Dissection of the Sheep Brain
Laboratory Exercise 30
Background
Mammalian brains have many features in common. Because human brains may not
be available, sheep brains often are dissected as an aid to understanding
mammalian brain structure. However, as in the cat, the adaptations of the sheep
differ from the adaptations of the human, so comparisons of their structural features
may not be precise.
Materials Needed
Textbook
Dissectible model of the human brain
Preserved sheep brain
Dissecting tray
Dissection instruments
Purpose of the Exercise
Observe the major features of the sheep brain and compare these features with
those of the human brain.
Procedure
1. Obtain a preserved sheep brain and rinse it thoroughly in water to remove as
much of the preserving fluid as possible.
2. Examine the surface of the brain for the presence of meninges. (The outermost
layers of these membranes may have been lost during removal of the brain from
the cranial cavity.) If meninges are present, locate the following: dura mater
(thick, opaque outer layer), arachnoid mater (delicate, transparent middle layer
that is attached to the undersurface of the dura mater), pia mater (thin, vascular
layer that adheres to the surface of the brain).
3. Remove any remaining dura mater by pulling it gently from the surface of the
brain.
4. Position the brain with its ventral surface down in the dissecting tray. Study
figure 30.1 and locate the labeled structures on the specimen of the brain.
5. Gently separate the cerebral hemispheres along the longitudinal fissure and
expose the transverse band of white fibers within the fissure that connects the
hemispheres. This band is the corpus callosum.
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6. Bend the cerebellum and medulla oblongata slightly downward and away from
the cerebrum (figure 30.2). This will expose the pineal gland in the upper midline
and the corpora quadrigemina, which consists of four rounded structures
associated with the midbrain.
7. Position the brain with its ventral surface upward. Study figure 30.3 and locate
the labeled structures on the brain specimen.
8. Although some of the cranial nerves are quite small, missing, or are difficult to
find, locate as many of the nerves as possible.
9. Using a sharp scalpel, cut the brain along the midline to produce a midsagittal
section. Refer to figure 30.4 as a guide to locate the following structures:
cerebrum
• cerebral hemisphere
• cerebral cortex
• white matter
• gray matter
olfactory bulb
corpus callosum
cerebellum
• white matter
• gray matter
third ventricle
fourth ventricle
diencephalon
• optic chiasma
• infundibulum
• pituitary gland (may be missing)
• mammillary bodies
• thalamus
• hypothalamus
• pineal gland
midbrain
• copora quadrigemina
• cerebral peduncles
pons
medulla oblongata
10. Complete part A.
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Figure 30.1 The sheep brain with meninges.
Figure 30.2 Gently bend the cerebellum and medulla oblongata away from the
cerebrum to expose the pineal gland and the corpora quadrigemina.
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Figure 30.3 Ventral surface of the sheep brain.
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Figure 30.4. Midsagittal section of the right half of the sheep brain dissection.
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Part A
Answer the following questions:
1. How do the relative sizes of the sheep and human cerebral hemispheres differ?
2. How does the amount of convolutions and sulci of the sheep cerebrum compare
with the human cerebrum?
3. What is the significance of the differences you noted in your answers for
questions 1 and 2?
4. What difference did you note in the structures of the sheep cerebellum and the
human cerebellum?
5. How do the sizes of the olfactory bulbs of the sheep brain compare with those of
the human brain?
6. Based on their relative sizes, which of the cranial nerves seems to be most highly
developed in the sheep brain?
7. What is the significance of the observations you noted in your answers for
questions 5 and 6?
8. Prepare a list of at least five features to illustrate ways in which the brains of
sheep and humans are similar.
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