Dissection Guide
for the Human Brain
Vesalius: “ De Humani Corporis Fabrica”, 1543
Dr. Jan G. Veening
Dept of Anatomy
UMC St Radboud
University of Nijmegen
the Netherlands
2
1 OVERVIEW OF THE DISS ECTION COURSE: 2
1.1 Introductory remarks 3
2 Meningeal membranes 4
2.1 Meninges and (sub)arachnoidal space 4
2.2 Clinical Notes 6
3 Blood supply of the brain 8
3.1 Arterial supply of forebrain and hindbrain 8
3.2 Venous drainage of the brain 11
3.3 Clinical Notes 12
4 Basal surface of the brain and cranial nerves 13
4.1 Subdivisions and cranial nerves 13
4.2 Clinical Notes 16
5 Medial view of the bisected brain 17
5.1 Subdivisions of the brain 17
5.2 Clinical Notes 22
6 Lateral aspect of the hemisphere 23
6.1 Topographical and functional localization 23
6.2 Clinical Notes 27
7 Subcortical structures and relationships 28
7.1 ‘Limbic System’ (Temporal lobe,
hippocampus and amygdala) 29
7.2 Visual System, Basal Ganglia
and Internal Capsule 32
7.3 Clinical Notes 38
8 Cerebellum and Brainstem 39
8.1 Structure and topography 39
8.2 Clinical notes 41
9 Spinal Cord 43
9.1 Structure and topography 43
9.2 Clinical Notes 43
10 Frontal and horizontal sections of the human brain 45
11 List of abbreviations 50
12 Index and Glossary 51
3
1.1 Introductory remarks
The Central Nervous System (CNS), composed of Brain and Spinal Cord, is
the most complicated organ in the human body. It may contain 10
11
neurons and its
weight ranges between 1.2 and 1.6 kg. Despite the size of the brain, forming only
about 2% of the human body, 20% of cardiac output is aimed at the brain to provide
it with sufficient amounts of oxygen and energy for proper and continued functioning.
The CNS is completely surrounded by a covering consisting of membranes
(meninges) and CerebroSpinal Fluid (CSF) or - liquor. Before dissecting the brain
itself, attention will be paid to the arterial and venous vascularisation of the brain and
to the membranes and spaces surrounding it.
All participants of the dissection course are urgently requested to follow the
indications, provided by the dissection-guide, as closely as possible.
Every step to be taken during the dissection course is indicated in italics.
When in doubts about one of the steps to be taken, e.g. about a plane of sectioning,
please approach one of the assistants for additional information.
Anatomical structures, printed bold, are supposed to be known at the end of
the dissection course.
Equipment required for dissection of the brain:
- surgical gloves
- surgical knife
- brain dissection knife
- scissors
- forceps
- ……..
- ………
- ………
-
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2) MENINGEAL MEMBRANES
2.1 Meninges and (sub)arachnoidal space.
The brain and spinal cord are surrounded by three membranes of mesodermal
origin (meninges): the dura mater, the arachnoid and the pia mater. The space
between the arachnoid and pia mater is the (sub)arachnoidal space (Fig 1). This
space is filled with cerebrospinal fluid (CSF) (Fig 1), which serves as a cushion
between the soft nervous tissue and the bony skull. The meninges have important
protective and supportive functions, play a role in circulation and absorption of CSF
and convey venous blood to the internal jugular veins. The meninges and their
relationships to the brain and the CSF system are of great relevance in a number of
clinical conditions including head injuries, intracranial hemorrhages, infections and
hydrocephalus (Heimer, 1995).
Fig 1. In this figure, the following structures can be identified:
- meningeal layers, dural folds and – sinus, arachnoidal granulations
- subarachnoidal vessels and parts of the medial cerebral artery
- several parts of the ventricular system
- cerebral cortex, basal ganglia, thalamus and cerebellum
Dura mater (pachymeninx) (Fig 1)
The dura is a tough and fibrous membrane, attached to the bones forming the
cranial cavity. On autopsy, when the brain is removed from the skull, the lower part
of the dura is usually left behind, to be studied in situ, on the base of the skull.
The dura consists of 2 layers: an outer “periosteal” layer, functioning as a
periosteum for neurocranial bones, and an inner “meningeal” layer. The inner layers
may join to form dural folds that subdivide the cranial cavity into compartments. The
falx cerebri (Fig 1), between the left and right hemisphere, and the tentorium
cerebelli (Fig 1), between cerebrum and cerebellum, are the main folds contributing
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to this compartmentalization. Potential spaces exist between dura and cranial bones:
the epidural space, and between dura and arachnoidea: the subdural space.
Bleeding may occur in each of these spaces. See: Clinical Notes
In between the inner and outer layers of the dura, endothelium lined spaces
occur, called: sinus (Figs 1,7). A sinus contains venous blood, and the largest one is
the superior sagittal sinus, collecting blood from the superior cerebral veins on the
convexity of the cerebral hemispheres (Figs 1,3,7). At the confluence of sinuses,
this blood stream is joined by the inferior sagittal sinus and the sinus rectus (Fig
7Re), contributing blood collected from the medial and the inner parts (v. cerebri
magna) of the hemispheres, respectively. From the confluence of sinuses, the
transverse and sigmoid sinuses carry the venous blood to the internal jugular
vein, via the jugular foramen (Fig 7).
The branches of the paired middle meningeal arteries (Fig 1) are running
between the inner and outer layers of the dura, providing the arterial vascularisation
of the dura. Their distribution pattern with anterior and posterior branches is visible in
the dura as well as in the inner surface of the skull. Damage to these arteries may
lead to epi- or extradural bleeding. See: Clinical Notes.
Arachnoidea & Pia mater (Leptomeninx) (Fig 1)
The arachnoid and pia mater can be considered as an entity, the “pia-
arachnoid”, because they are extensively connected via fine connective tissue
strands: trabeculae. The main difference is, however, that the arachnoid follows the
dura mater, bridging all irregularities of the brain surface, whereas the pia follows the
contours of the brain and dips into all irregularities of its surface.
The space between arachnoid and pia, the subarachnoid space, contains
cerebro-spinal fluid (CSF) flowing in from the single median and two lateral
apertures of the fourth ventricle. Alongside the superior sagittal sinus, arachnoid
granulations can be observed consisting of aggregations of arachnoid villi,
protruding into the sinus through openings in the dura, to secrete CSF in the venous
blood stream (Figs 1,3).
Where the subarachnoid space widens to form cavities, these are called
subarachnoid cisterns. Most important are: cerebello -medullary cistern (cisterna
magna) (Fig 3) and the lumbar cistern (Figs 3,30). The latter is occupied by the
nerve roots of the caudal medulla (cauda equina). Both can be tapped for analysis
of CSF (cisterna- or lumbar puncture) and the lumbar cistern can be used to deliver
a spinal anesthetic.
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A B
Fig 2. Observe the dislocation of the dural folds, and the herniation of the cerebellar
tonsils, occurring after supratentorial (A) vs infratentorial (B) tumorgrowth.
2.2 Clinical Notes
- Extra- (epi-)dural hematomas: middle meningeal artery.
- Subdural hematomas: superficial brain veins entering dural sinus.
- Intracranial pressure: supra- and infra-tentorial compartments, tonsillar herniation
(Fig 2B).
- Hydrocephalus: increased production or decreased absorption of CSF.
- Meningitis: viral or bacterial infection of pia-arachnoid.
- Lumbar puncture: between vertebrae L3 – L5.
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Fig 3. Liquorcirculation of the brain and spinal cord;
relationships with arterial and venous vessels.
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3) BLOOD SUPPLY OF THE BRAIN.
The extensive blood supply reaches the brain via two arterial systems: the
internal carotid arteries (about 70% of total blood supply) (Figs 3,4), entering the
skull through the carotid canal, and the vertebral arteries (about 30%) (Figs 3,4),
entering through the foramen magnum.
Fig 4. Arterial supply of the brain. Basal view.
3.1 Arterial supply of the Forebrain.
- Internal Carotid System (Figs 3,4)
After entering the skull through the carotid canal, each internal carotid artery
reaches the subarachnoidal space, just caudal to the optic chiasm. After giving
off some collateral branches (anterior choroidal arteries (Fig 4), running in a
caudolateral direction to supply part of the choroid plexus in the inferior horn of
the lateral ventricle, and posterior communicating arteries (Fig 4), running in a
caudal direction to join the posterior cerebral artery), the internal carotid artery
divides into two main terminal branches: anterior and middle cerebral arteries.
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Fig 5. Arterial supply of the brain. Lateral and medial view, showing the territories of
ACA, MCA and PCA.
The Anterior Cerebral Artery (ACA) (Figs 4,5,6):
- passes above the optic chiasm
- gives of the medial striate artery (= recurrent artery of Heubner) (? rostral
part of the internal capsule) (Fig 6-10)
- is connected with the opposite ACA, via the short anterior communicating
artery (Fig 6-4)
- provides perforating arteries in the anterior perforated substance (?
hypothalamus, rostral/basal forebrain) (Fig 6-8)
- frontal and pericallosal branches run rostrally and caudally above the
corpus callosum and supply the medial surfaces of the hemispheres, back to
the parieto-occipital sulcus (Figs 5,6-2)
The Middle Cerebral Artery (MCA) (Figs 4,5,6):
- is the largest branch of the internal carotid artery (60 – 80 % of carotid blood)
- gives off many perforating central branches: the lateral striate arteries (via
anterior perforated substance ? thalamus, basal ganglia, internal capsule)
(Fig 6-9); See: Clinical Notes.
- its lateral branch passes along the depth of the lateral fissure, to provide the
insula, continuing laterally to emerge on the lateral surface of the hemisphere
(Fig 6-3)
- branches of the MCA (frontal-, central-, parietal-, temporal- and temporo-
occipital branches) supply at least two-thirds of the lateral surface of the brain
(Figs 4,5).
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- The Basilar system
The basilar artery (Figs 4, 6-5), arising from the fusion of the paired vertebral
arteries (see below) forms two terminal branches, the paired posterior cerebral
arteries (Figs 4, 6-6), that curve around the lateral aspect of the midbrain to
reach the medial and inferior surfaces of the temporal and occipital lobes.
The Posterior Cerebral Artery (PCA):
- gives off proximal branches (via posterior perforated substance ?
hypothalamus and thalamus) (Fig 6-12,6-14)
- gives off lateral (temporal) and medial (occipital, calcarine and callosal)
branches (Figs 4,5)
- is connected to the internal carotid artery through the posterior
communicating artery (Fig 4,6-7).
Arterial supply of the Hindbrain
The brainstem and cerebellum are supplied by the vertebral and basilar arteries and
their branches. The two vertebral arteries (Fig 4) (? for. magnum ? forr.
transversaria of the upper 6 cervical vertebrae ? subclavian artery) unite at the
lower border of the pons to form the basilar artery.
Vertebral arteries (paired):
- ? posterior inferior cerebellar artery (PICA) (Fig 4), supplying dorsolateral
medulla oblongata and posterior/inferior parts of cerebellum;
- ? anterior spinal artery, unpaired, supplying (para)median parts of the
medulla oblongata, before descending in vertebral canal.
Basilar artery (unpaired):
- ? anterior inferior cerebellar artery (AICA), supplying upper medulla and
inferior surface of cerebellum;
- ? pontine and internal auditory (labyrinthine) arteries (Fig 4), supplying
pons and membranous labyrinth of inner ear;
- ? superior cerebellar artery (SCA) (Fig 4), along the upper border of the
pons to superior surface of cerebellum;
- (? posterior cerebral arteries, the 2 terminal branches supplying part of the
forebrain, see above)(Figs 4,6).
Fig 6. The circle of Willis surrounding the optic chiasm and pituitary stalk and some of its
anatomical variations.
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Circle of Willis
The anterior and posterior communicating arteries connect the carotid and the
basilar system and help form a complete arterial circle at the base of the brain: the
circulus arteriosus cerebri or circle of Willis. This circle is of great clinical
importance, providing for the possibility of collateral circulation (anastomosis) in the
event of occlusion (thrombosis) in one of the contributing arteries. However, this
circular arterial configuration is very variable (Fig 6) and an effective collateral
circulation may not always be possible.
Numerous short perforating branches (Fig 6-8,6-12) from the circle of Willis
supply the mesencephalon and hypothalamus.
Cut through the superior cerebellar arteries as well as the posterior and middle
cerebral arteries and the anterior cerebral arteries, where they disappear in the
longitudinal fissure. Remove the circle of Willis together with the basilar and vertebral
arteries and study its configuration again, in comparison with known anatomical
variations (Fig 6).
3.2 Venous drainage of the brain
Fig 7. The venous drainage of the human head and brain.
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The superficial cerebral veins (Figs 1,7) also lie in the subarachnoid space and
empty into intracranial venous sinuses.
A deep cerebral vein appears under the caudal corpus callosum (great cerebral
vein or v. cerebri magna of Galen) Fig 7-CM), draining the internal cerebral veins
into the straight sinus (sinus rectus) (Fig 7-Re).
3.3 Clinical Notes
- Cerebrovascular accidents (freq uent involvement of the arteries providing
the internal capsule, with serious consequences!).
- Stroke (atherosclerosis, ischemia, e.g. from internal carotid artery, effects
dependent on size and location).
- Occlusion; embolism.
- Hemorrhage.
- Angiography, (by injecting contrast medium via a catheter into a vertebral or
carotid artery).
- Aneurysms (in the circle of Willis ? extensive subarachnoid hemorrhage).
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4) BASAL SURFACE OF THE BRAIN AND CRANIAL NERVES
The rest of the arachnoid membranes can now be removed from the basal
surface of the brain, taking care to save the cranial nerves. Try to localize the
following structures at the basal surface of the brain:
Fig 8. Basal surface of the human brain
4.1 Basal surface and cranial nerves
TELENCEPHALON
The telencephalic contribution to the basal surface of the brain consists of the
frontal and temporal lobes (Fig 8). On the frontal lobe, the irregular orbital gyri
can be observed, lateral to the gyrus rectus. In between these gyri, the olfactory
bulb can be observed in the olfactory sulcus, with its caudal continuation: the
olfactory tract (Fig 8). This tract can be followed, caudally, unto the anterior
perforated space or – substance (Fig 8)(with the numerous blood vessels
penetrating the ventral brain surface). Here, the olfactory tract splits in a medial and
a lateral branch. The lateral branch of the olfactory tract conveys information to
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the most medial extension of the temporal lobe: the uncus or primary olfactory
cortex (Fig 8). Below the surface of the uncus, we will later observe the amygdaloid
body, as another recipient of the olfactory information. Caudal to the uncus, the most
medial gyrus on the temporal lobe: the parahippocampal gyrus, can be observed,
covering the hippocampus, both forming important constituents of the limbic system.
The other longitudinal gyri at the ventral surface of the temporal lobe (the occipito-
temporal gyri) are mostly difficult to define.
DIENCEPHALON
The hypothalamus is the ventral part of the diencephalon. Rostrally, the
optic chiasm (Fig 8) is formed by the crossing components of the optic nerves
(Figs 8,9-26). Fibers from the nasal half of each retina cross the median plane here
and enter the contralateral optic tract (Fig 8), joining the fibers from the ipsilateral
temporal retina. (Question: does the left optic tract carry information about the Left or
the Right part of the visual field?) We will observe later that the optic tract swings
around the cerebral peduncle to the lateral geniculate body in the thalamus.
The hypophysis (pituitary gland) usually remains in the sella turcica below
the dura, at autopsy, and cannot be observed in our brain specimen (but take a look
at the available models of the brain and base of the skull, to apprehend its size and
location!). The pituitary stalk, however, can be observed, extending from the tuber
cinereum (Fig 8)(hypothalamic base, surrounding third ventricle). As the caudal
border of the hypothalamus, the paired mammillary bodies (Figs 8,9-10) can be
observed, which are connected to the hippocampus via an elongated fiber
connection, the fornix.
MESENCEPHALON
At the ventral surface, two massive cerebral peduncles (Fig 9-9) can be
observed, bordering the interpeduncular fossa. The cerebral peduncles contain the
fibers descending from cortical areas (motor cortex and other areas) to brainstem
(corticobulbar-), pons (corticopontine-) and spinal cord (corticospinal fibers).
The floor of the interpeduncular fossa is perforated by many small blood vessels, the
posterior perforated space (Fig 9-8).
The oculomotor nerves (III) (Figs 8,9 -25) emerge from the medial sides of
the cerebral peduncles. The trochlear nerves (IV) (Figs 8,9-24) pass between the
peduncles and the uncus of each temporal lobe.
METENCEPHALON
The pons (Figs 8,9-6) is composed of numerous transverse fibers, crossing
the median plane towards the contralateral cerebellar hemisphere. Running dorsally,
these fibers form the middle cerebellar peduncle (MCP)(Fig 27-2). Laterally, at the
transition from pons to MCP, the trigeminal nerve (V) (Fig 8) emerges, with its
minor (motor) root (Fig 9-23) and its major (sensory) root (Fig 9-22).
From a ventral point of view, the lateral parts of the cerebellar hemispheres
(Fig 8) can be observed. In the angle between the medulla oblongata and the
cerebellum, choroid plexus (Figs 9,27-9) (“flower bouquet of Bochdalek”) is
protruding through the lateral apertures (of Luschka), where CSF enters the
subarachnoid space.
At the caudal border of the pons, the abducens nerves (VI) (Figs 8,9-21)
and, more laterally, the (intermedio- ) facial (VII) (Figs 8,9 -20) and the vestibulo-
cochlear (VIII) nerves (Figs 8,9-21) emerge.
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Fig 9. Basal surface of the brainstem.
MYELENCEPHALON OR MEDULLA OBLONGATA
Alongside the anterior median fissure (Figs 8,9-1) , the pyramids or
pyramidal tracts (Figs 8,9-4) are running in parallel, composed of corticospinal
fibers. In the pyramidal decussation (Fig 9-3), about 2 cm caudal to the pons,
about 80% of the pyramidal fibers cross the median fissure, to descend
contralaterally in the dorsolateral funiculus of the spinal cord. (If the decussating
fibers are hard to detect visually, a probe can be placed in the median fissure, close
to the pons. Pushing the probe caudally through the fissure, an obstruction can be
felt at the level of the pyramidal decussation).
Lateral to the pyramidal tract, the olive (Figs 8,9-5) can be observed,
containing the inferior olivary nucleus, an important relay-center from where ‘climbing
fibers’ arise to influence the Purkinje-cells in the contralateral cerebellar cortex.
Lateral to the olive, the inferior cerebellar peduncle (ICP) (Fig 27-3) can be
observed.
Between the pyramidal tract and the olive (medial paraolivary fissure), the
many roots, converging into the hypoglossal nerve (XII) (Figs 8,9-14), emerge.
Between the olive and the ICP (lateral paraolivary fissure), the roots of the
glossopharyngeal (IX), vagus (X) and the cranial part of the accessory nerve
(XI) (Figs 8,9-15/17/18) appear. The spinal part of the accessory nerve (XI) (Fig 9-
16) arises from the spinal cord and ascends through the foramen magnum to join the
cranial accessory.
(If the cranial nerves cannot be studied appropriately in the available brain
specimen, which is often the case, spend some time to study the available models of
the brain. This is especially helpful in combination with a model of the base of the
skull, showing the openings in the dura and the location of the foramina used by the
cranial nerves to leave the cranial cavity.)
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4.2 Clinical Notes
- Head injuries, resulting in fractures of the base of the skull (may involve
several cranial nerves).
- Pituitary adenomas, (arising from the adenohypophysis in the sella turcica,
may compress the optic chiasm and basal hypothalamus).
- Cerebello-pontine angle tumors, (schwannoma’s arising from the
vestibulocochlear nerve in the internal auditory canal. May grow into the
space between the lateral side of the pons, the cerebellum and the posterior
surface of the petrous bone (Figs 8,9). The expanding tumor may influence
Vth, VIIth, VIIIth, IXth and Xth cranial nerves as well as cerebellar function).
Midsagittal section
The brain has to be bisected carefully, using a moistened brain-dissection-
knife. The knife can be placed on the corpus callosum and pulled (not pressed!)
gently through the brain, taking care to stay in the midsagittal plane. If the brain
specimens are slightly misformed, during storage, it is preferable to start the
bisection at the ventral side of the brain, after pushing the midline structures back in
the proper position, with the help of a colleague-student.)
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5) THE MEDIAL VIEW OF THE BISECTED BRAIN
The distribution of the anterior cerebral artery (ACA) (Fig 5) can now be
studied. It courses in a wide curve backward on the dorsal side of the corpus
callosum (Fig 11) and supplies blood to the medial surface of the frontal and parietal
cortices, back to the parieto-occipital sulcus (Figs 5,11). (Question: which parts of
the sensory and motor ‘homunculi’ are relying on the ACA-supply?) The distribution
of the posterior cerebral artery (PCA) (Fig 5) can now be studied on the medial
sides of the temporal and occipital lobes, supplying e.g. most of the visual cortex.
All vessels may now be removed, together with the arachnoid, from the medial
surface of the hemisphere, but leave the membranes below the corpus callosum
intact!
5.1 Subdivisions of the brain
Try to identify now the five main divisions of the brain:
- for the transition between spinal cord and myelencephalon (Figs 10,11), no
clearcut landmarks are available. The border plane is generally indicated at
the level of the foramen magnum, perpendicular to the brainstem axis;
- between myelencephalon and metencephalon (Figs 10,11) the border
plane is at the caudal margin of the pons, perpendicular to the brainstem
axis;
- between metencephalon and mesencephalon the border plane runs from
the rostral margin of the pons to the caudal margin of the inferior colliculus
(Fig 10-22), still perpendicular to the brainstem axis;
- between the mesencephalon and diencephalon the border plane is no
longer running in parallel to the caudal mesencephalic border plane, reflecting
the forward bending of the neuraxis in the human brain. The anatomical
landmarks for this border plane are the caudal margins of the mammillary
bodies (Figs 10-15,11) ventrally, and the posterior commissure dorsally
(Figs 10-8,11);
- the border plane between diencephalon and telencephalon takes an almost
vertical position, due to the continued forward bending of the neuraxis. This
border is located along the line from optic chiasm (Figs 10-18,11) via lamina
terminalis (Figs 10-14,11) to anterior commissure (Figs 10-11,11). From a
position rostral to this plane, the telencephalic subdivision in the human brain
has expanded enormously, in the form of two hemispheres that have
overgrown the rest of the brain to a great extent.
Ascending now from caudal to rostral, the following structures can be identified in the
median plane:
MYELENCEPHALON (MEDULLA OBLONGATA)
Ventrally, caudal to the pons, the pyramidal tract is descending towards its
decussation. Dorsally, from the caudal tip of the 4
th
ventricle, (obex)(Figs 10,11,27-
13), the posterior (inferior) medullary velum (Fig 11) extends as a roof towards
the cerebellum. Because this velum is very thin and surrounding the median
aperture (of Magendie (Fig 3-M); CSF? subarachnoid space), it may be easily
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damaged and hardly recognizable. Caudally, the 4
th
ventricle continues as the
central canal (Figs 10,11) of the spinal cord. Rostrally, the 4
th
ventricle becomes
wider, due to its rhomboid form.
METENCEPHALON
In the pons, many transverse fib er bundles (fibrae transversae) can be
observed, crossing the midline on their way to the contralateral middle cerebellar
peduncle (MCP) (Fig 27-2). The gray matter in between these bundles contain the
pontine nuclei, from which the crossing fibers originate.
The cerebellum (Figs 10,11)(weighing about 150 g) is positioned on the
brainstem and the median section shows a thin outer layer of gray matter, the
cerebellar cortex, surrounding a tree-like outline of white matter, the ‘arbor vitae’.
The cerebellum is very important for the control and coordination of movements. Its
outer surface is characterized by deep fissures, separating the narrow folia . Try to
localize the primary fissure (Figs 10-29,26A), that separates the anterior and
posterior lobes, and the postero-lateral fissure (Figs 10-39,26A). The latter fissure
separates the nodulus (Fig 10-40), and, more laterally, the flocculus, together
forming the flocculo-nodular lobe or vestibulocerebellum , from the posterior lobe.
All other transverse fissures bear no relationship with functional subdivisions of the
cerebellum, because these are organized along a medio-lateral axis. The median
part of the cerebellum, the vermis (Fig 26B), (most conspicuous on the inferior
surface) can be referred to as spinocerebellum (Fig 26C), while the two large
lateral parts, the cerebellar hemispheres (Fig 26C), have been referred to as
cerebro- or ponto-cerebellum, reflecting the extensive relationships with these
particular parts of the CNS. The most caudomedial parts of the cerebellar
hemisphere, adjoining the brainstem, are the cerebellar tonsils (Figs 2,10-
45,11,26A), lying directly above the foramen magnum of the skull. (see Clinical
Notes).
The 4
th
ventricle reaches its greatest extent under the cerebellum, with an
apex, referred to as fastigium (Fig 10-42). Narrowing anteriorly, towards the
cerebral aqueduct, the 4
th
ventricle is roofed by the anterior (or superior)
medullary velum (Figs 10-23,11), which is extending between the two superior
cerebellar peduncles (SCP) (Fig 26-1), and covered by a thin layer of cerebellar
cortex, the lingula (Fig 10-24). The widest point of the rhomboid 4
th
ventricle is found
at the lateral recess (Fig 26-9), from where choroid plexus may extend further
laterally through the lateral aperture (of Luschka), mentioned before.
MESENCEPHALON
The subdivision of the mesencephalon is based on the location of the
cerebral aqueduct (of Sylvius) (Figs 10,11), a narrow passage for the CSF flowing
from the 3
rd
to the 4
th
ventricle.
Dorsal to the aqueduct, the tectum consists of the superior (Fig 10-21) and
inferior colliculi (Fig 10-22), together with their contralateral counterparts, also
referred to as ‘corpora quadrigemina’. The superior colliculus is an important visual
reflex center, whereas the inferior colliculus is an important reflex and relay center
for auditory information.
Ventral to the aqueduct, the tegmentum of the mesencephalon contains a
number of brain areas (red nucleus, substantia nigra etc), that will be studied later on
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a series of transverse sections. The same holds for the Central Gray Substance or
PeriAqueductal Gray- (PAG) - region, surrounding the aqueduct.
Along the ventrolateral side of the tegmentum, the cerebral peduncles
descend towards the pons.
Fig 10. Medial view of the brainstem and diencephalon.
DIENCEPHALON
The 3
rd
ventricle (Fig 11) has been opened by the midsagittal section. In its lateral
wall, the hypothalamic sulcus (Figs 10-13,11) can be observed, subdividing the
diencephalon in an upper part, the thalamus, and a lower part, the hypothalamus.
The roof of the 3
rd
ventricle consists of a thin membrane, with choroid plexus
(Figs 10-9,11) (producing CSF) attached to it. Caudally, this membrane connects to
an outstanding part of the thalamus, the epith alamus (Fig 10-4), containing the
habenular nuclei. Below the habenular commissure (Fig 10-5), the pineal gland
or pineal body (epiphysis) (Figs 10-6,11) extends caudally over the upper part of
the mesencephalon. This gland is involved in day-night rhythms and the production
of melatonin.
Below the pineal body, the posterior commissure (Figs 10-8,11) can be
observed, ‘roofing’ the entrance of the cerebral aqueduct from the 3
rd
ventricle,
rostral to the superior colliculus. Both the habenular and posterior commissures
contain commissural and/or decussating fibers, connecting several diencephalic and
mesencephalic brain areas.
In the floor of the 3
rd
ventricle, we observe the mammillary body (Figs 10-
15,11), extending as the caudal part of the hypothalamus. Rostral to the mammillary
body, the floor of the ventricle (infundibulum, median eminence) is frequently
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damaged or even lacking, due to the hypophysis (pituitary gland), torn off during
brain autopsy. The infundibular recess (Fig 10-19), (extending into the
infundibulum and the pituitary stalk, Figs 10-20,11) can, however, be noted as a
ventral extension of the ventricular space. More rostrally, the optic recess (Fig 10-
17) extends towards the optic chiasm (Figs 10-18,11).
The hypothalamus is strongly involved in behavioral control, as well as in the
neuronal and hormonal control of the “internal state” of the body (“homeostasis”, all
kinds of autonomic and endocrine functions). As such, it plays a pivotal role in the
“limbic system”.
With the retinal fibers crossing here, we have reached the rostral wall of the
3
rd
ventricle. Following this wall, dorsally, along the lamina terminalis (Figs 10-
14,11), forming the telo-diencephalic borderline, we reach the anterior commissure
(Figs 10-11,11). This commissure connects the left and right amygdaloid bodies as
well as other medial temporal regions (Fig 20).
Along the anterior commissure, we see the fornix (Figs 10-12,11), entering
the diencephalon after bending downwards from a dorsal position. It contains
hippocampal fibers on their way to the mammillary bodies, as well as to other parts
of the hypothalamus. A little dorsal to the adjoining fornix and anterior commissure,
the connection between the third and the lateral ventricle can be observed, the
interventricular foramen (of Monro)(Figs 10,11).The choroid plexuses of the 3
rd
and lateral ventricles are continuous via this foramen.
Dorsal to the hypothalamic sulcus, the medial wall of the thalamus can be
observed. The left and right thalami are in about 2/3 of human brains in contact
across the 3
rd
ventricle via the thalamic adhesion or massa intermedia (Figs 10-
10,11).
In the human brain, the thalamus is much larger than the hypothalamus, as
we will see, on further dissection. It contains many thalamic nuclei, most of them
connected (reciprocally) with neocortical areas, serving a variety of sensory and
motor functions.
Before proceeding with the study of the medial telencephalic surface, this is
an appropriate moment to transect the rostral brainstem, in order to expose the
medial aspect of the temporal lobe more completely. The transection has to be made
perpendicular to the longitudinal axis of the brainstem, through the rostral
mesencephalon, at the level of the superior colliculus, see Figs 10,11. (In case of
doubt, please ask for assistance!) Care has to be taken that no other parts of the
brain are involved in this transection of one half mesencephalon. The caudal part of
the brainstem and the cerebellum can be laid aside, to be studied later again. The
examination and dissection of the brain are to be continued at the telo-diencephalic
part.
TELENCEPHALON
At the median view, the corpus callosum (Fig 11)(the main commissure
between both hemispheres, containing about 300 million fibers!) has to be located
first, for a proper orientation. It consists of several parts, the names of which are
frequently used to indicate a rostro-caudal position in the brain: the splenium (Fig
11)(wide caudal part), the body (Fig 11)(or corpus, large midportion), the genu (or
“knee”) (Fig 11) (rostral part, reflecting downward) and its ventral extension, the
rostrum (Fig 11), which is continuous with the lamina terminalis, described earlier.
The corpus callosum forms the main road along which information is transferred from
21
one hemisphere to the other, and male-female differences in size (‘sexual
dimorphism’) have been described.
Fig 11. Medial view of the human brain.
Caudally below the corpus callosum, the fornix (Fig 11) appears, running
rostrally, but bending away from the corpus callosum ventrally to touch the anterior
commissure, before entering the hypothalamus (see above). The fornix constitutes a
major connection between the hippocampus in the temporal lobe and several areas
in basal tel - and diencephalon, in particular the mammillary bodies (Figs 19,20).
Underneath the corpus callosum, the fornices from both sides are joined for a while
(the body), until they start curving downward. Then they separate in two columns,
each of them disappearing in a wall of the 3
rd
ventricle, in front of the interventricular
foramen (Figs 10-12,11). (In one half of the dissected brain, some gray matter can
be removed from the wall of the 3
rd
ventricle by blunt dissection, to disclose the fornix
on its course to the mammillary body). The length and the form of the fornix reflect
the hippocampal displacement induced by the telencephalic expansion and
transformation of the human brain.
In between the fornix and the anterior part of the corpus callosum, the septum
pellucidum (Fig 11) is stretched as a double-layered membranous separation of
both lateral ventricles. (Mostly, the septum is present in one half of the bisected
brain, the other half allowing a view into the lateral ventricle itself). So, there is no
communication between both lateral ventricles, as both of them drain their CSF on
the 3
rd
ventricle via the interventricular foramina. The dorsal part of the septum
pellucidum is mainly composed of fibers and glial cells, but the ventral part (the
22
precommissural septum) contains several septal nuclei, participating in the
circuitry of the limbic system.
Surrounding the corpus callosum, we observe the cingulate gyrus (Fig 11),
starting in the subcallosal area underneath the rostrum of the corpus callosum, and
continuing caudally and ventrally into the parahippocampal gyrus, which runs
rostrally on the medial aspect of the temporal lobe, until reaching the uncus (see
above). The cingulate and parahippocampal gyri together are considered as the
‘cortical ring’ of the limbic system, (surrounding the ‘subcortical’ ring, see later).
Several textbooks describe these areas together also as the 5
th
cortical lobe, or the
limbic lobe (Fig 21).
At the upper border of the medial cortical surface, the central sulcus (Figs
11,14,16) comes to end, surrounded by the paracentral lobulus (Fig 11), containing
the medial extensions of the pre- and postcentral gyri or motor- and somatosensory
cortex. (Question: which parts of the sensory and motor homunculi are represented
in the paracentral lobulus?) More caudally, the deep parieto-occipital sulcus (Fig
11) forms a conspicuous border between these lobes. The cuneus (Fig 11), forming
part of the occipital lobe, is wedged between the parieto-occipital sulcus and another
deep sulcus, the calcarine sulcus (Fig 11). The latter sulcus is surrounded by the
visual cortex (Brodmann’s Area 17, or V1)(Figs 5,12,14).
5.2 Clinical Notes
- Commissurotomy: (“split-brain-surgery”), provided a wealth of information
about lateralization and other forms of hemispheric specialization.
- blood supply: remember the blood supply of the paracentral lobulus (ACA)
and the visual cortex (PCA)(Fig 5).
- vascular accidents in the thalamus: may lead to various sensory, motor,
cognitive or memory disturbances.
- pineal gland: involved in circadian rhythms, producing melatonin; may calcify
at later age and become detectable on radiographs of the skull.
- pituitary tumors (adenomas): may compress optic chiasm? visual field
defects.
- vascular accidents or tumor growth in or adjoining the hypothalamus:
may lead to a large variety of “hypothalamic syndromes”, with symptoms
including complex behavioral, sensory (visual), autonomic, endocrine and
cognitive functions like: hypothermia, eating disorders, ‘diabetes insipidus’, or
‘post-traumatic-stress disorders’.
- “bicommissural plane”: the horizontal ‘reference plane’ through both the
anterior and the posterior commissures, as used in human brain imaging
studies.
23
6) LATERAL ASPECT OF THE HEMISPHERE
The remaining parts of the pia-arachnoid can now be removed from the lateral
aspect of the hemisphere, including the blood vessels running in the subarachnoidal
space. Take care to leave the cortical surface itself undamaged.
6.1 Topographical and functional localization.
The convex lateral surface of the hemisphere reveals a complicated pattern of
gyri and sulci. Before we start studying the cortical surface in more detail, it is
important to keep in mind that a basic pattern can be observed in any hemisphere,
but that its details are so variable that no two human brains are fully identical. Even
the two hemispheres of a single brain may show remarkable differences!
Fig 12: Parcellation of the cortical surface in area’s, according to Brodmann (1909).
About a century ago, in 1909, Brodmann subdivided the human cortex in more
than 50 different areas. This parcellation was made on the basis of a histological
analysis of the structure and composition of the cortical layers forming the superficial
gray matter of the cortex (Fig 12). In the areas composing the neocortex or
isocortex 6 layers can be observed. The allocortex comprises cortical areas with
only 5 or less different layers. Frequently, the location of these areas does not
conform to the morphological pattern of specific gyri or sulci.
Concerning the localization of specific cortical functions, sometimes reference
is made to (parts of) specific gyri but more frequently to specific area numbers.
However, functional studies, using modern imaging techniques, have shown that a
considerable variability exists between individuals in the distribution of cortical
24
activity related to a specific function. In addition, recent findings show that an
identical cortical damage may have different effects in the male or in the female
brain!
Fig 13. Functional localization on the cortical surface. A: Lateral view; B: Medial view; C:
“perisylvian-language-zone”.
Based on the position of the major sulci: the lateral sulcus (Sylvian fissure)
(Figs 12,14,15, the central sulcus (Rolandic fissure)(Figs 12,14,15) and the
parieto-occipital sulcus (see above), the hemisphere can be divided into 4 lobes:
the frontal lobe, the parietal lobe, the temporal lobe and the occipital lobe (Figs
14,15). The insula, by some authors referred to as the fifth lobe, can be observed in
the depth of the lateral sulcus, overgrown by the opercula of the frontal, parietal and
temporal lobes (See Fig 21, and frontal and horizontal sections).
THE FRONTAL LOBE.
The frontal lobe is divided into 4 main gyri: the precentral gyrus (area 4,
“primary motor cortex, M1”, homunculus!) and 3 frontal gyri: the superior, middle
and inferior frontal gyri (Figs 13,14,15). These frontal gyri comprise the “premotor
25
cortex” (areas 6,8,44), the “frontal eye field” (area 8, middle FG)(Fig 13A), and the
“supervisory attentional system”, bordering it rostrally (area 9, essential for
cognitive functions and conscious learning, ‘working memory’). In the (triangular and
opercular parts of the) inferior FG, (areas 44 and 45) the “motor speech area” (of
Broca) (Fig 13A) is located, usually (95%) in the left hemisphere.
Generally speaking, the caudal parts of the frontal lobe are more involved in
motor functions. The rostral parts serve more complex functions. Destruction of
these rostral parts may even induce “personality-changes”.
Fig 14. Lateral surface of the brain.
THE PARIETAL LOBE.
Running parallel to the central sulcus, the postcentral gyrus (Figs 13,14,15)
can be observed (areas 1, 2 and 3, “primary somatosensory cortex, S1”,
homunculus!) Caudal to this gyrus, the parietal lobe can be divided in an upper part,
the superior parietal lobule (Fig 14), and a lower part, the inferior parietal lobule
(Fig 15). The upper lobule (areas 5, 7) seems to participate in the “where?-visual-
pathway”, extending upward from the visual cortex. In the inferior parietal lobule two
important gyri can be recognized: the supramarginal gyrus (area 40, capping the
posterior tip of the lateral sulcus,) and the angular gyrus (area 39) (Fig 13A), which
caps the posterior tip of the superior temporal gyrus. Both gyri (at the left side of the
brain) form part of the “perisylvian-language-zone” (Fig 13C), between Wernicke’s
area 22 (temporal lobe) and Broca’s areas 44 and 45. The angular gyrus (area 39)
seems to contain a dictionary of words and syllables. Area 40 of the inferior parietal
lobule seems to contain, in addition, a “body scheme” (awareness of the existence
and spatial relationships of body parts).
26
THE OCCIPITAL LOBE
The occipital lobe is located behind the parietal lobe, separated by an
imaginary line extending from the parieto-occipital sulcus (see above, Fig 11) and
the preoccipital notch. Several occipital gyri can be observed on the lateral side.
The deep calcarine sulcus (Fig 11) on its medial side (see above) sometimes
curves around the occipital pole to the lateral surface.
The occipital lobe is primarily involved in visual functions. From the “primary
visual cortex” (V1, area 17, surrounding the calcarine sulcus), visual information is
processed via secondary (V2, area 18) and tertiary (V3-V5, area 19) visual areas.
Together these areas form a “where?-visual pathway”, upward into the parietal
cortex, and a “what?-visual pathway”, downward into the temporal lobe.
Fig 15. Lateral view showing the main functional areas.
THE TEMPORAL LOBE
On the lateral surface of the temporal lobe, 3 gyri can be discerned: the
superior-, the middle- , and the inferior temporal gyrus (Figs14,15). The inferior
temporal gyrus participates in the “what?-visual pathway” (area 20), extending
downward from the visual cortex, and is important for the recognition of objects and
faces.
If the lateral sulcus is carefully spread apart, the extensive dorsal surface of
the superior temporal gyrus becomes exposed. Try to locate the transverse
temporal gyrus (Heschl’s gyrus) (Fig 16B), comprising Brodmann’s areas 41 and
42, “primary auditory cortex”, and behind it the temporal plane (planum
temporale) (Fig 16B). In about 60% of the human brains, the left temporal plane is
considerably larger than the right one (Fig 16B).
27
In the superior temporal gyrus, the secondary auditory cortex, area 22,
surrounds areas 41 and 42. The caudal part of area 22, under the temporal plane, is
known as “Wernicke’s-language-area”. All of these areas form part of the
“perisylvian-language-zone” (Fig 13C).
Fig 16A. showing the insula, in a lateral view, surrounded by the frontal (1), parietal
(2) and temporal (3) opercula.
Fig 16B. showing the Left-Right differences on the dorsal temporal surface and the
planum temporale.
THE INSULA
After spreading the lateral sulcus and carefully bending away the frontal,
parietal and temporal opercula, the insular cortex, with its short and long gyri (Fig
16A) is exposed. Parts of the insular cortex have been referred to as “cortical taste
area”, “visceral cortex” and “cortical pain center”. In agreement with these “visceral
functions”, the insular cortex is extensively connected with “limbic” brain areas.
6.2 Clinical Notes
- stroke: sudden appearance of neurological deficits, due to blockade of or
hemorrhage from a cerebral artery;
- Transient Ischemic Attack (TIA): temporary local interruption of cerebral
blood flow, often foreboding a major stroke;
- aphasia (inability to speak; motor-: Broca, sensory-: Wernicke;),
- alexia ( - - read), amnesia ( - - remember), anomia (- - name common
objects), anopsia (- - visual perception), apraxia (- - voluntary movement),
astereognosis (- - identify common objects by touch): instances of
neurological deficits resulting from vascular accidents or caused by other
kinds of damage to specific cortical areas.
28
7) SUBCORTICAL STRUCTURES AND RELATIONSHIPS
During the next phase of the dissection, the lateral ventricle will be opened,
in order to study its configuration and to expose some important subcortical parts of
the “limbic system”. (Remember that some cortical parts of this system, the cingulate
and the parahippocampal gyri, were studied earlier on the midsagittal section. In
addition the prominent fiber bundle, the fornix, originating from the hippocampus,
and the hypothalamus were studied already on the median plane, Fig 11).
The central part (body or corpus) of the lateral ventricle (Figs 17A,B) can
now be opened as follows: after wetting the brain dissection knife, the brain is placed
on its lateral side , with the median plane in a horizontal position. Cut the
hemisphere now halfway between the cortical surface and the corpus callosum,
parallel to the latter’s rostrocaudal axis. (Using ‘brain terminology’: this results in a
horizontal brain section! Do you agree? If not, or doubtful, please ask for
assistance!) The next (horizontal!) cuts are to be made each time a few mm’s lower,
resulting in a few horizontal cortical slices, the lowest one touching upon the corpus
callosum, without transecting it.
Fig 17A. The ventricular system, projected upon a horizontal plane;
Fig 17B. The section lines along which the lateral ventricle should be opened.
Observe in the brain slices: the cortical outer layer, consisting of a
continuous layer of gray matter, 2-4 mm thick, completely surrounding the white
A
B
29
matter. The white matter, covering the lateral ventricle, is referred to as semioval
center and consists of numerous associative, commissural and projection fibers.
Try to find the dorsal tip of the lateral ventricle by palpating the semioval
center. Using a surgical knife, a small hole can be cut in the roof of the lateral
ventricle. Now the roof can be removed completely by expanding the hole step by
step in the rostral and caudal direction. Take care not to disturb the medial part of
the corpus callosum, the fornix and other brain structures bordering the lateral
ventricle.
Inspection of the opened lateral ventricle shows that its central part
continues rostrally as the anterior (or frontal) horn (Fig 17A), rostral to the
interventricular foramen, and caudally as the posterior (or occipital) horn (Fig
17A), extending into the occipital lobe. In the lateral wall of the ventricle, rostrally the
head (caput) (Fig 24) and caudally the tail (cauda) of the caudate nucleus can be
observed. The floor of the lateral ventricle is formed by the thalamus (Figs
11,24)(covered by a thin telencephalic ependymal layer, the lamina affixa) and the
fiber bundle running ‘on top of it’, the fornix. In addition, the choroid plexus (Fig
17A) is abundantly present in the lateral ventricles, covering partially the structures
forming the ventral wall.
Make sure that the hole in the roof of the lateral ventricle is large enough to
observe the dead end in the anterior horn and the caudal extension in the posterior
horn. In the medial wall of the latter horn, an eminence may protrude, the calcar
avis, caused by the deep calcarine sulcus.
7.1 ‘Limbic system” (temporal lobe, hippocampus, amygdala)
After removal of the frontal and parietal opercula, the insula and the upper
surface of the temporal lobe (the details of which have been inspected already, see
above) are fully exposed. BEFORE STARTING THE NEXT STEP IN THE
DISSECTION, study Figs 17 and 18 carefully to make sure that you understand
how to make the following incisions. If doubtful, PLEASE ASK ASSISTENCE!
The dissection continues by inserting the scalpel in the caudal part of the superior
temporal sulcus (Figs 17,18), until the tip appears in the ventricular space. After
extending the cut caudally upward (Fig 17A), the cut continues following the superior
temporal sulcus, rostrally. By bending the temporal lobe a bit, the tip of the scalpel
remains continuously visible, protruding in the ventricular space of the inferior
(temporal) horn (Figs 17,18). After reaching the rostral tip of the temporal lobe, the
superior temporal gyrus can be removed to expose the full extent of the temporal
horn, and the conspicuous structure lining it medially: the hippocampus (Figs
19,20).
Fig 18. The lines of sectioning to open the lateral ventricle.
30
As the next step in the dissection, the opened temporal lobe can be taken
apart from the rest of the brain, by separating the uncus-region of the temporal lobe
(containing the amygdaloid body)(Fig 20) from the frontal lobe, and by cutting the
fornix and the corpus callosum in the dorsal part of the lateral ventricle, at about the
level of the interventricular foramen. (From the fact that circumcision of the uncus
and transaction of these two major fiber bundles is sufficient to separate the
temporal lobe from the rest of the brain, it can be conceived how the temporal lobe
expanded and how its contents were displaced relative to the rest of the
telencephalon!) Now we can start with the inspection of the subcortical structures in
the temporal lobe.
Fig 19. Dorsal view of the brain after opening the lateral ventricle. Observe the
location and the spatial relationships of amygdala, hippocampus, fornix, lateral ventricle and
corpus callosum.
The hippocampus (Figs.19,20) is a cortical structure. In the human brain it
is ‘folded inward’ from the medial wall of the temporal lobe, and therefore no longer
visible at the outer surface. It can be observed as an elongated structure located in
the floor and medial wall of the inferior (temporal) horn of the lateral ventricle. Its
superficial layer (alveus) consists of white matter, myelinated fibers accumulating
caudally as the fimbria hippocampi, the flattened temporal part of the fornix (Figs
19,20).
The hippocampus is of utmost importance for memory functions, and may be
severely affected in case of Alzheimer’s disease, loosing up to 70% of its neuronal
population. In the human brain, its hypothalamic connections via the fornix are
extremely elongated, due to its location in the floor of the temporal lobe.
31
Fig 20. In addition to the structures shown in Fig 19, in this lateral view the olfactory bulb,
the septal region, the anterior commissure and the olfactory tract can be observed, the latter
two heading for the amygdala located in the uncus of the temporal lobe.
Fig 21. Schematic lateral view of the brain areas composing the limbic system. Observe the
central position of the hypothalamus in the limbic circuitry.
32
After removal of the pia-arachnoid and some small blood vessels from the
groove between the fimbria and the superior surface of the parahippocampal gyrus,
the dentate gyrus (Figs 19,20), with its serrated surface, becomes visible, partially
covered by the fimbria hippocampi. The dentate gyrus forms an important part of the
intrinsic hippocampal circuitry. The parahippocampal gyrus (Figs 19,20,21) can
now be studied again, located alongside and covering the hippocampus. The basal
surface of the parahippocampal gyrus widens rostrally, surrounding the uncus (Fig
8). This part of the gyrus contains the entorhinal cortex (Brodmann’s area 28),
characterized by irregularities in its surface. The entorhinal cortex is an important
point of convergence in the communication between neocortical areas and the
hippocampal and amygdaloid regions.
The amygdaloid body (corpus amygdaloideum, or amygdala) (Figs
20,21) is located in front of the hippocampus and consists of a subcortical nuclear
complex of gray matter, ‘filling’ the uncus and bordering the entorhinal cortical
region. (Cutting a few thin slices, horizontally or transversally through the rostral tip
of the temporal lobe, will reveal more of its size and location!) The amygdala is
connected with diencephalic and other subcortical regions, either directly (via fibers
to/from the rostromedial tip of the temporal lobe) or via another elongated fiber
connection, the stria terminalis, (following the same extended course as the fornix
and the cauda of the caudate nucleus). Via these connections, the amygdala exerts
its important function, to bring (emotional) experiences to expression in behavior
and, via the hypothalamus, also in autonomic nervous activity. Both the
hippocampus and amygdala are extensively related to hypothalamic areas and
together these regions form the major components of the subcortical part of the
‘limbic system’.
The whole temporo-occipital lobe can now be cut in a series of frontal slices.
Stay aware of the orientation of the slices (medial vs. lateral, etc), especially in the
occipital lobe. Make sure to cut at least the following rostrocaudal levels:
- uncus (subcortical position of amygdala, entorhinal cortex);
- hippocampus (superficial white matter (alveus, fimbria), dentate and
parahippocampal gyrus);
- calcarine sulcus (Fig 8) (1 cm deep, surrounded by visual cortex, with
myelinated fibers forming a thin band of white matter in layer 4, hence:
“striate cortex” containing the “visual stria of Gennari”).
7.2 Visual system, Basal Ganglia and Internal Capsule
Before starting the further dissection of the remaining telo-diencephalic part
of the brain, we will study some structures related to the “visual system”, which
have come into full view after the removal of the occipito-temporal lobe. Try to locate:
optic nerve (N II), optic chiasm, optic tract, (Figs 22,23) encircling the cerebral
peduncle (Fig 22) on the way to its thalamic destination: the lateral geniculate
body or - nucleus (Fig 22-4). Nearby, the (auditory) medial geniculate body (Fig
22-7) and the upper part of the superior colliculus (Fig 23) can be located. Now it
can be observed that the thalamus, with its lateral and caudal extensions, is by far
the largest part of the human diencephalon.
33
Fig 22. Ventral view of the visual system: optic nerve (1), optic chiasm (2), optic tract (3),
lateral geniculate body (4). In addition: medial geniculate body (7), cerebral peduncle and
mammillary bodies.
Fig 23. visual system (A), brain areas en visual field defects (B) occurring after lesions in
the system at particular places.
A
B
34
After separating (carefully!) the optic chiasm from the anteroventral
hypothalamus, the optic tract fibers can be torn loose from the cerebral peduncle,
until where they reach their thalamic destination, the Lateral Geniculate Body. This
separation of the optic tract fibers from the cerebral peduncle will provide us with a
better view of the descending internal capsule-cerebral peduncle fiber-stream, during
a later phase of the dissection.
The “basal ganglia” (or corpus striatum) (Fig 24) comprise some large
nuclear masses: the caudate nucleus, the putamen (together also referred to as
neostriatum, or even shorter: striatum ) and globus pallidus (also referred to as
paleostriatum or pallidum). During ontogeny, the descending corticofugal fibers
‘wedge’ themselves, as internal capsule (Figs 24,25), between the developing brain
areas of tel - and diencephalon. While, e.g. in the rat brain, there is no detectable
borderline between caudate- and putamen-parts of the striatum, in the human brain
we have eventually the situation of a separate caudate nucleus, bordering the lateral
ventricle and located medial to the internal capsule, and a putamen, lateral to the
internal capsule. The putamen is joined by the globus pallidus, and together they are
also referred to as lentiform nucleus (Figs 24,25), the ‘cone-shaped’ mass lateral to
the internal capsule. Only the rostral part of the putamen remains connected to the
head of the caudate nucleus, via ‘cellular bridges’ traversing the internal capsule
(see Fig 24A). The elongated tail of the caudate nucleus ends close to the central
nucleus of the amygdaloid complex (Fig 24A).
(The terminology of the “basal ganglia” is rather confusing! From a
developmental point of view, the amygdala (Fig 24A,B) also originates from a
‘ganglion’ in the floor of the lateral ventricle (hence the term: “basal ganglia”), but
because of its extensive relationships with other brain areas, it is considered part of
the “limbic system”, and not of the “basal ganglia” in its functional sense. On the
other hand, based on a functional point of view, the diencephalic subthalamic
nucleus, and the mesencephalic substantia nigra are generally included in the
“basal ganglia” because of their extensive mutual relationships with striatum and
pallidum. Adding to the confusion, these functionally defined “basal ganglia” have
also been referred to as “extrapyramidal system”.
Finally, the striatal areas mentioned so far have been referred to as the
“dorsal striatal system”, to make a distinction with ‘ventral striatal areas” (like nucleus
accumbens, “ventral tegmental area”) which have much stronger connections with
the “limbic system”).
Functionally, the basal ganglia play an important role in the organization and
planning of movement and behavior, and their disintegration, e.g. as a result of
Parkinson’s or Huntington’s disease leads to serious motor and cognitive deficits.
Before taking the next step in the dissection, try to locate all of the
structures, mentioned above, in Figs 24 and 25 and in the horizontal and frontal
brain slices, available in the dissection room. Pay attention, especially in the
horizontal slices, to the ‘bent-leg-shaped’ form of the Internal Capsule (Fig 25), with
its anterior and posterior limbs, the genu and a retrolenticular part. The
sublenticular part, running below the lentiform nucleus in a medio-lateral direction
can be observed better in the frontal slices (Fig 24B) or in the preparation itself, after
removal of the Lentiform Nucleus (see below). ( Questions: What is the location of
these parts relative to caudate and lentiform nucleus and the thalamus? Where and
how is a ‘motor-homunculus’ located in the internal capsule, composed of
35
descending fibers from the motorcortex? Concerning the reciprocal connections
between (different parts of) the thalamus and the neocortex, most of the visual and
somatosensory fibers are running through the ……………………………? part of the
Internal Capsule and most of the auditory fibers are running through the
…………………………? part.) In the frontal slices, pay especially attention to the
caudal continuation of the Internal Capsule into the Cerebral Peduncle (Fig 25A),
located bilaterally at the ventral surface of the mesencephalon. (Question: Which
brain areas are bordering the Internal Capsule- and Cerebral Peduncle fibers during
their descending course?)
Fig 24 Basal ganglia, internal capsule and thalamus. A) Lateral view, showing section
planes of figs B and C. In addition, the caudate nucleus , including its tail (cauda), the
putamen, the internal capsule fibers, the thalamus and the amygdala have been indicated
schematically. B) Frontal view showing caudate and lentiform (= putamen + globus pallidus)
nuclei bordering the internal capsule. C) Horizontal view, showing the relationship between
the thalamus, the caudate and lentiform nuclei and the (different parts of the) internal
capsule.
36
Fig 25. The internal capsule. A) Lateral view, showing the descending stream of
corticofugal fibers, traversing the corona radiata, internal capsule, cerebral peduncle (= crus
cerebri) and pyramidal tract successively. B) A schematic horizontal view showing the
internal capsule and some thalamocortical connections running through the retrolenticular
part of it. C) A more detailed horizontal view of the internal capsule, showing its contents and
the bordering brain areas.
The brain is now placed on its (flat) dorsal side, with the median plane
standing up. We may start cutting thin (about 3 mm) sagittal slices (parallel to the
median plane!), beginning laterally in the insular cortex, until a few mm’s lateral to
the Lateral Geniculate Body. (If any doubt exists about this plane of
sectioning, please ask for assistance and explanation! Choosing the wrong
plane at this point will completely disturb the rest of the dissection program!)
In the last cutting plane, the Putamen will become clearly visible as a mass of gray
matter, surrounded by white matter: Internal Capsule fibers. By blunt dissection,
using the opposite end of the scalpel, the combination of Putamen and Globus
Pallidus (together: the Lentiform Nucleus) can be removed from the lateral side of
37
the Internal Capsule. The medial part of the Lentiform Nucleus, thus obtained, can
be placed on its flat lateral side, to be sectioned vertically into two ‘halves’.
On the new cutting planes through the Lentiform Nucleus (see Figs 24,25),
color differences become visible between the darker Putamen part and the lighter
Globus Pallidus, containing more myelinated fibers. Sometimes, even the
differentiation between the internal and external parts of the Globus Pallidus is
clearly visible.
In the cone-shaped lateral surface of the Internal Capsule, we can observe:
- in the anterior limb of the Internal Capsule: patches of gray matter,
penetrating the fiber layer, forming a continuity between the head of the
Caudate Nucleus and the rostral part of the Putamen;
- traversing the sublenticular part of the Internal Capsule: fibers of the anterior
commissure (Figs 11,21), on their way to and from the (removed) amygdala,
temporal lobe and some olfactory areas;
- the caudal continuation of the Internal Capsule is formed by the Cerebral
Peduncle. From its 3 components, the corticobulbar- and corticopontine
fibers reach their destinations at brainstem levels, the corticospinal fibers
continue to descend into the Spinal Cord.
After making a superficial transverse cut across the surface of the Cerebral
Peduncle, it is possible to remove the superficial fibers by blunt dissection, using the
opposite end of the scalpel. Continuing this blunt dissection upward, we arrive in the
Internal Capsule and expose the funnel-shaped transition between both parts of the
descending corticofugal fiber system. Proceeding the blunt dissection of the Internal
Capsule itself in the frontal, parietal and occipital direction reveals how all parts of
the neocortex send their descending fibers converging into the Internal Capsule
(‘Corona radiata’)(Fig 25A). The further removal of fibers from the anterior part
(‘limb’) of the Internal Capsule, will eventually expose the Caudate Nucleus from its
lateral side.
The dissection of the Forebrain will now be finished by cutting a series
of thin (3-5 mm) frontal slices, using the wetted brain-dissection-knife. Make sure
that one transection is at the plane of the Anterior Commissure, and another at
the mid-thalamic level (Fig 11).
Try to locate the following structures:
- in the section through the Anterior Commissure:
o anterior commissure
o rostral hypothalamus (Optic Chiasm-level)
o internal capsule
o caudate nucleus
o location of the (removed) lentiform nucleus
o lateral ventricle
o corpus callosum
o septum pellucidum and fornix
- in the section through the mid-thalamus:
o third ventricle with hypothalamic sulcus
o thalamus, with lamina medullaris interna (the fiber layer subdividing the
thalamus in different parts)
o hypothalamus, with fornix
o internal capsule/cerebral peduncle
o caudate nucleus
38
7.3 Clinical Notes
Limbic system:
- olfaction, anosmia
- temporal lobe epilepsy
- memory functions
- dementia (‘Alzheimer’s disease’)
- nocicepsis (insula and anterior cingulate cortex)
- autonomic regulatory functions
- emotional and motivational functions
- “reward systems” (dopamine, drug dependency, addiction)
Visual system:
- lesions affecting the areas of the optic nerve, - chiasm or – tract, (caused by
pituitary tumors, meningiomas and aneurysms of the carotid artery or the
circle of Willis);
- scotoma, hemianopsia, quadrantanopsia
- pupillary light reflex (see Fig 23A)
- Horner’s syndrome (see Fig 23A): miosis (pupil constriction), drooping eyelid
and unilateral anhydrosis.
Basal Ganglia:
- “extrapyramidal” disorders (involuntary movements, difficulties in
movement initiation, - organization and – control, changed muscle tone)
- tremor, rigidity, bradykinesia, hemiballism, orofacial dyskinesias
- Parkinson’s disease
- Huntington’s chorea
Internal Capsule:
- capsular (‘lacunar’) infarct
- spastic paresis, (mono- or hemiplegia), (“pyramidal tract syndrome”)
39
8) CEREBELLUM AND BRAINSTEM
8.1 Structure and topography
The brainstem-cerebellum preparation was removed from the forebrain by a
transverse cut through the Superior Colliculus, perpendicular to the neuraxis (see p
20) At this cutting plane, the following structures can be recognized:
- mesencephalic tectum, superior colliculus, (function?)
- cerebral aqueduct
- periaqueductal gray (PAG), (functions?)
- substantia nigra, (function? disease?)
- cerebral peduncles, (origin?, destination?)
The following structures can be localised at the median plane (see Fig 11):
- fourth ventricle with fastigium and obex
- anterior (superior) medullary velum, with lingula
- posterior (inferior) medullary velum, with median aperture, (Foramen of
Magendie, cerebrospinal fluid, CSF!)
- lateral recess and – aperture, (Foramen of Luschka, cerebrospinal fluid, CSF!)
- cerebellum
- “arbor vitae”
- vermis and cerebellar hemispheres
- primary fissure, separating the anterior and posterior lobes
- posterolateral fissure, separating the posterior and flocculonodular lobes
- cerebellar tonsil
- brainstem
- inferior colliculus (function?)
- pons ? middle cerebellar peduncle (MCP)
- pyramidal tract, (origin?, destination?)
- central canal.
Fig 26 C shows how the cerebellum can be subdivided in 3 functionally different
parts:
- ponto- or cerebro-cerebellum: the cerebellar, hemispheres communicating
extensively with the contralateral forebrain, via the pons;
- spinocerebellum: vermis with adjoining medial zone, communicating
extensively with the spinal cord;
- vestibulocerebellum: flocculonodular lobe, communicating extensively with
the vestibular system.
By making a “horizontal cut” through the cerebellum (= parallel to the dorsal surface
of the brainstem!)(see Fig 26A), the “stem” of the “arbour vitae” is transected, to
expose the ‘deep cerebellar nuclei’, with the large dentate nucleus and some
smaller medial nuclei, of which the fastigial nucleus can be usually recognized, as
a separate entity. By cutting additional thin (1-2 mm) slices from the (remnants of
the) cerebellum towards the dorsal surface of the brainstem, but without cutting or
even touching the brainstem itself, the cerebellum can be removed step by step
from the brainstem, to expose the 3 cerebellar peduncles. It may be necessary to
remove the last parts of the cerebellar lobi carefully by hand or using a small
dissection knife.
40
Fig 26. Fig A shows a schematic midline view of the cerebellum and brainstem (for details
see Figs 10,11); Fig B shows the deep cerebellar nuclei as observed in a horizontal section,
as indicated in fig A; Fig C shows a map of the cerebellum, as used in functional anatomical
studies.
Fig 27. Dorsal view of the brainstem (and diencephalon) after removal of the cerebellum.
Now, try to recognize the 3 cerebellar peduncles, connecting the cerebellar
white matter with the brainstem and surrounding the rhomboid 4
th
ventricle, opened
from its dorsal side:
- superior cerebellar peduncle (SCP, brachium conjunctivum), containing
mainly cerebellar efferents towards brainstem and diencephalon; (no ...)
- middle cerebellar peduncle (MCP, brachium pontis), the largest and most
lateral peduncle, containing afferent fibers from the contralateral pontine
nuclei; (no …)
- inferior cerebellar peduncle (ICP, restiform body), containing mainly
afferent fibers from brainstem and spinal cord; (no …)
In between the cerebellar peduncles, the borders of the 4
th
ventricle (rhomboid
fossa) can be observed. Try to localize the following structures:
on the dorsal aspect of the brainstem:
- inferior colliculus (function?) (no …)
- anterior medullary velum, (with emerging n.IV, n. trochlearis; function?)(no …)
- gracile and cuneate tubercles (function?) (no … and no …)
on the floor of the 4
th
ventricle:
- median sulcus and lateral recess (? lateral aperture) (no … and no …)
- striae medullares (‘acousticae’) (no …)
- vestibular area (no …)
- vagal trigone and area postrema (‘circumventricular organ’, lacking a ‘Blood-
Brain-Barrier’) (no …)
obex (no …)
Fig
27
41
The brainstem can now be dissected further as a series of thin (1-2 mm) transverse
sections, which have to be kept in proper order and position.
Try to localize the following brain areas again, at the level of:
Mesencephalon:
- tectum
- tegmentum
o nucleus ruber (‘red nucleus’)
o substantia nigra (hardly detectable in brains with Parkinson’s disease!)
- PAG
- cerebral peduncle
rostral pontine level:
- transverse fibers (? middle cerebellar peduncle)
- locus coeruleus (pigmented nucleus, located laterally along the border of the
expanding 4th ventricle; transmitter substance?)
- (raphe nuclei, invisible macroscopically, but consider its location; transmitter
substance?)
medulla oblongata:
- pyramidal tract (descending fibers; origin? destination?)
- inferior olivary nucleus ( ? contralateral cerebellum)
- dorsal funiculi (ascending fibers, origin? destination?)
8.2 Clinical Notes
- obstructive hydrocephalus;
- tonsillar herniation (Fig 2);
- vascular accidents (? various syndromes, comprising a variety of sensory
and motor symptoms, depending on the specific vessels involved, (see Fig
28) e.g. syndromes of Wallenberg, Weber etc., frequently combining
involvement of ipsilateral cranial nerve nuclei and contralateral long fiber
systems).
42
Fig 28. Main arteries contributing to the vascularisation of the brainstem.
43
9) SPINAL CORD
9.1 Structure and topography of the Spinal Cord
Fig 29. Transverse section through the spinal cord, showing the distribution of gray and
white matter and dorsal and ventral roots.
If preparations of the spinal cord are available for demonstration, they can be
used. If not available, Figs 29 and 30 or additional textbook figures can be used to
study the following structural aspects of the spinal cord:
- length (about 50 cm) and thickness (1-1.5 cm)
- dorsal and ventral roots (‘radix’, functional specificity?)
- gray matter (with dorsal, ventral and lateral horns or cornua; specific
functions?)
- white matter (with dorsal, ventral and lateral columns or funiculi, containing
ascending or descending fibers; specific functions?)
- segmental organisation, dermatomes
- cervical enlargement (‘intumescentia’, C3 – T1, brachial plexus)
- lumbar enlargement (‘intumescentia’, L1 – S2, lumbosacral plexus)
- spinal ‘ascent’ (‘ascensus medullae’)
- cauda equina
- medullary cone (L1/L2)
- meningeal covering (spinal dura mater)
- lumbar cistern (? lumbar puncture, CSF)
9.2 Clinical Notes
- spina bifida
- spinal anesthesia
- demyelinating diseases (multiple sclerosis etc)
- sensory ataxia (posterior column function)
- cordotomy ( pain relief)
- “upper motor neuron disease” (pyramidal tract lesion; from flaccid paralysis
? spasticity and hyperreflexia, “Babinski-sign”)
- “lower motor neuron disease” ( weakness, wasting and areflexia)
- hemi-, para- and tetra-plegia.
44
Fig 30. The longitudinal segmental structure of the spinal cord related to meninges,
vertebral column and dermatomes.
45
10) FRONTAL AND HORIZONTAL SECTIONS OF THE
HUMAN BRAIN
To test your knowledge of the human brain, the following sections can be
studied. Try to label the indicated structures.
Frontal 1
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
46
Frontal 2
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
47
Frontal 3
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
48
Horizontal 1:
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
49
Horizontal 2
1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
50
11) LIST OF ABBREVIATIONS
ACA : anterior cerebral artery
AICA : anterior inferior cerebellar artery
CSF : cerebrospinal fluid
ICP : inferior cerebellar peduncle
MCA : middle cerebral artery
MCP : middle cerebellar peduncle
PAG : periaqueductal gray
PCA : posterior cerebral artery
PICA : posterior inferior cerebellar artery
SCA : superior cerebellar artery
SCP : superior cerebellar peduncle
TIA : transient ischemic attack
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11) INDEX AND GLOSSARY
OF THE BRAIN DISSECTION GUIDE
