A Verbose Guide to Dissection of the Sheep`s Brain H

A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
TO START...
You will need a sheep brain, and a human half-brain.
This guide begins by discussing structures that are
visible in an intact brain. We will then proceed to
take apart the sheep brain, revealing various internal
structures. Please wear gloves!
COORDINATE SYSTEMS
For vertebrates other than primates, the upper coordinate system shown in Fig. 1 applies throughout the
CNS. It works fine for the primate forebrain (cerebral hemispheres and diencephalon), but not very
well for the brain stem and spinal cord. The reason is
that, due to upright primate posture, the spinal cord
is frequently vertical, while the long axis of the forebrain is always horizontal. Thus the primate CNS
contains a permanent flexure that is not present in
other vertebrates. For primate brain stem and spinal
cord we use the lower coordinate system:
Fig. 1: Human CNS and two coordinate systems. Upper
one is used for forebrain, lower one is used for brain
stem and spinal cord.
Fig. 2 would look much the same but shifted downward. Factors other than body size do play a role, but
exactly what factors is less than clear. Diet may
make some difference - leaf-eating primates tend to
have smaller brains than other primates, for example.
Probably for any given species, brain size is a compromise between forces that tend to increase it, and
forces pushing for a decrease. A large brain has
drawbacks, being metabolically expensive, and (in
some species) causing difficulties at birth. A flying
animal also has a strong interest in keeping brain
weight down.
dorsal (towards the back) = posterior
ventral (towards the belly) = anterior
rostral (towards the nose) = superior
caudal (towards the tail) = inferior
BRAIN SIZE
Brain size among mammals varies over about 4
orders of magnitude. Bat brains may weigh less than
1 g, and the blue whale brain, 9 kg. The main determinant of brain size is body size (see Fig. 2). If we
were considering fish and reptiles, the scatter plot of
Fig. 2: Brain size vs body
size for 15 orders of mammals (data based on 883
species).
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A Verbose Guide to Dissection of the Sheep’s Brain
thalamus
s tr
ro
interventricular
foramen
H. Sherk
al
anterior
hypothalamus
Fig. 3: On left, medial view of human brain (right half). On right, lateral view of human brain stem.
to the brain stem, one to the diencephalon, and one
to the olfactory bulb. They are rather similar across
all vertebrate classes - see the correspondence
between the alligator and horse in Fig. 5. As you
read through the following discussion, try to find the
various nerves on your sheep brain. Some brains
have more nerves intact than others. You can also
look for them on human brains, but again you may
have to hunt to find a brain with relatively intact
nerves. Use Nolte's Fig. 3-14 to locate human cranial
nerves
MAJOR BRAIN SUBDIVISIONS
On your sheep brain and on your human brain, find
the following major brain subdivisions:
cerebral cortex
cerebellum
brain stem (medulla, pons, midbrain)
diencephalon (thalamus and hypothalamus;
very little diencephalon is visible in an intact
whole brain - only the ventral surface of the
hypothalamus)
INPUT AND OUTPUT
Information enters and leaves the brain via the spinal
cord and the cranial nerves. In your sheep brain, look
at the cut end of the spinal cord (slice off the end if
you can't see the distinction between gray and white
matter - see Note 1, p. 22). Sensory information from
the body travels to the brain in 3 major tracts located
in the white matter (look at Fig. 4, and try to find the
corresponding locations in the sheep spinal cord).
This information is all somatosensory (e.g., concerning touch, pressure, pain, temperature, etc.). Most of
the rest of spinal white matter is occupied by tracts
descending from the brain, carrying motor instructions for control of motor neurons. These tracts are
located in the lateral funiculus and ventral funiculus.
Some cranial nerves are straightforward in function,
being devoted to a single task. Others are “mixed”,
i.e. containing both sensory and motor axons, and
Fig. 4: Cross-section through human spinal cord, cervial
level. Ascending tracts (shown in gray on right side)
carry somatosensory information up to brain. Descending
tracts (not shown) carry motor informatin from brain to
spinal cord. They occupy the open white matter, except
for a rim surrounding the gray matter, which carries local
axon traffic.
Cranial nerves are responsible for all other information flow to and from the brain. They are attached to
various sites on the ventral surface of the brain, ten
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
a good-sized optic nerve. In species that rely more
on other sensory modalities, the optic nerve is punier
(see alligator). In the Ganges River dolphin, "this
nerve is as thin as a thread" (Pilleri & Gihr, 1970), as
vision is of little use in its turgid environment.
The 5th cranial nerve, the trigeminal, actually consists of two nerves running together, one motor and
one sensory, and so qualifies as a mixed nerve.
However, the sensory trigeminal is far larger than the
motor. It consists of axons of somatosensory receptors innervating the face and head structures. In
many species this is a big nerve, larger than the
optic. In the sheep the somatosensory system is
dominated by input from the muzzle, tongue, and
mouth, all carried by the trigeminal nerve. The elephant (Fig. 6B) takes the prize for largest trigeminal
nerve, with whales close behind. The elephant presumably needs a big nerve to innervate its sensitive
trunk. In whales, the territory of the sensory trigeminal nerve is huge - in some species the head makes
Fig. 5: Ventral view, alligator and horse brains showing
the 12 cranial nerves (except for #1, the olfactory).
sometimes more than one sensory modality or motor
function.
cerebral cortex
olfactory bulb
Afferent Crainal Nerves
Vision and olfaction are dealt with very simply, by
one nerve apiece. The invisible 1st nerve, the olfactory, consists of the axons of olfactory receptor neurons. These axons enter the olfactory bulb in little
bundles, not as a proper nerve at all, and synapse
there. The importance of olfaction, and hence the
size of the olfactory nerve, varies widely among different species. The oppossum (Fig. 6A) is an example of a macro-osmotic animal. In absolute terms, the
elephant has the largest olfactory bulb and presumably the largest olfactory nerve (Fig. 6B). Humans
approach the other extreme, but there are some still
more deficient species that lack any olfactory nerve
or bulb at all, such as dolphins.
A
olfactory tract
optic chiasm
The 2nd cranial nerve, the optic, arises from the retina and runs to the optic chiasm, partially crosses,
and continues on to synapse in the thalamus (part of
the diencephalon), and also in the midbrain. [In fish,
reptiles, amphibians and birds, all optic axons cross
at the chiasm - look at the alligator.] Not surprisingly, the optic nerve is huge in highly visual animals such as primates and birds. The sheep also has
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B
Fig. 6: A. Medial view of right half of oppossum brain
showing large olfactory bulb. B. Ventral view of elephant brain showing olfactory bulb and all cranial
nerves.
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
cles, but also pupil size and lens accommodation
(that is, it sets lens curvature appropriately for the
current viewing distance). The abducens nerve is
easy to identify if it is still attached. The little trochlear nerve violates the general rule and emerges
from the dorsal surface of the brain stem. (It is visible on many of these sheep brains, though rarely on
the human brains.) Although these same 3 nerves are
found in all vertebrate classes, they can be modified
to suit a particular niche. In the Ganges River dolphin, for example, they are missing.
up 1/3 of the body. This nerve also innervates the
"melon", a unique whale structure that has been
called an accoustic lens because it is thought to focus
sound to aid in echolocation.
The 8th cranial nerve, the vestibulo-cochlear (or
stato-accoustic), carries auditory and vestibular
information. ("Cochlear" refers to the auditory part
of the nerve, which carries information from the
cochlea, the structure containing primary auditory
receptor neurons.) Its relative size reflects the importance of the auditory system for a given species. It is
of respectable size in humans, but is one of the largest of cranial nerves in the sperm whale (Fig. 7) and
bat, both species that perform echolocation.
The 5th nerve, the trigeminal, has a motor component that innervates chewing muscles.
The 7th cranial nerve, the facial, has a motor component that innervates muscles of the face. In humans
you can see that the 7th nerve is clearly smaller than
the 8th, but this is by no means the rule among vertebrates or even mammals. Note that in the sperm
whale, which has an enormous 8th nerve, the 7th
appears to be equally big. Since whales have few if
any tastebuds, the astonishing size of the 7th is due
entirely to its complex motor function: it appears to
control musculature of the blowhole, and probably
of air sacs used (perhaps) for sound production. The
7th nerve is decidedly bigger than the 8th in the elephant, probably due to an enlargement of the motor
component for fine control of the trunk. The alligator, which is not noted for its facial mobility or
expressiveness, appears to lack any 7th cranial nerve
(Fig. 5).
The 7th cranial nerve, the facial, has a sensory component that relays taste information to the brain, as
well as a motor component (see below). However,
in fish the facial nerve is dominated by another sensory modality, input from the lateral line organ.
The 9th cranial nerve, the glosso-pharyngeal, like the
7th nerve transmits taste information but also has a
motor function (see below).
Efferent Cranial Nerves
Three cranial nerves are devoted to controlling eye
muscles: they are the 3rd (oculomotor), the 4th (trochlear), and the 6th (abducens). The oculomotor
nerve is quite sizable in the sheep brain, and you
should be able to find it emerging from the midbrain.
This nerve controls not only 4 of the 6 ocular mus-
Fig. 7: Ventral views of humpback whale (left) and sperm whale (right) brains.
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
The motor component of the 9th nerve, the glossopharyngeal, innervates muscles of the pharynx and
larynx. It does only part of the job; the 10th cranial
nerve does the rest.
The 10th cranial nerve, the vagus, emerges from the
medulla as a series of rootlets. Collectively, they add
up to a sizable nerve. This nerve contains axons carrying out all sorts of functions, the most important of
which is parasympathetic. Preganglionic axons go
to parasympathetic ganglia that serve abdominal and
thoracic organs, including the heart. (Parasympathetic action causes slowing of the heart.) You can
identify the vagus nerve as a fringe of rootlets.
gray matter
white matter
The 12th cranial nerve, the hypoglossal, also
emerges as a row of rootlets. It has a simple function, motor control of the tongue.
CEREBRAL CORTEX
The cerebral cortex is found only in mammals, and
varies widely in size and appearance. Some species,
usually those with small brains, have a smooth (lissencephalic) cortex while others have a highly
folded cortex (gyrencephalic). [Each bulge is a
gyrus, and each crease is a sulcus.] The oppossum
brain (Fig. 6) is lissencephalic, as is the rat brain
(Fig. 13). Curiously, the brains of Sirenia (dugongs
and manatees) are lissencephalic despite large body
size. The most gyri occur in whale brains, which are
also the largest. However, you can see that even a
brain of moderate size like that of the sheep generally is well endowed with gyri.
Fig. 8: Top, coronal section through mouse brain showing cortical gray and white matter. Below, horizontal
section through left cerebral hemisphere of elephant
brain showing same thing.
What is the significance of the pattern of gyri and
sulci? While gyration is essential for packing a large
cortex efficiently into the skull, the particular
arrangement of the gyri may be of no functional consequence. If you look at a whole sheep or human
brain, you will notice that there is usually a fair bit of
variation in the gyral pattern between the left and
right hemispheres. Even cats, with fewer gyri overall, show this kind of random variation.
The complex appearance of highly folded cortices
obscures their essentially simple structure. There are
two distinct cortical hemispheres, left and right, that
are connected by a huge fiber tract, the corpus callosum, which we will look at later. Every cerebral cortex is built on the same plan, being a sheet of gray
matter (neuronal cell bodies) some 1-3 mm thick,
encapsulating white matter (axons). A cross section
through any cortex immediately reveals this structure (Fig. 8). This arrangement, which is inside out
relative to the original vertebrate design, permits
growth of huge cortical hemispheres. The gray matter is subdivided into layers or laminae, which need
not concern us yet.
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Some related species show similarities in the general
layout of gyri. Two good examples are carnivores
and primates. Compare the fox brain (Fig. 9) with
that of the cat - the fox shows a rather simple pattern
of sulci that somewhat resembles a set of nested
crescents, and other carnivores are variants of this.
Larger brains, as in bears, add more gyri that obscure
this simple pattern.
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
Olfactory
bulb
Fig. 9: Lateral view of fox brain (left) and cat brain (right). Cat brain is missing brain stem and cerebellum.
fissure) in every primate brain. Another primate sulcus that you need to learn to identify in human brains
is the central sulcus (also called Rolandic fissure). It
is not strikingly deep or easy to pick out in humans,
but it is important as a landmark for dividing the primate brain into regions (called "lobes" for some
Primate brains run the gamut from almost entirely
lissencephalic (owl monkey, lemur) to rather heavily
gyrated (human). Even the lemur does have one
respectable sulcus, the lateral (or Sylvian) fissure.
This is a primate trademark, the largest sulcus (or
Fig. 10: Lateral view of some primate brains (not to scale).
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
ing to the unusual spectacle of raw sensory information being fed directly into cortex. The rule for the
rest of cortex is that sensory information is relayed to
cortex from the thalamus.
obscure reason). Curiously, another landmark sulcus, the lunate, seems to be present in all gyrencephalic primate brains except humans.
The pattern of gyri in the order Artiodactyla (clovenhooved animals, including sheep) is said to resemble
that of carnivores, although on the whole it looks
mostly confused (look at Fig. 11). Although one
sheep brain looks more or less like another sheep
brain, it is difficult to find a common underlying pattern among different species. Compare your sheep
to the pig brain...matching up homologous sulci is a
challenge.
Another readily visible region of allocortex is the
piriform ("pear-shaped") cortex, which has only 3
layers. The rostral part of piriform cortex is a major
target of axons from olfactory bulb, which you can
see travelling as a wide white bundle, the olfactory
tract (see Figs. 5, 6, and 12A). The caudal part of
piriform cortex, on the other hand, is non-olfactory,
being connected to the hippocampus. (The hippocampus, another region of allocortex, is not visible
without cutting open the brain.) Caudal piriform
cortex is referred to as entorhinal, the reason being
that it lies medial to ("internal to") the rhinal sulcus.
Think of the rhinal sulcus as the boundary between
neocortex and allocortex. Even lissencephalic brains
have a rhinal sulcus, though it may be more of a dent
than a sulcus. It is clearly visible in non-primate
brains (e.g., Figs. 9, 11, and 12A). Find it on your
sheep and human brains (see Fig. 12B, next page).
Sheep
rhinal sulcus
Ibex
The sheep has a respectable olfactory bulb, though
not so large in proportion to its brain as in many species. Primates, on the other hand, have very small
olfactory bulbs (look at human and baboon brains).
What, you may ask, becomes of the piriform cortex
in an animal with such a pitiful olfactory bulb? In
humans there is still a bulge called the piriform cor-
rhinal sulcus
Fig. 11: Lateral view of sheep and ibex brains (ibex is
missing its brain stem and cerebellum).
olfactory tract
On your sheep brain, start by distinguishing between
neocortex and other kinds of cortex. Neocortex,
which is phylogenetically most recent, includes most
of visible cortex. Neocortex is defined as having 6
layers, though there are many variations in different
regions. Non-neocortex, which I shall refer to collectively as allocortex, is visible only on the ventral
aspect of the intact brain. Here first identify the
olfactory bulbs, which have a laminar structure
(though with only 3 layers) and thus qualify as a
simple kind of cortex. The first cranial nerve enters
the olfactory bulb as bundles of axons penetrating
the bulb's rostral and ventral surfaces. These axons
arise directly from olfactory receptor neurons, lead-
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piriform
cortex
rhinal
sulcus
Fig. 12A: Ventral view of cat brain.
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
Sheep
rhinal
sulcus
somatosensory
Rat
Cat
Fig. 12B: Ventral view of human brain. Cortex lying
medial to the rhinal sulcus is allocortex rather than neocortex. The uncus is the most medial bulge of entorhinal
cortex; its anterior portion gets input from the olfactory
bulb.
tex, but only a small patch of it gets direct input from
the olfactory bulb, and might be considered strictly
olfactory in nature (see Fig. 12B).
Human
Turning to neocortex, look first at your sheep brain
and make a guess as to the locations of the two sensory cortical areas that have been identified in sheep
(see Fig. 13). Visual cortex is situated posterior, and
somatosensory, anterior, in accordance with the universal mammalian pattern. Somewhere between
them is auditory cortex, which I think has not been
studied in sheep. The cat, rat, and human show other
versions of the same pattern.
Note that in the human brain there are vast territories
of cortex intervening between purely visual, auditory, and somatosensory areas. Unlike the sheep (but
like the cat) in humans much of visual cortex is on
the medial aspect of the hemisphere. A substantial
part of primary visual cortex (that is, the first stage
of visual cortex, also called V1) is buried inside the
calcarine sulcus, a distinctive primate feature (find
NBio 401, 2012
Fig. 13: Sensory cortical areas in 4 species. Sheep brain
is shown in dorsal view, others are lateral views. In
sheep, 2 distinct areas of visual cortex are identified. The
other species also have multiple distinct areas of visual
cortex.
this sulcus on your human brain). Auditory cortex in
humans is not really visible on the cortical surface,
but is buried inside the lateral sulcus - you can see it
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A Verbose Guide to Dissection of the Sheep’s Brain
Sperm whale
H. Sherk
corpus callosum
Elephant
Fig. 14: Medial view of right cerebral hemisphere of sperm whale and elephant, approximately to
same scale. The brain stems and cerebelli have been cut off.
of the cortical hemispheres varies from species to
species. In humans it is fairly large, and you will see
that the sheep also has a decent-sized callosum when
you cut that brain in half. In dolphins and whales,
however, it is surprisingly puny (see Fig. 14 above).
The cross-sectional area of the corpus callosum
should scale with the volume of cortical gray matter,
which obviously doesn't happen in the Cetaceans.
Now look back at the oppossum (Fig. 6A); notice
anything missing? Marsupials have no corpus callosum at all.
by putting your fingers into the sulcus and gently
prying it open. Somatosensory cortex is simple to
find: it occupies the gyrus just behind the central sulcus, thus named the postcentral gyrus.
Cortical lobes As mentioned above, the human brain
is divided into regions called lobes. Using Nolte's
Fig. 3-5 for guidance, find the frontal, parietal, temporal, and occiptal lobes. Note that you can do this
on the basis of 3 sulci (the lateral, central, and parieto-occipital) plus a little indentation called the preoccipital notch. Subdividing most primate brains in
a similar fashion takes only a little imagination, but
doing so for other species really is a matter of guesswork.
CEREBELLUM
The cerebellum looks at first glance even more complex than the cerebral cortex. In reality its structure
is quite similar: it is a sheet of gray matter encapsulating white matter. The sheet is highly folded in
species with a large cerebellum, but is perfectly
smooth in species with a small cerebellum (e.g., the
codfish, Fig. 15). The oppossum (Fig. 6A) is inbetween, with plenty of folds (called folia) but clearly
not approaching the number and depth boasted by
primates, elephants, whales, etc.
Looking at the medial aspect of the human brain,
find the limbic lobe. It is delimited by the cingulate
sulcus, which continues as the collateral and then
rhinal sulci. Where exactly the transition from collateral to rhinal occurs I don't know. Finally, there is
a buried "lobe" called the insula which is hidden
under the overhanging gyri of the frontal lobe. Look
at the dissected human brain and at Nolte's Fig. 3-6
to see the insula.
Functionally the cerebellum is simpler than the cerebral cortex, at least to the extent that the whole thing
has more or less the same function. It is part of the
motor system, acting indirectly on other structures
that in turn may synapse on motor neurons. Phylogenetically, the cerebellum is ancient, appearing in
approximately the same form in all vertebrates. Its
In most mammals, the two cerebral hemispheres are
connected together by the largest tract of the CNS,
the corpus callosum. Find it in your human halfbrain (for some reason it never looks like white matter). The size of the callosum relative to the volume
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
Fig. 15: Lateral view of
codfish brain showing
smooth cerebellum.
size "is correlated with the intricacy of bodily movements", according to Romer, so that it is highly
developed in some fishes as well as in birds and
mammals, but pretty negligible in reptiles and
amphibians (have a look at the cerebellum of the
Gecko). The extraordinary size of the cerebellum in
whales and dolphins doesn't at first glance seem to
fit Romer's rule, but perhaps it is important for generating the complex sounds these creatures produce.
Compare the cerebelli of the human and sheep brain,
and then compare the size of the pons in each brain.
(Look at the elephant (Fig. 6B) and whales (Fig. 7)
also.) There is a good correlation between the size
of the pons and the size of the cerebellum, because
most of the pons is occupied by a relay nucleus that
transmits information from cerebral cortex to the
cerebellum. [Notice that there is no clearly distinct
pons in the alligator, Fig. 5, or codfish, Fig. 15.
Why?] Although the cerebellum appears to be snuggled up against the cortex, in fact they are not even
in contact in a living brain, being separated by a
sheet of dura (or even bone in some species). The
cerebellum is connected solely to the brain stem. It
is attached via three stalks (peduncles), which are
actually fiber tracts. The largest of these is the middle one (conveniently named the middle cerebellar
peduncle), and connects to the pons. You can easily
find it in both your sheep and human brains; it is the
bulge that extends dorsally from the pons up into the
cerebellum. It consists entirely of axons going from
the relay nucleus in the pons up to the cerebellum.
The other two peduncles, one caudal and one rostral
to the middle cerebellar peduncle, are much smaller,
and cannot be seen in an intact brain.
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Before cutting your sheep brain in half, have a look
at the vermis (the worm), the central strip that girdles
the cerebellum along its rostrocaudal equator. It is
rather prominant in the sheep but in species with
large cerebelli, including humans, it is dwarfed by
the enormous cerebellar hemispheres on either side.
Using a long knife, cut your sheep brain in half in the
midsagittal plane (see Note 2, p. 22). You can now
see the distinction between gray matter and white
matter in the cerebellum. You may also be able to
see the large ovoid nuclear mass forming the base of
the pons (it occupies most of the pons): this mass is
the pontine nuclei, which receives input from cerebral cortex, and sends axons up to cerebellum. Compare to the human brain.
Fig. 16: Dorsal view of human brain stem and thalamus
with half of the cerebellum attached.
With a scalpel, carefully cut the cerebellum free
from the brain stem (see Note 3, p. 22). In doing so,
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
Fig. 17: Medial view
of sheep brain.
corpus callosum
thalamus
cerebral aqueduct
4th ventricle
human and sheep brains. The interconnection
between the ventricles is essential because CSF
(cerebrospinal fluid) is continuously made inside all
of them, but can only exit the ventricular system
from the 4th ventricle. Exit holes are located just
under the cerebellum on the midline (the foramen of
Magendi = the medial foramen) and laterally (the
foramen of Luschka = the lateral foramen). Having
escaped from the ventricles, CSF bathes the brain
and eventually is taken up into the venous drainage
via arachnoid granulations. These are part of the
middle layer of meninges (tissue coverings of the
brain), the arachnoid, which should still be visible on
your sheep brain.
you cut through 3 tracts. The largest, as already
noted, is the middle cerebellar peduncle. You also
cut through the inferior cerebellar peduncle, composed of axons carrying proprioceptive somatosensory information (i.e., from muscle and joint
receptors) and axons coming from the inferior olive,
a large nucleus located in the medulla. (The olive
forms a distinctive elongated bulge on the lateral
side of the medulla, which you can readily locate in
both human and sheep brains.) You also cut through
the superior cerebellar peduncle, which is the output
of the cerebellum, and attaches to the midbrain. As
you can see in Fig. 16, the three cerebellar peduncles
all fuse together as they enter the cerebellum.
MIDBRAIN
We will skip over the medulla and pons, which are
packed with nuclei that are interesting but not visible
without sectioning and staining the brain. (A nucleus
is a group of neurons involved in a common function.) The midbrain, however, has three rather prominant bulges: the cerebral peduncle and the superior
and inferior colliculi.
VENTRICLES
By cutting your brain in half, you have revealed two
of the 4 ventricles (internal spaces), the 3rd and 4th.
The 4th ventricle is the triangular space between the
pons and the cerebellum. The 3rd ventricle is the
space between the left and right diencephalons. Find
the cerebral aqueduct (the canal connecting the 3rd
and 4th ventricles) in sheep and human brains. The
other two ventricles are the lateral ventricles,
enclosed within the cerebral hemispheres. There is
one per hemisphere, and for some mysterious reason
the left and right ones are numbered 1 and 2 (or perhaps it's the other way round). Both are connected to
the 3rd ventricle by way of a hole called the interventricular foramen (see Fig. 3) - look for this on
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Cerebral peduncle
The cerebral peduncle is one stage in the great
descending tract of the CNS. In the human brain the
cerebral peduncle is huge, and in the sheep, rather
modest. Look at it in your half-brains, and also in
the isolated human brain stem. This descending tract
has a different name as it passes through each major
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A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
through the midbrain as the cerebral peduncle, and
then through the pons as the corticobulbar fibers (see
Fig. 18, next page). Most (in humans, about 85%)
stop here and synapse on the pontine nuclei (which
send their axons to...?) The sizes of the cerebral
peduncle, the pons, and the cerebellum are all well
correlated. In the medulla,the remaining axons continue as the pyramidal tract or pyramid. In the
human brain, the pyramids form two distinct ridges
running along the ventral surface of the medulla
(look at them in a whole human brain). They are less
prominant in sheep. At the junction between
medulla and spinal cord, most axons cross to the
other side and continue on down the cord as the corticospinal tract, the largest of descending spinal
tracts.
midbrain
midbrain
Colliculi ("little hills")
The two colliculi are phylogenetically ancient, the
superior colliculus in particular being "one of the
most conservative structures in the brains of vertebrates" (Butler & Hodos, 1996) (in Fig. 19A, compare superior colliculus (= optic tectum (OT)) in an
amphibian, a reptile, and a fish). “Optic tectum” is
pons
mudpuppy
medulla
turtle
medulla
Fig. 18: Transverse sections through human brain stem
(transverse = orthogonal to long axis) showing large
descending cortical tract. Sections are stained for
fibers, so tracts and nerves are dark. These drawings are
not accurately to scale.
teleost fish
CNS subdivision. It originates in cerebral cortex: as
the cortical axons funnel down past the thalamus,
they are called the internal capsule (we will see them
a little later in horizontal sections). They travel
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Fig. 19A: Dorsal views of brains of 3 non-mammalian
species showing locations of optic tectum (OT), telencephalon (T), cerebellum (Cb), and olfactory bulb (OB).
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A Verbose Guide to Dissection of the Sheep’s Brain
4th ventricle
optic tectum
H. Sherk
to their lateral line system: they generate an electric
field, and detect perturbations in it caused by other
animals or objects. This information also is dealt
with in a particular part of the torus semicircularis.
torus
semicircularis
In all vertebrates, the optic tectum gets direct input
from the retina. It is the major visual structure in
most vertebrates; only in mammals and some birds
has the dominant site of visual processing shifted to
the forebrain. The optic tectum is used for orienting
in response to visual cues. It gets auditory and
somatosensory as well as visual input - this makes
sense, since obviously you can orient to a sound or
touch as well as to a visual cue. In animals possessing a lateral line, the torus semicircularis relays lateral line information, both mechano- and electroreceptive, to the optic tectum. In addition to brain
stem input, the optic tectum receives input from the
telencephalon; in the case of mammals, from a huge
Fig. 19B: Coronal section through left half of fish
midbrain (Icgalurus). Visible are 3 divisions of torus
semicircularis (fish inferior colliculus). Stippling =
electrosensory. Diagonal lines = mechanosensory.
Gray = auditory.
an alternative name for the superior colliculus
because it processes visual information; in nonmammals, this term is used exclusively. The inferior
colliculus processes auditory information. Its nonmammalian homologue (the torus semicircularis)
has a different location, but still is auditory, at least
in part (see Fig. 19B). In some animals, much of it
has been co-opted by lateral line sensation, which is
evidently more important than hearing for various
fish and tadpoles. The lateral line system has a
mechanoreceptive component that is sensitive to
pressure changes, and allows the fish to detect the
motion of other animals. This information has its
own processing zone in the torus semicircularis.
Many fish also have an electroreceptive component
Bluegill
anterior
lateral
posterior
Cat
4th ventricle
Fig. 21: Dorsal view of right optic tectum of 2 species,
the bluegill (a teleost fish) and cat. Each right tectum
maps the left eye’s visual field (shown in gray at top).
Lines of latitude and longitude in visual field are drawn
on each tectum at 10 deg intervals. Upper visual field is
mapped medial to horizontal merdian, lower visual field
is mapped lateral to horizontal meridian.
3rd nerve
Fig. 20: Coronal section through turtle midbrain showing layers of optic tectum. S = superficial, C = central,
P = periventricular.
NBio 401, 2012
13
A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
extent of neocortex, including both sensory and
motor areas.
megabats are basically "flying primates", only very
distantly related to microbats.
The optic tectum is divided into cell layers, visible in
stained sections (see Fig. 20). The visual field of the
contralateral eye (that is, the region seen by this eye)
is mapped across the surface of the tectum. This
map looks remarkably similar across different vertebrate classes (e.g., Fig. 21). There are two mammalian groups, however, that don't exactly follow the
general pattern: primates and megachiropterans
("megabats", one of the two suborders of bats). In
these, only half of the contralateral retina sends
axons to the optic tectum, resulting in a map that is
shifted forward so that the vertical meridian of the
visual field (the line dividing the visual field into
right and left halves) coincides with the front edge of
the tectum. The function of this arrangement is
unknown. It is, however, of considerable interest
because it is one of the major pieces of evidence that
In the owl, it has been found that auditory input to
the optic tectum forms a map of space that is aligned
with the visual map. This sensible organization
means that activation of a given tectal site by either
sensory modality - visual or auditory - should result
in orientation of the owl to the same spatial location.
In fish, tadpoles, and even one adult toad (Xenapus),
information originating from the lateral line (relayed
by a couple of intervening nuclei) also is mapped in
the optic tectum in an orderly topographic fashion,
presumably in register with the visual map.
The inferior colliculus has a very different structure
and function than the superior colliculus. It is not
layered, nor does it receive auditory information
straight from the cochlea, but rather from auditory
nuclei located more caudally in the brain stem. The
pineal body
Dolphin
Sheep
superior colliculus
inferior colliculus
tectal commissure
Microbat
Megabat
inferior colliculus
Fig. 22: Colliculi in 4 species of mammal (not to scale!). At top left, medial view of right half of brain of bottlenose
dolphin. Note huge inferior colliculus. Top right, medial view of sheep brain - note huge superior colliculus. A tract
(the tectal commissure) connects the left and right colliculi, and this tract is remarkably large in the sheep. At bottom,
dorsal views of bat brains. On left is megachiropteran brain, in which superior colliculus is larger than inferior, and
neither is big enough to extend beyond the cerebral cortex. On right is microchiropteran brain - note enormous inferior
colliculi.
NBio 401, 2012
14
A Verbose Guide to Dissection of the Sheep’s Brain
Fig. 23: Horizontal section through
sheep brain (dorsal surface of section). This view is just below (inferior to) the corpus callosum.
Through the lateral ventricles you
can see the head of the caudate
nucleus, and part of the hippocampus.
anterior
H. Sherk
choroid plexus
caudate n.
hippocampus
posterior
fornix
inferior colliculus is huge relative to the superior colliculus in microbats, who use high frequency sonar
(Fig. 22). Toothed whales and dolphins, another
echolocating group, show a similar morphology.
One might predict that the reverse relationship, big
superior colliculus and small inferior colliculus,
would characterize primates because they make frequent and precise saccadic eye movements, and
these orienting movements are controlled by the
superior colliculus. However, superior and inferior
colliculi are about the same size in primates (look at
human brain). It is ungulates who have a really
impressive superior colliculus - look at your sheep
brain. Does this mean they are making saccadic eye
movements hither and yon? We don't know...but
they do have a remarkably large oculomotor nerve,
as you have already seen, which implies pretty good
control of eye movements.
TELENCEPHALON - CUT 1
Using a long knife, make a horizontal cut through
one of your sheep hemispheres in the plane of the
corpus callosum, just along its upper edge. Now you
can see the cortical gray and white matter, and the
lateral ventricle. Inside the lateral ventricle is a strip
of darker stuff, the choroid plexus, which makes
CSF (see Fig. 23).
TELENCEPHALON - CUT 2
Make a second cut parallel to the first at a level just
above the pineal body, and through the upper part of
the thalamus. This cut will look something like Fig
24 (next page).
Anteriorly, the floor of the lateral ventricle is formed
by the surface of a large nucleus, the caudate, which
we will see clearly with the next cut. (You can actually see it already by looking at the medial side of
the hemisphere, just below the corpus callosum and
above the fornix. You are looking into the lateral
ventricle through a window that would, in the living
NBio 401, 2012
brain, be covered by a membrane, the septum pellucidum. The latter is still intact in many of the human
half-brains.) More posteriorly, the ventricular floor
is the surface of the hippocampus, which is actually
part of the cortex. Its surface (i.e., the part you can
see) is covered by a thin fiber sheet that connects the
hippocampus with various structures in the diencephalon. This bunch of fibers has, alas, three
names, but for convenience I will refer to it simply
as the fornix (technically, it is called the alveus when
it is a sheet on the surface of the hippocampus, then
is the fimbria when the fibers are no longer in contact with the hippocampus; as they coalesce into a
bundle on their downward journey, they become the
fornix). Look at the medial surface of the sheep and
human brains to see the fornix as it travels anterior
and down, heading for the diencephalon. Some
axons will make it all the way to the mammillary
bodies, on the ventral surface of the hypothalamus.
15
HIPPOCAMPUS
Although not particularly impressive in this horizontal section, the hippocampus often has a striking
appearance because of its rolled-up layers. You can
see this in gross brains, particularly human, but it is
most obvious when cells are stained (look at slide
stained with cresyl violet). In addition to its aesthetic appeal, the hippocampus is attractive because
of its known role in memory formation (see below).
A Verbose Guide to Dissection of the Sheep’s Brain
Fig. 24: Horizontal section
through sheep brain, slightly
deeper than Fig. 23. The
brain stem caudal to the
superior colliculus is missing. The lateral ventrical
appears twice, but each time
just as a thin slit.
H. Sherk
claustrum
putamen
hippocampus
internal capsule
LGN
thalamus
superior colliculus
caudate
anterior
pineal body
posterior
lateral
ventricle
lateral ventricle
septal region
fornix
It is not the locus of memory storage, but is the
mechanism by which long-term memories are laid
down. This process may or may not involve longterm potentiation.
.
The location and overall shape of the hippocampus is
often rather confusing. In mammals, it is a 3-layered
cortex that adjoins entorhinal cortex. In brains of at
least moderate size (e.g., cat-size or greater), it is
fairly long (in humans it is about 8 cm), and thus
overall is sausage-shaped. It lies along the medial/
ventral wall of the lateral ventricle. Recall that, posteriorly, this ventricle curves downward; thus in
many animals the long sausage of the hippocampus
also curves downward. In a horizontally cut brain,
you generally get approximately transverse crosssections through the hippocampus, but you will see
that in coronal cuts through the sheep brain, you can
get an extended longitudinal section of the hippocampus. However, this won't happen in the human
brain because, due to the greatly expanded temporal
lobe of the cortex, the entire hippocampus lies along
the temporal horn of the lateral ventrical (see Fig.
25) and so is relatively straight.
Memory is obviously a useful function for any animal, and thus not surprisingly all vertebrates have
some homologue of the hippocampus. Although in
non-mammalian species the hippocampus doesn't
NBio 401, 2012
look much like the mammalian version, all seem to
possess a layer of small, densely packed, intensely
staining neurons that resemble the dentate gyrus.
The first demonstration that the hippocampus is
essential for the storage of long-term memory came
inadvertently in humans when the hippocampus was
destroyed bilaterally in an attempt to alleviate severe
epilpeptic seizures. The most famous of these
patients, HM, was studied for several decades thereafter: he had severe anterograde amnesia (loss of
ability to lay down new memories), though he was
lateral ventricle
3rd ventricle
temporal horn
4th ventricle
Fig. 25: Ventricles of human brain, seen from left side.
Hippocampus lies along ventral surface of temporal horn
of lateral ventricle.
16
A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
This is a rather large area of cortex, and it shows
more than a two-fold variation in size between individuals. But there is no correlation between size of
V1, and brain size.]
still capable of certain kinds of learning. Loss of a
single cell population, the pyramidal cells in the
CA1 portion of the hippocampus, causes the same
deficit. These neurons are particularly vulnerable to
ischemia, so that patients who suffer some ischemic
catastrophe such as carbon monoxide poisoning
often show anterograde amnesia.
In many animals the hippocampus seems to be
largely devoted to spatial memory. An elegant demonstration of the relationship between a brain structure and behavior involves the hippocampus of birds
and their talent for spatial memory. In some bird
families (titmice and crows) certain species store
food in scattered locations, and are capable of
retrieving food from "many thousands of sites in
their home range" (Harvey & Krebs, 1990). These
species have substantially larger hippocampi than
species that don't store food. [This story has a puzzling corrolary: species with extra-large hippocampi
do not have a larger telencephalon overall. Has
some other structure shrunk? There is a similar puzzle in humans regarding primary visual cortex, V1.
BASAL GANGLIA
In your horizontal section through the sheep brain
you can see the caudate nucleus, the largest of the
five nuclei making up the basal ganglia. [This term
is a misnomer, as these nuclei obviously belong to
the CNS (a ganglion is a collection of neurons that is
part of the peripheral nervous system).] The basal
ganglia are a major component of the motor system,
performing some vital but poorly understood function. Their importance becomes most obvious when
they do not work properly, as in various neurological
diseases (e.g., Parkinson's disease, Huntington's chorea). Although the nuclei of the basal ganglia are
scattered across different subdivisions of the brain,
they are tightly linked into a single functional system.
In a slightly deeper section you will see the nucleus
lateral ventricle
corpus callosum
3rd
ventricle
thalamus
lateral ventricle
hippocampus
MGN
brain stem
LGN
Fig. 26: Hippocampus in coronal sections through portion of left
entorhinal
hemisphere of rat (on left) and human (on right). In rat you can see
cortex
subdivisions of hippocampus, the fields of CA pyramids (CA stands
for cornu Ammonus, the horn of Ammon, an old name for hippocampus). Heavy black stripe shows small, densely staining cells of dentate gyrus. Human shows same organization, but
everything has been pushed down and medial by growth of temporal lobes. Note thalamic nuclei, the lateral and medial
geniculate nuclei [LGN & MGN], nearby.
NBio 401, 2012
17
A Verbose Guide to Dissection of the Sheep’s Brain
adjoining the caudate, the putamen (Figs. 24 and 27).
The two are separated by the internal capsule (its
anterior limb), which has a very streaky appearance
here and has given rise to another widely-used term
for these two nuclei, the striatum. The caudate and
putamen are functionally very similar, and perhaps
really should be considered a single nucleus that has
been fortuitously split by the internal capsule. Just
medial to the putamen sits a third nucleus, the globus
pallidus (the "pale globe").
The connections of these three nuclei are captured
succinctly in Fig. 19-6 of Nolte. One important
detail not evident in this diagram is the fact that the
globus pallidus is divided into two parts. You can
often see this when looking at human slices. The
external globus pallidus gets input from the caudate
and putamen, and in turn sends axons to the internal
globus pallidus. In non-primate mammals the external globus pallidus goes by a different name, the
entopeduncular nucleus. The internal globus pallidus also gets direct input from the caudate and putamen. It provides the sole output of the basal ganglia
(with one exception noted below), which is to the
thalamus. Remarkably, this output is inhibitory.
H. Sherk
that sits just beneath the thalamus. In the midbrain,
just above the cerebral peduncle, is the substantia
nigra. Look at this nucleus in a section through
human brain - it is the easiest one in the brain to spot
because it is conveniently pigmented black (hence
the name) by melanin. (You will not see it in your
sheep midbrain - I am not sure why). These two
nuclei are reciprocally connected to the caudate and
putamen. Part of the substantia nigra breaks the rule
that the only output of the basal ganglia is from the
globus pallidus: the substantia nigra, pars reticulata,
sends axons to the superior colliculus, that ubiquitous collector of input from practically everywhere.
THALAMUS
In your horizontal section of sheep brain you can see
two almost separate regions of gray matter that are
both part of the thalamus. The small, lateral piece is
the lateral geniculate nucleus (LGN), which is the
visual relay nucleus of the thalamus. It appears to
have come adrift from the rest of the thalamus due to
the intervention of the optic radiation (axons going
from LGN to visual cortex, and vice versa). The
optic radiation merges into the internal capsule.
The mammalian thalamus is made up of approximately 12 nuclei. Of these, you can rarely distinguish more than a couple in gross brains. The visual
and auditory relay nuclei (LGN and MGN) can be
The other two nuclei of the basal ganglia are located
some distance away. In the diencephalon is an
almond-shaped nucleus, the subthalamic nucleus,
caudate
putamen
globus pallidus
thalamus
posterior
NBio 401, 2012
Fig. 27: Human basal ganglia. Left, horizontal section with caudate & putamen dotted. Note external and internal divisions of globus pallidus on left side. Right, ghost brain with caudate and
putamen in gray, substantia nigra in black. Caudate has a long tail that
follows the dorsal-lateral surface of the lateral ventricle’s temporal horn.
18
A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
lateral ventricle
VPL
VPM
LGN
MGN
hippocampus
Fig. 27: Coronal sections through thalamus of raccoon (left) and human (above). The raccoon sections
are ordered from rostral, at top, to caudal. The human
section is an idealized drawing showing distinct thalamic nuclei.
CG central gray
Pit pituitary
CP cerebral peduncle
Pul pulvinar
Hy hypothalamus
Rt reticular n.
IC internal capsule
LD lateral dorsal n.
LP lateral posterior n.
MD medial dorsal n.
identified by location - see human brain slices and
Figs. 26 and 28. The LGN is also visibly layered in
primates and some other species (see raccoon, bottom section, Fig. 28). The third big sensory relay
nucleus, the ventral posterior, is not visible as a distinct entity. You should have some idea, however, of
the location of the two parts of this nucleus, VPL
(ventral posterior lateral) and VPM (ventral posterior
medial); they are shown in Fig. 28 in coronal sections through raccoon and human brains. In many
mammals VPL and VPM are merged together into
NBio 401, 2012
the ventrobasal complex.
Every thalamic nucleus but one has its own target
territory in the ipsilateral cortex, and each target territory in turn sends axons back to its thalamic
nucleus. As you know, the cortex relies almost
exclusively upon the thalamus for its subcortical
input. Every so often the question arises, can you
perceive with your thalamus alone, or is perception a
strictly cortical phenomenon? Perhaps this is not a
meaningful question, because it turns out that the
19
A Verbose Guide to Dissection of the Sheep’s Brain
thalamus is as reliant upon the cortex as vice versa.
If the cortical territory to which a thalamic nuclus
projects is destroyed, the projection neurons in that
nucleus (which is the great majority of neurons)
degenerate and disappear. Thus one cannot ask, for
example, whether after a stroke that destroys all of
somatosensory cortex, the surviving somatosensory
thalamic nuclei can mediate somatosensory perception...there won't be any surviving somatosensory
nuclei. Presumably the reason is that, in mammals,
the cortex is the only target of axons from thalamic
nuclei, and when the entire axonal arbor of a CNS
neuron is destroyed, the neuron eventually dies.
Take your intact half sheep brain, and make a series
of coronal cuts through it. Try to identify the various
structures that we have mentioned. You can use the
sheep atlas, which has coronal sections - just bear in
mind that they are myelin-stained, so dark and light
are reversed relative to the actual brain. Following
are some suggestions.
The thalamic reticular nucleus does not project to the
cortex, or indeed out of the thalamus at all. It forms a
shell largely encapsulating the thalamus (see Fig.
27), and provides negative feedback to thalamci
nuclei.
TELENCEPHALON - CUT 3
Make your last horizontal cut through the anterior
commissure (this looks like a white dot on the
medial surface of the hemisphere anterior to the thalamus, at the tip of the fornix). You should now see a
good segment of anterior commissure bending anteriorly and seemingly merging with the white matter
of the internal capsule. The anterior commissure,
despite its diencephalic location, serves to link the
left and right cerebral cortices, specifically the temporal lobes.
Look closely just medial to the putamen - you will
see a pale but distinct region of gray matter. This is
the globus pallidus.
Just anterior to the anterior commissure is a substantial region of gray matter that, in coronal sections,
appears to represent a fusion of the caudate and putamen. However, it seems to have a distinct function
(and, naturally, a distinct name, the nucleus accumbens). N. accumbens receives a dopaminergic input
from the midbrain; in schizophrenic patients this
input is thought to be pathologically elevated, leading to excessive activation of n. accumbens. There is
an abnormally high level of one variety of dopamine
receptor in the nucleus accumbens of these patients.
CORONAL CUT 1
NBio 401, 2012
H. Sherk
Cut at the level of the anterior commissure. You can
see the gray matter of the septal region, which is
heavily interconnected with the hippocampus. Hopefully you will see the columns of the fornix (e.g., the
part of the fornix that dives steeply downward into
the diencephalon). These pass just posterior to the
anterior commissure. Medial to the putamen is the
extremely pallid globus pallidus (it is darker in
human brains than in sheep). Underlying the globus,
and extending medially, is a thin strip of gray matter,
the basal nucleus of Meynert. These cholinergic neurons project extensively to cerebral cortex (including
hippocampus), and are interesting because most of
them degenerate in Alzheimer's disease, causing a
huge loss of cortical cholinergic activity.
CORONAL CUT 2 (aprox = S7 in sheep atlas)
Cut about 1/3 of the way through the thalamus (from
its anterior edge). Notice that the caudate has shrunk
dramatically (we are into the "tail" here). The thalamus, on the other hand, is quite imposing. The fornix
appears to be suspended below the corpus callosum.
The optic tract is easy to find. If you are lucky, you
may have caught the subthalamic nucleus, a clearlydemarkated, almond-shaped nucleus next to the
optic tract.
What appears to be temporal lobe cortex, but with a
curious absence of white matter, is a huge nucleus
called the amygdala. Its connections are complex
and its function poorly understood. It seems to be
related to emotional state. Why it should be so strikingly large in the sheep is a mystery.
CORONAL CUT 3
Cut through anterior part of mammillary body. Probably mostly what you will see, excluding cortex, is
thalamus. You can also see the internal capsule
merging into the cerebral peduncle.
CORONAL CUT 4
20
A Verbose Guide to Dissection of the Sheep’s Brain
do
H. Sherk
l
r sa
arachnoid
dorsal root
ganglion
spinal nerve
dura
Fig. 28: Human spinal cord. Above is a chunk encompassing two segments
through cervical cord. At right is a complete cord, cut into two pieces - dorsal
view.
Cut just posterior to the mammillary body. Now you
should be able to see LGN up at the dorso-lateral
corner of the thalamus, with the optic tract entering it
from below. Beneath the optic tract is the MGN. The
hippocampus should appear twice in this section,
both above the thalamus and ventro-lateral to it.
These slices of hippocampus will be connected by
the fornix.
CORONAL CUT 7
Cut through the medulla, where the 8th nerve
attaches. I hope that you can see a thin crescent of
gray matter sitting on top of the bulge of white matter here. The white matter is the inferior cerebellar
peduncle, and the gray matter, the dorsal cochlear
nucleus.
SPINAL CORD
If you have not yet examined a spinal cord, get one
of the human cords and look at the following.
Dorsal root ganglion (most have been torn off,
but you can usually find one or two still
attached).
Dorsal and ventral roots (which are motor?
which sensory?)
Cauda equina - the prolonged dorsal and ventral roots that exit below mid-thoracic vertebral
levels. Where would you find the dorsal root
ganglia of these long roots?
Cervical enlargement and lumbosacral enlargement.
CORONAL CUT 5
Cut through the middle of the superior colliculus.
With a little imagination you can see its layers - most
dorsally, a thin layer of gray, and beneath it, a thicker
white layer, which contains axons entering from retina and cortex. Surrounding the cerebral aqueduct is
the central gray (or periaqueductal gray, PAG),
which is made up of very small neurons and is
involved in regulating pain perception.
CORONAL CUT 6
Cut through the pons. You should see the middle cerebellar peduncle, which is hardly as impressive as in
a human brain but is still a good-sized tract. Medial
to it you may see a bulge that is the superior cerebellar peduncle. (What axons travel in each of these
peduncles?)
NBio 401, 2009
As you know, sections through the spinal cord look
somewhat different at different levels. Look at
Nolte's Fig. 10-6 for a beautiful series spanning the
length of the human spinal cord.
21
A Verbose Guide to Dissection of the Sheep’s Brain
H. Sherk
cutting exactly on the midline.
REFERENCES
3. When you cut off the cerebellum, try cutting from
the lateral side, and don't worry if you cut through a
bit of cerebellar cortex. It is best if you can do it in a
single cut.
Butler, A.B. & Hodos, W. (1996) Comparative Vertebrate Neuroanatomy. Wiley-Liss.
Harvey, P.H. & Krebs, J.R. (1990) Comparing
brains. Science 249: 140-146.
Haug, H. (1970) Der makroskopische Aufbau des
Grobhirns. Erg. Anat. Entwickl. 43: 1-70.
Morgane, P.J. & Jacobs, M.S. (1972) Comparative
anatomy of the Cetacean nervous system. In
Functional Anatomy of Marine Mammals, ed.
R.S. Harrison, Academic Press.
Pearson, R. & Pearson, L. (1976) The Vertebrate
Brain. Academic Press.
Pettigrew, J.D., Jamieson, B.G.M., Robson, S.K.,
Hall, L.S., McAnally, K.I. & H.M. Cooper
(1989) Phylogenetic relations between microbats, megabats and primates (Mammalia: Chiroptera and Primates). Phil. Trans. R. Soc.
Lond. B 325: 489-559.
Pilleri, G. & Gihr, M. (1970) The central nervous
system of the Mysticete and Odontocete
whales. Investigations on Cetacea, vol. 2.
Romer, A.S. (1966) The Vertebrate Body. W.B.
Saunders Co.
Yoshikawa, T. (1967) Atlas of the Brains of Domestic Animals. U. of Tokyo Press.
4. When you are done for the day, get a zip-lock
bag, write your name on it with waterproof marker,
and put the sheep brain in it. Pour in some "Moistening Fluid" and zip up; put it in one of the tupperware boxes. Return the human brain to the box it
came from.
--------------------------------------------------------------NOTES
1. When you slice off the end of the spinal cord and
look at the gray matter/white matter distinction, be
warned that often the gray matter looks white, and
the white matter, darker. Same thing with human
spinal cords. I don't know why.
2. When you cut your sheep brain in half, try the following. At the anterior end, carefully pry apart the
two cerebral hemispheres. You will have to tear the
arachnoid (the middle of the layers of meninges,
which is still present on the sheep brain). Work your
way posteriorly until you have pried apart the cortical hemispheres along their entire length and can get
a good look between them at the top of the corpus
callosum. Then use a long knife (the black-handled
ones work well) to make the cut, starting from the
anterior end. You can use various landmarks on the
ventral surface of the brain to make sure that you are
NBio 401, 2012
22