Instructions for Sheep Brain Dissection
Mammalian brains have many features in common. Human brains are not readily
available, so one brain of choice for dissection, that closely resembles the human brain, is
that of the sheep. Sheep brains are removed at the time the animal is slaughtered and then
preserved for later use. However, the adaptations of the sheep differ from the adaptations
of the human, so comparisons of their structural features may not be precise. The sheep is
quadruped, therefore the spinal cord is horizontal, unlike the vertical orientation in a
bipedal human. Preserved sheep brains have a different appearance and are firmer than
those removed from the cranial cavity, caused by the preservatives used. Observe the
major features of the sheep brain and compare them with those of the human brain
throughout the dissection.
Begin the dissection by gathering the equipment and supplies listed here.
(sheep brain, dissecting pan, scissors, scalpel, probe, paper towels and lab manual) .
PART 1: External Anatomy of the Brain
Step 1: Observe the surface of the brain. Surrounding the brain are a series of membranes called
meninges. The three layers of the meninges are the outer dura mater, the middle arachnoid
mater, and the inner pia mater. You can see that the sheep brain is completely enclosed in that
tough outer covering called the dura mater. You can also still see some structures on the brain
before you remove the dura mater. Take special note of the pituitary gland on the underside of
the brain. This gland is responsible for controlling growth, development, reproduction, and many
other things for the body. Also note the optic chiasma which looks like an “X” shape right
where the two optic nerves leaving the back of the eye cross over one another. And finally, note
the olfactory bulb which will be discussed in more detail in the next unit but these structures
carry dissolved chemicals (smells) to the brain. These two structures will likely be pulled off
when you remove the dura mater.
Brain with Dura Mater Intact
External Structures
Step 2: Using your scissors or scalpel, carefully remove the dura mater from the outside of the
brain. Be very careful on the underside of the brain to not pull off the pituitary gland or any
cranial nerves. You may need to cut around some of these structures leaving behind a little bit of
the dura mater. As you cut off the dura mater, a tough chunk will come out from the longitudinal
fissure. This is known as the falx cerebri. Another tough chunk will come out from between the
cerebrum and cerebellum, known as the tentorium cerebelli. This one may need to be cut
slightly on each side of the cerebellum in order to remove it from the brain.
Removal of the Dura Mater
External Structures
This image on the right shows the ventral surface of the sheep's brain with most of the dura
mater removed. The pituitary gland and the optic chiasma are still intact. (A = pituitary gland, B
= optic chiasma, C = olfactory bulb)
Step 3: Now that the dura matter has been completely removed, you can see the infundibulum
(attached to the pituitary gland) in the center. Careful dissection also reveals a few other large
nerves: the olfactory nerves (CI), the optic nerves (CII), and the oculomotor nerves (CIII).
Often these nerves are removed with the dura mater, but in this image they are still intact. You
can also visibly see the rounded bulge on the brain stem (pons), the medulla oblongata, and the
below that.
spinal cord
Olfactory Nerves
Step 4: If you flip the brain over to the other side, you can see the cerebellum, it will be loosely
attached to the cerebrum in most cases. If you did not carefully remove the dura mater you may
have accidentally pulled the entire cerebellum away from the brain. The lobes of the brain are
visible, as well as the transverse fissure, which separates the cerebrum from the cerebellum.
The convolutions of the brain are also visible as bumps (gyri), as are the grooves of the brain
(sulci).
Step 5: The gap between the cerebrum and the cerebellum at the transverse fissure can reveal
some internal parts of the brain. In this image, a student is bending the cerebellum down to show
the superior and inferior colliculi. Just behind the colliculi, (you may have to pull down firmly)
the pineal gland is just barely visible. This is an important gland in hormone secretions for the
body.
PART 2: Internal Anatomy of the Brain
Step 6: Using a scalpel, cut the brain into left and
right hemispheres using the longitudinal fissure as
a guide. Once it’s opened up, you’ll be able to see
several structures. Notice that the left side of the
brain is a mirror image to that of the right side.
Step 7: Locate the arbor vitae (tree of life) found
within the cerebellum. This is the branching, treelike area of white matter. You can also still see the
transverse fissure between the cerebrum and the
cerebellum. The fourth ventricle sits in front of the
cerebellum where it loosely attached to the brain stem. You should also be able to follow the
cerebral aquaduct down from the fourth ventricle to the middle of the spinal cord. Remember
that both of these structures are usually filled with cerebrospinal fluid (CSF).
Step 8: Several structures can also be located in the cerebrum. The corpus callosum is a bundle
of white fibers that connects the two hemispheres of the brain, providing coordination between
the two. Right beneath this piece of tissue lies one of the lateral ventricles (remember that you
have a left and a right lateral ventricle). Right below that, a piece of the corpus callosum
separates the lateral ventricle from the third ventricle. You can also see the region known as the
thalamus, and then below that, the hypothalamus. The thalamus acts as a 'relay station' for
sensory information. It receives messages from the nerve axons and then transmits them to the
appropriate parts of the brain. Connecting the thalamus down to the brain stem is a small little
region known as the midbrain. The rest of the brainstem (pons and medulla oblongata) sit below
that.
Step 9: Finally, cut a small section off the very side of the cerebrum to examine the difference
between white matter and gray matter (see the picture below).
PART 3: Review
Step 10: With your lab group, so back through the whole brain and identify all major
structures, both externally and internally. Use the pictures below to ensure you’ve
covered all the major structures so you’re all ready for the brain quiz!
PART 4: Clean Up
Step 11: Please dispose of your brain as told by the teacher. Then, hold any loose or
extra brain pieces in one hand and peel that one glove off so that it is inside out with the
extra brain pieces inside. Hold that glove in the other hand, and peel the other glove off
so that it is inside out with the first glove inside. Tie a knot in your glove and place it in
the garbage can.
Step 12: Rinse your tools in the sink using soap, and place them on a clean piece of
paper towel near your lab bench. Using damp paper towel and soap, thoroughly wipe
down your lab counter, and any other surfaces that may be contaminated. Dispose of the
paper towels in the garbage can.
Step 13: Wash your hands with soap and water. Return to your desk and begin working
on your lab report.