Lesson No 18
The
telencephalon includes the cerebral hemispheres with their cavities, the
lateral ventricles.
Each
cerebral hemisphere may be divided into three fundamental parts: the pallium,
the rhinencephalon, and basal nuclei, also fornix and corpus callosum.
Cortex of the Cerebrum
A hemisphere is covered by cortex and has inferior, dorsolateral and
medial surfaces. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of
the temporal lobe, the temporal pole.
The right and left hemispheres are separated medially by a deep cleft, named
the longitudinal cerebral fissure.
The surfaces of the hemispheres are molded into a number of irregular
eminencies, named gyri or
convolutions, and separated by furrows termed fissures and sulci. The
hemispheres consist of 5 lobes: frontal, perietal, occipital, temporal and
insula (located in depth of lateral sulcus).
On
the dorsolateral surface of the hemisphere can be finding the Central sulcus [Rolandi] that separates
frontal and parietal lobes. The Lateral
sulcus [Sylvii] that separates temporal lobe from the frontal and parietal
lobes. Parietooccipital sulcus
passes between parietal and occipital lobes on the medial surface.
There
are some sulci on the dorsolateral surface of the frontal lobe: precentral sulcus,
superior frontal sulcus, and inferior frontal sulcus. They separate: precentral
gyrus, superior frontal gyrus, middle frontal gyrus, and inferior frontal
gyrus. The inferior frontal gyrus is divided into opercular, triangular and
orbital parts by anterior and ascending rami.
The
lateral surface of the parietal lobe
is cleft by a well-marked furrow, the intraparietal sulcus and the postcentral
sulcus. There are postcentral gyrus, superior parietal lobule and inferior
parietal lobule. The last contains the supramarginal and angular gyri.
The
temporal lobe is divided into
superior, middle, and inferior gyri by the superior and middle temporal sulci.
Three or four gyri will be seen springing from the depth of the hinder end of
the lateral sulcus, these are named the transverse
temporal gyri (Heschl).
The
occipital lobe is small and
pyramidal in shape. It is traversed by the transverse occipital sulci that
border occipital gyri.
On
the insula (it is located in depth
of lateral sulcus) they distinguish circular and central sulci which border
longi and breve gyri.
There
are some sulci on the medial surface of the hemisphere:
sulcus corporis callosi, cingulate sulcus, hippocampal sulcus, parietooccipital
and calcarine sulcus. They separte corporus callosum from gyrus cinguli,
superior frontal gyri, gyrus parahippocampalis, dentate gyrus, paracentral
lobule (of Bets), precuneus and cuneus. Gyrus cinguli and gyrus
parahippocampalis with isthmus form fornicate gyrus that it is the central part
of the rhinencephalon.
On
the inferior surface of the
hemisphere can be finding: collateral sulcus, occipitotemporal sulcus, rhinal
sulcus and orbital sulcus. They separate the lateral occipitotemporal gyrus,
medial occipitotemporal gyrus, lingual gyrus, gyrus rectus and orbital gyri.
Cortical Fields
We
distinguish some specific cortical fields that control motor,
sensory, language and others functions. They can be divided into more general
motor area, sensory area and also some specific sensory centres.
Motor area involved
with the control of voluntary muscles. It is located
in precentral gyrus and
paracentral lobule (motor homunculus).
Centre
of conjugate deviaton of the
eyes to the opposite side is located in
posterior part of the middle frontal gyrus.
Sensory area
responsible for cutaneous and muscular sensations (temperature, pain and
tactile), located in postcentral gyrus and paracentral lobule (sensory homunculus).
Centre
of stereognosia is located in
superior parietal lobule closely to intraparietal sulcus. It
responsible for the body
schema: the right can be differentiated from the left.
Auditory
centre located in the transverse temporal gyri (Heschl). Function is
interpretation of auditory sensations.
Visual
cortical centre situated in calcarine sulcus. Function of the occipital lobe is
connscious perception of vision.
Smell
and tasting centre located in part of limbic system - uncus.
Centre
of praxia is located
supramarginal gyrus.
Language
areas:
motor
-
1.
Motor speech area (Broca's centre). Damage in
the region of the lower frontal convolution produces motor aphasia.
2.
Writing (graphic) area located in middle frontal gyrus.
sensory
-
3.
Auditory language centre (Wernicke`s area) located in superior temporal
gyrus. It responsible for understanding of spoken language.
4.
Reading centre situated in angular gyrus
(responsible also for reading, writing, counting and
calculating).
The telencephalon includes:
(1) the cerebral hemispheres with their cavities, the lateral
ventricles; and (2) the pars optica hypothalami and the anterior portion
of the third ventricle (already described under the diencephalon). As
previously stated (see page 744), each cerebral hemisphere may be divided into
three fundamental parts, viz., the rhinencephalon, the corpus striatum, and the
neopallium. The rhinencephalon, associated with the sense of smell, is the
oldest part of the telencephalon, and forms almost the whole of the hemisphere
in some of the lower animals, e. g., fishes, amphibians, and reptiles.
In man it is rudimentary, whereas the neopallium undergoes great development
and forms the chief part of the hemisphere.
The Cerebral Hemispheres.—The cerebral
hemispheres constitute the largest part of the brain, and, when viewed together
from above, assume the form of an ovoid mass broader behind than in front, the
greatest transverse diameter corresponding with a line connecting the two
parietal eminences. The hemispheres are separated medially by a deep cleft,
named the longitudinal cerebral fissure, and each possesses a central
cavity, the lateral ventricle.
The Longitudinal Cerebral Fissure (fissura cerebri
longitudinalis; great longitudinal fissure) contains a sickle-shaped
process of dura mater, the falx cerebri. It front and behind, the
fissure extends from the upper to the under surfaces of the hemispheres and
completely separates them, but its middle portion separates them for only about
one-half of their vertical extent; for at this part they are connected across
the middle line by a great central white commissure, the corpus callosum.
In a median sagittal section (720) the cut corpus callosum presents the appearance of a broad, arched
band. Its thick posterior end, termed the splenium, overlaps the mid-brain,
but is separated from it by the tela chorioidea of the third ventricle and the
pineal body. Its anterior curved end, termed the genu, gradually tapers
into a thinner portion, the rostrum, which is continued downward and
backward in front of the anterior commissure to join the lamina terminalis.
Arching backward from immediately behind the anterior commissure to the under
surface of the splenium is a second white band named the fornix: between
this and the corpus callosum are the laminæ and cavity of the septum
pellucidum.
Surfaces of the Cerebral Hemispheres.—Each
hemisphere presents three surfaces: lateral, medial, and inferior.
The lateral surface is convex in adaptation to the
concavity of the corresponding half of the vault of the cranium. The medial
surface is flat and vertical, and is separated from that of the opposite
hemisphere by the great longitudinal fissure and the falx cerebri. The inferior
surface is of an irregular form, and may be divided into three areas:
anterior, middle, and posterior. The anterior area, formed by the orbital
surface of the frontal lobe, is concave, and rests on the roof of the orbit and
nose; the middle area is convex, and consists of the under surface of the
temporal lobe: it is adapted to the corresponding half of the middle cranial
fossa. The posterior area is concave, directed medialward as well as downward,
and is named the tentorial surface, since it rests upon the tentorium
cerebelli, which intervenes between it and the upper surface of the cerebellum.
These three surfaces are separated from each other by the
following borders: (a) supero-medial, between the lateral
and medial surfaces; (b) infero-lateral, between the lateral and
inferior surfaces; the anterior part of this border separating the lateral from
the orbital surface, is known as the superciliary border; (c) medial
occipital, separating the medial and tentorial surfaces; and (d) medial
orbital, separating the orbital from the medial surface. The anterior end
of the hemisphere is named the frontal pole; the posterior, the occipital
pole; and the anterior end of the temporal lobe, the temporal pole.
About
The surfaces of the hemispheres are moulded into a number of
irregular eminences, named gyri or convolutions, and separated by
furrows termed fissures and sulci. The furrows are of two kinds, complete
and incomplete. The former appear early in fetal life, are few in number,
and are produced by infoldings of the entire thickness of the brain wall, and
give rise to corresponding elevations in the interior of the ventricle. They
comprise the hippocampal fissure, and parts of the calcarine and collateral
fissures. The incomplete furrows are very numerous, and only indent the
subjacent white substance, without producing any corresponding elevations in
the ventricular cavity.
The gyri and their intervening fissures and the sulci are
fairly constant in their arrangement; at the same time they vary within certain
limits, not only in different individuals, but on the two hemispheres of the
same brain. The convoluted condition of the surface permits of a great increase
of the gray matter without the sacrifice of much additional space. The number
and extent of the gyri, as well as the depth of the intervening furrows, appear
to bear a direct relation to the intellectual powers of the individual.
Certain of the fissures and sulci are utilized for the
purpose of dividing the hemisphere into lobes, and are therefore termed interlobular;
included under this category are the lateral cerebral, parietoöccipital,
calcarine, and collateral fissures, the central and cingulate sulci, and the
sulcus circularis.
Lateral surface
of left cerebral hemisphere, viewed from the side.
The Lateral Cerebral Fissure (fissura cerebri
lateralis [Sylvii]; fissure of Sylvius) (726) is a well-marked cleft on the inferior and lateral surfaces of the
hemisphere, and consists of a short stem which divides into three rami. The stem
is situated on the base of the brain, and commences in a depression at the
lateral angle of the anterior perforated substance. From this point it extends
between the anterior part of the temporal lobe and the orbital surface of the
frontal lobe, and reaches the lateral surface of the hemisphere. Here it
divides into three rami: an anterior horizontal, an anterior ascending, and a
posterior. The anterior horizontal ramus passes foward for about
The Central Sulcus (sulcus centralis [Rolandi];
fissure of Rolando; central fissure) ( 725,
726) is situated about the middle of the lateral surface of the hemisphere,
and begins in or near the longitudinal cerebral fissure, a little behind its
mid-point. It runs sinuously downward and forward, and ends a little above the
posterior ramus of the lateral fissure, and about
The Parietoöccipital Fissure (fissura parietoöccipitalis).—Only a small part of this fissure is seen on the lateral surface of the
hemisphere, its chief part being on the medial surface.
The lateral part of the parietoöccipital fissure
(726) is situated about
The Sulcus Circularis (circuminsular fissure) (731) is on the lower and lateral surfaces of the hemisphere: it surrounds
the insula and separates it from the frontal, parietal, and temporal lobes.
Lobes of the Hemispheres.—By means of
these fissures and sulci, assisted by certain arbitrary lines, each hemisphere
is divided into the following lobes: the frontal, the parietal,
the temporal, the occipital, the limbic, and the insula.
Frontal Lobe (lobus frontalis).—On
the lateral surface of the hemisphere this lobe extends from the frontal pole
to the central sulcus, the latter separating it from the parietal lobe. Below,
it is limited by the posterior ramus of the lateral fissure, which intervenes
between it and the central lobe. On the medial surface, it is separated from
the cingulate gyrus by the cingulate sulcus; and on the inferior surface, it is
bounded behind by the stem of the lateral fissure.
Principal
fissures and lobes of the cerebrum viewed laterally.
The lateral surface of the frontal lobe (726)
is tranversed by three sulci which divide it into four gyri:
the sulci are named the precentral, and the superior and inferior frontal; the
gyri are the anterior central, and the superior, middle, and inferior frontal.
The precentral sulcus runs parallel to the central sulcus, and is
usually divided into an upper and a lower part; between it and the central
sulcus is the anterior central gyrus. From the precentral sulcus, the superior
and inferior frontal sulci run forward and downward, and divide the
remainder of the lateral surface of the lobe into three parallel gyri, named,
respectively the superior, middle, and inferior frontal gyri.
The anterior central gyrus (gyrus centralis
anterior; ascending frontal convolution; precentral gyre) is bounded in front
by the precentral sulcus, behind by the central sulcus; it extends from the
supero-medial border of the hemisphere to the posterior ramus of the lateral
fissure.
The superior frontal gyrus (gyrus frontalis
superior; superfrontal gyre) is situated above the superior frontal sulcus
and is continued on to the medial surface of the hemisphere. The portion on the
lateral surface of the hemisphere is usually more or less completely subdivided
into an upper and a lower part by an antero-posterior sulcus, the paramedial
sulcus, which, however, is frequently interrupted by bridging gyri.
The middle frontal gyrus (gyrus frontalis medius;
medifrontal gyre), between the superior and inferior frontal sulci, is
continuous with the anterior orbital gyrus on the inferior surface of the
hemisphere; it is frequently subdivided into two by a horizontal sulcus, the medial
frontal sulcus of Eberstaller, which ends anteriorly in a wide bifurcation.
The inferior frontal gyrus (gyrus frontalis
inferior; subfrontal gyre) lies below the inferior frontal sulcus, and
extends forward from the lower part of the precentral sulcus; it is continuous
with the lateral and posterior orbital gyri on the under surface of the lobe.
It is subdivided by the anterior horizontal and ascending rami of the lateral
fissure into three parts, viz., (1) the orbital part, below the anterior
horizontal ramus of the fissure; (2) the triangular part (cap of
Broca), between the ascending and horizontal rami; and (3) the basilar
part, behind the anterior ascending ramus. The left inferior frontal gyrus
is, as a rule, more highly developed than the right, and is named the gyrus
of Broca, from the fact that Broca described it as the center for
articulate speech.
The inferior or orbital surface of the frontal
lobe is concave, and rests on the orbital plate of the frontal bone (729). It is divided into four orbital gyri by a well-marked H-shaped orbital
sulcus. These are named, from their position, the medial, anterior,
lateral, and posterior orbital gyri. The medial orbital gyrus
presents a well-marked antero-posterior sulcus, the olfactory sulcus,
for the olfactory tract; the portion medial to this is named the straight
gyrus, and is continuous with the superior frontal gyrus on the medial
surface.
Temporal Lobe (lobus temporalis).—The
temporal lobe presents superior, lateral, and inferior surfaces.
The superior surface forms the lower limit of the
lateral fissure and overlaps the insula. On opening out the lateral fissure, three
or four gyri will be seen springing from the depth of the hinder end of the
fissure, and running obliquely forward and outward on the posterior part of the
upper surface of the superior temporal gyrus; these are named the transverse
temporal gyri (Heschl) (730).
The lateral surface (726) is bounded above by the posterior ramus of the lateral fissure, and by
the imaginary line continued backward from it; below, it is limited by the
infero-lateral border of the hemisphere. It is divided into superior, middle,
and inferior gyri by the superior and middle temporal sulci. The superior
temporal sulcus runs from before backward across the temporal lobe, some
little distance below, but parallel with, the posterior ramus of the lateral
fissure; and hence it is often termed the parallel sulcus. The middle
temporal sulcus takes the same direction as the superior, but is situated
at a lower level, and is usually subdivided into two or more parts. The superior
temporal gyrus lies between the posterior ramus of the lateral fissure and
the superior temporal sulcus, and is continuous behind with the supramarginal
and angular gyri. The middle temporal gyrus is placed between the
superior and middle temporal sulci, and is joined posteriorly with the angular
gyrus. The inferior temporal gyrus is placed below the middle temporal
sulcus, and is connected behind with the inferior occipital gyrus; it also
extends around the infero-lateral border on to the inferior surface of the temporal
lobe, where it is limited by the inferior sulcus.
Section of brain
showing upper surface of temporal lobe.
The inferior surface is concave, and is continuous
posteriorly with the tentorial surface of the occipital lobe. It is traversed
by the inferior temporal sulcus, which extends from near the occipital
pole behind, to within a short distance of the temporal pole in front, but is
frequently subdivided by bridging gyri. Lateral to this fissure is the narrow
tentorial part of the inferior temporal gyrus, and medial to it the fusiform
gyrus, which extends from the occipital to the temporal pole; this gyrus is
limited medially by the collateral fissure, which separates it from the lingual
gyrus behind and from the hippocampal gyrus in front.
The Insula (island of Reil; central lobe) (731) lies deeply in the lateral or Sylvian fissure, and can only be seen
when the lips of that fissure are widely separated, since it is overlapped and
hidden by the gyri which bound the fissure. These gyri are termed the opercula
of the insula; they are separated from each other by the three rami of the
lateral fissure, and are named the orbital, frontal, frontoparietal, and
temporal opercula. The orbital operculum lies below the anterior
horizontal ramus of the fissure, the frontal between this and the
anterior ascending ramus, the parietal between the anterior ascending
ramus and the upturned end of the posterior ramus, and the temporal
below the posterior ramus. The frontal operculum is of small size in those
cases where the anterior horizontal and ascending rami of the lateral fissure
arise from a common stem. The insula is surrounded by a deep circular sulcus
which separates it from the frontal, parietal, and temporal lobes. When the
opercula have been removed, the insula is seen as a triangular eminence, the
apex of which is directed toward the anterior perforated substance. It is
divided into a larger anterior and a smaller posterior part by a deep sulcus,
which runs backward and upward from the apex of the insula. The anterior part
is subdivided by shallow sulci into three or four short gyri, while the
posterior part is formed by one long gyrus, which is often bifurcated at
its upper end. The cortical gray substance of the insula is continuous with
that of the different opercula, while its deep surface corresponds with the
lentiform nucleus of the corpus striatum.
The insula of
the left side, exposed by removing the opercula.
The term limbic lobe was
introduced by Broca, and under it he included the cingulate and hippocampal
gyri, which together arch around the corpus callosum and the hippocampal
fissure. These he separated on the morphological ground that they are
well-developed in animals possessing a keen sense of smell (osmatic animals),
such as the dog and fox. They were thus regarded as a part of the
rhinencephalon, but it is now recognized that they belong to the neopallium;
the cingulate gyrus is therefore sometimes described as a part of the frontal
lobe, and the hippocampal as a part of the temporal lobe.
On the medial
surface of the brain students should find:
· Cerebral hemispheres
communicate each other by corpus callosum.
Corpus callosum anteriorly carries a genu
that passes into rostrum. Last
continue as a lamina rostralis and lamina terminalis. Back part of the corpus
callosum called splemium
Lateral surface
of left cerebral hemisphere, viewed from above.
·
· Above
the corpus callosum there is fornix
cerebri posteriorly passes into crura
fornicis, anteriorly continue as a columna
fornicis
· Lamina septi pellucidi is
tightened between corpus callosum and columna fornicis
Thalamus
positioned under fornix
· Backward
from the thalamus there is quadrigeminal
plate with two upper and two lower hillocks, the superior and inferior colliculi
Medial surface of left
cerebral hemisphere.
The medial part of the parietoöccipital fissure (727) runs downward and forward as a deep cleft on the medial surface of the
hemisphere, and joins the calcarine fissure below and behind the posterior end
of the corpus callosum. In most cases it contains a submerged gyrus.
The Calcarine Fissure (fissura calcarina) (727) is on the medial surface of the hemisphere. It begins near the
occipital pole in two converging rami, and runs forward to a point a little
below the splenium of the corpus callosum, where it is joined at an acute angle
by the medial part of the parietoöccipital fissure. The anterior part of
this fissure gives rise to the prominence of the calcar avis in the
posterior cornu of the lateral ventricle.
The Cingulate Sulcus (sulcus cinguli; callosomarginal
fissure) (727) is on the medial surface of the hemisphere; it begins below the
anterior end of the corpus callosum and runs upward and forward nearly parallel
to the rostrum of this body and, curving in front of the genu, is continued
backward above the corpus callosum, and finally ascends to the supero-medial
border of the hemisphere a short distance behind the upper end of the central
sulcus. It separates the superior frontal from the cingulate gyrus.
The Collateral Fissure (fissura collateralis) (727) is on the tentorial surface of the hemisphere and extends from near the
occipital pole to within a short distance of the temporal pole. Behind, it lies
below and lateral to the calcarine fissure, from which it is separated by the
lingual gyrus; in front, it is situated between the hippocampal gyrus and the
anterior part of the fusiform gyrus.
·
· Cerebral aqueduct [Silvii] passes
under mesencephalic tectum. It communicates front with the 3d ventricle, back with the 4th ventricle
· The
4th ventricle is bordered
upper and lower by the superior
medullary velum and inferior medullary velum
· The
floor of the IVth ventricle, the rhomboid fossa, occupies the dorsal surface of the medulla
oblongata and the pons
· Cerebellum
consists of two hemispheres which connected by vermis
Cerebral
hemispheres are separated by median
longitudinal fissura that passes to the corpus callosum. Cerebrum is separated
from the cerebellum by transverse
fissura.
The medial surface of the frontal lobe is occupied by
the medial part of the superior frontal gyrus (marginal gyrus) (727). It lies between the cingulate sulcus and the supero-medial margin of
the hemisphere. The posterior part of this gyrus is sometimes marked off by a
vertical sulcus, and is distinguished as the paracentral lobule, because
it is continuous with the anterior and posterior central gyri.
Parietal Lobe (lobus parietalis).—The
parietal lobe is separated from the frontal lobe by the central sulcus, but its
boundaries below and behind are not so definite. Posteriorly, it is limited by
the parietoöccipital fissure, and by a line carried across the hemisphere from
the end of this fissure toward the preoccipital notch. Below, it is separated
from the temporal lobe by the posterior ramus of the lateral fissure, and by a
line carried backward from it to meet the line passing downward to the
preoccipital notch.
Orbital surface
of left frontal lobe.
The lateral surface of the parietal lobe (726) is cleft by a well-marked furrow, the intraparietal sulcus of
Turner, which consists of an oblique and a horizontal portion. The oblique part
is named the postcentral sulcus, and commences below, about midway
between the lower end of the central sulcus and the upturned end of the lateral
fissure. It runs upward and backward, parallel to the central sulcus, and is
sometimes divided into an upper and a lower ramus. It forms the
hinder limit of the posterior central gyrus.
From about the middle of the postcentral sulcus, or from the
upper end of its inferior ramus, the horizontal portion of the
intraparietal sulcus is carried backward and slightly upward on the parietal
lobe, and is prolonged, under the name of the occipital ramus, on to the
occipital lobe, where it divides into two parts, which form nearly a right
angle with the main stem and constitute the transverse occipital sulcus.
The part of the parietal lobe above the horizontal portion of the intraparietal
sulcus is named the superior parietal lobule; the part below, the inferior
parietal lobule.
The posterior central gyrus (gyrus centralis
posterior; ascending parietal convolution; postcentral gyre) extends from
the longitudinal fissure above to the posterior ramus of the lateral fissure
below. It lies parallel with the anterior central gyrus, with which it is
connected below, and also, sometimes, above, the central sulcus.
The superior parietal lobule (lobulus parietalis superior)
is bounded in front by the upper part of the postcentral sulcus, but is usually
connected with the posterior central gyrus above the end of the sulcus; behind
it is the lateral part of the parietoöccipital fissure, around the end of
which it is joined to the occipital lobe by a curved gyrus, the arcus
parietoöccipitalis; below, it is separated from the inferior parietal
lobule by the horizontal portion of the intraparietal sulcus.
The inferior parietal lobule (lobulus parietalis
inferior; subparietal district or lobule) lies below the horizontal portion
of the intraparietal sulcus, and behind the lower part of the postcentral
sulcus. It is divided from before backward into two gyri. One, the supramarginal,
arches over the upturned end of the lateral fissure; it is continuous in front
with the postcentral gyrus, and behind with the superior temporal gyrus. The
second, the angular, arches over the posterior end of the superior
temporal sulcus, behind which it is continuous with the middle temporal gyrus.
The medial surface of the parietal lobe (727) is bounded behind by the medial part of the parietoöccipital
fissure; in front, by the posterior end of the cingulate sulcus; and below, it
is separated from the cingulate gyrus by the subparietal sulcus. It is
of small size, and consists of a square-shaped convolution, which is termed the
precuneus or quadrate lobe.
Occipital Lobe (lobus occipitalis).—The
occipital lobe is small and pyramidal in shape; it presents three surfaces: lateral,
medial, and tentorial.
The lateral surface is limited in front by the
lateral part of the parietoöccipital fissure, and by a line carried from
the end of this fissure to the preoccipital notch; it is traversed by the
transverse occipital and the lateral occipital sulci. The transverse
occipital sulcus is continuous with the posterior end of the occipital
ramus of the intraparietal sulcus, and runs across the upper part of the lobe,
a short distance behind the parietoöccipital fissure. The lateral
occipital sulcus extends from behind forward, and divides the lateral
surface of the occipital lobe into a superior and an inferior gyrus,
which are continuous in front with the parietal and temporal lobes. 125
The medial surface of the occipital lobe is bounded
in front by the medial part of the parietoöccipital fissure, and is
traversed by the calcarine fissure, which subdivides it into the cuneus and the
lingual gyrus. The cuneus is a wedge-shaped area between the calcarine fissure
and the medial part of the parietoöccipital fissure. The lingual gyrus
lies between the calcarine fissure and the posterior part of the collateral
fissure; behind, it reaches the occipital pole; in front, it is continued on to
the tentorial surface of the temporal lobe, and joins the hippocampal gyrus.
The tentorial surface of the occipital lobe is
limited in front by an imaginary transverse line through the preoccipital
notch, and consists of the posterior part of the fusiform gyrus (occipitotemporal
convolution) and the lower part of the lingual gyrus, which are separated
from each other by the posterior segment of the collateral fissure.
The cingulate gyrus (gyrus cinguli; callosal convolution) is
an arch-shaped convolution, lying in close relation to the superficial surface
of the corpus callosum, from which it is separated by a slit-like fissure, the callosal
fissure. It commences below the rostrum of the corpus callosum, curves
around in front of the genu, extends along the upper surface of the body, and
finally turns downward behind the splenium, where it is connected by a narrow isthmus
with the hippocampal gyrus. It is separated from the medial part of the
superior frontal gyrus by the cingulate sulcus, and from the precuneus by the
subparietal sulcus.
The hippocampal gyrus (gyrus hippocampi) is
bounded above by the hippocampal fissure, and below by the anterior part of the
collateral fissure. Behind, it is continuous superiorly, through the isthmus,
with the cingulate gyrus and inferiorly with the lingual gyrus. Running in the
substance of the cingulate and hippocampal gyri, and connecting them together,
is a tract of arched fibers, named the cingulum (page 843). The anterior
extremity of the hippocampal gyrus is recurved in the form of a hook (uncus),
which is separated from the apex of the temporal lobe by a slight fissure, the incisura
temporalis. Although superficially continuous with the hippocampal gyrus,
the uncus forms morphologically a part of the rhinencephalon.
Scheme of
rhinencephalon.
The Hippocampal sulcus begins immediately behind the
splenium of the corpus callosum, and runs forward between the hippocampal and
dentate gyri to end in the uncus. It is a complete fissure (page 819), and
gives rise to the prominence of the hippocampus in the inferior cornu of the
lateral ventricle.
The Rhinencephalon, associated with the sense of smell, is the oldest
part of the telencephalon, and forms almost the whole of the hemisphere in some
of the lower animals, e. g., fishes, amphibians, and reptiles. In man it is
rudimentary. It divides into central and peripheral parts. The central part includes the hippocampus,
gyrus fornicatus (gyrus cinguli + gyrus hyppocampi), gyrus dentatus, septum
pellucidum and uncus. The peripheral
part includes the olfactory bulb, olfactory tract, olfactory trigone and
anterior perforated substance. Cortical smell analyzer located in the uncus. Rhinencephalon is a center
of emotional colouring of sensible perception of external environment (Limbic system). Together from all
subcortical centers it is by energy source for cortex and answers for vitally
important man reactions regulates activity of internal organs: hunger feeling
and thirst, sounds perceptions and smells. Here are the memory mechanisms.
Scheme of rhinencephalon.
The rhinencephalon comprises the olfactory lobe, the uncus,
the subcallosal and supracallosal gyri, the fascia dentata
hippocampi, the septum pellucidum, the fornix, and the hippocampus.
1. The Olfactory Lobe (lobus olfactorius) is
situated under the inferior or orbital surface of the frontal lobe. In many
vertebrates it constitutes a well-marked portion of the hemisphere and contains
an extension of the lateral ventricle; but in man and some other mammals it is
rudimentary. It consists of the olfactory bulb and tract, the olfactory
trigone, the parolfactory area of Broca, and the anterior
perforated substance.
(a) The olfactory bulb (bulbus olfactorius) is an oval,
reddish-gray mass which rests on the cribriform plate of the ethmoid and forms
the anterior expanded extremity of the olfactory tract. Its under surface
receives the olfactory nerves, which pass upward through the cribriform plate
from the olfactory region of the nasal cavity.
(b) The olfactory tract (tractus
olfactorius) is a narrow white band, triangular on coronal section, the
apex being directed upward. It lies in the olfactory sulcus on the inferior
surface of the frontal lobe, and divides posteriorly into two striæ, a
medial and a lateral. The lateral stria is directed across the lateral
part of the anterior perforated substance and then bends abruptly medialward
toward the uncus of the hippocampal gyrus. The medial stria turns
medialward behind the parolfactory area and ends in the subcallosal gyrus; in
some cases a small intermediate stria is seen running backward to the
anterior perforated substance.
(c) The olfactory trigone (trigonum
olfactorium) is a small triangular area in front of the anterior perforated
substance. Its apex, directed forward, occupies the posterior part of the olfactory
sulcus, and is brought into view by throwing back the olfactory tract.
(d) The parolfactory area of Broca (area
parolfactoria) is a small triangular field on the medial surface of the
hemisphere in front of the subcallosal gyrus, from which it is separated by the
posterior parolfactory sulcus; it is continuous below with the olfactory
trigone, and above and in front with the cingulate gyrus; it is limited
anteriorly by the anterior parolfactory sulcus.
(e) The anterior perforated substance (substantia
perforata anterior) is an irregularly quadrilateral area in front of the
optic tract and behind the olfactory trigone, from which it is separated by the
fissure prima; medially and in front it is continuous with the
subcallosal gyrus; laterally it is bounded by the lateral stria of the
olfactory tract and is continued into the uncus. Its gray substance is
confluent above with that of the corpus striatum, and is perforated anteriorly
by numerous small bloodvessels.
2. The Uncus has already been described (page 826) as
the recurved, hook-like portion of the hippocampal gyrus.
3. The Subcallosal, Supracallosal, and Dentate
Gyri form a rudimentary arch-shaped lamina of gray substance extending over
the corpus callosum and above the hippocampal gyrus from the anterior
perforated substance to the uncus.
(a) The subcallosal gyrus (gyrus
subcallosus; peduncle of the corpus callosum) is a narrow lamina on the
medial surface of the hemisphere in front of the lamina terminalis, behind the
parolfactory area, and below the rostrum of the corpus callosum. It is
continuous around the genu of the corpus callosum with the supracallosal gyrus.
(b) The supracallosal gyrus (indusium
griseum; gyrus epicallosus) consists of a thin layer of gray substance in
contact with the upper surface of the corpus callosum and continuous laterally
with the gray substance of the cingulate gyrus. It contains two longitudinally
directed strands of fibers termed respectively the medial and lateral
longitudinal striæ. The supracallosal gyrus is prolonged around the
splenium of the corpus callosum as a delicate lamina, the fasciola cinerea,
which is continuous below with the fascia dentata hippocampi.
(c) The fascia dentata hippocampi (gyrus
dentatus) is a narrow band extending downward and forward above the
hippocampal gyrus but separated from it by the hippocampal fissure; its free
margin is notched and overlapped by the fimbria—the fimbriodentate fissure
intervening. Anteriorly it is continued into the notch of the uncus, where it
forms a sharp bend and is then prolonged as a delicate band, the band of
Giacomini, over the uncus, on the lateral surface of which it is lost.
The remaining parts of the rhinencephalon, viz., the septum
pellucidum, fornix, and hippocampus, will be described in connection with the
lateral ventricle.
White Matter of the Cerebrum. The
external
capsule located between putamen and claustrum. The extrema capsule separates
the claustrum and cortex of the insula.
The corpus
callosum connect right and left cerebral
hemispheres. Corpus callosum anteriorly carries a genu that passes into rostrum.
Last continue as a lamina rostralis
and lamina terminalis. Back part of
the corpus callosum called splemium.
Anterior fibers of the corpus callosum form the frontal forceps, posterior fibers of the corpus callosum - the occipital
forceps.
The Corpus Callosum (733) is the great transverse commissure which unites the cerebral
hemispheres and roofs in the lateral ventricles. A good conception of its
position and size is obtained by examining a median sagittal section of the
brain (720), when it is seen to form an arched structure about
Corpus callosum from
above.
The anterior end is named the genu, and is
bent downward and backward in front of the septum pellucidum; diminishing
rapidly in thickness, it is prolonged backward under the name of the rostrum,
which is connected below with the lamina terminalis. The anterior cerebral
arteries are in contact with the under surface of the rostrum; they then arch
over the front of the genu, and are carried backward above the body of the
corpus callosum.
The posterior end is termed the splenium and
constitutes the thickest part of the corpus callosum. It overlaps the tela
chorioidea of the third ventricle and the mid-brain, and ends in a thick,
convex, free border. A sagittal section of the splenium shows that the
posterior end of the corpus callosum is acutely bent forward, the upper and
lower parts being applied to each other.
The superior surface is convex from before backward,
and is about
The inferior surface is concave, and forms on either
side of the middle line the roof of the lateral ventricle. Medially, this
surface is attached in front to the septum pellucidum; behind this it is fused
with the upper surface of the body of the fornix, while the splenium is in
contact with the tela chorioidea.
On either side, the fibers of the corpus callosum radiate in
the white substance and pass to the various parts of the cerebral cortex; those
curving forward from the genu into the frontal lobe constitute the forceps
anterior, and those curving backward into the occipital lobe, the forceps
posterior. Between these two parts is the main body of the fibers which
constitute the tapetum and extend laterally on either side into the temporal
lobe, and cover in the central part of the lateral ventricle.
The fornix
cerebri located under corpus
callosum and has a body, columna fornicis
(anteriorly) and crura fornicis
(posteriorly). Crus fused with the hippocampus and form the fimbria hippocampi. Anterior commissura positioned closly to the columna fornicis.
The Fornix (720,
747,
748) is a longitudinal, arch-shaped lamella of white substance, situated
below the corpus callosum, and continuous with it behind, but separated from it
in front by the septum pellucidum. It may be described as consisting of two
symmetrical bands, one for either hemisphere. The two portions are not united
to each other in front and behind, but their central parts are joined together
in the middle line. The anterior parts are called the columns of the
fornix; the intermediate united portions, the body; and the posterior
parts, the crura.
Diagram of the tracts
in the internal capsule. Motor tract red. The sensory tract (blue) is not
direct, but formed of neurons receiving impulses from below
in the thalamus and transmitting them to the cortex. The optic radiation
(occipitothalamic) is shown in violet.
The body (corpus fornicis) of the fornix is
triangular, narrow in front, and broad behind. The medial part of its upper
surface is connected to the septum pellucidum in front and to the corpus
callosum behind. The lateral portion of this surface forms part of the floor of
the lateral ventricle, and is covered by the ventricular epithelium. Its
lateral edge overlaps the choroid plexus, and is continuous with the epithelial
covering of this structure. The under surface rests upon the tela chorioidea of
the third ventricle, which separates it from the epithelial roof of that
cavity, and from the medial portions of the upper surfaces of the thalami.
Below, the lateral portions of the body of the fornix are joined by a thin
triangular lamina, named the psalterium (lyra). This lamina
contains some transverse fibers which connect the two hippocampi across the
middle line and constitute the hippocampal commissure. Between the
psalterium and the corpus callosum a horizontal cleft, the so-called ventricle
of the fornix (ventricle of Verga), is sometimes found.
The columns (columna fornicis; anterior pillars;
fornicolumns) of the fornix arch downward in front of the interventricular
foramen and behind the anterior commissure, and each descends through the gray
substance in the lateral wall of the third ventricle to the base of the brain,
where it ends in the corpus mammillare. From the cells of the corpus mammillare
the thalamomammillary fasciculus (bundle of Vicq d’Azyr) takes
origin and is prolonged into the anterior nucleus of the thalamus. The column
of the fornix and the thalamomammillary fasciculus together form a loop
resembling the figure 8, but the continuity of the loop is broken in the corpus
mammillare. The column of the fornix is joined by the stria medullaris of the
pineal body and by the superficial fibers of the stria terminalis, and is said
to receive also fibers from the septum pellucidum. Zuckerkandl describes an olfactory
fasciculus which becomes detached from the main portion of the column of
the fornix, and passes downward in front of the anterior commissure to the base
of the brain, where it divides into two bundles, one joining the medial stria
of the olfactory tract; the other joins the subcallosal gyrus, and through it
reaches the hippocampal gyrus.
Diagram of the fornix.
The fornix and corpus
callosum from below.
The crura (crus fornicis; posterior pillars)
of the fornix are prolonged backward from the body. They are flattened bands,
and at their commencement are intimately connected with the under surface of
the corpus callosum. Diverging from one another, each curves around the
posterior end of the thalamus, and passes downward and forward into the
inferior cornu of the lateral ventricle (750). Here it lies along the concavity of the hippocampus, on the surface of
which some of its fibers are spread out to form the alveus, while the
remainder are continued as a narrow white band, the fimbria hippocampi,
which is prolonged into the uncus of the hippocampal gyrus. The inner edge of
the fimbria overlaps the fascia dentata hippocampi (dentate gyrus)
(page 827), from which it is separated by the fimbriodentate fissure;
from its lateral margin, which is thin and ragged, the ventricular epithelium
is reflected over the choroid plexus as the latter projects into the chorioidal
fissure.
Interventricular Foramen (foramen of Monro).—Between the columns of the fornix and the anterior ends of the thalami,
an oval aperture is present on either side: this is the interventricular
foramen, and through it the lateral ventricles communicate with the third
ventricle. Behind the epithelial lining of the foramen the choroid plexuses of
the lateral ventricles are joined across the middle line.
The Anterior Commissure (precommissure) is a
bundle of white fibers, connecting the two cerebral hemispheres across the
middle line, and placed in front of the columns of the fornix. On sagittal
section it is oval in shape, its long diameter being vertical and measuring
about
The Septum Pellucidum (septum lucidum) (720) is a thin, vertically placed partition consisting of two laminæ,
separated in the greater part of their extent by a narrow chink or interval,
the cavity of the septum pellucidum. It is attached, above, to the under
surface of the corpus callosum; below, to the anterior part of the fornix
behind, and the reflected portion of the corpus callosum in front. It is
triangular in form, broad in front and narrow behind; its inferior angle
corresponds with the upper part of the anterior commissure. The lateral surface
of each lamina is directed toward the body and anterior cornu of the lateral
ventricle, and is covered by the ependyma of that cavity.
The cavity of the septum pellucidum (cavum septi
pellucidi; pseudocele; fifth ventricle) is generally regarded as part of
the longitudinal cerebral fissure, which has become shut off by the union of
the hemispheres in the formation of the corpus callosum above and the fornix
below. Each half of the septum therefore forms part of the medial wall of the
hemisphere, and consists of a medial layer of gray substance, derived from that
of the cortex, and a lateral layer of white substance continuous with that of
the cerebral hemispheres. This cavity is not developed from the cavity of the
cerebral vesicles, and never communicates with the ventricles of the brain.
The septum pellucidum is tightened between corpus callosum and fornix.
It consists of the 2 laminae and cavity between them.
Lateral ventricles
The two lateral ventricles are cavities situated in
the lower and medial parts of the cerebral hemispheres. They are separated from
each other by a median vertical partition, the septum pellucidum, but communicate with the third ventricle and
indirectly with each other through the interventricular
foramen. Each lateral ventricle consists of a central part, and three
prolongations from it, termed anterior, posterior and inferior cornua.
The anterior horns passes forward into the
frontal lobe. It bordered:
· medially - by lamina of septum pellucidum
· laterally - by head of caudate nucleus
· anteriorly and roof - by corpus callosum
Central
part is found in parietal lobe. It is limited:
·
below - by body of caudate nucleus and dorsal surface of the thalamus;
·
upwards and laterally - by fibers of corpus callosum, which form a roof.
The
posterior horns localised in occipital lobes and cover by white
matter ‘tapetum’. They carry the bulb and the calcar avis on medial wall, and a
collateral triangle on the floor.
The
inferior horns are found in temporal lobe. They are boundered:
·
medially - by hippocampus;
·
below - by white matter, which forms collateral eminence;
·
superolaterally - by white matter;
·
superomedially - by a tail of caudate nucleus.
The
central part and temporal horn of lateral ventricle contain choroid plexus of lateral ventricle
generated of penetration pia mater by vessels. Choroid plexus passes to the III
ventricle through interventricular foramen. Choroid plexus takes part in
formation of larger half of cerebrospinal fluid.
A cerebrospinal
fluid passes from lateral ventricles through the interventricular foramen
into third ventricle, where its amount increases. Then it flows from third
ventricle through the cerebral aqueduct into fourth ventricle. In the fourth
ventricle a cerebrospinal fluid passes the subarachnoid space through the
median (of Magendie) and lateral (of Luschka) aperture and also to the central
canal of the spinal cord. Cerebrospinal fluid of the subarachnoid space returns
to the venous blood in the venous sinuses through the Pachioni’s arachnoid
granulation.
Basal
nuclei are the paired masses of gray matter located deep
within the white matter in base of the forebrain. Basal nuclei include 1)
corpus striatum, 2) claustrum and 3) nucleus amygdaloideus.
Corpus
striatum is composed of caudate
nucleus (it has a head, body and tail) and lentiform nucleus (it consists of medial
and lateral globus pallidus and putamen). There is a thick lamina of
white substance, the internal capsule between caudate
nucleus and globus pallidus. It has the prominence of the curve is called the genu, the frontal crus and the occipital
crus. The occipital crus separates the lentiform nucleus from the thalamus
and carries optic and acoustic radiation.
The nucleus
amygdaloideus is an ovoid gray mass situated at the lower end of the
inferior cornu of the lateral ventricle (in temporal lobe).
The corpus striatum has
received its name from the striped appearance which a section of its anterior
part presents, in consequence of diverging white fibers being mixed with the
gray substance which forms its chief mass. A part of the corpus striatum is
imbedded in the white substance of the hemisphere, and is therefore external to
the ventricle; it is termed the extraventricular portion, or the lentiform
nucleus; the remainder, however, projects into the ventricle, and is named
the intraventricular portion, or the caudate nucleus (737).
The caudate nucleus (nucleus caudatus; caudatum)
(741,
742) is a pear-shaped, highly arched gray mass; its broad extremity, or head,
is directed forward into the anterior cornu of the lateral ventricle, and is
continuous with the anterior perforated substance and with the anterior end of
the lentiform nucleus; its narrow end, or tail, is directed backward on
the lateral side of the thalamus, from which it is separated by the stria
terminalis and the terminal vein. It is then continued downward into the roof
of the inferior cornu, and ends in the putamen near the apex of the temporal
lobe. It is covered by the lining of the ventricle, and crossed by some veins
of considerable size. It is separated from the lentiform nucleus, in the
greater part of its extent, by a thick lamina of white substance, called the
internal capsule, but the two portions
of the corpus striatum are united in front (743,
744).
Horizontal section of right cerebral
hemisphere.
The lentiform nucleus
(nucleus lentiformis; lenticular nucleus; lenticula) (741) is lateral to the caudate nucleus and thalamus, and is seen only in
sections of the hemisphere. When divided horizontally, it exhibits, to some
extent, the appearance of a biconvex lens (742), while a coronal section of its central part presents a somewhat
triangular outline. It is shorter than the caudate nucleus and does not extend
as far forward. It is bounded laterally by a lamina of white substance called
the external capsule, and lateral to this is a thin layer of gray
substance termed the claustrum. Its anterior end is continuous with the
lower part of the head of the caudate nucleus and with the anterior perforated
substance.
In a coronal section through the middle of the lentiform
nucleus, two medullary laminæ are seen dividing it into three
parts. The lateral and largest part is of a reddish color, and is known as the putamen,
while the medial and intermediate are of a yellowish tint, and together
constitute the globus pallidus; all three are marked by fine radiating
white fibers, which are most distinct in the putamen (744).
The gray substance of the corpus striatum is traversed by
nerve fibers, some of which originate in it. The cells are multipolar, both
large and small; those of the lentiform nucleus contain yellow pigment. The
caudate and lentiform nuclei are not only directly continuous with each other
anteriorly, but are connected to each other by numerous fibers. The corpus
striatum is also connected: (1) to the cerebral cortex, by what are termed the corticostriate
fibers; (2) to the thalamus, by fibers which pass through the internal
capsule, and by a strand named the ansa lentiformis; (3) to the cerebral
peduncle, by fibers which leave the lower aspect of the caudate and lentiform
nuclei.
Coronal section through anterior cornua
of lateral ventricles.
The claustrum
(742,
744) is a thin layer of gray substance, situated on the lateral surface of
the external capsule. Its transverse section is triangular, with the apex
directed upward. Its medial surface, contiguous to the external capsule, is
smooth, but its lateral surface presents ridges and furrows corresponding with
the gyri and sulci of the insula, with which it is in close relationship. The
claustrum is regarded as a detached portion of the gray substance of the
insula, from which it is separated by a layer of white fibers, the capsula
extrema (band of Baillarger). Its cells are small and spindle-shaped,
and contain yellow pigment; they are similar to those of the deepest layer of
the cortex.
The nucleus amygdalæ (amygdala) (741), is an ovoid gray mass, situated at the lower end of the roof of the
inferior cornu. It is merely a localized thickening of the gray cortex,
continuous with that of the uncus; in front it is continuous with the putamen,
behind with the stria terminalis and the tail of the caudate nucleus.
The internal capsule (capsula interna) (745,
746) is a flattened band of white fibers, between the lentiform nucleus on
the lateral side and the caudate nucleus and thalamus on the medial side. In
horizontal section (742) it is seen to be somewhat abruptly curved, with its convexity inward;
the prominence of the curve is called the genu, and projects between the
caudate nucleus and the thalamus. The portion in front of the genu is termed
the frontal part, and separates the lentiform from the caudate nucleus; the
portion behind the genu is the occipital part, and separates the lentiform
nucleus from the thalamus.
Coronal section of brain through
anterior commissure.
The frontal part
of the internal capsule contains: (1) fibers running from the thalamus to the
frontal lobe; (2) fibers connecting the lentiform and caudate nuclei; (3)
fibers connecting the cortex with the corpus striatum; and (4) fibers passing
from the frontal lobe through the medial fifth of the base of the cerebral
peduncle to the nuclei pontis. The fibers in the region of the genu are named
the geniculate fibers; they originate in the motor part of the cerebral
cortex, and, after passing downward through the base of the cerebral peduncle
with the cerebrospinal fibers, undergo decussation and end in the motor nuclei
of the cranial nerves of the opposite side. The anterior two-thirds of the
occipital part of the internal capsule contains the cerebrospinal fibers,
which arise in the motor area of the cerebral cortex and, passing downward
through the middle three-fifths of the base of the cerebral peduncle, are
continued into the pyramids of the medulla oblongata. The posterior third of
the occipital part contains: (1) sensory fibers, largely derived from the
thalamus, though some may be continued upward from the medial lemniscus; (2)
the fibers of optic radiation, from the lower visual centers to the cortex of
the occipital lobe; (3) acoustic fibers, from the lateral lemniscus to the
temporal lobe; and (4) fibers which pass from the occipital and temporal lobes
to the nuclei pontis.
The fibers of the internal capsule radiate widely as they
pass to and from the various parts of the cerebral cortex, forming the corona
radiata (745) and intermingling with the fibers of the corpus callosum.
The external capsule (capsula externa) (742) is a lamina of white substance, situated lateral to the lentiform
nucleus, between it and the claustrum, and continuous with the internal capsule
below and behind the lentiform nucleus. It probably contains fibers derived
from the thalamus, the anterior commissure, and the subthalamic region.
Dissection showing the course of the
cerebrospinal fibers.
The substantia
innominata of Meynert is a stratum consisting partly of gray and partly of
white substance, which lies below the anter
ior part of the thalamus and lentiform nucleus. It consists of three
layers, superior, middle, and inferior. The superior layer is named the ansa
lentiformis, and its fibers, derived from the medullary lamina of the lentiform
nucleus, pass medially to end in the thalamus and subthalamic region, while
others are said to end in the tegmentum and red nucleus. The middle
layer consists of nerve cells and nerve fibers; fibers enter it from the
parietal lobe through the external capsule, while others are said to connect it
with the medial longitudinal fasciculus. The inferior layer forms the
main part of the inferior stalk of the thalamus, and connects this body with
the temporal lobe and the insula.
The stria terminalis (tænia semicircularis)
is a narrow band of white substance situated in the depression between the
caudate nucleus and the thalamus. Anteriorly, its fibers are partly continued
into the column of the fornix; some, however, pass over the anterior commissure
to the gray substance between the caudate nucleus and septum pellucidum, while
others are said to enter the caudate nucleus. Posteriorly, it is continued into
the roof of the inferior cornu of the lateral ventricle, at the extremity of
which it enters the nucleus amygdalæ. Superficial to it is a large vein,
the terminal vein (vein of the corpus striatum), which receives
numerous tributaries from the corpus striatum and thalamus; it runs forward to
the interventricular foramen and there joins with the vein of the choroid
plexus to form the corresponding internal cerebral vein.
Pathways of the brain and spinal
cord subdivide
into: associative, commissural and projection fibers.
Associative pathways
are the tracts communicating functional areas of one hemisphere. They can be
divided into long associative fibres and short associative fibres. The long
associative pathways include:
1.
Superior longitudinal fascicle communicates frontal, parietal and
occipital lobes;
2.
Inferior longitudinal fascicle communicates parietal, occipital,
temporal lobes;
3.
Uncinate fascicle - communicates frontal, parietal and temporal lobes;
4.
Fornicate fascicle - communicates central areas of rhinencephalon;
To
short associative tracts are arcuate
fibres of the cerebrum, which communicate neighbouring convolutions within
hemisphere.
associationis
breves).
The
Commissural pathways communicate
symmetric areas of both hemispheres of cerebrum and both halves of spinal cord
for co-ordination of their activity. There are:
1.
corpus callosum;
2.
anterior cerebral commissura;
3.
posterior cerebral commissura;
4.
habenular commissura ;
5.
fornicate commissura ;
6.
interthalamic adhesion;
7.
posterior white commissura of spinal cord.
The
Projecting pathways are the tracts,
which communicate the cerebrum and spinal cord with working organs. They
subdivided into ascending (sensible) and descending (motor). The sensible
projecting tracts into its turn subdivide into exteroreceptive,
interoceptive and proprioreceptive. The motor projecting fibers tracts
are pyramidal³ and extrapyramidal.
To
exteroreceptive (E) tracts belong the tracts temperatural, palpable,
pain sensivity, also vision, hearing, olfaction. The proprioreceptive (P)
tracts subdivided into tracts of cortical direction (Goll tracts and Burdach)
and cerebellar directions (Flechsig and Gowers tracts). Anterior and lateral
corticospinal tracts and also corticonuclear tract belong to pyramidal
pathways. The extrapyramidal tracts include rubrospinal, vestibulospinal,
reticulospinal, olivospinal pathways.
(E)
Pain and temperature sensation pathway (Lateral
spinothalamic tract).
1.
Body of first neuron is found in spinal
ganglion. The peripheral process of this neuron terminates by exteroreceptor in
skin, and central - on the strength of posterior rootlets into posterior horns
of spinal cord, where contacts with second neuron.
2.
The second neuron localised in proper nucleus of posterior horns of
spinal cord. Its axons make a crossing in grey commissura of spinal cord and
pass in lateral funiculus (lateral spinothalamic tract). Axons within the
medial of lemniscus reach the thalamus, where terminate by synapse with body of
third neuron.
3.
The third neuron is in lateral nucleus of thalamus, and its axons ‘thalamocortical tract’ pass to postcentral
gyrus (cortical analyser of skin sensation) through the back third of internal
capsule.
(E)
Pressure and touch sensation pathway (Anterior
spinothalamic tract).
1.
Body of first neuron is found in spinal
ganglion. The peripheral process of this neuron terminates by exteroreceptor in
skin, and central – in the gelatinose substance of spinal cord, where contacts
with second neuron.
2.
The second neuron axons make a crossing in grey commissura of spinal
cord and pass in lateral funiculus (anterior spinothalamic tract). Axons reach
the thalamus, where terminate by synapse with body of third neuron.
3.
The third neuron is in thalamus,
it passes to postcentral gyrus (cortical analyser of skin sensation) through
the back third of internal capsule:
Proprioreceptive
pathway (cortical direction) - Bulbothalamic tract.
1.
The first neuron of this tract lies in spinal ganglion. Its peripheral process
terminates by proprioreceptor in muscles, tendoms, jonts capsules and
ligaments. The central process passeswith posterior rootlets into spinal cord
and form in white matter Goll tract - fascicle (upper 11 segments). The process
passes to medulla oblongata, where
terminate by synapse with second neuron:
2.
The second neurons are in gracilis and cuneate nuclei of medulla oblongata. Axons this neuron
form bulbothalamic tract, which composes a base of medial lemniscus. Axons of
second neuron cross (deccusation of lemniscus) and reach the thalamus, where terminate by synapse
with third neuron.
3.
The third neurons pass through the internal capsule and form the
thalamocortical tract:
(P)
Proprioreceptive pathways with cerebellar direction (Spinocerebellar tracts).
Posterior spinocerebellar tract
(Flechsig tract)
1.
The first neuron is in spinal
ganglion. Its peripheral process terminates by proprioreceptor and central with
the posterior rootlets of spinal cord passes to the grey matter.
2.
The second neuron lies in thoracic nucleus of posterior horns
(Clarc-Steiling). Its axons pass in lateral funiculus. Then it reaches the
cortex of the cerebellar vermis
through the inferior cerebellar pedunculi and dentate nucleus. Nervous impulse passes to the ruber nucleus:
Anterior spinocerebellar tract
(Gowers tract). Two-neurons way. This tract differs from previous by its second
neuron, a body of which is found in medial intermediate nucleus of spinal cord.
Axons of the second neuron get across and pass into lateral funiculus, reaching
the superior cerebral velum. They cross here and pass to the vermis and dentate
nucleus. Nervous impulse also passes to the ruber nucleus.
Lateral
corticospinal (pyramidal) tract
1.
A Body of first neuron is found in gigantic pyramidal cells (Bets) of
top and middle third of precentral gyrus of the cortex. Axons pass through anterior third part of internal capsule,
pons, medulla oblongata; part of fibres gets across forming the pyramids. The
crossed fibres of first neuron pass in lateral funiculus of the spinal cord
‘lateral corticospinal (pyramidal) tract’ and terminate in anterior horns of
spinal cord.
2.
The second neuron lies in motor nucleus of anterior horns, and its axons
pass with the anterior rootlets and spinal
nerves as far as skeletal muscles of the trunk and extremities:
Anterior corticospinal (pyramidal)
tract
1.
Major part of fibres of first neurons of anterior corticospinal tract
does not cross in pyramids of medulla oblongata, and passes in anterior
funiculus of the spinal cord. Axons cross in grey commissura terminate by
synapse with bodies of second neurons.first
2.
The second neuron lies in motor nucleus of anterior horns, and its axons
pass within the anterior rootlets and spinal nerves as far as skeletal muscles
of the trunk and extremities.
Corticonuclear tract
1.
The first neuron of this tract is found in gigantic pyramidal cells of
cortex in lower third of precentral gyrus. Axons pass through the genu of
internal capsule, base of cerebral peduncles and terminate in motor nucleus of
rhomboid fossa and midbrain, previously passing across partially on opposite
side.
2.
The second neuron lies in motor nucleus of rhomboid fossa and midbrain
of opposite side, and its axons with cranial nerves pass as far as stripped
muscles of the head and superficial neck muscles:
Extrapyramidal
pathways transmit impulses providing muscles tone and reflexes
of equilibrium and execution of automated motions. They include rubrospinal,
tectospinal, vestibulospinal, reticulospinal and olivospinal tracts.
Rubrospinal
tract
1.
The first neuron is found in red nucleus (n. ruber). Its axons make tegmental
deccusation and pass through the base of cerebral peduncles, pons, and medulla
oblongata. They form tract in lateral funiculus and reach motor nuclei in
anterior horns of spinal cord.
2.
The second neuron lies in motor nucleus of anterior horns of spinal
cord, and its axons on the strength of anterior rootlets and spinal nerves
reach trunk and extremities muscles.
3.
This
2-neuron tract is descending link for reflex link of unconscious motion
coordination. Spinocerebellar tracts (Flechsig and Gowers tracts) are the
ascending links for this regulation. Spinocerebellar tracts send impulses for
rubrospinal tracts through the intermediate link (from cerebellar vermis and
dentate nuclei to the ruber nuclei).
Corticopontocerebellar
tract (way of cortical correction of the cerebellum)
This
2-neuron tract starts from all lobes of cerebral hemispheres. Axons run through
the internal capsula to the proper pontini nuclei. Second neurons cross and
pass to the cerebellar vermis through the middle cerebellar pedunculi. Some
links directed also to dentate and ruber nuclei. This pathway materializes
cortical correction of unconscious motion co-ordination.
1. epidermis (superficial layer);
Also a soft
connective tissue there is situated.
Prepared
by
Reminetskyy B.Y.