DISSECTION 3
Exposure of the Cranial Cavity and Brain
References: M1 822-844, 865-878, 1054-1060, N 4-7, 94-98, 103, 137; N 4-7, 99-104, 109, 144;
R 24-27, 34-36, 85-91
AT THE END OF THIS LABORATORY PERIOD YOU WILL BE RESPONSIBLE FOR THE
IDENTIFICATION AND DEMONSTRATION OF THE STRUCTURES LISTED BELOW:
l.
Bones and bony features: frontal, parietal, occipital, temporal, sphenoid, ethmoid; glabella,
supraorbital notch, frontal crest, grooves for the middle meningeal artery, groove for the superior
sagittal sinus, pits for arachnoid granulations (granular fovea), parietal foramen, foramen magnum,
external occipital protuberance, internal occipital protuberance, groove of the transverse sinus,
internal occipital crest, occipital condyles, hypoglossal canal, jugular foramen, groove for the
inferior petrosal sinus, clivus, internal acoustic meatus, groove for the sigmoid sinus, carotid canal,
dorsum sellae, posterior clinoid processes, hypophyseal fossa, tuberculum sellae, anterior clinoid
processes, foramen ovale, foramen spinosum, foramen rotundum, superior orbital fissure, optic
canal, greater and lesser wings of the sphenoid, cribriform plate of the ethmoid, crista galli, foramen
cecum, pericranium, outer table, diploë, inner table. (NOTE: some texts use the terms sulcus instead
of groove, fovea instead of pits, lamina instead of table.) coronal suture, sagittal suture, lambdoid
suture.
2. Layers of the scalp: Skin, tela subcutanea (Connective tissue), galea Aponeurotica, Loose
connective tissue, and the Pericranium.
3. Nerves: trochlear, trigeminal, facial, vestibulocochlear, glossopharyngeal, vagus, spinal accessory,
and hypoglossal. (NOTE: you should be able to identify each of these nerves at their origin from
the brain and where they penetrate the dura or otherwise leave the cranial cavity.)
4. Meninges: dura mater, falx cerebri, tentorium cerebelli, arachnoid, arachnoid granulations,
arachnoid trabeculae, subdural space, subarachnoid space, cisterna magna (cisterna cerebellomedullaris), and pia mater.
5. Vessels: middle meningeal and vertebral arteries, the superior sagittal, straight, and transverse
sinuses, the confluence of the sinuses, one or more emissary veins, and one or more cerebral veins.
YOU SHOULD ALSO BE ABLE TO DO THE FOLLOWING THINGS:
1.
Give a definition of emissary veins.
2.
State the differences between the cranial dura and the spinal dura and give the innervation of both
types of the dura.
3.
State the location of nerves and vessels of the scalp, and explain why scalp wounds bleed so freely.
4.
State the locations where the anterior and posterior fontanels would be present in an infant. State
the normal time of closure of these fontanels.
5.
State the vascular route by which blood flows from the heart to the brain and back to the heart and
state which vessels produce hemorrhage in the various intracranial spaces.
6.
Draw and label a diagram of an arachnoid granulation and trace the route taken by cerebrospinal
fluid from its point of formation to the arachnoid granulation.
Dissection 3, Exposure of Cranial Cavity and Brain
The Scalp
Before beginning the dissection, identify on a
skull the bones and bony features listed on page
one. Then make a mid-sagittal incision across the
top of the skull from the GLABELLA (G7.1, 7.2;
N2, N2) to the EXTERNAL OCCIPITAL
PROTUBERANCE, cutting all the way to the bone.
Reflect a small portion of the SKIN OF THE SCALP
away from the TELA SUBCUTANEA, and then,
using sharp dissection, separate the tela from the
underlying
GALEA
APONEUROTICA
(the
aponeurosis of the epicranius muscle). Look for
nerves and vessels in the tela subcutanea, and
note the ease with which the galea aponeurotica
can be separated from the skull. Identify the layer
of LOOSE CONNECTIVE TISSUE between the galea
and the PERICRANIUM (the periosteum of the
outer surface of the skull).
Removal of the Calvaria
Next strip the scalp away from the skull down
to the level of the line shown in the illustration.
This is the line at which the skull has its
maximum circumference, the so-called line of
maximum cranial length.
Page 2
skull cap has been completely separated from the
rest of the skull, a scalpel handle or forceps may
be used to loosen the dura attached to its inner
surface. If unusual difficulty is experienced, be
sure to ask for help from one of the instructors.
Place the body in the prone position and
reflect the remnants of the suboccipital muscles
from the OCCIPITAL BONE and use the pencil to
mark lines from the lateral margins of the
FORAMEN MAGNUM to the cut margin of the
skull. These lines should intersect the line of
maximum cranial length about one inch lateral to
each lambdoid suture. Cut through the outer table
along these lines with the saw, and then use the
mallet and chisel to break through the inner table.
Take care not to damage the dura as this is done.
The wedge of bone may then be removed by
prying it loose from the dura.
Examine the surface of the dura near the
midline
and
look
for
ARACHNOID
GRANULATIONS. On the inner surface of the
calvaria, identify the GRANULAR FOVEA, small
pits produced by the arachnoid granulations
(G7.14; N95; N100).
Identify examples of
EMISSARY VEINS which pass through the parietal
foramina identified in the calvaria.
As usual, gloves should be worn during
dissection; this is imperative whenever the brain
is handled.
The Meninges and Meningeal Spaces
The Dural Venous Sinuses
Identify the CORONAL, SAGITTAL, and
LAMBDOID SUTURES and look for the PARIETAL
FORAMINA (A756, G7.3; N4-7; N4-7). Place a
rubber band around the skull along the line of
maximum cranial length. Use it as a guide for
marking the skull with a pencil. Then saw
through the outer table ONLY of the skull along
this line. Use a chisel and mallet to gently break
through the inner table. Use great care not to
damage the underlying dura or brain. When the
Incise the SUPERIOR SAGITTAL SINUS and the
TRANSVERSE SINUSES and then remove the dura
covering them. The junction of the superior
sagittal sinus and the transverse sinuses is called
the CONFLUENCE OF THE SINUSES. Identify the
STRAIGHT SINUS. (A768; G7.27; N97, 98; N103,
104) Into which transverse sinus does it empty?
Does the superior sagittal sinus drain
predominantly to one side?
Taking care not to
remove the
the transverse sinuses.
vertebral canal, the dura
ARACHNOID,
injure the underlying
dura that is inferior to
Note that, as in the
and arachnoid are not
Dissection 3, Exposure of Cranial Cavity and Brain
fused but the intervening SUBDURAL SPACE is
very narrow, filled in life with only a molecular
layer of fluid. Note the thickness of the arachnoid
in this region and compare it with the arachnoid
in the vertebral canal. Open the arachnoid and
note the ARACHNOID TRABECULAE which pass
through the SUBARACHNOID SPACE between the
arachnoid and the PIA MATER. The large space
that you have opened inferior to the cerebellum is
called the CISTERNA CEREBELLO–MEDULLARIS
or CISTERNA MAGNA, and it is the largest of the
intracranial cerebrospinal fluid cisterns. The
arachnoid covering the cisterna magna may now
be removed. (See G7.66; N103; N109)
Identify the MIDDLE MENINGEAL ARTERY
(G7.16; N95; N100) in the dura and look in the
calvaria to observe the GROOVES in the skull
associated with it. Then open the DURA ONLY
by incisions parallel to the superior sagittal sinus
and the transverse sinuses.
Identify the
CEREBRAL VEINS deep to the surface of the
arachnoid. Observe other cerebral vessels in the
subarachnoid space. Through what meningeal
spaces do the cerebral veins pass? Where are
they most likely to be torn and into what space
are they most likely to bleed?
Cut through all the veins entering the superior
sagittal and transverse sinuses. Then pull the
cerebral hemispheres away from the midline and
identify the FALX CEREBRI and the TENTORIUM
CEREBELLI. (See A767, 777; G7.15A; N97, 98;
N103, 104).
Page 3
Identification of the Cranial Nerves
Cut through the cerebellum in the midsagittal
plane and then make a parasagittal cut about 5
mm to the right. Remove the narrow slice of
cerebellum and push the two cerebellar
hemispheres gently apart to see the brainstem and
the floor of the fourth ventricle. Make a second
parasagittal cut about one inch from the lateral
margin of the cerebellum and remove this lateral
segment of the right hemisphere. Then carefully
slice away small portions of the remaining tissue
until the TROCHLEAR NERVE can be identified
(G7.20; N108, 109; N114, 115).
CAREFULLY
retract
the
cerebellar
hemispheres away from the floor of the posterior
cranial fossa and identify the TRIGEMINAL,
VESTIBULOCOCHLEAR, GLOSSOPHARYNGEAL,
VAGUS, SPINAL ACCESSORY, and HYPOGLOSSAL
NERVES. The FACIAL NERVE is usually hidden
from view by the vestibulocochlear. DO NOT
break these nerves by forcible retraction of the
brain. DO NOT cut these nerves, but spend some
time studying their attachments to the brainstem,
their relationships to the vessels in the area, and
the location where they disappear into the dura.
Also note the VERTEBRAL ARTERIES as they
enter the cranial cavity by passing through the
foramen magnum.
When you are finished, replace the skullcap
and scalp over the cranial cavity to keep the
tissues moist.
STUDY QUESTIONS ARE ON THE NEXT PAGE
Dissection 3, Exposure of Cranial Cavity and Brain
Page 4
Study Questions
1.
What are the layers of the scalp?
1. Skin, tela subcutanea, galea aponeurotica
(epicranius), a layer of loose connective tissue,
and the pericranium.
2.
Which of these layers make up the
movable portion of the scalp?
2.
3.
In which layer are the nerves and
vessels of the scalp found?
3.
The nerves and vessels are in the tela
subcutanea.
4.
Into which layer then should a local
anesthetic be injected?
4.
Into the tela subcutanea, where the nerves are.
5.
Why are scalp wounds usually
very bloody?
5.
The arteries of the scalp are numerous and they
anastomose freely with each other on the same
side and across the midline. In addition, they
are tightly embedded in the dense connective
tissue of the tela subcutanea, so that they are
unable to retract, but remain open when they are
cut.
6.
What is the "danger space" of the scalp?
6.
The layer of loose connective tissue between
the epicranius and the pericranium allows easy
movement of the outer layers of the scalp.
However, it also readily permits the spread of
infections or fluids. Since it is traversed by
emissary veins which connect the dural venous
sinuses with veins in the scalp, abscesses in this
layer may result in intracranial infection.
Therefore, this layer is called the danger space
or danger zone of the scalp.
7.
What is the calvaria?
7.
The calvaria is the skull cap or roof of the skull.
It is formed by the frontal bone, the parietal
bones, and part of the occipital bone.
Name the bones which form the calvaria.
8.
Name the layers which are in direct
contact with the bone of the calvaria.
The outer three layers -- the skin, the tela
subcutanea, and the galea aponeurotica -- are
tightly bound together and move as a unit. Some
authors (e.g. Hollingshead and Grant) limit their
definition of the term scalp to these three layers.
8.
The calvaria is covered with periosteum on its
inner and outer surfaces. The outer periosteum
is called the pericranium, while the inner
periosteum forms the outer layer of the dura.
Dissection 3, Exposure of Cranial Cavity and Brain
9.
The bones of the calvaria have what layers?
9.
Page 5
The bones of the calvaria have three layers:
The outer table (external lamina), the
cancellous diploë, and the inner table (internal
lamina).
10. Of these layers, which is the thinnest and
most liable to fracture?
10. The inner table is thinner than the outer, and it
may be fractured by blows that do not break the
outer table.
11. What are the fontanels?
11. The fontanels are membranous spaces between
the incompletely developed cranial bones of the
fetus and the newborn. These spaces are
bridged by the pericranium externally and the
dura mater internally. Since most of the bones
of the skull are formed either partially or
entirely as membrane bones and since
ossification is not complete at the time of birth,
the bones forming the sides and roof of the skull
are still united by membrane.
12. What are the names of the two most easily
felt fontanels?
12. The anterior fontanel and the posterior fontanel.
13. Where is the anterior fontanel and when
does it close?
13. The anterior fontanel lies at the intersection of
the sagittal and coronal sutures, that is, at the
common corner of the frontal and parietal
bones. It closes during the second year.
14. Where is the posterior fontanel?
14. The posterior fontanel is located at the
intersection of the sagittal and lambdoid
sutures.
When does it close?
It closes approximately 2 months after birth.
15. What are the major subdivisions of
the floor of the cranial cavity?
15. The floor of the cranial cavity is divided into
three large, depressed areas, the anterior,
middle, and posterior cranial fossae.
16. What is the boundary between the
anterior and middle cranial fossae?
16. The anterior and middle cranial fossae are
separated by the sphenoid ridges made up for
the most part by the posterior margin of the
lesser wing of the sphenoid bone. In the
midline, the anterior border of chiasmatic
sulcus separates the anterior fossa from the
middle fossa.
17. What is the boundary between the
middle and posterior cranial fossae?
17. The posterior cranial fossa is separated from the
middle cranial fossa by the petrous ridges of
the temporal bones and by the dorsum sellae in
the midline.
Dissection 3, Exposure of Cranial Cavity and Brain
18. Name the cranial fossa in which each of the
following openings is found and list the
structures passing through each opening.
Page 6
18.
a.
foramen cecum
a.
anterior cranial fossa – frontal emissary
vein
b.
openings in the cribriform plates of the
ethmoid bone
b.
anterior cranial fossa – filaments of the
olfactory nerve
c.
optic canal
c.
middle cranial fossa – optic nerve and
ophthalmic artery
d.
superior orbital fissure
d.
middle cranial fossa –ophthalmic vein,
ophthalmic division of the trigeminal
nerve,
the oculomotor nerve, the
trochlear nerve, the abducens nerve
e.
foramen rotundum
e.
middle cranial fossa – the maxillary
division of the trigeminal nerve
f.
foramen ovale
f.
middle cranial fossa – the mandibular
division of the trigeminal nerve
g.
foramen spinosum
g.
middle cranial fossa – the middle
meningeal artery
h.
foramen lacerum
h.
middle cranial fossa – nothing of
consequence. (It is filled with cartilage in
life which forms the floor of the anterior
end of the carotid canal.) The greater
petrosal nerve passes through the upper
part of the foramen lacerum but enters the
pterygoid canal before completely
traversing the foramen lacerum.
i.
internal acoustic meatus
i.
posterior cranial fossa – facial nerve,
vestibulocochlear nerve, labyrinthine
artery
j.
jugular foramen
j.
posterior cranial fossa –glossopharyngeal
nerve, vagus nerve, accessory nerve,
inferior petrosal sinus, and sigmoid sinus
k.
hypoglossal canal
k.
posterior cranial fossa – hypoglossal
nerve
l.
foramen magnum
l.
posterior cranial fossa – brainstem,
vertebral arteries, spinal roots of the
accessory nerves, anastomotic
connections between the basilar plexus
and the internal vertebral venous plexus
Dissection 3, Exposure of Cranial Cavity and Brain
Page 7
19. What are the meninges and what
is their function?
19. The meninges are the three membranes, the
dura mater, the arachnoid, and the pia mater,
which envelop the central nervous system and
separate it from the walls of the bony cavities
in which it lies. They function to protect,
support, and nourish the central nervous
system.
20. How does the cranial dura mater
differ from the spinal dura?
20. The cranial dura, instead of being separated
from the bone and the periosteum by an
extradural (epidural) space, is fused to the
periosteum of the inner surface of the skull, so
that this periosteum becomes an outer layer of
the dura. In addition, the cranial dura contains
endothelial lined venous sinuses which are
responsible for the return of almost all the
blood that enters the cranial cavity. Also, the
inner layer of the cranial dura has several
duplications or folds which partition the cranial
cavity.
21. What are the names of these dural folds?
21. The dural folds are:
1) the falx cerebri,
2) the tentorium cerebelli,
3) the falx cerebelli, and
4) the diaphragma sellae.
22. To what bones is the tentorium cerebelli
attached?
22. The outer border of the tentorium is attached
on each side to the posterior clinoid process of
the sphenoid bone, to the petrous ridge of the
temporal bone, and to the transverse ridges of
the occipital bone. The inner border of the
tentorium attaches to the anterior clinoid
process of the sphenoid bone.
23. Name and locate the dural venous
sinuses that are closely associated
with the tentorium cerebelli.
23. The superior petrosal sinus lies in the outer
border of the tentorium where it is attached to
the petrous ridge of the temporal bone. The
transverse sinus is in the outer border of the
tentorium where it attaches to the groove for
the transverse sinus on the occipital bone. The
straight sinus is along the line of intersection
of the tentorium and the falx cerebri.
24. What is the tentorial notch?
24. The tentorial notch or tentorial incisure is the
opening bounded by the anterior free border of
the tentorium cerebelli and the posterior
surface of the dorsum sellae.
What part of the brain is partially
encircled by the tentorial notch?
The upper portion of the midbrain is found
within the tentorial notch.
Dissection 3, Exposure of Cranial Cavity and Brain
Page 8
25. In addition to the midbrain, what
other structures pass through the
tentorial notch?
25. The posterior cerebral arteries.
26. What is the diaphragma sellae?
26. The diaphragma sellae is a small circular fold
of dura with a hole in its center that is attached
to the margins of the hypophyseal fossa of the
sella turcica. Its inferior surface covers the
hypophysis.
27. Severe bleeding from the middle
meningeal artery usually results in
a collection of blood in what space?
27. In the extradural (epidural) space, a space that
is produced by the blood as it strips the dura
away from the inner surface of the skull.
28. What is the usual site of bleeding that
produces a subdural hematoma?
28. Subdural hematomas are usually the result of
bleeding from cerebral veins at their point of
entry into the dural venous sinuses.
29. Trace blood from the motor cortex
to the heart, naming in order all
vessels traversed.
29. Superior cerebral vein draining the precentral
gyrus
superior sagittal sinus
confluence
of the sinuses
transverse sinus
sigmoid
sinus
internal jugular vein
brachiocephalic vein
superior vena cava
right atrium
30. What is an emissary vein? Give several
examples.
30. An emissary vein is a vein which connects a
dural venous sinus with veins outside of the
cranial cavity. Some of these are connected
through the veins of the diploë with the
outside, while others pass directly through the
skull. The named emissary veins are the
frontal emissary vein, parietal emissary veins,
mastoid emissary veins, condylar emissary
veins, and occipital emissary vein. There are
other veins and venous plexuses which are not
usually called emissary veins, but which fit the
general definition as given above and can
serve as important channels by which blood
can leave the cranial cavity. The superior
ophthalmic vein, which communicates with the
cavernous sinus and the facial vein and the
anastomoses between the basilar plexus and
the internal vertebral venous plexuses fall into
this category.
31. How does most of the blood normally
get out of the cranial cavity?
31. Through the dural sinuses which drain into the
internal jugular vein.
Dissection 3, Exposure of Cranial Cavity and Brain
Page 9
32. What is a lateral lacuna?
32. A lateral lacuna is an endothelial lined cleft
between the two layers of the dura which
communicates with or is a lateral extension of
one of the dural sinuses, usually the superior
sagittal sinus. The arachnoid granulations
project into these lacunae.
33. What is an arachnoid granulation?
33. An arachnoid granulation is a structure
composed of arachnoid cells which penetrates
the dural wall of a venous sinus or lacuna and
which is separated from the blood only by the
endothelial lining of the sinus or lacuna. They
are the principal sites of reabsorption of
cerebrospinal fluid into the blood.
34. Draw and label a diagram of an arachnoid
granulation.
34.
35. Trace the route of cerebrospinal fluid flow
from the point of its formation to its site of
reabsorption into the blood.
35. Choroid plexus of lateral ventricle
interventricular foramen
third ventricle
cerebral aqueduct
fourth ventricle
lateral
or median apertures of the fourth ventricle
subarachnoid space
arachnoid granulation
blood.
36. What is a cerebrospinal fluid cistern?
36. A cistern is an enlargement of the subarachnoid
space.
37. Name and locate one major cerebrospinal
fluid cistern.
37. The cisterna magna (cerebellomedullaris) is
located in the angle between the inferior surface
of the cerebellum and the posterior surface of the
medulla.
Dissection 3, Exposure of Cranial Cavity and Brain
Page 10
38. Where does the subarachnoid space extend beyond the confines of the cranial cavity?
39. What is the usual source of bleeding into the subarachnoid space?
40. In general terms, where may extravasated blood collect within the cranial cavity?
41. Be able to trace the route of blood flow from any general region of the brain to the heart.
42. How is the cranial dura innervated? Where are the cell bodies of afferent fibers to the tentorium cerebelli?
43. What is the cranium? Of how many bones does it consist? Name them.
44. The great cerebral vein drains into what structure?
45. What other cavities in the skull are in direct relation to the middle cranial fossa and separated from it by thin walls
of bone?
46. What is a cephalhematoma? How could a cephalhematoma be distinguished from a hematoma of the scalp?
47. What is the metopic suture?
48. What is the clivus? What parts of the brain are resting on the clivus?
LJ:bh revised 06/18/09