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 Head or front endCaudal, Posterior Tail or hind endDorsal Back or top sideVentral Belly or bottom side Lateral Away from the midlineMedial Toward the midlineProximal CloserDistal Farther away
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.
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, semi-transparent, 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.
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 IIIrd
(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 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.
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 spaghetti-like trochlear nerve (IV) as it
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.
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 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 rostral-caudal 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 "Other
View").
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 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.
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.
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 lateral-most 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 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.
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 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 (see "Other
View").
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.
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 2
o'clock from the massa intermedia (assuming the brain is horizontal with the
ventral side up), locate 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 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.
Callosal
Sulcus: The
sulcus on the medial surface of the cerebral hemisphere which separates the
corpus callosum from the cingulate gyrus.
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.
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.
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)
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
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 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 ventral wall of the 3th 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.
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.