B Brain Dissection - Welcome to Lake Pend Oreille School District_

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B Brain Dissection - Welcome to Lake Pend Oreille School District_ Powered By Docstoc


                          Sheep Brain Dissection
For the past two weeks we have examined the anatomy of the human brain. Since the
basic plan of all mammalian brains is the same, we can expand our understanding of the
structure of the human brain by studying the structure of the brain of some other
mammal. Because of its size and relative availability, the sheep brain is often used for
comparative dissection. The sheep brain is somewhat smaller than the human brain but
the structures are essentially the same and the sheep brain is quite easy to work with. In
this activity, we will dissect the sheep brain and examine those structures common to
both the sheep and the human brain.


Dissecting tray               Dissecting kit
Sheep brain            Sheep dissection brain pictures (Packet)



1. Examine the surface of the brain. You may be able to locate a tough sac-like protective
layer around the brain. This is the dura mater (literally, “tough mother”), the outermost
of the meninges, the three protective connective tissue membranes that surround the
brain and the spinal cord. It is possible that the dura mater has been removed. If the dura
mater is still present, follow the directions in part B to carefully remove it.

2. Regardless of whether you have found a dura mater, you probably will not be able to
locate the second meningeal layer, the arachnoid layer, or arachnoid membrane. The
arachnoid layer is sometimes called the arachnoid matter (literally, “spider mother”).
This web-like structure does not withstand preservation well. (If you notice tufts of
cottony tissue across certain brain fissures, however, you may have located remnants of
the arachnoid mater.)

3. You should be able to locate the inner layer, the pia mater (literally, “gentle mother”),
which tightly adheres to the sulci and gyri of the brain. Gently separate at least part of
the pia mater from the surface of the brain, by lifting it up with your forceps.

4. Describe the meningeal layers you were able to locate.

Examine the sheep brain. Hold the brain so that the superior side is up. Notice the thin
outer covering on the brain. What is this
Make 3 observations about this outer covering.
Hold the brain so that the inferior side is up. What are the visible parts on this side?
 You are now going to carefully take the dura mater off the brain. Hold the brain so the
superior side is facing up. Find the opening located near the frontal lobe. Using your
scissors cut the dura mater from front to back, off to one side of the brain. Now cut up the
other side. Peel back and cut off the dura mater.

Describe the texture of the cerebral cortex (3 observations)

What are the valleys called?______________
What are the ridges called?_______________

What is the purpose of all the valleys and ridges?______________________________

Turn your brain so the inferior side is up. You are now going to remove the dura mater on
that side. Cut between the optic nerve and the pituitary gland. Carefully cut around the
brain stem and cerebellum. Since this tissue is very fragile, it is important to use care at
this point.

5. Turn your sheep brain so that you are viewing its left lateral aspect. Compare the
various areas of the sheep brain (cerebrum, brain stem, cerebellum) to the photo of the
human brain in the sheep dissection brain pictures.

6. Relatively speaking, which of these structures is obviously larger in humans?

7. What does this indicate about the mental abilities of a sheep when compared to those
of a human?

D. THE CEREBRUM                                                     Diagram I

8. Use the diagram to the right and the
Intact Sheep Brain (ventral and Dorsal
view) and the Dorsal Midbrain
Structures of the Sheep Brain pictures
on the Brain pictures to help you
identify the following structures.

9. Begin with your sheep brain ventral
surface down in your dissecting pan.
The most prominent feature of the
dorsal surface of the brain is the
cerebrum. The longitudinal fissure
divides the cerebrum into the right and
left cerebral hemispheres. Note the
numerous sulci and gyri forming the
surface of the brain. You are actually
looking at the cerebral cortex, the
gray matter of the brain where most
voluntary thought processes take

10. Observe that each cerebral hemisphere is divided into four lobes: frontal, parietal,
temporal and occipital. These lobes are directly inferior to the corresponding structures
in the cranial bones. Identify these lobes on your sheep brain. Check them off as you find

Frontal______ Parietal______ Temporal______ Occipital______

11. How do these lobes compare with the human cerebral lobes?

12. Locate the cerebellum by finding the transverse fissure posterior to the cerebrum
that separates it from the cerebellum. Note that the sulci and gyri of the cerebellum seem
to be much more tightly packed that the corresponding structures in the cerebrum.

13. Note the protruding central area of the cerebellum, the vermis. (This midline
structure is also present in humans. In humans, however, the vermis is reduced in size and
the cerebellum is divided into hemispheres separated by the flax cerebelli.)

14. What does the cerebellum control in humans?

15. Describe the exterior of the cerebellum (3 observations)

16. Locate the spinal cord, inferior to the cerebellum. Depending on the condition of the
brain and how much of the spinal cord is present, you may be able to find remnants of
one or more of the cervical (spinal) nerves.

17. Describe the appearance of the spinal cord.

18. With your hands, bend the specimen so you separate the cerebrum from the
cerebellum along the transverse fissure. Identify the corpora quadringemins, the four
prominent swellings immediately anterior to the cerebellum. The paired swellings closest
to the cerebellum are the inferior colliculi, which are involved with auditory reflexes.
Centrally located at the anterior end of the superior colliculi , which are involved with
visual reflexes. Centrally located at the anterior end of the superior colliculi is the pineal
body, the endocrine gland which secretes the hormone megaton. Use the dorsal Midbrain
Structures of the Sheep Brain picture to help you identify these parts.

19. Check off the colliculi as you locate them.

Inferior colliculi_______    Superior colliculi________

20. How do the colliculi and pineal body of the sheep compare with the colliculi and
pineal body of the human?

21. Observe the structure that seems to originate under the optic chiasma, the
infundibulum, or the stalk of the pituitary gland (see diagram II). In actuality, the
infundibulum arises from the hypothalamus. Some specimens may still have the pituitary
gland attached to the infundibulum. (we will examine the hypothalamus further when we
study the sagittal anatomy of the brain.) If you sheep brain is overly preserved, you may
find the infundibulum a bit mushy and thus easily destroyed. Posterior to the
infundibulum is the single-lobed mammillary body. In humans the mammillary body is
doubled-lobed. The mammillary body controls the feeding reflexes.

22. How do these structures compare with the corresponding human structures?


23. Find the cerebral peduncles, posterior to the mammillary body; these are fibrous
tracts connecting the cerebrum and the medulla.

24. Describe the cerebral peduncles of the sheep brain.

25. Locate the pons, posterior to the cerebral peduncles. The trochlear nerves, cranial
nerve IV, can be found at the junction of the midbrain and the pons.

26. Describe the pons of the sheep brain.

27. Find the medulla oblongata, posterior to the pons. This structure is more commonly
called the medulla.

28. What does the medulla do in the human brain?


29. To explore the internal structures of the brain, it will be necessary to separate the
brain into left and right halves. Take the knife and carefully cut down the longitudinal
fissures. Refer to diagram II and the brain packet to identify the structures described.

                                        Diagram II

30. Manually move the left and right cerebral hemispheres apart from each other. Note
the thick fibrous tissue holding the hemispheres together. This is the corpus callosum,
the nervous tissue that functions as a communicating band between the cerebral

31. Describe the corpus callosum. What is its job in the human brain?

32. Examine either half of the brain. Locate the corpus callosum. Follow this fibrous
band around posteriorly. The posterior end of the corpus callosum is known as the
spleniuium of the corpus callosum. The fibrous band inferior to the corpus callosum is
the fornix. Between the corpus callosum and fornix is an opening. If your incision was
good, this opening is covered by a thin membrane known as the septum pellucidum. If
you destroyed the septum pellucidum, note where it should be.

33. The area behind the septum pellucidum is the lateral ventricle. Put you blunt probe
into the ventricle and get a feel for the size of this cavity. The brain has a lateral ventricle
in each cerebral hemisphere.

34. Inferior to the fornix you will find a large round area known as the intermediate mass
of the thalamus. Run your probe along the underside of the thalamus and see if you can
locate a cavity. This is the third ventricle. Refer to diagrams II and page 2 of the packet to
help you visualize this third ventricle. The third ventricle is sometimes difficult to
recognize as a cavity in preserved brains. Between the lateral ventricles and also
communicating with this third ventricle is the interventricular foramen, also known as the
foramen of Munro. You will probably not be able to locate this foramen.

35. Relocate on the sagittal section those structures you found when you examined the
dorsal and ventral views of the brain. Note particularly the hypothalamus, which is
directly inferior to the thalamus and the cavity of the third ventricle. If your brain is well
preserved, the hypothalamus will be a distinct V-shaped structure. The infundibulum
extends from the hypothalamus. The pituitary gland is the knob-like structure at the end
of the infundibulum.


1. Examine Sheep Brain Picture #1 in your packet. Match the parts below with the correct
letter from the diagram.

__________Cerebellum                               __________Cerebrum

__________Gyri                                     __________Longitudinal fissure

__________Medulla oblongata                        __________Sulci

2. Examine Sheep Brain Picture #2 in your packet. Match the parts below with the correct
letter from the diagram.

__________Olfactory bulb                           __________Optic nerve

__________Optic chiasma                            ___________Optic tract

__________Mammillary body                          ___________Pons

__________Medulla oblongata                        __________Spinal cord

__________Cerebrum                                 __________Cerebellum

3. Examine Sheep Brain Picture #3 in the packet. Match the parts below with the correct
letter from the diagram.

__________Pineal body                              __________Cerebellum

__________Fornix                                   __________Corpus callosum

__________Olfactory bulb                           __________Thalamus

__________Pons                                     __________Medulla oblongata

4. Examine Sheep Brain Picture #4 in the packet. Match the parts below with the correct
letter from the diagram.

__________Spinal cord                               __________Cerebellum


5. Examine Sheep Brain Picture #5 in the packet. Match the parts below with the correct
letter from the diagram.

__________Cerebral peduncle                         __________Medulla oblongata

__________Mammillary body                           _________Spinal cord

__________Olfactory bulb                            __________Pons

6. Examine Sheep Brain Picture #6 in the packet. Match the parts below with the correct
letter from the diagram.

__________Cerebellum                                __________Cerebral Hemisphere

__________Medulla oblongata                         __________Corpus callosum

__________Pineal body                               __________Pons

__________Thalamus                                  __________Spinal cord

7. Identify the part of the meninges that is being described below. Use the key provided
to indicate your answers.

A = Arachnoid                 P = Pia mater         D = Dura mater

__________Outer most meninges                 __________Provides support to brain

__________Delicate connective tissue          __________Middle layer

__________Directly covers & is attached       __________Inner most layer
          to the surface of the brain.

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