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Dissection Frogs Central Nervous System


Dissection of an adult bullfrog's central nervous system (CNS) Experiment
consists of the dissection and analyzation of a bullfrog’s nervous system.

Dissection consists of the isolation of the CNS consisting of the brain and
spinal cord. It also consists of analyzing the nervous tissue under the
microscope. Materials needed: frog, scalpel, razor blade, dissecting tray, pins,
forceps, scissors, microscope, slides, slide covers, water, blue dye, diagram of
frog’s brain, paper towel. The bullfrog’s CNS is composed of the brain,
which is further divided into the olfactory bulb, cerebrum, optic tract, optic
lobe, pituitary gland, and the cerebellum. Also contained within the CNS is the
spinal cord which in contained within the spine The brain was about an inch in
length and ¾ of an inch in diameter. The spinal cord was about 2.5 inches in
length and the diameter of the spine was about 1/8 of an inch. The cerebellum in
the frogs brain is very small in comparison to the rest of the brain and also to
the size of the cerebellum in other animals compared to the size of their
brains. Place the frog in the dissecting tray so the ventral section faces
upwards with the head pointing away from you. This will allow for easier access
to the CNS since you don’t have to cut through the skull. Pin each of its legs
down to the tray using the pins. Using the scalpel start the incision at the
base of the jaw. This will be a superficial cut just enough to cut through the
skin. Pull the scalpel in a caudal direction towards the tail. After you’ve
done this make another incision lateral to where you started the first one and
another one where you ended the cut. Pull the skin back and either cut it off or
pin it down. Muscle of the breastbone and abdomen will be exposed. Using the
scissors make a deep cut through this, again cutting in a caudal direction to
the tail. Cut off the arms of the bullfrog and remove all of the muscle mass.

You must now remove the lower jaw of the frog. Using the scissors cut through
the area where the jaw connects to the skull. This will expose a membrane
covering the brain. Make a small superficial cut using the scalpel in this
membrane. Using the scalpel, lift up the cut and then use the scissors to remove
all of this membrane. The brain should now be exposed. Now you must remove all
the internal organs. There may or may not be an egg sac, if there is try not to
puncture it when you remove it otherwise it can get messy, but if you do it is
ok. Remove all the internal organs until you can see the spinal cord. Now that
you have isolated the nervous system you can remove pieces of it to view under
the microscope. To do this, first identify the section you are going to remove
using a diagram of the brain such as the one shown in latter pages or one out of
a lab manual. Using a razor blade remove a small section of the brain. Set this
on the table and using the blade cut away as thin of a section as you can. Place
a drop or two of water on a slide and place the thin section of brain tissue on
the slide. Place a cover slip over this. Put 1-2 drops of the dye right next to
the cover slip. Place the paper towel on the side opposite where you placed the
dye. This should pull the dye through and stain the tissue. After you’ve done
this place 2-3 drops of water on the same side you placed the dye and pull that
through with the paper towel to remove the excess dye. Place this under the
microscope and observe. Any thin red, squiggly lines are vascular tissue. The
stringy bundles of fiber would be the nerve tracks. After doing all of this you
should clean up! Remove the pins and rinse those off. Throw away the frog and
all the large pieces of it. Clean off the tray and let it air dry. The intact,
isolated brain matter appeared to be gray, signifying that it was unmyelinated
nerve tissue. It was narrow and each part of the brain was somewhat easily
distinguishable. Under the microscope the tissue had easily distinguishable
nerve fibers and in some cases it contained vascular tissues distinguishable by
the small capillaries. The tissue felt smooth and rubbery. It was somewhat soft
yet firm. Based on my observations from the microscope I saw how the soma and
dendrites and axons all went together to form nervous tissue.

Bibliography
http://k-2.stanford.edu/creatures/InfoFrames/CNS.1.1.html - various information
http://www.neurocomputing.org/comparative_neuroanatomy.htm - picture of frog’s
brain