Biology 2401
The brain : Exercise 25 (pages 347-358;362-365)
You need to know ( structures and location) Fig. 25.1 , 25.2, 25.3, 25.4, 25.5, 25.7, 25.11, ,
25.13
The cranial nerves will be discussed later and only the Optic nerve and Olfactory nerve are to
be known on the sheep brain.
Read the pages and study the figures. This handout is a brief summary of what is in these pages. It does not
indicate that you should not read your labmanual. Remember that Pictures speak volumes !!
See Figure 14.1 in your textbook. : Shows the development of the neural tube and how it develops into the
main structures of the brain
Primary brain vesicles :
ProEncephalon
MesEncephalon
Rhombencephalon
Secondatry brain vesicles
becomes
remains
becomes
Telencephalon and Diencephalon
Mesencephalon
Metencephalon and Mylencephalon
Know the primary and secondary brain vesicles and what they become !
The adult brain is mushroom shaped and can be divided into 4 principal parts
 The Cerebrum with right and left hemispheres (develops from telencephalon)
 The Diencephalon
 Contains the Thalamus and Hypothalamus, Epithalamus
 The Brain stem
 Contains the midbrain (Mesencephalon), pons (Metencephalon) and medulla oblongata
(Myelencepahon)
 The medulla oblongata is continuous with the Spinal Cord
 The Cerebellum (develops from Metenecephaon), inferior to the cerebrum and posterior to the
brainstem.
The Cerebrum and Diencephalon are also grouped together under the name "forebrain"
The Forebrain ( see fig. 25.1, 25.5 and 25.7 )
The cerebral hemispheres make up the bulk of the brain mass and sit like a mushroom cap on the brain stem. The
surface ( 2 to 4 mm thick) is composed of gray matter called the cerebral cortex, containing billions of neuron cell
bodies. Beneath the cortex lies the cerebral White matter, consisting out of myelinated axons and dendrites with
islands of gray matter situated deep in the white matter e.g. the subcortical nuclei.
During embryonic development, there is a rapid increase in brain size in which the gray matter enlarges faster than
the white matter. The result is that the cortex starts to fold over upon itself, creating many convolutions
Definitions :
 Gyri ( gyrus) = elevated ridges of brain tissue
 Sulci ( sulcus) = shallow grooves in-between the ridges
 Fissures = deeper grooves, separating large regions of the brain
 Nuclei = collections of neuron cell bodies within the brain
 Tracts = bundles of axons within the brain
Fissures, sulci and gyri form landmarks that can be used to locate structural and functional areas of the brain.
 longitudinal fissure, separating the two hemispheres.
 transverse fissure , separates the cerebral hemispheres from the cerebellum below.
Sulci divide each hemisphere in 5 lobes and 4 of
the lobes are named after the cranial bone that
cover them. Frontal lobe, parietal lobe, occipital
lobe and temporal lobe .
An important sulcus is the Central sulcus. It
separates the frontal lobe from the parietal lobe
 the gyrus anterior of the central sulcus =
pre-central gyrus
 the gyrus posteriorly of the central sulcus =
post-central gyrus
Using your labmanual, can you identify the
different numbers (important names/function) and
letters (lobes) on the brain above ?
The Isolated Sheep Brain
Whole brain

The brain is covered by a sheet of white connective tissue. This is a part of the meningeal membranes.
Question : ( see Lab activity 1, page 348)
o What are the meninges ? How many are there ? Which one are you seeing ?
o What is the falx cerebri and falx cerebelli ?

Compare your sheep brain with Figure 25.11 and 25.12. Make sure you understand what the superior
(top) and inferior sides of the brain are. Proceed and locate the following structures
o Cerebrum with the Two hemispheres and the Longitudinal fissure
o An approximate location of the 4 lobes
o The Cerebellum
o Midbrain
o Pons
o Medulla oblongata
o Spinal Cord and Central Canal
o Optic nerves and Optic chiasm
o Olfactory bulb and olfactory tract
o Pituitary gland (if present) and mammillary body
o Cerebral peduncles
Note : Midbrain, pons and medulla make up what is called the brainstem.
Look carefully at figure 25.11 Gently hold the brain such that you are looking down on the hemispheres and
cerebellum. Gently bend the brain at the interface between hemispheres and cerebellum ( do not damage the
brain).
You will see a set of two bumps. These are the Corpora quadrigemini. They are part of the Midbrain
 Upper two bumps are the Superior colliculi
 Lower two bumps are the Inferior colliculi
Superior to the Superior colliculi and deep between to the two hemispheres lies a little pin-like
structure. This is the pineal gland and part of the epithalamus
What are the functions of the Superior and Inferior colliculi and what is the function of the pineal gland ?
Sagittally Sectioned Brain
We are now ready to proceed with the dissection. Follow the directions on page 362 …. !
You will have to make a clean mid-sagittal section all the way from superior to inferior direction.
This section allows you to see the structures inside the brain. You should be able to find all the structures
mentioned above. In addition, the following structures should be recognized.
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Elements of the diencephalon:
o Epithalamus ( contains the choroid plexus and pineal gland)
o Intermediate mass of the Thalamus
o Hypothalamus, infundibulum and pituitary gland
o What is the infundibulum ?
Locate the optic chiasma : immediate posterior to it is the hypothalamus , and the mammillary body
is immediately posterior to the hypothalamus. In other words, the hypothalamus is below the thalamus
and sandwiched between optic chiasma and mammillary body.
If you did your dissection correct, you should see the pituitary gland as a mass of tissue ‘hanging’
under the brain and pointing in the direction of the spinal cord. ( unfortunately, this is removed from
most sheep brains and we do not get to see it ) .
Corpus callosum : white fiber tracts that connect the two hemispheres ( a nice strip of white matter
located above the lateral ventricle)
Elements of the brain ventricles: the brain has ventricles that are filled with cerebrospinal fluid.
See Figures 25.3 and 25.4,
o Two lateral ventricles ( one in each hemisphere)
 The opening into the lateral ventricle can be seen right under the corpus collosum.
It is covered with a membrane : septum pellucidum. ( most often damaged during
dissection)
o The third ventricle that runs between the two lateral ventricles and towards the pons ( is hard
to see in a sagittal section)
o The third ventricle connects with the fourth ventricle via the cerebral aquaduct. It is located
on the dorsal side of the midbrain inferior to (under) the inferior colliculi
o The fourth ventricle is located on the posterior side of the pons and medulla oblongata and
anterior to the cerebellum. ( so between medulla and cerebellum)
o The fourth ventricle is continuous with the central canal of the spinal cord.
Locate the elements of the brain stem : midbrain, pons and medulla oblongata
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Find the pineal gland again. Right under the pineal gland and on the dorsal side of the midbrain are two
bumps: the upper bump is the superior colliculus of the corpora quadrigemina, the lower bump is the
inferior colliculus (obviously, you only see one of each since the brain has been cut in half)
Element of the Limbic system ( emotional brain)
o Fornix : located between the intermediate mass of the thalamus and the lateral ventricle
(located immediately deep to the lateral ventricle)
Elements of Cerebellum
o Arbor Vitae (white matter of cerebellum)
o Folia cerebelli (the ‘leaflets’ of the cerebellum , gray matter)
Make a table of each structure seen and provide the function of that structure !
Do the work problems pages 367-371 (relating to the Brain). This is not homework, but
good preparation !