DISSECTION 4
The Brain and Floor of the Cranial Cavity
References: M1 878-888, 1054-1060; N 82, 97-98, 131-133; N 86, 103-104, 114-115, 138-140;
R 66-75, 87, 90-97
AT THE END OF THIS LABORATORY PERIOD YOU WILL BE RESPONSIBLE FOR THE
IDENTIFICATION AND DEMONSTRATION OF THE STRUCTURES LISTED BELOW:
1. Nerves: the 12 cranial nerves [olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal
(V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), spinal
accessory (XI), hypoglossal (XII)] by both name and number at their origin from the brain and
where they penetrate the dura mater or otherwise exit the cranial cavity, the trigeminal ganglion, the
motor root of the trigeminal nerve, and the ophthalmic, maxillary and mandibular divisions of the
trigeminal nerve.
2. Vessels: Identify or demonstrate the internal carotid, ophthalmic, vertebral, anterior spinal,
posterior inferior cerebellar, anterior inferior cerebellar, basilar, pontine, superior cerebellar,
posterior cerebral, posterior communicating, anterior cerebral, middle cerebral, anterior
communicating, anterolateral central arteries (lenticulostriate), the circulus arteriosus cerebri.
superior sagittal sinus, the transverse sinus, straight sinus, the confluence of the sinuses, and the
sigmoid, inferior petrosal, superior petrosal, and cavernous sinuses.
3. Meninges: the falx cerebri, tentorium cerebelli, tentorial notch, and the diaphragma sellae (with
associated infundibulum and hypophysis), denticulate ligaments.
4. Bones and bony features: cribriform plate of the ethmoid, posterior clinoid process.
5. Structures of the Central Nervous System: medulla oblongata (olive, pyramid), pons,
mesencephalon (inferior and superior colliculi), diencephalon, cerebellum, cerebral hemisphere,
olfactory tracts, olfactory bulbs, spinal cord.
YOU SHOULD ALSO BE ABLE TO DO THE FOLLOWING THINGS:
1. Demonstrate your knowledge of the cerebral arterial circle and its connections by tracing the route
of blood flow from the heart to various parts of the brain assuming that there are occlusions in one
or more major vessels.
2. Demonstrate understanding of the pupillary changes that occur with increasing intracranial pressure.
3. List the types and origins of the nerve fibers in the oculomotor, trochlear, and abducens nerves, and
name the structures which they supply.
4. Label a diagram of a frontal section through the cavernous sinus showing all structures within and in
direct relation to the sinus. List and explain the symptoms and signs associated with cavernous
sinus thrombosis or carotid-cavernous fistula.
5. Identify structures on cross-sectional drawings, CT scans or tissue sections through the head.
Dissection 4, Removal of Brain and Floor of Cranial Cavity
Removal of the Brain
Review the posterior cranial nerves and their
attachments to the floor of the cranial cavity. Be
sure you can identify the TROCHLEAR,
TRIGEMINAL,
VESTIBULOCOCHLEAR,
GLOSSOPHARYNGEAL,
VAGUS,
SPINAL
ACCESSORY,
and HYPOGLOSSAL NERVES.
Remember that the FACIAL NERVE is usually
hidden from view by the vestibulocochlear. Then
cut both trochlear nerves as far laterally as
possible. The fifth, seventh, eighth, ninth, tenth,
and twelfth cranial nerves should be cut on both
sides about half way between their attachment to
the brainstem and the point where they penetrate
the dura. DO NOT cut the spinal root of the
spinal accessory nerve as it does not attach to the
brain stem. The VERTEBRAL ARTERIES should
both be cut, and the spinal cord should be severed
(A775; G7.20; N108, 109; N114, 115).
The cadaver should then be placed in the
supine position with the head propped up. The
falx cerebri should be separated from the cerebral
hemispheres and cut from its attachment to the
crista galli. The falx may now be lifted from
between the cerebral hemispheres. The brain
should be supported with a hand until it is
removed. Gently retract the frontal lobes and
identify the OLFACTORY TRACTS and BULBS,
which should be dislodged from the CRIBRIFORM
PLATES. What is the difference between the
olfactory tracts and the olfactory nerves? Next
identify the INFUNDIBULUM (the stalk of the
pituitary gland) and cut it. Then identify and cut
the OPTIC NERVES and then the INTERNAL
CAROTID ARTERIES.
The OCULOMOTOR
NERVES should then be identified and cut. The
brain should be lifted back to identify the
ABDUCENS NERVES and to cut them. Finally
identify again the TENTORIUM CEREBELLI and
the opening in its median aspect through which
the brain passes, the TENTORIAL NOTCH. Make a
lcm cut in the margin of the tentorial notch on
each side at the attachment to the petrous ridge.
The brain should now be free of all attachments to
the cranium except for a few minor veins, which
may be torn free as the brain is removed. Pull the
brain gently backward and deliver the brainstem
and remaining cerebellar hemisphere through the
Page 2
tentorial notch. DO NOT REMOVE the falx and
tentorium cerebelli, but leave the tentorium
intact except for the l cm cuts just made.
Structures Piercing the Dura
On the skull be able to locate the openings
through which each cranial nerve leaves the
cranial cavity. Then, examine the floor of the
cranial cavity in your cadaver and identify the cut
ends of each of the cranial nerves. Note where
they enter the dura (A777, A684; G4.39B, 7.4,
7.23; N9, 10, 98; N9, 11, 104). Identify the
INTERNAL
CAROTID
ARTERIES,
the
OPHTHALMIC ARTERIES, the INFUNDIBULUM OF
THE HYPOPHYSIS, the HYPOPHYSIS, and the
DIAPHRAGMA SELLAE (A773, A777; N98;
N104). In the foramen magnum, identify the cut
surface of the SPINAL CORD, the two VERTEBRAL
ARTERIES, the SPINAL ACCESSORY NERVES, and
the highest dentation of the DENTICULATE
LIGAMENTS (G4.45).
The Dural Venous Sinuses
Attempt to put the FALX CEREBRI and the
TENTORIUM CEREBELLI back into their proper
position and follow the course of blood flow
through the SUPERIOR SAGITTAL SINUS, the
CONFLUENCE OF THE SINUSES, and the
TRANSVERSE SINUSES. Place a probe into the
STRAIGHT SINUS.
Open the dura over the SIGMOID SINUS all the
way to the jugular foramen (A777; G7.16A,
G7.18; N95-97; N100-103). Then do the same
with the INFERIOR PETROSAL SINUS. Open the
SUPERIOR PETROSAL SINUS also.
Remove
enough of the dura covering these sinuses so that
they stay open.
Note the relationship of the oculomotor nerve
to the POSTERIOR CLINOID PROCESS and the free
edge of the tentorium cerebelli (A773, A777;
G7.23, G7.21; N98; N104). Is there a sharp edge
here in your cadaver against which the
oculomotor nerve could be compressed by the
brain? What would you notice about a patient
whose oculomotor nerve was compressed here?
Dissection 4, Removal of Brain and Floor of Cranial Cavity
The Trigeminal Nerve and Ganglion
Now identify the TRIGEMINAL NERVE and
remove the dura covering the TRIGEMINAL
GANGLION and the OPHTHALMIC, MAXILLARY,
and MANDIBULAR DIVISIONS of the trigeminal
nerve (A777; G7.22A; N82; N86). Follow each
division of the trigeminal nerve to the point where
it leaves the cranial cavity taking care to only
dissect the lateral aspect of the ophthalmic
division which lies in the lateral wall of the
cavernous sinus. Pull the trigeminal ganglion
laterally and identify the MOTOR ROOT OF THE
TRIGEMINAL NERVE (A773) as it joins the
mandibular division distal to the ganglion.
The Cavernous Sinus
Identify the TROCHLEAR NERVE. Incise the
dura above it as you follow it into the
CAVERNOUS SINUS (A773; G7.22A; N 82, 98;
N86, 104). Identify the OCULOMOTOR NERVE
within the sinus (A771, A773; G7.23; N82; N86)
and note the fibrous trabeculae which cross the
sinus. Follow the ABDUCENS NERVE into the
cavernous sinus and observe its relationship to the
INTERNAL CAROTID ARTERY. Look for the layer
of endothelium which covers the internal carotid
artery and the abducens nerve.
The Blood Supply and
Anatomy of the Brain
Now turn to the brain that you removed
earlier in the dissection and identify the origins of
the cranial nerves from the surface of the brain
stem (A773; G7.18; N130-136; N136-143).
Identify the three regions of the brainstem, the
MEDULLA
OBLONGATA,
PONS,
and the
MESENCEPHALON. Locate the OLIVE, PYRAMID,
and the INFERIOR and SUPERIOR COLLICULI.
Next identify the DIENCEPHALON, CEREBELLUM,
and the CEREBRAL HEMISPHERES (N106, 114115). If the vertebral arteries have been cut low
enough, identify the ANTERIOR SPINAL and the
POSTERIOR INFERIOR CEREBELLAR ARTERIES
(A775; N 136; N143). The ANTERIOR INFERIOR
CEREBELLAR ARTERIES are usually branches of
Page 3
the BASILAR ARTERY. Note the small PONTINE
BRANCHES of the basilar artery entering the
substance of the pons.
The SUPERIOR
CEREBELLAR ARTERIES arise from the basilar
near the anterior margin of the pons and pass
laterally to reach the superior surface of the
cerebellum. The basilar artery then bifurcates to
form the two POSTERIOR CEREBRAL ARTERIES.
What is the relationship of the oculomotor nerve
to the superior cerebellar and posterior cerebral
arteries? In addition to supplying the occipital
lobe and part of the lower surface of the temporal
lobe, the posterior cerebral arteries also give rise
to perforating branches (posteromedial and
posterolateral central branches) which supply
posterior parts of the thalamus. The posterior
cerebral arteries also receive the POSTERIOR
COMMUNICATING BRANCHES of the internal
carotid arteries. The internal carotid ends by
dividing into the ANTERIOR and MIDDLE
CEREBRAL ARTERIES.
The anterior cerebral
arteries are joined across the midline by the
ANTERIOR COMMUNICATING ARTERY. What is
the CIRCULUS ARTERIOSUS CEREBRI? (A775
G7.26, p624; N 132, 133; N139, 140)
The middle cerebral artery supplies most of
the lateral surface of the cerebral hemisphere, but
it also has important perforating branches, the
ANTEROLATERAL
CENTRAL
BRANCHES
(sometimes
called
"striate
arteries"
or
"lenticulostriate arteries") which supply the basal
ganglia and the internal capsule (N134; N141).
These vessels are particularly prone to rupture in
hypertensive disease states and have been called
the "arteries of cerebral hemorrhage." Sometimes
an anterior choroidal artery can be identified
arising from the internal carotid artery near its
termination (N132; N139).
What is the
importance of this artery?
Before leaving the laboratory be sure to
place the brain in the storage tank provided
with your table number on it. Then put the
container in the cabinet beneath one of the
sinks.
Dissection 4, Removal of Brain and Floor of Cranial Cavity
Page 4
STUDY QUESTIONS
1.
What is the first intracranial branch
of the internal carotid artery?
1.
The ophthalmic artery.
2.
What are the terminal branches of
the internal carotid?
2.
The anterior and middle cerebral arteries.
3.
Where does the internal jugular
vein begin?
3.
The internal jugular vein is directly continuous
with the sigmoid sinus in the posterior
compartment of the jugular foramen.
4.
Trace the route of blood flow from
the heart to the posterior inferior
surface of the left half of the cerebellum,
assuming that both vertebral arteries are
occluded in the neck.
4.
Heart aorta brachiocephalic artery right
common carotid
internal carotid
posterior
communicating artery posterior cerebral
artery basilar artery left vertebral artery
left posterior inferior cerebellar artery
cerebellum.
Why might you find the right pupil
dilated in a patient who has an
expanding extradural hematoma
caused by rupture of the right middle
meningeal artery?
5.
The oculomotor nerve is closely related to the
posterior clinoid process and the free edge of the
tentorium cerebelli. As the brain is deformed and
shifted by the hematoma the inferior medial
portion of the temporal lobe (the uncus) is
compressed against the free edge of the tentorium
cerebelli and pressure is placed upon the
oculomotor nerve, which is caught between these
structures. The preganglionic fibers in the
oculomotor nerve end in the ciliary ganglion
whose postganglionic fibers innervate the
sphincter of the pupil. When the pressure on the
oculomotor nerve becomes great enough, the
preganglionic fibers cease to function and the
pupil becomes dilated and does not react to light.
6.
Where are the cell bodies of the
preganglionic fibers in the
oculomotor nerve?
6.
Nucleus of Edinger-Westphal.
7.
What other types of fibers are found
in the oculomotor nerve and where
are their cell bodies located?
7.
Efferent fibers to skeletal muscle. Their cell
bodies are in the oculomotor nucleus.
8.
What muscles are supplied by
these fibers?
8.
All the extraocular muscles except the superior
oblique and the lateral rectus.
9.
What nerves supply the superior
oblique and the lateral rectus?
9.
The trochlear nerve supplies the superior oblique
and the abducens nerve supplies the lateral rectus.
5.
Dissection 4, Removal of Brain and Floor of Cranial Cavity
10. What is the name of the cleft in the
dura in which the trigeminal ganglion
is located?
Page 5
10. The trigeminal cave (also called Meckel's cave).
11. Name the nerves which pass through the cavernous sinus? (A770, A773; G7.22C, 7.39A)
12. From your knowledge of the anatomy and relations of the cavernous sinus, list and discuss the signs of
cavernous sinus thrombosis.
13. What is pulsating exophthalmos? With what serious pathological condition is this associated?
14. Be able to trace blood from the heart to any general area of the brain.
LJ:bh
revised
06/18/09