-1Introduction/Background
The plan in approaching the problem of dissecting the sheep's brain is the following: First, an
examination of the external features of the specimen will be undertaken. Second, the brain will be
bisected along the longitudinal midline, and the several features of the medial face of the hemisphere
considered. Third, the brain will be sectioned in a coronal manner, and the deep structures of the
brain studied from that point of view. (For your dissection you will use only the right hemisphere of
the brain for your coronal sections).
Directional Terminology
This guide will use a number of terms that refer to position and direction in describing the brain. In
quadrupeds (virtually everyone but us) directional terms are based on four basic reference points:
head - up, tail - behind, back - top, and belly - down. In bipeds (e.g., humans), the neuraxis bends
through a 90ø angle in the skull and human directional references shift with this bend. The
directional terminology used here is largely appropriate for the anatomy of quadruped brains.
Most users of this software do not have an intrinsic interest in this species. Rather, the sheep brain
serves as a less expensive alternative to the human brain. For their benefit, some poetic license has
been taken in labeling certain structures and human terminology has been inaccurately incorporated.
Our apologies to the purists. (You can appear sage by pointing out our errors.)
Directional Terms
Rostral, Anterior
Caudal, Posterior
Dorsal
Ventral
side
Head or front end
Tail or hind end
Back or top side
Belly or bottom
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Lateral
Medial
Proximal
Distal
Away from the midline
Toward the midline
Closer
Farther away
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Three directional planes exist in the brain: rostral/caudal, dorsal/ventral, and medial/lateral. When
sectioning (cutting) the brain, which planes are visible is determined by the type of section and two
of these are used in this guide. In the sagittal section (which is made parallel to the midline, dorsal to
ventral) the rostral/caudal and dorsal/ventral planes can be seen. In the coronal or cross section
(made perpendicular to the midline, dorsal to ventral) the medial/lateral and dorsal/ventral planes can
be seen.
Subdivisions of the Brain
Forebrain
Telencephalon: cerebral cortex, corpora striata (caudate nucleus, internal capsule, putamen), and
rhinencephalon (olfactory brain, e.g., olfactory bulb, hippocampus, amygdala, septal region, and
cingulate cortex)
Diencephalon: thalamus, hypothalamus, pineal gland, and posterior lobe of the pituitary (a migrated
portion of the hypothalamus)
Midbrain
Mesencephalon: corpora quadrigemia (tectum--inferior and superior colliculi), tegmentum
Hindbrain
Metencephalon: cerebellum, pons
Myelencephalon: medulla oblongata
The Brain Stem is composed of the thalamus, hypothalamus, midbrain, pons and medulla.
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Dissection Instructions 2:
2
On the ventral side of the brain,
about midway between rostral and
caudal ends, locate the pituitary
gland. It is a midline mass of tissue
lying caudal to the optic chiasm. If
your brain still has all of its
membranes intact, a number of
types of tissue should be visible.
For example, the grayish, semitransparent, tough membrane is the
dura mater, while the dark brown,
spongy material on either side of
the pituitary are capillary beds.
Rostrally, you may also find large
globs of yellowish fat that are
associated with the eyes. Buried in
all this is the bulbous pituitary
itself.
2A
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Dissection Instructions 3:
Carefully dissect connective tissues
from the caudal aspect of the
pituitary and gently lift the pituitary
mass from its caudal end. You
should be able to see the III rd
(oculomotor) cranial nerve pair
attached to the ventral surface of
the brain, on either side of the
midline. These nerves are fairly
broad, but quite flat, and may be
difficult to see if they are lying
down directly on the brain. Directly
on the midline, anterior to the
oculomotor nerves, you may find
the thin stalk of the pituitary (the
infundibulum) which connects the
3
body of the pituitary to the base of
the brain. Keeping the pituitary
lifted away from the ventral surface of the brain, use a pair of small scissors to sever the two nerves
(III) and the infundibulum as far away from the brain as you can. Carefully interrupt any other
connective tissue present, lift the pituitary away, mark the caudal or rostral aspect of the pituitary
(you'll forget), and set it aside. You can examine it later to see the difference between its anterior
(rostral) and posterior (caudal) lobes. Continuing with the ventral aspect of your specimen, at its
very rostral limit, locate the two light colored pad-like flaps of tissue which are the olfactory bulbs.
Caudal to the olfactory bulbs, with a little removal of dura and fat, you should be able to find the cut
stumps of the optic nerves (II). Follow these back and you'll see that they blend into an "X" on the
midline. The fused part of the X is
the optic chiasm. Caudal to the
chiasm are the optic tracts, which are
part of the ventral surface of the
brain.
Stay on the Ventral Surface 4--
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Just caudal to the optic tracts, two
very large bundles of fibers form the
ventral surface of the brain. These are
the cerebral peduncles. On the medial
part of each you should find the
oculomotor nerve (III) that we saw
earlier. While carefully removing the
membranes between the cerebral and
cerebellar cortices, look up between
them to find to see the fine spaghettilike trochlear nerve (IV) as it
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4B
proceeds ventralwards from its exit
point on the dorsal aspect of the
brain stem. The large trigeminal
nerve (V) runs rostrally through this
region. It is usually quite difficult to
remove its protective membrane,
consequently, it is often severed
fairly close to the brain stem, leaving
only a stump on the side of the pons.
Stay on the Ventral Surface 5-Several cranial nerves can be seen exiting
from the medulla. At its rostral end, you
may be able to see the thin (and easily
ripped off, see "Other View") abducens
nerve (VI). The abducens exits from the
trapezoid body (a thin transverse band of
fibers on the surface of the brain just caudal
to the pons, see Plate 8) at a point just
lateral to the longitudinally coursing
pyramidal tracts (see Plate 8). Lateral to the
abducens is the more substantial facial
nerve (VII). Lateral to this, tucked up just
under the cerebellum, you may see the tuft
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5B
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that forms the vestibulocochlear nerve (VIII).
Caudal to the vestibulocochlear nerve you
may see another light tuft of nerves that are
made up of the glossopharyngeal nerve(IX)
blending into the more caudal vagus nerve
(X). Caudal to this you may see the spinal
accessory nerve (XI) which appears as a line
of connected filaments running in a rostralcaudal direction (they may take on the
appearance of a broad yard rake). The
hypoglossal nerve (XII) exits in several
branches from the lateral edge of the caudal
medulla (see "5B").
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A New Perspective 6-Place the brain on its ventral surface.
Look down from the top at the most
ventrocaudal point of the cerebellum.
If your thin arachnoid membrane is
still in place (it isn't in Plate 6), note
how the arachnoid forms the roof of a
space, the cisterna magna, as it
sweeps caudally from the cerebellum
to the medulla. Carefully separate the
caudal part of the cerebellum from the
medulla; as you lift the cerebellum the
arachnoid will rupture, and you
should be able to see yet another
membrane (or fragments of it); this is
the tela chorioidea, forming the
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posterior roof of the 4th ventricle.
Separate the cerebellum from the medulla until that membrane ruptures; the internal space revealed
by this maneuver is the 4th ventricle. The caudal point at which the two sides of the tela choroidea
come together is called the obex. This can be seen on the dorsal surface of the medulla and forms the
caudal boundary of the 4th ventricle. Looking into the 4th ventricle, you may see some dark spongy
tufts; these are pieces of choroid plexus.
Another Perspective 7--
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Leaving the brain on its ventral surface,
we will now look down over the rostral
end of the cerebellum. Looking down
from the top, careful bending of the
cerebellum in a caudal direction will
reveal a second cistern, the superior
cistern, under the rostral surface of the
cerebellum (not visible in this Plate).
Looking further down the brain stem,
you may be able to see the white
membrane forming the rostral roof of
the 4th ventricle, the anterior medullary
velum. Rostrally, this membrane
attaches to the caudal inferior
colliculus. Caudally, it continues under
the cerebellum.
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Back to the Ventral Surface 8--
8
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After you have located the cranial
nerves, and the cisterns just
described, carefully strip away
any remaining dura and arachnoid
from the medulla. On the ventral
aspect of the medulla a number of
surface features can be readily
seen: Note the longitudinal ridges
coursing immediately on either
side of the midline (marked by the
ventral median sulcus); these are
the pyramidal tracts. At the rostral
end of the medulla, locate the
band of transverse fibers
paralleling the pons that form the
trapezoid body.
Ventral and Lateral Perspectives
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The very prominent swelling just
anterior to the trapezoid body; this
is the pons (L. bridge). Three
cerebellar peduncles attach the
Cerebellum to the brain stem. The
large middle cerebellar peduncle
(brachium pontis) can be seen from
the lateral perspective.
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Another look between Cerebral
and Cerebellar Cortices 10
Looking down from the top, once
again, gently pull back on the
cerebellum to reveal the dorsal
surface of the brain stem. Here you
should be able to easily see the
corpora quadrigemina, that is, the
superior (rostral) and inferior
(caudal) colliculi. The superior
colliculi are considerably larger
than the inferior. Collectively, this
area is also called the tectum.
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Ventral-Rostral Features 11
Just rostral to the pons, the ventral
surface of the brain is formed by
the cerebral peduncles, which run
up to the optic tracts (seen earlier).
The IIIrd cranial nerves can be seen
exiting from the peduncles. The
interpeduncular cistern lies in the
"V" on the midline where the
caudal-medial aspects of the
peduncles meet.
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Ventral-Rostral Features 12
Just anterior to the interpeduncular
cistern, locate the small, but
distinct, protuberance lying on the
midline; this is the mammillary
body, and it marks the caudal limit
of the hypothalamus, as seen from
the ventral approach. The rostral
border of the hypothalamus is
marked by the optic chiasm, and the
lateral boundaries are the medial
edges of the cerebral peduncles. The
general outline of the hypothalamus
from the ventral aspect, takes on
something of a diamond
configuration. The remainder of the
diencephalon (i.e., the thalamus)
cannot be seen without sectioning
the brain.
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Ventral-Rostral Features 13
Still viewing the brain from the
ventral aspect, notice the fairly
large, relative smooth masses of
cortical tissue just lateral to the
cerebral peduncles, extending, at
the caudal limit, from the lateralmost part of the pons rostrally to
include the olfactory bulbs. This
mass of tissue is the
rhinencephalon. The large rhinal
fissure marks the lateral boundary
of this region. The larger part,
beginning at about the
rostral/caudal level of the optic
chiasm and proceeding caudally is
13
the hippocampal gyrus. Within this
gyrus resides the amygdala and part of the hippocampus (not visible without sectioning the brain).
Rostral to the hippocampal gyrus you will find the lateral olfactory gyrus. Running along the medial
surface of the lateral olfactory gyrus, locate the lateral olfactory stria, a band of fibers originating in
the olfactory bulbs and coursing
caudally. The medial olfactory stria
is seen as a fiber bundle coursing
medially, disappearing into the
longitudinal fissure just anterior to
the optic chiasm.
13B
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Dorsal-Rostral Perspective-The two cerebral hemispheres are
separated by the longitudinal
fissure. Each hemisphere is divided
into four major lobes. The frontal
lobes are limited caudally by the
cruciate fissure. The area caudal to
the cruciate fissure is the parietal
lobe whose line of separation from
the more posteriorly placed
occipital lobe is ill-defined. The
temporal lobe in the sheep is very
little developed in contrast to
primates. In the sheep, it is
represented by a slight bulge
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superior to the hippocampal gyrus.
The cruciate fissure is somewhat
variable in the sheep, and sometimes difficult to locate. However, the superior frontal sulcus is easily
located. Parallelling the longitudinal fissure, the superior frontal sulcus divides the frontal poles into
approximately equal left- and right-halves, and, if traced caudally, it is seen to "T-end" into the
cruciate fissure.
14B
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The Medial Face--
If you are very fortunate, you may
be able to see the central canal of
the caudal medulla and spinal cord
as it moves rostrally and opens up
under the cerebellum, becoming the
4th ventricle (cerebrospinal fluid
actually tends to run caudally in the
ventricular system). Under the
rostral cerebellum, the relatively
thick anterior medullary velum
should be easily located, It forms
the anterior roof of the 4th ventricle
and the rostral limit of the 4th
ventricle is marked by the
attachment of the anterior
medullary velum to the caudal
inferior colliculus. The 4th ventricle
is continuous with the cerebral aqueduct of the midbrain. Tissue dorsal to the middle of the aqueduct
is the tectum, and that ventral to its middle, excluding the cerebral peduncles, is the tegmentum.
The tectum consists of the corpora quadrigemina and the underlying lamina quadrigemina; the latter
can be seen as the cut surface immediately superior to the aqueduct, appearing somewhat like the
end of a piece of plywood (which is a laminated structure). The cerebral aqueduct opens up into the
3rd ventricle (discussed further in Plate 16) which in turn is continuous with the two lateral
ventricles that run out into each cerebral hemisphere.
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Rostral Medial Face--
The relationship between the 3rd
ventricle and the thalamus is
somewhat unusual. Most of the
medial portions of the two thalami
are fused in a structure called the
massa intermedia (appropriately
named, if you remember your Latin
roots). Where this fusion exists, of
course, there can be no ventricular
space. Consequently, the 3rd
ventricle must run around the
massa intermedia. In the vicinity of
the massa intermedia, the lateral
walls of the 3rd ventricle are
formed by the unfused medial
nuclei of the two thalami. Ventrally
the lateral walls of the 3rd ventricle are formed by the medial nuclei of the hypothalami. At about 10
o'clock from the massa intermedia (assuming the brain is horizontal with the ventral side up), locate
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the small piece of tissue that is just
outside the 3rd ventricle. This is the
pineal body. Looking at the most
ventral part of the 3rd ventricle, you
should be able to appreciate how it
extends down into the hollow stalk
attaching the pituitary and
hypothalamus (the infundibulum).
Just caudal to this region, you will
find the cut surface of the
mammillary body. Just rostral to
this region, you will find the cut
surface of the optic chiasm. The
corpus callosum is the very
prominent collection of axons that
17
extends for some distance along the
medial face of the cerebral hemispheres. At the rostral end, it curves ventrally and caudally, virtually
making a 180ø turn. The area of turning is called the genu, and if you look carefully (with a perfectly
cut brain), you will see that the caudal extending portion of this bend comes to a point and ends; this
is the rostrum. At the caudal end of the corpus callosum it can be seen that a similar 180ø turn is
made; this bend is the splenium. The main "body" of the corpus callosum runs between these two
turns.
Immediately dorsal to the body of the corpus callosum (i.e., that part between the splenium and
genu) locate the crease that forms the callosal sulcus. The cortical outfolding just dorsal to the
callosal sulcus is the cingulate gyrus, which, in turn, is bounded dorsally by a crease, the cingulate
sulcus. Follow the cingulate gyrus caudally, and you will find that it courses ventrally and laterally,
eventually becoming continuous with the hippocampal gyrus that we saw on the ventral surface of
the rostral brain. Just inferior to the genu and rostrum of the corpus callosum is a region of medial
face cortex called the septal area. As you will see in cross-sections, this region does not extend very
deeply from the midline. At the caudal edge of the septal area, you should be able to locate the small,
light-colored dot which is a cross-section of the anterior commissure. Just inferior to the attachment
point of the pineal body, there is a much smaller (you won't be able to see it here, but it may appear
in cross-section) bundle of decussating fibers called the posterior commissure. If the bisection of the
brain was exactly on-center, there will be a membrane extending ventralwards from the body of the
corpus callosum, the septum pellucidum, which separates the two lateral ventricles from each other.
The fornix is a bundle of fibers that parallels the corpus callosum through much of its course. You
can find the body of the fornix lying about midway between the massa intermedia and the body of
the corpus callosum. You'll get a better look at the fornix in Plate 18.
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Time for a Little Digging--
You are about to scrape away part
of the medial face of one of your
brain halves. Choose your half
wisely. If your cut is off center, be
certain that the structures we are
interested in are still in the half you
choose to scrape, they don't lie far
from the midline. The most caudal
part of the fornix is made up, in
part, of decussating fibers called the
hippocampal commissure (also
known as the commissure of the
fornix), which interconnect the two
hippocampi. The more rostral part
of the structure is made up of fibers
largely originating in the hippocampi, paralleling the midline, and these are called the body of the
fornix. Just dorsal to the anterior commissure, the fibers of the body of the fornix separate into two
distinct bundles of fibers which move just lateral to the midline, forming the columns of the fornix.
Some cell bodies in the mammillary body send their axons to the dorsal anterior thalamus through
the mammillothalamic tract, which lies just beneath the medial face of the hemisphere. Use a scalpel,
and, holding the blade at a right angle to the medial face of the hemisphere, very carefully scrape
away the tissue between the anterior commissure and the mammillary body in order to uncover the
column of the fornix, which will appear as a distinct white line arcing ventrocaudally from the body
of the fornix. Next, do the same thing in the area between the mammillary body and the dorsal
anterior part of the massa intermedia, uncovering the mammillothalamic tract.
The Tree of Life (Internal
Structure of the Cerebellum)-Note the internal structure of the
cerebellum. The thick core of
myelinated axons branching out
into separate folia, reminiscent of a
tree (arbor vitae), frosty window
pane (for those of you in northern
latitudes), or cauliflower florets
(any analogy to food is a good
analogy)
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Dissection Instructions 20:
Now you get to do some serious
slicing. You will be making a series
of coronal sections using one of
your hemi-brains. Typically, you
should use the half that you haven't
scraped to see the
mammillothalamic tract and
columns of the fornix . Once again,
you should minimize cutting
artifacts by making one clean pass
through the tissue, rather than
sawing through it. You might also
consider the following: As you are
asked to make sections of the brain,
rather than attempting to make the
section at the exact level in one
single cut, make a number of
thinner cuts until the cut-face of
your
specimen resembles the
20B
relevant figure. If you are skilled at
cutting you can end up with a deck
of brain slices each about as thick
as a robust slice of bologna (don't
shuffle this deck). Plate 20 shows
the results of a cut that passes
through the genu of the corpus
callosum. Try to have the angle of
your cuts match that of the sagittal
section that is shown prior to each
coronal section. Substantial
deviations from this angle will
make it somewhat challenging for
you to match our plates with your
sections. Looking at the cut surface
of your first section, the darker
material is mostly cell bodies, and
the lighter tissue is mostly axons. Working from the ventral aspect of your section, look laterally
until you find the first infolding, this is the rhinal fissure. To appreciate the full extent of this fissure,
either look at the ventral lateral surface of your uncut hemi- brain or look back at Plate 13.
Obviously, you should be able to see the rhinal fissure in many of your coronal sections, we will
only label it here, however. Impress your friends by finding it elsewhere. Just medial to the rhinal
fissure you may be able to locate the lateral olfactory stria. Moving to the medial face of the ventral
part of your section, the gray matter here is the septal region. You should have no trouble locating
the corpus callosum lying under the ventral surface of the medial cerebral cortex. As you did for the
rhinal fissure, either look at the uncut hemi- brain (medial face) or look back at Plate 16 to
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appreciate the full extent of the corpus callosum. Obviously, the corpus callosum will show up in
many of your coronal sections, we have only labeled it here. You may also see part of the rostrum of
the corpus callosum as it extends ventrally and caudally from the genu. The thin space above the
corpus callosum is the callosal sulcus, the cortex above that is the cingulate gyrus , and the space
above that is the cingulate sulcus. We have only labeled them here, but, as you would expect, these
structures will occur in many of your crossections (Examine either the medial face of the uncut
hemi-brain or Plate 17). Off the midline, the space below the corpus callosum is lateral ventricle.
The caudate nucleus forms the lateral wall of the lateral ventricle. Just lateral and ventral to the
caudate nucleus, find the stippled band of white and gray matter. The white matter is the internal
capsule. Ventral and lateral to the internal capsule you should be able to see the putamen (a nuclear
group). This arrangement of nuclei and fibers (caudate nucleus, internal capsule, and putamen) gives
the area a striated appearance, and thus led to terming this group the corpus striatum. Immediately
lateral to the putamen, you should be able to see a solid streak of white matter, the external capsule.
Just lateral to this, typically, you can see the triangularly shaped claustrum. The caudate, internal
capsule, putamen, external capsule, claustrum and septal area will appear in subsequent sections of
this guide, however they will not always be labelled.
21
Notice how the septal area has become larger on the ventral medial face of the brain. The caudate is
still prominent and the internal capsule should be easier to see. You may be able to see the beginning
of the globus pallidus, which is contiguous with the putamen on its ventral medial face. The globus
pallidus has a lighter and more homogeneous coloration than the putamen. If you had trouble seeing
the claustrum in the last plate, it should be clear here.
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Anterior Optic Chiasm 22
In this section you can see the
anterior part of the optic
chiasm, which is stuck to the
ventral surface of the brain.
You can also see the
decussating fibers of the
anterior commissure running
across the midline of the brain.
You may also see some fibers
of the columns of the fornix as
they form
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Caudal Optic Chiasm 23
On the midline, immediately
superior to the chiasm, you will
find a vertical space; this is the
most rostral part of the 3rd
ventricle. Here, the medial face
of the anterior hypothalamus
forms the lateral wall of the 3rd
ventricle. Note that the dorsal
part of the 3rd ventricle extends
out under the body of the fornix.
(The "T" shape isn't clear here
because the left side of the fornix
has twisted down from its natural
positioning.) The lateral
ventricles take on the shape of
horns, extending out and up over the caudate. Here too, you can see the effects of the "C" shape of
the fornix, with the body visible dorsally and the columns visible ventrally. The gray area just
ventral and medial to the caudate is your first glimpse of the thalamus in crossection; this is the most
dorsal and rostral region. The medial face of the thalamus forms the lateral walls of the dorsal third
ventricle (see Plates 15-17).
24
Just caudal to the Optic
Chiasm 24
On the ventral surface of the brain
you will see that the optic chiasm
no longer extends across the
midline (the ventral hypothalamus
has taken its place) and the optic
tracts are proceeding laterally. The
massa intermedia of the thalamus
makes its appearance here,
obstructing the middle of the 3rd
ventricle. Other subdivisions of the
thalamus can be seen lateral to the
massa intermedia. Our old friend
the caudate nucleus is still visible
here, but is much reduced from rostral sections. At the junction between the caudate and dorsal
thalamus you may be able to spot a streak of white matter, this is the stria terminalis. Like the fornix,
the stria terminalis takes on a modified "C" structure as it courses between the amygdala and
hypothalamus. The fornix is fairly flattened here because we are seeing its central body as well as
lateral fibers of the fimbria that are joining the body. You'll get a better feel for the origin of the
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fimbria in Plate 28. Notice that, in addition to the preceding, you can still see the columns of the
fornix.
25
The optic tracts have moved more
laterally and, lateral to them and
buried in the cortex, you should be
able to see the amygdala (more
appropriately, amygdaloid
complex). It has an elongated
teardrop shape. There may still be
a hint of the putamen located
above the dorsal end of the
amygdala, with the external
capsule and claustrum just lateral
to the putamen. The caudate is still
visible as well. The thalamus
makes up most of the medial
section. The hypothalamus extends
through about the bottom third of
the diencephalon at this point. Four white dots are visible. The top two are the mammillothalamic
tracts and the ventral are the columns of the fornix (approaching their endpoint in the mammillary
bodies). Look back at Plate 18 to get a perspective on these pathways.
26
Rostral Mammillary Body 26
Not much has changed from the last
section.
We have indicated some tufts of
choroid plexus in the third and lateral
ventricles.
At this point, you should have fibers
crossing between the two
hippocampi, forming the commisure
of the fornix (hippocampal
commisure). They aren't distinctly
visible because they are blending
with fimbria and fornix fibers. The
internal capsule is very prominent.
The ventral aspect of the 3rd
ventricle is fairly small.
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Caudal Mammillary Body-The most interesting development
here is that you can see another
demonstration of the modified "C"
we've encountered previously. Here
you can see dorsal and ventral
aspects of the lateral ventricle and
the hippocampus as they are cut
twice through their "C" course. The
optic tracts are meeting up with their
major target, the lateral geniculate
nucleus of the thalamus. You can
also see the cerebral peduncles
forming on the ventral surface of the
brain.
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Here you can more clearly see the
"C" shape of the hippocampus as
well as the white fibers of the
fimbria. The habenula may also be
seen on the dorsal surface of the
diencephalon. The pulvinar of the
thalamus may be seen just medial to
the lateral geniculate body.
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Right Through the Pineal
Body 29
Here we have captured the pineal
gland sitting in the notch between
the two lobes of the thalamus. The
thin 3rd ventricle can be seen (we
have passed caudal to the massa
intermedia). The medial geniculate
nucleus may be visible in your
section.
The Superior Colliculus 30
30
This section catches the rostral part of
the superior colliculi. If you look
carefully, you should be able to see that
they have a subtle striated (striped)
appearance. The cerebral aqueduct can
be see here on the midline (no other
ventricular space can be seen at this
point). It is fairly small at this point, it
will broaden out more caudally.
Surrounding the cerebral aqueduct is
the periaqueductal gray region (check
those latin roots). The cerebral cortex
that is visible at this point is the
occipital cortex. Note that there is no
interconnecting corpus callosum at this
level.
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Just Rostral to the Cerebellum
31
Here you can see the caudal part of
the superior colliculus (notice the
striations) and the beginning of the
inferior colliculus (which will not
appear striated). The cerebral
aqueduct has become much larger.
The rostral part of the pons can be
seen at the ventral edge of the
section. The unsectioned rostral
part of the cerebellum can be seen
behind the cut brain stem (this is a
caudally directed view).
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Immediately Rostral to the
Cerebellum 32
This section through the pons
shows the pontine fibers running
up the lateral edge of the pons
forming the brachium pontis.
You can see both the 4th ventricle
and the anterior medullary velum
that forms the roof of rostral part of
this ventricular space (this is a
caudally directed view).
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34
At the Level of the Trapezoid
Body 33
This section shows the massive
peduncular attachments of the
cerebellum as well as some of the
nuclei that are found deep within
the cerebellum. You should be able
to see that the majority of the
lateral medulla is covered by a thin
sheet of fibers, the trapezoid body.
You may also see some striations
on the ventral midline as these
trapezoid fibers decussate.
Caudal to the Cerebellar
Peduncles 34
In the ventral lateral medulla,
you should be able to see the
inferior cerebellar peduncle. In
this caudally directed view, you
can see that the 4th ventricle is
beginning to close down as the
ventricle approaches the obex.
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Caudal Cerebellum—
In this caudally directed view, you
can see the 4th ventricle closing
down at its caudal end, and the
black dot at the floor of this
ventricle which is the beginning of
the central canal. In the ventral
medial medulla you may be able to
see a zebra pattern created by the
inferior olivary nuclei. In this
section, you can see a tuft of
glossopharyngeal (IX) fibers on the
lateral edge of the medulla. Note-The lateral cerebellar cortices
appear to curve away from you,
they do not. These are actually flat
cut surfaces that have fallen back
because they do not have
underlying cortical tissue to hold
them up.
We're Not in Kansas Anymore—
This section through the cervical
spinal cord shows the classic
butterfly shape formed by the gray
matter of the cord. (We know, it
looks white, but it's gray, trust us.)
You can also see dorsal and ventral
rootlets on the outside of the cord.
The central canal is very small and
is not readily visible here.
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Glossary
Abducens (Cranial Nerve VI): An efferent nerve which connects to the abductor muscles of the
eye.
Amygdala: A division of the basal ganglia which is located in the temporal lobe. Amygdala
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Anterior Commissure: A bundle of nerve fibers which crosses the median plane in the upper part
of the lamina terminalis and which contains connections between the olfactory bulbs,
amygdaloid nuclei, and parts of the temporal lobe cortices.
Anterior Medullary Velum: A thin layer of tissue between the cerebellar peduncles which forms
the roof of the rostral part of the fourth ventricle.
Body of the Fornix: The segment of the fornix which projects rostrally and ventrally in the edge of
the septum pellucidum from the hippocampal commissure to a point just above the anterior
commissure.
Brachium Pontis (Middle Cerebellar Peduncle): A bundle of nerve fibers connecting the pons and
cerebellum.
Brain Stem: Collectively, the midbrain, pons, and medulla.
Callosal Sulcus: The sulcus on the medial surface of the cerebral hemisphere which separates the
corpus callosum from the cingulate gyrus.
Caudate Nucleus: One of the basal ganglia, it consists of a head, which forms the anterior lateral
wall of the lateral ventricle, a body, lying above the dorso-lateral thalamus, and a tail, which is
located above the lateral ventricle's inferior horn.
Central Canal: A narrow channel which extends the length of the spinal cord from the caudal tip of
the fourth ventricle.
Cerebellar Cortex: The outer layer of the cerebellum, it consists of three layers cells: molecular
(the outermost), Purkinje, and granular (the innermost).
Cerebellum: From the Latin for "little brain," the cerebellum is located behind the cerebral
hemispheres. Its functions include coordination, equilibrium, and muscle tone. The cerebellum
consists of a medial part, the vermis, and two cerebellar hemispheres.
Cerebral Aqueduct: A narrow channel through the midbrain which connects the third and fourth
ventricles.
Cerebral Cortex: The outer layer (gray matter)of the cerebral hemispheres. It consists of six layers:
the molecular(outermost; layer I); external granular (layer II); external pyramidal (III); internal
granular (IV); internal pyramidal (V); and multiform (innermost; layer VI).
Cerebral Peduncle: A peduncle is a large band of nerve fibers in the brain, usually composed of
multiple tracts. The cerebral peduncle is the ventral part of each half of the midbrain (not
including the tectum).
Choroid Plexus: These membranes are the primary source of cerebrspinal fluid. They can be found
in the lateral, third, and fourth ventricles.
Cingulate Gyrus (Collosal Gyrus): A gyrus is a ridge (outfolding) of the surface of the cerbrum.
The cingulate gyrus is located on the medial surface of the cerebral hemisphere between the
cingulate sulcus and the sulcus of the corpus collosum.
Cingulate Sulcus: A sulcus (plural sulci) is s groove (infold) of the surface of the brain or spinal
cord. The cingulate sulcus is on the medial surface of the cerebrum.
Claustrum: A thin sheet of gray matter located between the external capsule and
putamen.(medially; between the insula and extreme capsule laterally)
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Column of the Fornix: (See Fornix) The column of the fornix is the segment which turns backward
and downward behind the anterior commissure into the hypothalamus. It ends mainly in the
mamillary body.
Commissure of the Fornix (Hippocampal Commissure): (See Fornix) A commissure is a bundle
of nerve fibers which crosses the median plane, sometimes, but not always, connecting similar
structures on the two sides of the brain. The commissure of the fornix is just beneath the
splenium of the corpus callosum.
Corpus Callosum: A large band of nerve fibers which mainly connects corresponding areas of the
two cerebral hemispheres. It consists of four parts which form a rostrally-projecting “hook”
shape: the rostrum, genu, body, and splenium.
External Capsule: Layer of nerve fibers between the putamen and the claustrum.
Facial Nerve (Cranial Nerve VII): The facial nerve arises from the ventrolateral surface of the
pons at its junction with the medulla. It is composed mostly of motor fibers which supply the
muscles of the face, including those that close the eyes, parasympathetic relays to the lacrimal
gland (which produces tears) and the salivary glands, taste fibers from the anterior two-thirds of
the tongue, and some cutaneous fibers from the external ear.
Fimbria: The main bundle of efferent nerve fibers arising from the hippocampus.
Frontal Lobe: The part of the cerebral hemisphere anterior to the central sulcus.
Genu: From the Latin for knee, genu refers to a bend. The most anterior part of the corpus callosum
is the genu.
Globus Pallidus: Part of the basal ganglia
Glossopharyngeal Nerve (Cranial Nerve IX): This cranial nerve arises from the ventrolateral
surface of the medulla, just caudal to the facial nerve. It contains several afferent nerve fibers
important to the regulation of blood pressure and respiration, taste fibers from the back of the
tongue, and sensory fibers from the palate and pharynx. It also contains parasympathetic fibers
for several glands, and motor fibers for the stylopharyngeus muscle.
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