakdown of hemoglobin in the liver are discharged as feces by the digestive system.  Refer to figure 6.

    Pass a blunt probe or a pair of blunt forceps through the pharynx and down the esophagus.  Feeling with your fingers, you can tell when the probe enters the stomach.  The esophagus is quite short since the frog lacks a neck.

    The stomach is a large, saccular organ in which food is stored and digestion continues.  It curves toward the right side of the body and is usually J shaped.  Cut it open.  If it is not completely filled with remains of food, its lining forms conspicuous longitudinal folds.  The posterior termination is a thick muscular pyloric sphincter, the contraction of which keeps feed in the stomach until it is ready to move into the small intestines.  Feel the pyloric sphincter with your fingers.

    Digestion is completed and food is absorbed in the small intestines.  The first part, the duodenum, curves toward the liver and is the principle site of digestion.  The rest of it is for the absorption of nutrients into the blood.  Slit open a part of the small intestines and observe the irregular folds of the muscular wall.  Most of the secretions that inter the intestine come from the pancreas and liver; some enzymes and mucus are secreted by cells lining the small intestines.

    The large intestine, or colon, is a short segment from which water and ions are absorbed.  Bacteria reside in the large intestines.  The undigested residue and many of the bacteria form feces that are stored temporarily and finally eliminated by the large intestine.  Cut open the colon and observe its contents.  The colon becomes narrow as it passes through the pelvic canal to enter the cloaca.  Carefully cut through the pelvic girdle and spread the legs apart.  Cut only on the midventral line and do not injure the urinary bladder.  The cloaca is the terminal part of the digestive system which also receives the urinary and genital ducts.  The cloaca opens to the outside of the cloacal aperture.

Question 10:  Define:
                    metabolism-
           
                    digestion-

                    nutrient-

Question 11:  What is the advantage of the folds in the walls of the stomach and small intestines?

Question 12:  Which has a thicker wall, the stomach or small intestines?  Why?

Question 13:  What difference is there in the appearance of the contents of the stomach and small intestines?  

                    of the stomach and large intestines?

                    Why are the contents different?

Question 14:  What is the role of the bacteria found in the large intestine?

Question 15:  Is a cloaca found in humans?

                    What accomplishes the function of the cloaca in humans?

VII.  The Respiratory System
    Living organisms oxidize materials obtained from digestion using oxygen and releasing carbon dioxide.  A part of the oxygen used by the frog enters the circulatory system through the capillaries in the skin, a part through the capillaries in the mucous membrane of the mouth and pharynx, and a part through the capillaries in the lungs.  Refer to figure 7.

    In your specimen locate the glottis again.  The larynx, situated just dorsal and anterior to the heart, is a chamber reinforced by cartilage between the glottis and the lungs.  To expose it, dissect away all the muscles from the underside of the lower jaw so as to expose the body of the hyoid cartilage.  Note the bony thyroid processes of the hyoid which extend posteriorly.  The larynx is enclosed between these processes, and a short bronchus connects each lung with the larynx.  Cut a window in the floor of the larynx and find the vocal cords, a pair of folds of the walls of the larynx.  Slit open one lung and observe the air sacs on the inner surface.  Small capillaries surround these sacs and here the respiratory exchange takes place.

Question 16:  For what purpose does a living organism need oxygen?

Question 17:  What is the source of carbon dioxide released by a living organism?

Question 18:  What anatomical difference(s) do you notice between the respiratory system of the frog and that of man?

VIII.  The Circulatory System

    A.  The Frog Heart - refer to figure 8 for help in locating these structures.  The heart lies in the pericardial cavity beneath the pectorial girdle.  Carefully remove the thin pericardium surrounding the heart.  There are three main divisions of the heart:  the right atrium, the left atrium, and the ventricle.  Refer to figure 11 for comparisons of vertebrate hearts.

    The ventricle is the thick-walled conical portion pointing posteriorly.  The conus arterious (bulbus cordis) is a large vessel arising from the right anterior corner of the ventricle.  It extends forward over the ventral side of the right anterior corner of the ventricle and right atrium.  Anteriorly, it divides into a truncus anteriosus on each side.  Note the two thin-walled sacs, the right and left atria, which are anterior and dorsal to the ventricle.  Entering the left atrium are the two pulmonary veins.  The upper (anterior) margin of the heart may be lifted to see the veins more easily.  Lift the ventricle and identify the large, thin-walled sinus venosus below.  Two precaval veins and a large postcaval vein unite to form this structure.  Blood from the sinus venosus enters the right atrium.

Question 19:  Make comparative observations of the frog's heart and the human heart.

Question 20:  Give the function of:
                    arteries:

                    veins:

                    capillaries:

IX.  The Urogenital Ssystem - Refer to figures 9 & 10.  Because the excretory and reproductive systems of the vertebrates are so closely associated, they are usually considered together.  Be sure to see both male and female.

    A.  The Male Urogenital System

        The kidneys are long, narrow, reddish (often covered by blue latex) organs lying against the dorsal body wall external to the peritoneum.  The testes are yellow, oval organs attached to the anterior portions of the kidneys by a mesentery.  From each testis several small ducts, the vasa effentia, pass through the mesentery into the kidney.

    Sperm pass through the vasa efferentia into the kidneys.  Yellow fat bodies with long fingerlike processes are attached to the testes.  Find the ureter along the lateral margin of each kidney.

    B.  The Female Urogenital System

    The kidneys are long, narrow, reddish (often covered by blue latex) organs lying against the dorsal body wall external to the peritoneum.  Find the ureter along the lateral margin of each kidney and trace it to the cloaca.

    The ovaries are paired and are attached to the dorsal body wall by a mesentery.  When filled with eggs, the ovaries are conspicuous; when empty, they are small.  The fat bodies are attached to the ovaries at the anterior ends.  The oviducts are conspicuous, coiled, white tubes.  Eggs enter into the oviducts through funnel-shaped openings, the ostia, near the bases of the lungs.  Posteriorly, each oviduct widens into a uterus which serves as a temporary storage area for eggs.  Find where each uterus opens to the cloaca.  Eggs produced in the ovaries break out into the coelom and are carried into the oviducts by ciliary currents produced by ciliated cells covering the openings of the oviducts.

Question 21:  How is the human female's reproductive system different from that of the frog?

Question 22:  Give the function of:
                    testes:

                    ovaries:

                    uterus:

                    kidneys:

Question 23:  Define:
                    gametes:

                    gonads: