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					Stuyvesant High School                                             Department of Biology & Geo-Science

                                      LABORATORY EXERCISE #12

                                    EXTERNAL FEATURES AND
                               LEG MUSCULOSKELETON OF THE FROG

We share a pedigree with the frog. Both of us are members of the Subphylum Vertebrata, animals with
backbones. Although we are mammals and frogs are amphibians, we are close enough to share many
anatomical and physiological characteristics. This laboratory exercise is the first in a series that will help
you discover many of these similarities but also many of the differences between the frog and ourselves.
In this exercise you will examine various external features and relate them to how they help the frog in its

1 Observe the external features of the frog.
2 Relate the external structures to their functions.
3 Observe the structure of a leg muscle and leg bone of the frog.
4 Compare amphibious structures with those of a human.
5 Demonstrate a mature and responsible attitude when using preserved specimens for biological study.

1. Describe three characteristics of a typical mammal and three characteristics of a typical amphibian.
2. Describe three similarities and three differences between a mammal and an amphibian.

Preserved frog, dissecting pan lined with a paper towel, dissection kit, toothpick, hand lens, dissection
manual, charts or pictures of various types of frogs, liter-size plastic bag with twist-tie, plastic gloves,
safety glasses or goggles, and video of frog dissection.

Work in pairs. If the smell of the preservative is very strong, rinse the frog under running water at the sink
and pat it dry. Lay the frog in the dissecting pan, belly side (ventral) side down.

I. Examination of External Features
1. Review with your teacher the terms which relate to body orientation: dorsal, ventral, anterior,
    posterior, lateral, proximal and distal.
2. Touch the surface of the frog’s skin. In life it is covered with a mucous secretion, which keeps the skin
    moist. Try to determine if a fat layer is present.
3. Observe the coloration on the frog’s dorsal side. Flip the frog over and note the difference in
    coloration on the ventral side.
4. Observe the frog’s fore legs and hind legs. Note the differences in their sizes, and the function of
    each skeletal muscle. Examine a picture of a live frog and note how its legs are folded up.
5. Observe the fore legs and hind legs. Note the differences in their structure, and the differences in the
    size of the digits and the webbing between the digits.
6. Contrast the shape of the “hands” of your frog with those of other frogs in the room. Male frogs can
    sometimes be distinguished from female frogs by the pads on the male “thumbs”. These pads are
    used in the mating process.
    The body of the frog can be divided into the head and trunk region. Note that it has no neck. Observe
    how the eyeball bulges at the top of the head. Note the two outer eyelids and the movable,
    translucent NICTITATING membrane (“Nictitating” comes from the latin word meaning “to wink”).

Regents Living Environment                              1                               Laboratory Manual
Stuyvesant High School                                            Department of Biology & Geo-Science

7. Observe the oval TYMPANIC MEMBRANES on each side of the head. These act as external
   eardrums. The males have larger membranes than the females. Observe the EXTERNAL NARES

II. Examination of the Mouth
1. Carefully force open the mouth of the frog. The jaws should be stiff due to rigor mortis in the muscles.
    It might be necessary to break the jaw joint. Your teacher will demonstrate this. Prop the mouth open
    with a toothpick which has been broken in half.

2. Observe how the forked tongue is attached to the anterior of the mouth.
   It is usually covered with a sticky secretion. The tongue is thrust out only
   in the presence of moving objects. The frog does not respond to non-
   moving prey!

3. With your finger, feel along the edges of the jaws for the small conical
   TEETH, and on the upper part of the mouth find the two additional teeth,
   the VOMERINES. In frogs, teeth are replaced as needed.

4. Use your finger or your probe to find the soft opening to the esophagus
   at the back of the mouth. Distinguish this from the slit-like, cartilaginous
   GLOTTIS, the opening to the respiratory system.

5. Use your probe to fine the EUSTACHIAN TUBE openings located on the
   sides of the upper jaw. These lead to the tympanic membrane. Find the
   internal opening of the NARES. Note the bulge of the eyeballs on the
   roof of the mouth. When the frog swallows, it closes its eyelids. This causes the eyeballs to press
   down into the mouth and thus helps to push the food into the esophagus.

Regents Living Environment                             2                             Laboratory Manual
Stuyvesant High School                                          Department of Biology & Geo-Science

III. Examination of the Thigh Muscles and Calf
1. Carefully strip the skin off the frog. (Your
     teacher will demonstrate this procedure.)
2. Study the pictures of the frog musculature
     during the dissection. Gently separate each
     muscle of the thigh with a blunt probe. Be
     careful not to shred the muscles with the
     teasing needle. If you have to cut a muscle to
     see the underlying deeper muscles, cut
     cleanly across the belly (the middle) of the
     muscle. Identify as many muscles as possible.
3. Continue separating and identifying the
     muscles of the calf.

IV. Observation on the Sciatic Nerves and
Bones of the Leg
1. On the other leg, you will find the sciatic nerve.
     The sciatic nerve is a nerve that comes from
     the dorsal spinal cord. One SCIATIC NERVE,
     innervates or goes into each leg. It is quite
     easy to locate one sciatic nerve. Separate the
     muscles of the thigh from the dorsal side. The
     sciatic nerve runs near the bone. After you
     have found the nerve, carefully remove the muscles from this     MUSCLES OF FROG HIND LEG
    second leg. Continue removing the muscles from the calf and
    follow the nerve.
2. Identify the bones of the leg using charts and the manual.

NOTE: Please save the used frog for the second lab on the frog.

Regents Living Environment                            3                          Laboratory Manual