Download DISSECTION OF THE SHEEP BRAIN DAY 1 Includes Part 1 – Part 3

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DISSECTION OF THE SHEEP BRAIN
In this dissection, we will be comparing the sheep brain to a human brain. Many
of the structures are similar so you will be able to see the structures we have been
studying in class. This lab will be performed in “chunks”. We will study each division in
class and then follow each up with the comparison of the sheep brain.
Your dissection grade will be based on three qualities:
1. Class participation and use of time / answers to questions in the lab
2. Neat cuts on the brain / intact structures
3. Lab practical
In addition to each of the brain divisions, we will be studying the CNS protectors- mainly
the meninges and the cranial nerves that we can find.
DAY 1 Includes Part 1 – Part 3
PART 1: CNS PROTECTORS
In day 1 of this lab you will identify the 3 meninge layers and remove them from the
sheep brain.
Identify the structures in bold:
1. Dura mater- The dura mater is a foggy, tough covering on the outside of the
brain. Identify it. Feel its consistency and notes its toughness.
2. Cut through the dura mater along the line of the longitudinal fissure (which
separates the brain into right and left sides) and carefully remove it from the
brain. BE CAREFUL ON THE INFERIOR SIDE NOT TO DAMAGE THE CRANIAL NERVES!
3. Arachnoid- The arachnoid spaces gets compressed during the preservation
process. You can see it mostly in the fissures of the brain. It will appear as a
delicate “cottony” material spanning the fissures.
4. Pia mater- Use toothed forceps to “lift up” a bit of this thin layer covering the
brain tissue. You do not need to remove this membrane! Just identify it.
PART 2: THE MAIN DIVISIONS OF THE BRAIN
In this section of the lab, you will identify the main divisions. You will not make any cuts
in this part of the lab.
Identify the structures in bold:
1. Find the right and left cerebral hemispheres. Remember the right and left
directions belong to the sheep, not to you. The right hemisphere controls the left
side of the body and the left hemisphere controls the right side of the body.
2. Examine the cerebellum. Notice that, in contrast to the human cerebellum, it is
not divided longitudinally, and that the fissures are oriented differently.
3. Flip the brain over to the inferior side. Locate the brain stem which then leads to
the spinal cord.
4. The final division, the diencephalon, we will locate later after making the sagittal
cut on the brain.
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PART 3: CRANIAL NERVES
Some of the cranial nerves will have been damaged by harvesting the brain and
removing the dura mater. Find as many as you can and share your finds with other
groups. One trick to remembering the order of these nerves from anterior to posterior
position is the following mnemonic device: “On occasion our teacher feels vicious” (like
in giving lab practicals ☺) Use your atlas and the picture below to help you identify the
nerves. The chart on the following page tells you more about each and how doctors
test patients to assess their “nerve health”.
Identify the structures in bold: (Use atlas figure 49 and your textbook p. 502-508)
1. Study the inferior surface of the brain.
2. Look for the club-like olfactory bulbs anteriorly, on the inferior surface of the
cerebral hemispheres. These are large and “spongy”. Axons of the olfactory
bulbs run from the nasal mucosa through the ethmoid bone to synapse with the
olfactory bulbs. These send sensory information concerning your sense of smell.
3. The optic nerve (II) carries sensory impulses from the retina of the eye. Thus this
cranial nerve is involved in the sense of vision. Identify the optic nerves, optic
chiasma (large “X”), and optic tracts.
4. Moving more posteriorly, identify the next set of nerves, the oculomotor nerves.
These are wide and flat and arise from the midbrain surface. These provide
motor fibers to extrinsic muscles of the eyeball.
5. Continue to move posteriorly and try to find the remainder of the cranial nerves.
These nerves are more fragile so you may not have every one on your sheep
brain. Pay special attention to the location and compare them to the drawing
below and to your atlas.
• Trigeminal nerves- very large set of nerves that have several branches. It
runs on either side of the pituitary and attaches to the brain along the
sides of the pons, close to the medualla. They are involved in chewing
and sensations of the head and face
• Facial nerves- medium-large nerves involved in taste sensations, salivary
gland function and facial expression.
• Vagus nerves- comes from the medulla lateral surfaces; these are called
“wanderers” which serve many organs in the head, thorax and abdomen.
Questions:
1. How does the size of the olfactory bulbs on the sheep compare to those on the
human photo in your atlas (figure 49)?
2. Using your observations about the size of the olfactory bulbs, infer on how the size
of this structure influences the functional needs of the organisms. (HINT: Do
humans needs more or less of a sense of smell than sheep? Why?
3. What is the difference between the mammilary body of the sheep and that of
the human?
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Day 2
PART 4: CEREBRAL HEMISPHERE
In day 2 of this lab you will identify the external and internal structures of the cerebrum.
Follow the procedure and be sure to locate all the external structures first before
making any cuts.
Identify the structures in bold:
1. Find the longitudinal fissure that divides the cerebrum into right and left sides.
Gently force the cerebral hemispheres apart laterally to expose the corpus
callosum deep down inside.
2. Examine the superior surface of the brain. Notice its surface is thrown into
convolutions (fissures, sulcus and gyri). Distinguish between these two structures.
3. Identify the transverse fissure that separates the cerebrum from the cerebellum.
4. Using figure 48 of your atlas, identify the following lobes of the cerebrum. Review
their function by lisiting it next to the names below, as well.
•
Frontal lobe
•
Parietal lobe
•
Temporal lobe
•
Occipital lobe
•
Insula (carefully separate the temporal lobe from the frontal lobe along
the lateral sulcus in order to view the insula)
5. Locate the following divisions on the cerebrum. Again, review their function by listing
it next to each structure.
• Precentral gyrus (primary motor cortex)
•
Central sulcus (divides the frontal lobe from the parietal lobe) this is the
division between the areas of the brain that control motor function and
interpret sensory information.
•
Postcentral gyrus (primary somatosensory cortex)
6. Look at the most anterior end of the brain and locate the two olfactory lobes.
On one side, cut ½ an inch off the front of the cerebrum (you’ll destroy one of
the olfactory bulbs).
7. Identify the gray matter and the white matter from this cut. Also, identify which
area includes the cerebral cortex.
8. Place the brain ventral side down on the dissecting tray and make a cut
completely through it in a superior to inferior direction. Cut through the
longitudinal fissure, corpus callosum and midline of the cerebellum. Be careful to
make this cut as clean as possible.
9. Identify the corpus callosum, again. Note its position and shape.
10. Identify the thin nervous tissue membrane immediately ventral to the corpus
callosum that seperates the lateral ventricles is the septum pellucidum.
11. Pierce the septum pellucidum and probe the lateral ventricle cavity.
12. Identify the fiber tract ventral to the septum pellucidum and anterior to the third
ventricle. This is the fornix.
Questions:
1. What function does the corpus callosum provide to communication between
the two hemispheres of the cerebrum?
2. What is the purpose of the brain to have fissures and gyri?
3. Label the following picture of the brain with each lobe. In addition, be sure to list
the functions of each cerebral lobe under procedure #4 in this section.
4. Label the precentral gyrus, central sulcus and postcentral gyrus on the picture
below. In addition, be sure to list the functions of these divisions under procedure
#5 in this section.
5. What color are the cerebral cortex neurons?
6. What makes the white matter a light (almost white) color?
7. What is the purpose of the lateral ventricle?
8. How does the relative size of the fornix in this brain compare with the human
fornix?
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Day 4 Includes Part 5 – Part 7
PART 5: DIENCEPHALON
As you locate these structures, review their functions as well.
Identify the structures in bold:
1. Looking on the inside of the brain, identify the thalamus, which forms the walls of
the third ventricle and is located posterior and ventral to the fornix. Remember
the roof of the third ventricle is called the choroid plexus.
2. The Mass intermedia (intermediate mass) of the thalamus spans the ventricular
cavity. It appears as an oval protrusion of the thalamic wall. In the sheep this is
much larger than in the human brain.
3. Identify the passageway from the third ventricle into the lateral ventricle. This is
the Foramen of Monroe (interventricular foramen).
4. The hypothalamus forms the floor of the third ventricle.
5. Identify the optic chiasma, infundibulum, pituitary gland (attached to the
infundibulum) and mammilary body on the external surface. The mammilary
body is usually covered by the pituitary gland where it is “stuck” to the
underneath of the brain by connective tissue. You can see it better if you loosen
the pituitary gland and look underneath on only one side of the sheep brain.
6. Identify the pineal (gland) body at the posterior end of the third ventricle, just
beneath the junction of the corpus callosum and fornix.
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PART 6: BRAIN STEM
The following structures in steps 1-3 are all part of the midbrain:
1. Locate the midbrain by identifying the corpora quadrigemina that form its dorsal
roof. The corpora quadrigemina are just above the cerebral aquaduct (narrow
canal connecting the third and fourth ventricles) and anterior to the cerebellum.
2. The most superior “bump” of the corpora quadrigemina is the superior colliculi
and just below it is the inferior colliculi. These are involved in reflexes.
3. Identify the cerebral peduncles which form the anterior walls of the fourth
ventricle. It is the anterior part of the midbrain, next to the thalamus and
hypothalamus. Fiber tracts that go through here connect the pons to the
cerebrum.
4. Identify the pons and the medualla oblongata, which lie anterior to the fourth
ventricle. The medulla continues into the spinal cord without any obvious
anatomical change, but the point at which the fourth ventricle narrows to a
small canal is generally accepted as the beginning of the spinal cord.
5. Identify the central canal. It will look like an indentation that goes through the
center of the medulla oblongata. This carries CSF into the spinal cord.
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PART 7: CEREBELLUM
Identify the structures in bold:
1. Identify the cerebellum posterior to the fourth ventricle. Notice its internal
treelike arrangement of white matter- the arbor vitae.
2. Identify the fourth ventricle that separates the cerebellum from the brain stem.