Dissection of the Sheep Brain
Sheep brains in Carosafe®, a formalin analog, will be supplied to
you for the following dissection exercises. The dissection must be carried
out on the side directed in order not to interfere with the later procedures.
Laboratory work should be supplemented by study of textbook, web and Sylvius.
I. Brain Membranes and Blood Vessels
To make the dissection easier, your specimen has had the brain membranes
(meninges) removed already. In this
lateral view and this
ventral view, with meminges intact, the dura mater is obvious. The pia mater
needs to be teased to retract into fissures (sulci), using this
view of the external anatomy of the cerebral cortex as an example. The arachnoid,
lying between these two membranes, is difficult to see.
Note the blood supply, especially the circle of Willis and the other vessels,
seen in this
ventral view. The hypophsis (pituitary) has probably already been removed
in your brain to permit better observation of the circle of Willis, but
it is intact in this
brain. Find the basilar artery; the posterior communicating arteries; the
posterior, middle, and anterior cerebral arteries; the internal carotid
arteries; and the anterior communicating artery.
II. Cranial Nerves
Using ventral views 1,
the roots of the twelve pairs of cranial nerves. Each is listed below and
its function identified. [Note, "motor" (below) includes parasympathetic
output for nerves 3, 7, 9 and 10.]
III. Surface Anatomy of the Brain
- The first "nerve" (olfactory bulb) is olfactory in
- The optic "nerves" run off from the optic chiasm.
- The oculomotor nerves are large and flat, emerging from the
cerebral peduncles, and running forward to supply four of the muscles of
the eyeball. Motor.
- The trochlear nerve emerges from the lateral surface, in the
angle between the occipital lobe of the cerebrum and the hindbrain. Motor-serves
the superior oblique muscle of the eye.
- The trigeminal nerve is behind the trochlear and is very large.
It emerges from the lateral border of pons. Mixed sensory and motor nerve.
It serves muscles of mastication and sensory fibers from the face.
- The abducens is a small flat nerve arising from the trapezoid
body. It is motor, serving the external rectus muscle of the eye.
- The facial nerve is lateral to the abducens and just behind
the trigeminal. It serves the muscles of the face.
- The acoustic nerve is just behind and slightly lateral to the
facial nerve. Sensory.
- The glossopharyngeal and vagus (X) nerves arise together,
behind and slightly ventral to the auditory nerve and separate later. The
glossopharyngeal is sensory and serves the mouth and tongue area.
- The vagus, as mentioned above, is both sensory and motor, serving
the heart, lungs and other internal organs.
- The spinal accessory nerve runs along the lateral surface of
the medulla and spinal cord, receiving fibers along the way. It is motor
in functions, serving the muscles of the neck.
- The hypoglossal nerve arises from the ventral surface of the
medulla, in a pair of more or less distinct roots. It is motor, innervating
the muscles of the tongue.
Examine carefully the external form of the sheep brain. On the dorsal
view, note the fissures (sulci) and convolutions (gyri); also identify the
cerebellar vermis and hemispheres, the medulla and the longitudinal fissure.
On the cerebral hemispheres
identify the lateral fissure, the rhinal fissure and the suprasylvian fissure.
Using ventral views 1,
the following structures:
IV. Brain Stem and Cortex
- Cerebral peduncles
- Interpeduncular nucleus
- Mammillary bodies
- Diagonal band
- Amygdaloid nucleus
- Pyriform area
- Hippocampal gyrus
- Lateral and medial olfactory gyri and stria
- Trapezoid body
- Anterior perforated substance
Observe the relations of the cerebral cortex to the cerebellar cortex.
Note especially the difference in the number of convolutions on each structure.
Use a scalpel to cut off a slice about one centimeter thick from the posterior
pole of the left cerebral cortex and a similar slice from the left lateral
border of the cerebellum. They should look like this.
Compare the cut surfaces and observe the relations of the gray matter to
the underlying white matter.
The rhombencephalon (or hindbrain) is composed of the metencephalon
(cerebellum and pons) and the myelencephaon (medulla oblongata). Note the
attachment of the cerebellum to the medulla. Cut these attachments (cerebellar
peduncles) on each side with the aid of a scalpel. Remove the cerebellum
and put it aside for future study. These peduncles should be cut as high
as possible, cutting into the substance of the cerebellum if necessary rather
than into the structures of the medulla. Take care not to injure the delicate
membranes beneath the cerebellum and forming the roof of the IVth
ventricle. The view should look like this.
The IVth ventricle is sometimes called the cavity of the rhombencephalon.
The cerebellum forms the roof of the ventricle but only for a short extent
between the cerebellar peduncles. Behind, the roof is formed by a thin non-nervous
membrane (tegmen) part of which is highly vascular and much folded. Note
the choroid plexus of the IVth ventricle, composed capillary
tufts and columnar epithelial tissue. The choroids plexi form a barrier
between the blood and the cerebrospinal fluid, and are thought to be the
source of the cerebrospinal fluid.
VI. Mid-sagittal view
Now cut the entire brain of the sheep into right and left halves. A long,
thin knife or large steel spatula is best suited to this purpose. The incision
should pass through the longitudinal fissure between the cerebral hemispheres
to cut through the corpus callosum in the floor of this fissure, and then
downward through the entire brain stem. Care should be taken to make this
cut smooth and exactly in the median plane. It should be made with a single,
long sweep of the knife. You may wish to locate the commissures as you cut
through them. Examine carefully the cut surfaces of the brain and identify
the structure brought into view. Refer to these photographs (1
of the median surface provided to you and locate the following structures
on your sheep brain:
- Fourth ventricle
- Mammillary bodies
- Lamina quadrigemina
- Superior colliculus
- Posterior commissure
- Pineal body
- Splenium, genu and body of the corpus callosum
- Massa intermedia of the thalamus
- Stria medullaris
- Body of the fornix
- Anterior commissure
- Septum pellucidum
- Hippocampal commissure
- Diagonal band
- Lamina terminalis
- Habenular commissure
There are three cerebellar peduncles: brachium conjunctivum, brachium
pontis, and corpus restiforme. Examine their cut surfaces on the dorsal
aspect of the medulla. Separate the fibers of the three peduncles from each
other on the cut surface. Then continue the separation of the conjunctivum
and middle peduncles for approximately one centimeter downward along the
dorso-lateral border of the medulla as they cross the spinal V tract superficially
to continue into the cord as the dorsal spino-cerebellar tract. Dissect
the fibers of the brachium pontis and the brachium conjunctivum. The latter
can be followed to its decussation in the cerebral peduncle beneath the
superior colleculus. Since you removed the cerebellum, you cannot do this
tract dissection: With gray matter removed, the lateral aspect of the cerebellar
peduncle is seen.
Cut the cerebellum into two halves along the medial sagittal plane. The
cerebellar gray and white matter seen in the median section of the vermis
has the appearance of arbor vitae.
VIII. Dissection of the Tracts
The dissection of the cerebrum outlined below is to be carried out on the
right half of the sheep brain. Locate the pyramid on the ventral surface
just caudal to the pons. Strip the pons fibers back from the cut median
surface for a short distance. Teasing them, you can see the direction the
axons go. Expose the longitudinal pyramidal fibers lying just dorsal to
the pontine fibers. Tease them and you see that they go at right angles
to the pons fibers. In the sheep, the pyramidal fibers interdigitate with
fibers of the trapezoid body which thus must be destroyed to expose the
pyramidal tract. By careful teasing, the pyramidal tract can be followed
spinalward only as far as the decussation. The fibers cannot be followed
any further. The pyramidal tract can be dissected cephalad from the pons
through the cerebral peduncle. This dissection should be deferred until
The arrangement of fibers in the subcortical white matter can be examined
by careful teasing. Only some of these fibers are to be studied at this
time. The dissection outlined in this section should not be carried farther
Refer to this
figure. Select a region along the dorsal border of the medial surface of
the hemisphere. Scrape away the gray matter covering two adjacent gyri to
expose the short associational (arcuate) fibers connecting these
gyri. Further teasing can reveal similar fibers lying deep in the white
matter, which connect the more remote gyri. Corona radiata is the name given
to those fibers running between the cortex and the brain stem via the internal
capsule of the corpus striatum. These include the thalamo-cortical sensory
projection fibers, other thalamo-cortical connections and the cortical efferent
tracts (the corticospinal, corticobulbar and corticopontile). These fibers
diverge from the upper border of the internal capsule like the rays of a
crown, hence "corona radiata."
The cingulum is a long associational tract close to the medial cortical
surface of the hemisphere. It runs parallel with the dorsal surface of the
corpus callosum in part of its course, beginning anteriorly in the gyrus
subcallosus beneath the head of the corpus callosum. It arches upward at
the genu of the corpus callosum, passes around the splenium of the callosum
posteriorly and then bends downward, forward and lateral-ward to the region
of the hippocampal gyrus. Begin its dissection at about the middle of the
corpus callosum, following it both anteriorly and posteriorly.
The corpus callosum are fibers that connect the neocortex of one
hemisphere with that of the opposite hemisphere. Tease the callosal fibers
a small distance and note that the texture tells you the direction the axons
IX. Mesencephalon and Prosencephalon
The mesencephalon includes all of the structures of the midbrain, such
as the superior colliculus, inferior colliculus, cerebral peduncles, and
the interpeduncular nucleus. The prosencephalon is divided into the diencephalon
and the telencephalon. The thalamus and hypothalamus (including the mammillary
bodies) are the major parts of the diencephalon, and the cerebral cortex,
corpus striatum (internal capsule, caudate nucleus, putamen and globus pallidus)
and rhinencephalon form the telencephalon.
Rhinencephalon and Limbic System
In older terminologies the rhinencephalon is often referred to as smell
brain. Although the olfactory bulb can be shown to have connections with
rhinencephalon, this is true for only some of its parts. More recently the
rhinencephalon has been divided into three interconnected parts: 1. Primary
olfactory structures directly related to the olfactory bulb. 2. Second system
receiving fibers from the first consisting primarily of septal region and
amygdaloid complex. 3. The third system consists of cingulate and entorhinal
cortex and the hypothalamus. These appear to be remotely, if at all, related
to olfactory afferents. Both the second and third systems send efferents
to the hypothalamus. The limbic system consists of the second and
third systems in addition to two diencephalic structures, the anterior thalamus
and the mammillary bodies. The structures of the rhinencephalon are relatively
more developed in the sheep, so that dissection is easier than in the human
brain. The primary olfactory neurons give rise to fibers that terminate
in the olfactory bulb, the primary olfactory center of the brain. From here
to the neurons of the second order (mitral cells), give rise to axons forming
the olfactory tracts or striae (lateral and medial) terminating in secondary
olfactory centers in the basal parts of the cerebral hemisphere.
Other tracts of the rhinencephalon usually named by hyphenated compound
words of which the second member designates the center into which the tract
discharges. For example (refer to
this figure) [Also note that, if your cut was a bit off of mid-sagittal,
these tracts may already be visible.]:
Mammillo-thalamic tract . This tract runs from the mammillary body
forward and dorsalward to the anterior nucleus of the thalamus. It can be
readily dissected by scrapping off the ependyma of the third ventricle,
beginning in the region of the mammillary body.
Habenulo-pendunclar tract . This tract also can be readily dissected
(although watch out, it is easy to scrape right through it). It runs from
the habenula into the ventral part of the cerebral peduncle immediately
behind the mammillary body.
The fornix can also be followed down toward the mammillary bodies.
The hippocampus and fornix.
The hippocampus (archipallium) is located in the floor of the posterior
horn of the lateral ventricle, together with the fimbria and hippocampal
commissure. Refer to
this figure. Now cut through the splenium of the corpus callosum to
the tip of the hippocampal gyrus. This is one of the 2 cuts you would make
to cut out a pie wedge of the cerebral cortex to reveal the hippocampus.
Peel apart this opening to reveal the hippocampus (refer to this
figure). Note that the hippocampus looks white. This is because it is covered
by the fimbria, fibers that form the fornix. The column of the fornix thus
is the efferent projection tract from the hippocampus to the mammillary
body (and habenula).
Teasing the fibers of the fimbria, you can tell by the texture what direction
they go. Here
is another view of the hippocampus; Note that, in this view, there is a
nick through the fimbria revealing the gray matter of the hippocampus.
Cut through the genu of the corpus callosum forward and downward towards
the olfactory bulb, to open up the anterior horn of the lateral ventricle.
In the sheep, unlike man, this is directly continuous with the ventricle
of the olfactory bulb.
The ventricular wall can be torn away along the border between the head
of the caudate nucleus and the hippocampus by careful pulling. Refer to
This reveals the internal capsule fibers passing downward and backward,
lateral to the caudate nucleus. You can tell, by the texture of the tearing,
the direction of the axons.
XI. Optic System
With trimming, caudate and hippocampus are clear in this
figure. Remove the hippocampus and identify lateral and medial geniculate
bodies (refer to this
figure). You can also see the inferior (auditory) and superior (visual)
colliculi on the lateral surface of the thalamus and midbrain. Follow the
optic tract from the chiasm to the lateral geniculate body.
XII. Further Dissection of the Pyramidal Tract
As the last step of the dissection, careful tearing down of the fibers
will permit you to follow out some of the internal capsule fibers into the
regions of the thalamus, midbrain, and medulla oblongata, especially the
cortico-spinal (pyramidal) tract. Although functionally a motor and thus
a descending tract, it can more easily be traced from the medulla upward
to the higher centers. It appears as an eminence (pyramid) on the ventral
surface of the medulla below the pons near the midline. Its fibers interlace
with those of the pons, where the cortico-spinal tract was located earlier.
It can then be followed along the ventral surface of the mesencephalon through
the cerebral peduncle into the internal capsule. The cell bodies for these
fibers lie in the superior frontal gyrus.
XIII. Dissection of the Left Half
The left half of the specimen may be cut into a series of transverse
or longitudinal slices.
A suggested method of making these sections is first to cut a "horizontal"
section through the neocortex, about three millimeters dorsal to the corpus
callosum and in the same plane as this structure. This section will
actually slope ventrally toward the frontal cortex, as does the corpus callosum.
Then cut laterally through both the genu and splenium of the corpus callosum
to the extremes of the anterior and inferior horns of the lateral ventricle,
respectively. After removing the corpus callosum, extend the cuts anteriorly
and posteriorly to expose the head of the caudate nucleus, and the hippocampus.
Refer to this
figure. Observe the relations of the lateral ventricle, the hippocampus,
and the head of the caudate nucleus to one another.
Another section can be made in the same plane, but at a somewhat smaller
angle from the horizontal, immediately superior to the pineal body, through
the hippocampus and the head of the caudate. Be careful to avoid cutting
through the dorsal surface of the thalamus. Refer to this
figure. Note the relation of the internal capsule to the adjacent structures.
The basal ganglia are made up of caudate, putamen and globus pallidus. Caudate
+ putamen = striatum (striated because strands of internal capsule make
it look striated), clearly seen in this section. Putamen + globus pallidus
= lentiform nucleus [lens shaped], clearly seen in this section. This section
is also excellent for seeing the hippocampus overlying the lateral geniculate
of the thalamus.
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This page was last updated 2/1/06