Biology 241 – Lab
LAB #18
(18th/20 Lab Sessions for Fall Quarter, 2008)
TOPICS TO BE COVERED:
»Discussion/Dissection of Sheep’s Brain
DESIRED OUTCOMES:
After completing the activities described for this lab session, students should:
»Be able to name and describe the three meninges that cover the brain; as well as the
various specific extensions of the outermost meninx.
»Be able to identify the four principle parts of the brain, the major regions of the brain and
present the basic functions of each.
»Be able to identify the surface features of each region of the brain.
MATERIALS NEEDED:
»Gloves
»Safety Glasses (or your own glasses)
»Dissection Trays (large, flat pans)
»Preserved Sheep’s Brains
»Dissection Tools (large forceps, scissors, blunt probes, dissection needles, scalpels)
»Colored pins
»Various Models of the Brain
»Human Brain Posters
»Photographic Atlas, Ch. 9
Activity #1: Discussion and Dissection of the Sheep Brain
As is true for all mammalian brains, the structure and function of the sheep brain is very similar to that
of the human brain. It will be of comparative benefit to note the significant differences as you study the two
brains. One of the chief differences between the human brain and that of other animals is the orientation of the
brainstem. Humans are vertical animals and walk on two legs. Thus, the spinal cord is perpendicular to the
ground; so the brainstem must also be vertical. In quadripeds animals, the spinal cord and brainstem are both
parallel to the ground. Another obvious major difference is that the frontal lobes of the cerebral hemispheres are
proportionately much smaller in the sheep than in the human brain.
The brain is surrounded by three layers of meninges. The outermost, the dura mater, is sometimes
partially or totally absent in our preserved specimens, as it can become detached during preparation. However,
more often, the dura mater is intact, and will be examined first.
A. Examination and Removal of the Meninges
1. Examine the intact dura mater on the sheep barin. Locate the falx cerebri and the tenorium cerebelli
(aka falx cerebelli) from the dorsal surface of the dura mater. The falx cerebri is nowhere near as extensive nor as “sickle-shaped” in the sheep brain as it is in the human brain. In the midline, note the thickened part of the dura covering the longitudinal fissure. If you cut through this with scissors, you will
enter the superior sagittal sinus.
2. Determine if your specimen still has a part of the ethmoid bone attached on the anterior frontal lobe. If
the bone is present, slip a blunt probe between the bone and the dura to carefully separate the bone away
from the specimen. Then, using your scissors, snip away any attached dura. Examine the removed ethmoid bone and identify the crista galli: crest of bone where that serves as the surface of attachment of the
meninges.
3. Insert a probe between the dura and the brain to gently separate the two. Use your scissors sparingly to
cut the dura away from the base of the brain, taking extra care not to cut or remove a cranial nerve.
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4. The dura will still be strongly anchored in place at the tentorium cerebelli. To detach the dura in one
piece, first loosen it from the rest of the brain, taking great care to cut around the pituitary gland, and
gather it at the tenorium. Grasp the dura at the tentorium deep between the cerebellum and cerebrum and
pull it straight out. Open the now detached dura (aka turn it inside out) and identify the falx cerebri and
tentorium cerebelli.
B. Identification of External Brain Anatomy
1. First, observe the gyri (singular: gyrus) and sulci (singular: sulcus) on the cortical surface of the cerebrum.
Second, examine the right and left cerebral hemispheres which are separated from each other by the longitudinal fissure. (In the sheep brain, the longitudinal fissure is nowhere near as deep as it is in the human brain.)
Third, examine each hemisphere of the cerebrum and identify the four superficial lobes: frontal, parietal,
occipital, and temporal. (A fifth lobe, the insula lies deep within the lateral sulcus and is not visible externally.)
(Additionally, in the sheep brain, the lateral sulcus, separating the flap-like temporal lobe from the overlying parietal
and frontal lobes is nowhere near as deep nor distinctive a landmark as it is in a human brain.)
Fourth, note that the cerebellum is separated from the cerebral hemispheres by the transverse fissure.
Fifth, examine the surface of each gyrus for the delicate arachnoid mater which is the second (aka middle) of the three meninges. It is most easily distinguished from the pia mater in the region overlying the
gyri, since the pia mater dips into the grooves and the arachnoid mater does not.
2. Using a blunt probe, gently remove a patch of the arachnoid mater (it will tear if you pull too vigorously; but
it will remain intact if you carefully elevate it with small strokes). You may now cut this elevated patch away.
3. Finally, by having removed a portion of the arachnoid mater, you will see a shiny inner covering: the
pia mater, directly touching the surface of the brain. Note that the pia mater follows the convolutions of
gyri and sulci, too; as did the arachnoid mater.
4. View the dorsal surface of the cerebellum. Compare the size of the folia with the gyri of the cerebrum.
Unlike the human brain, the sheep cerebellum is NOT divided medially into two lateral hemispheres.
The cerebellum is connected with the brainstem by three prominent fiber tracts or peduncles. Lift up the
lateral edge of the cerebellum. The middle cerebellar peduncle can be seen connecting the cerebellum
with the pons. Slightly posterior to this is the inferior cerebellar peduncle connecting the cerebellum to
the medulla. Finally, locate the superior cerebellar peduncle, which is composed of the fibers connecting
the midbrain to the cerebellum.
5. To examine the dorsal anatomy of the midbrain portion of the brainstem, hold the cerebrum in one hand
and the cerebellum in the other hand. Now gently flex your two hands away from each other, creating
more “space” between the two structures. The dorsal aspect of the midbrain will now be visible between
the cerebrum and cerebellum. You will visualize the pineal gland in the midline as well as four elevated
masses, which collectively are called the corpora quadrigemina. Distinguish between the superior colliculi and the inferior colliculi. Which are larger? What is the function of these masses of nervous tissue?
6. Turn the brain to view the ventral surface of the brain. A pair of olfactory bulbs can be seen beneath the
cerebral hemispheres. These bulbs lie over the cribriform plate of the ethmoid bone and receive the olfactory neurons from the nose. A white band, the olfactory tract, extends from each bulb along the ventral
surface of each of the cerebral hemispheres.
7. Posterior to the midbrain is the pons. This is composed primarily of white fibers, many of which run
transversely across the pons out to the cerebellum.
8. The medulla oblongata is posterior to the pons. The longitudinal bands of tissue at each side of the ventral median fissure on the ventral surface of the medulla are known as the pyramids.
9. Note how the spinal cord is a direct continuation of the medulla oblongata. Identify the pons and the
cerebral peduncles of the midbrain. (The cerebral peduncles resemble vertical pillars that seem to hold up the
cerebrum; hence their name meaning “little feet of the cerebrum”.) Locate the single mammillary body on the
hypothalamus. The mammillary body of the human brain is a paired mass.
10. If your specimen still has its pituitary gland intact, this will also be a prominent structure (although not part
of the brain) visible on the ventral surface of the brain. If the pituitary gland was inadvertently removed along with the dura mater, you should still be able to identify a “stub” of the infundibulum that attaches the
pituitary to the hypothalamus.
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11. As for the Cranial Nerves, we will be identifying these using Human Brain models, charts, and diagrams.
Just remember: Cranial Nerves I (Olfactory) and II (Optic) DO NOT ARISE from the brain; rather they
project to it from the olfactory mucosa bilaterally and retinas respectively; Cranial Nerves III (Oculomotor) and IV (Trochlear) arise from the Midbrain; Cranial Nerves V (Trigeminal), VI (Abducens), VII
(Facial) and VIII1 (Vestibular Division of Vestibulocochlear – for Equilibrium) arise from the Pons; and Cranial Nerves VIII2 (Cochlear Division of Vestibulocochlear – for Hearing), IX (Glossopharyngeal), X (Vagus),
XI (Spinal Accessory), and XII (Hypoglossal) arise from the Medulla Oblongata.
C. Identification of Internal Brain Anatomy
Before sectioning the brain, spread the cerebral hemispheres apart and observe deep in the longitudinal
fissure the thick transverse band of white fibers, the corpus callosum, that connects the right hemisphere to the
left hemisphere. You are viewing the superior aspect of the corpus callosum in this manner; and it can be seen
to be a structure that is continuous across the midline. Now, to view the remaining parts of the brain, a sagittal
section of the sheep brain will be used. Try to accomplish your cut just “a hair” off to either the right or left of
exact mid-sagittal plane – this will allow structures that lie exactly IN the midline to remain intact! Please observe the demonstration your Instructor gives, using the specialized brain knife, and lining up your cut.
IN SAGITTAL SECTION:
1. As stated above, the cerebral hemispheres are connected by a tract of white matter called the corpus callosum. It is easily identified as the curved white structure at the base of the cerebrum. The inferior portion of the corpus callosum is the fornix, a white tract connecting deep structures of the limbic system,
the “emotional” brain. The fornix narrows anteriorly and meets the anterior commissure, another tract of
white matter interconnecting the cerebral hemispheres.
2. A thin, vertical septum of tissue, the septum pellucidum, lies ventral to the corpus callosum. The right
and left lateral ventricles lie laterally adjacent to this septum. Of the two hemisections of your specimen,
one will have a lateral ventricle that you can see “into”; the other hemisection will have the septum pellucidum intact which will prevent viewing into the other lateral ventricle (unless you break through the
septum).
3. The diencephalon consists of the thalamus and hypothalamus, both visible on each of the hemisections.
Inferior to the fornix is the most distinctive feature of the diencephalon, the round massa intermedia.
(The thalamus is shaped like a bar-bell with two oval masses on each end connected by the massa intermedia in the middle.) This structure appears as a dull circular area not covered by epithelium. The slight
depression around the massa intermedia is the space of the third ventricle which passes through the diencephalon. Below the thalamus and massa intermedia is the hypothalamus. Just posterior to the hypothalamus is the mammillary body, the small elevation anterior to the narrowing of the midbrain. Inferior to
the posterior margin of the corpus callosum is the conical pineal gland.
4. The midbrain is clearly visible in the sagittal sections. Posterior to the pineal gland is the corpora quadrigemina which includes the superior colliculi and the inferior colliculi. A long canal called the cerebral
aqueduct passes anterior to these masses. Cerebral peduncles are located within the walls of the cerebral aqueduct. Their fibers extend to connect the pons with the cerebrum.
5. The cerebellum is positioned posterior to the pons and medulla. In a sagittal section, the white matter of
the cerebellum is apparent. Because this tissue is highly branched, it is called the arbor vitae, aka “tree
of life”. In the middle of the arbor vitae, are the cerebellar nuclei which function in the involuntary regulation of skeletal muscle contration. The cerebellum is divided into the anterior lobe, the posterior
lobe, and the flocculonodular lobe which faces the fourth ventricle. The gray matter of the cerebellum is
quite apparent on the outside.
6. The fourth ventricle lies above the medulla oblongata, below the cerebellum.
7. A quick word about the Ventricular System of the Brain: Deep in the brain are four chambers called
ventricles. The floor or one wall of each ventricle is lined with a specialized capillary bed called the
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choroid plexus. It is this choroid plexus structure that produces cerebrospinal fluid (CSF), as a filtrate
of the blood, which flows through the ventricles and enters the subarachoid space surrounding the brain
and spinal cord. CSF protects and nourishes the tissues of the brain. Using a dissection needle, carefully tease a bit of this choroid plexus vascular structure off the floor of one of the lateral ventricles and
pull it into view.
(A pair of lateral ventricles extend deep into the cerebrum as horseshoe-shaped chambers. At the midline of
the brain, the lateral chambers are separated from one another by a thin membrane, the septum pellucidum.
A brain sectioned just parasagittal to the exact midline exposes this membrane. Cerebrospinal fluid circulates from the lateral ventricles through the interventricular foramen and enters the third ventricle. This
small chamber surrounds the intermediate mass of the thalamus. CSF in the third ventricle passes through
the cerebral aqueduct of the midbrain and enters the fourth ventricle between the brainstem and the cerebellum. In the fourth ventricle, two lateral apertures and a single median aperture direct CSF laterally to the
exterior of the brain and into the subarachnoid space. CSF then circulates around the brain and the spinal
cord and then is reabsorbed via an arachnoid villus which projects into the largest vessels in the head: the
veins of the dural sinuses – specifically the superior sagittal sinus and the transverse sinuses.)
IN FRONTAL (aka CORONAL) SECTION:
When the brain is sectioned along a frontal plane (specifically near the level of the infundibulum), deep
structures of the cerebrum and diencephalon are visible. Notice how the corpus callosum transverses the brain
to connect the two cerebral hemispheres. Cerebral nuclei and the internal capsule form the lateral walls of the
lateral ventricles. These structures are involved in automating voluntary muscle contractions. Notice too, the
organization of gray matter and white matter in the brain. The covering of the cerebrum, the cerebral cortex, is
gray matter. The corpus callosum and other bridging structures of the brain consist of white matter.
It is also much easier to appreciate the extent of the two lateral ventricles as well as visualize the midline lying third ventricle from this plane of section.