Lesson No 22
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.
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 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,
when it is seen to form an arched structure about
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 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.
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
The Lateral Ventricles (ventriculus
lateralis) are irregular cavities situated in the
lower and medial parts of the cerebral hemispheres, one on either side of the
middle line. 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. They
are lined by a thin, diaphanous membrane, the ependyma, covered by
ciliated epithelium, and contain cerebrospinal fluid, which, even in health,
may be secreted in considerable amount. Each lateral ventricle consists of a central
part or body, and three prolongations from it, termed cornua.
The central part (pars centralis ventriculi
lateralis; cella) of the lateral ventricle extends from the
interventricular foramen to the splenium of the corpus callosum. It is an
irregularly curved cavity, triangular on transverse section, with a roof, a
floor, and a medial wall. The roof is formed by the under surface of the corpus
callosum; the floor by the following parts, enumerated in their order of
position, from before backward: the caudate nucleus of the corpus striatum, the
stria terminalis and the terminal vein, the lateral portion of the upper
surface of the thalamus, the choroid plexus, and the lateral part of the
fornix; the medial wall is the posterior part of the septum pellucidum, which
separates it from the opposite ventricle.
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 anterior cornu (cornu anterius; anterior
horn; precornu) (Fig.
736) passes forward and lateralward, with a slight inclination downward,
from the interventricular foramen into the frontal lobe, curving around the
anterior end of the caudate nucleus. Its floor is formed by the upper surface
of the reflected portion of the corpus callosum, the rostrum. It is
bounded medially by the anterior portion of the septum pellucidum, and
laterally by the head of the caudate nucleus. Its apex reaches the posterior
surface of the genu of the corpus callosum.
Drawing of a cast of the ventricular
cavities, viewed from the side. (Retzius.)
The posterior cornu (cornu posterius;
postcornu) passes into the occipital lobe, its direction being backward and
lateralward, and then medialward. Its roof is formed by the fibers of the
corpus callosum passing to the temporal and occipital lobes. On its medial wall
is a longitudinal eminence, the calcar avis (hippocampus minor),
which is an involution of the ventricular wall produced by the calcarine
fissure. Above this the forceps posterior of the corpus callosum, sweeping
around to enter the occipital lobe, causes another projection, termed the bulb
of the posterior cornu. The calcar avis and bulb of the posterior cornu are
extremely variable in their degree of development; in some cases they are
ill-defined, in others prominent.
Central part and anterior and posterior
cornua of lateral ventricles exposed from above
The inferior cornu (cornu inferior;
descending horn; middle horn; medicornu) the largest of the three,
traverses the temporal lobe of the brain, forming in its course a curve around
the posterior end of the thalamus. It passes at first backward, lateralward,
and downward, and then curves forward to within 2.5 cm. of the apex of the
temporal lobe, its direction being fairly well indicated on the surface of the
brain by that of the superior temporal sulcus. Its roof is formed chiefly by
the inferior surface of the tapetum of the corpus callosum, but the tail of the
caudate nucleus and the stria terminalis also extend forward in the roof of the
inferior cornu to its extremity; the tail of the caudate nucleus joins the
putamen. Its floor presents the following parts: the hippocampus, the fimbria
hippocampi, the collateral eminence, and the choroid plexus. When the choroid
plexus is removed, a cleft-like opening is left along the medial wall of the
inferior cornu; this cleft constitutes the lower part of the choroidal fissure.
Coronal section through posterior cornua of lateral
ventricle
Posterior and inferior cornua of left lateral
ventricle exposed from the side
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.
The caudate nucleus (nucleus caudatus; caudatum)
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.
Horizontal section of right cerebral hemisphere.
The lentiform nucleus
(nucleus lentiformis; lenticular nucleus; lenticula) 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.
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.
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
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) 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) 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 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) 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.
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.
Dissection showing the course of the cerebrospinal fibers.
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 association fibers unite different parts of
the same hemisphere, and are of two kinds: (1) those connecting adjacent gyri, short
association fibers; (2) those passing between more distant parts, long
association fibers.
The short association fibers lie immediately
beneath the gray substance of the cortex of the hemispheres, and connect
together adjacent gyri.
The long association fibers include the
following: (a) the uncinate fasciculus; (b) the cingulum; (c)
the superior longitudinal fasciculus; (d) the inferior longitudinal
fasciculus; (e) the perpendicular fasciculus; (f) the
occipitofrontal fasciculus; and (g) the fornix.
(a) The uncinate fasciculus passes
across the bottom of the lateral fissure, and unites the gyri of the frontal
lobe with the anterior end of the temporal lobe.
(b) The cingulum is a band of white
matter contained within the cingulate gyrus. Beginning in front at the anterior
perforated substance, it passes forward and upward parallel with the rostrum,
winds around the genu, runs backward above the corpus callosum, turns around
the splenium, and ends in the hippocampal gyrus.
(c) The superior longitudinal fasciculus
passes backward from the frontal lobe above the lentiform nucleus and insula;
some of its fibers end in the occipital lobe, and others curve downward and
forward into the temporal lobe.
Dissection of cortex and brain-stem
showing association fibers and island of Reil after removal of its superficial
gray substance
(d) The inferior longitudinal fasciculus
connects the temporal and occipital lobes, running along the lateral walls of
the inferior and posterior cornua of the lateral ventricle.
Deep dissection of cortex and brain-stem
(e) The perpendicular fasciculus runs
vertically through the front part of the occipital lobe, and connects the
inferior parietal lobule with the fusiform gyrus.
(f) The occipitofrontal fasciculus
passes backward from the frontal lobe, along the lateral border of the caudate
nucleus, and on the mesial aspect of the corona radiata; its fibers radiate in
a fan-like manner and pass into the occipital and temporal lobes lateral to the
posterior and inferior cornua. Déjerine regards the fibers of the
tapetum as being derived from this fasciculus, and not from the corpus
callosum.
(g) The fornix connects the hippocampal
gyrus with the corpus mammillare and, by means of the thalamomammillary
fasciculus, with the thalamus (see page 839). Through the fibers of the
hippocampal commissure it probably also unites the opposite hippocampal gyri.
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
corpus callosum connects 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 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,
when it is seen to form an arched structure about
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 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 pyramidalі and extrapyramidal.
Sensory Pathways from the Spinal Cord to the Brain.—The posterior root fibers conducting the impulses of conscious
muscle sense, tendon sense and joint sense, those impulses which have to do
with the coördination and adjustment of muscular movements, ascend in the
fasciculus gracilis and fasciculus cuneatus to the nucleus gracilis and nucleus
cuneatus in the medulla oblongata.
In the nucleus gracilis and nucleus cuneatus synaptic
relations are found with neurons whose cell bodies are located in these nuclei
and whose axons pass by way of the internal arcuate fibers, cross in the
raphé to the opposite side in the region between the olives and turn
abruptly upward to form the medial lemniscus or medial fillet. The medial
fillet passes upward in the ventral part of the formatio reticularis through
the medulla oblongata, pons and mid-brain to the principal sensory nucleus of
the ventro-lateral region of the thalamus. Here the terminals form synapses
with neurons of the third order whose axons pass through the internal capsule
and corona radiata to the somatic sensory area of the cortex in the
post-central gyrus.
Fibers conducting the impulses of unconscious
muscle sense pass to the cerebellum partly by way of the fasciculus
gracilis and fasciculus cuneatus to the nucleus gracilis and nucleus cuneatus,
thence neurons of the second order convey the impulses either via the dorsal
external arcuate fibers directly into the inferior peduncle of the cerebellum
or via the ventral external arcuate fibers which are continued from the
internal arcuate fibers through the ventral part of the raphé and after
crossing the midline emerge on the surface of the medulla in the ventral sulcus
between the pyramids or in the groove between the pyramid and the olive. They
pass over the lateral surface of the medulla and olive to reach the inferior
peduncle through which they pass to the cerebellum.
Other fibers conducting impulses of unconscious muscle
sense pass upward in the dorsal spinocerebellar fasciculus, which arises from
cells in the nucleus dorsalis. The posterior root fibers conducting these
impulses pass into the fasciculus cuneatus and the collaterals from them to the
nucleus dorsalis are said to come almost exclusively from the middle area of
the fasciculus cuneatus. They form by their multiple division baskets about the
individual cells of the nucleus dorsalis, each fiber coming in relation with
the bodies and dendrites of several cells. The axons of the second order pass
into the dorsal spinocerebellar fasciculus of the same side and ascend along
the lateral surface of the spinal cord and medulla oblongata until they arrive
at the level of the olive, they then curve backward beneath the external
arcuate fibers into the inferior peduncle and pass into the cerebellum. Here
they give off collaterals to the dentate nucleus and finally terminate in the
cortex of the dorsal and superior portion of the vermis, partly on the same
side, but to a great extent by way of a large commissure to the opposite side.
The fibers lose their myelin sheaths as they enter the gray substance and
terminate by end ramifications among the nerve cells and their processes. Some
of the fibers are said to end in the nucleus dentatus and the roof nuclei of
the cerebellum (the nucleus globosus, nucleus emboliformis and nucleus
fastigius) and others pass through them to terminate in the inferior vermis. A
few fibers of the dorsal spinocerebellar fasciculus are said not to enter the
inferior peduncle but to pass with the ventral
spinocerebellar fasciculus. The cerebellar reflex are is supposed to be
completed by the fibers of the superior peduncle which pass from the cerebellum
to the red nucleus of the mid-brain where some of their terminals and collaterals
form synapses with neurons whose axons descend to the spinal cord in the
rubrospinal fasciculus. The terminal and collaterals of this fasciculus end
either directly or indirectly about the motor cells in the anterior column.
The ventral spinocerebellar fasciculus, since most of
its fibers pass to the cerebellum, is also supposed to be concerned in the
conduction of unconscious muscle sense. The location of its cells of origin is
uncertain. They are probably in or near the dorsal nucleus of the same and the
opposite side; various other locations are given, the dorsal column, the
intermediate zone of the gray matter and the central portion of the anterior
column. The neurons of the first order whose central fibers enter the
fasciculus cuneatus from the dorsal roots send collaterals and terminals to
form synapses with these cells. The fibers which come from the opposite gray
columns cross some in the white and some in the gray commissure and pass with
fibers from the same side through the lateral funiculus to the marginal region
ventral to the dorsal spinocerebellar fasciculus. The fasciculus begins about
the level of the third lumbar nerve and continues upward on the lateral surface
of the spinal cord and medulla oblongata until it passes under cover of the
external arcuate fibers. It passes just dorsal to the olive and above this
joins the lateral edge of the lateral lemniscus along which it runs, ventral to
the roots of the trigeminal nerve, almost to the level of the superior
colliculus, it then crosses over the superior peduncle, turns abruptly backward
along its medial border, enters the cerebellum with it and ends in the vermis
of the same and the opposite side. Some of its fibers are said to join the
dorsal spinocerebellar fasciculus in the medulla oblongata and enter the
cerebellum through the inferior peduncle. A number of fibers are said to
continue upward in the dorsolateral part of the tegmentum as far as the
superior colliculus and a few pass to the thalamus. They probably form part of
the sensory or higher reflex path.
The posterior root fibers conducting impulses of pain
and temperature probably terminate in the posterior column or the
intermediate region of the gray matter soon after they enter the spinal cord.
The neurons of the second order are supposed to pass through the anterior
commissure to the superficial antero-lateral fasciculus (tract of Gowers) and
pass upward in that portion of it known as the lateral spinothalamic
fasciculus. This fasciculus lies along the medial side of the ventral
spinocerebellar fasciculus. It is stated by some authors that the pain fibers
pass upward in the antero-lateral ground bundles. In some of the lower mammals
this pathway carries the pain fibers upward by a series of neurons some of
which cross to the opposite side, so that in part there is a double path. In
man, however, the lateral spinothalamic fasciculus is probably the most
important pathway. On reaching the medulla these fibers continue upward through
the formatio reticularis in the neighborhood of the median fillet to the
thalamus, probably its ventro-lateral region. Whether higher neurons convey the
pain impulses to the cortex through the internal capsule is uncertain. The
pathway is probably more complex and Head is of the opinion that our sensations
of pain are essentially thalamic. The pain and temperature pathways in the
lateral spinothalamic fasciculus are not so closely intermingled but that one
can be destroyed without injury to the other.
Ransom suggests that the non-medullated fibers of the
posterior roots, which turn into Lissauer’s tract and ascend or descend for
short distances not exceeding one or two segments and finally end in the
substantia gelatinosa, are in part at least pain fibers and that the fasciculus
of Lissauer and the substantia gelatinosa represent part of the mechanism for
reflexes associated with pain conduction and reception while the fibers to the
higher centers pass up in the spinothalamic tract.
The fibers of tactile discrimination, according
to Head and Thompson, pass up in the fasciculus cuneatus and fasciculus
gracilis of the same side and follow the path of the muscle-sense fibers. The
axons of the second order arising in the nucleus cuneatus and gracilis cross
with the internal arcuate fibers and ascend to the thalamus with the medial
lemniscus, thence by neurons of higher order the impulses are carried to the
somatic sensory area of the cortex through the internal capsule. The other touch
fibers, shortly after entering the spinal cord, terminate in the dorsal
column or intermediate gray matter. Neurons of the second order send their
axons through the anterior commissure to pass upward in the antero-lateral
funiculus probably in the ventral spinothalamic fasciculus. In the
medulla they join or pass upward in the neighborhood of the medial lemniscus to
the thalamus and thence by neurons of higher order to the somatic sensory area
of the cortex.
The remaining ascending fasciculi form a part of the
complex known as the superficial antero-lateral fasciculus (tract of
Gowers). The spinotectal fasciculus, as its name indicates, is
supposed to have its origin in the gray matter of the cord and terminations in
the superior and inferior (?) colliculi of the mid-brain serving for reflexes
between the cord and the visceral and auditory centers of the mid-brain.
The spino-olivary fasciculus (olivospinal;
bulbospinal, Helweg’s bundle) is likewise of unknown constitution and
function; there is uncertainty even in regard to the direction of its fibers.
Sympathetic afferent fibers (visceral afferent; viscero-sensory; splanchnic afferent) enter
the spinal cord by the posterior roots of the thoracic and first two or three
lumbar nerves and the second to the fourth sacral nerves. The fibers pass to
these nerves from the peripheral sympathetic system through the white rami
communicantes. Some of the cell bodies of these afferent fibers are located in
the spinal ganglia and others are in the sympathetic ganglia. Some of the
afferent sympathetic fibers end about the cell bodies of somatic sensory
neurons and visceral impulses are thus transmitted to these neurons which
conduct them as well as their own special impulses to the spinal cord. Other
sympathetic afferent neurons whose cell bodies are located in the spinal
ganglia send collaterals to neighboring cells of somatic sensory neurons and
thus have a double path of transmission to the spinal cord. Such an arrangement
provides a mechanism for some of the referred pains.
To
sernsory
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:
The descending fasciculi which
convey impulses from the higher centers to the spinal cord and located in the
lateral and ventral funiculi.
The Motor Tract conveying voluntary impulses, arises from the pyramid cells situated in the
motor area of the cortex, the anterior central and the posterior portions of
the frontal gyri and the paracentral lobule. The fibers are at first somewhat
widely diffused, but as they descend through the corona radiata they gradually
approach each other, and pass between the lentiform nucleus and thalamus, in
the genu and anterior two-thirds of the occipital part of the internal capsule;
those in the genu are named the geniculate fibers, while the remainder
constitute the cerebrospinal fibers; proceeding downward they enter the
middle three-fifths of the base of the cerebral peduncle. The geniculate fibers
cross the middle line, and end by arborizing around the cells of the motor
nuclei of the cranial nerves. The cerebrospinal fibers are continued downward
into the pyramids of the medulla oblongata, and the transit of the fibers from
the medulla oblongata is effected by two paths. The
fibers nearest to the anterior median fissure cross the middle line, forming
the decussation of the pyramids, and descend in the opposite side of the
medulla spinalis, as the lateral cerebrospinal fasciculus (crossed
pyramidal tract). Throughout the length of the medulla spinalis fibers from
this column pass into the gray substance, to terminate either directly or
indirectly around the motor cells of the anterior column. The more laterally
placed portion of the tract does not decussate in the medulla oblongata, but
descends as the anterior cerebrospinal fasciculus (direct pyramidal
tract); these fibers, however, end in the anterior gray column of the
opposite side of the medulla spinalis by passing across in the anterior white
commissure. There is considerable variation in the extent to which decussation
takes place in the medulla oblongata; about two-thirds or three-fourths of the
fibers usually decussate in the medulla oblongata and the remainder in the
medulla spinalis.
The axons of the motor cells in the anterior column
pass out as the fibers of the anterior roots of the spinal nerves, along which
the impulses are conducted to the muscles of the trunk and limbs.
From this it will be seen that all the fibers of the
motor tract pass to the nuclei of the motor nerves on the opposite side of the
brain or medulla spinalis, a fact which explains why a lesion involving the
motor area of one side causes paralysis of the muscles of the opposite side of
the body. Further, it will be seen that there is a break in the continuity of
the motor chain; in the case of the cranial nerves this break occurs in the
nuclei of these nerves; and in the case of the spinal nerves, in the anterior
gray column of the medulla spinalis. For clinical purposes it is convenient to
emphasize this break and divide the motor tract into two portions: (1) a series
of upper motor neurons which comprises the motor cells in the cortex and
their descending fibers down to the nuclei of the motor nerves; (2) a series of
lower motor neurons which includes the cells of the nuclei of the motor
cerebral nerves or the cells of the anterior columns of the medulla spinalis
and their axiscylinder processes to the periphery.
The rubrospinal fasciculus arises from the
large cells of the red nucleus. The fibers cross the raphé of the
mid-brain in the decussation of Forel and descend in the formatio reticularis
of the pons and medulla dorsal to the medial lemniscus and as they pass into
the spinal cord come to lie in a position ventral to the crossed pyramidal
tracts in the lateral funiculus. The rubrospinal fibers end either directly or
indirectly by terminals and collaterals about the motor cells in the anterior column
on the side opposite from their origin in the red nucleus. A few are said to
pass down on the same side. Since the red nucleus is intimately related to the
cerebellum by terminals and collaterals of the superior peduncle which arises
in the dentate nucleus of the cerebellum, the rubrospinal fasciculus is
supposed to be concerned with cerebellar reflexes, complex motor
coördinations necessary in locomotion and equilibrium. The afferent paths
concerned in these reflexes have already been partly considered, namely, the
dorsal and ventral spinocerebellar fasciculi, and probably some of the fibers
of the posterior funiculi which reach the cerebellum by the inferior peduncle.
The tectospinal fasciculus arises from the
superior colliculus of the roof (tectum) of the mid-brain. The axons come from
large cells in the stratum opticum and stratum lemnisci and sweep ventrally
around the central gray matter of the aqueduct, cross the raphé in the
fountain decussation of Meynert and turn downward in the tegmentum in the
ventral longitudinal bundle. Some of the fibers do not cross in the
raphé but pass down on the same side; it is uncertain whether they come
from the superior colliculus of the same side or arch over the aqueduct from
the colliculus of the opposite side. The tectospinal fasciculus which comprises
the major part of the ventral longitudinal bundle passes down through the
tegmentum and reticular formation of the pons and medulla oblongata ventral to the
medial longitudinal bundle. In the medulla the two bundles are more or less
intermingled and the tectospinal portion is continued into the antero-lateral
funiculus of the spinal cord ventral to the rubrospinal fasciculus with which
some of its fibers are intermingled. Some of the fibers of the tectospinal
fasciculus pass through the red nucleus giving off collaterals to it, others
are given off to the motor nuclei of the cranial nerves and in the spinal cord
they terminate either directly or indirectly by terminals and collaterals among
the nuclei of the anterior column. Since the superior colliculus is an
important optic reflex center, this tract is probably
concerned in optic reflexes; and possibly also with auditory reflexes since
some of the fibers of the central auditory path, the lateral lemniscus,
terminate in the superior colliculus.
The vestibulospinal fasciculus (part of the
anterior marginal fasciculus or Loewenthal’s tract) situated chiefly
in the marginal part of the anterior funiculus is mainly derived from the cells
of the terminal nuclei of the vestibular nerve, probably Deiters’s and
Bechterew’s, and some of its fibers are supposed to come from the nucleus
fastigius (roof nucleus of the cerebellum). The latter nucleus is intimately
connected with Dieters’s and Bechterew’s nuclei. The vestibulospinal fasciculus
is concerned with equilibratory reflexes. Its terminals and collaterals end
about the motor cells in the anterior column. It extends to the sacral region
of the cord. Its fibers are intermingled with the ascending spinothalamic
fasciculus, with the anterior proper fasciculus and laterally with the
tectospinal fasciculus. Its fibers are supposed to be both crossed and
uncrossed. In the brain-stem it is associated with the dorsal longitudinal
bundle.
The pontospinal fasciculus (Bechterew)
arises from the cells in the reticular formation of the pons from the same and
the opposite side and is associated in the brain-stem with the ventral
longitudinal bundle. In the cord it is intermingled with the fibers of the
vestibulospinal fasciculus in the anterior funiculus. Not much is known about
this tract.
There are probably other descending fasciculi such as
the thalamospinal but not much is known about them.
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.
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.
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.
Theme
1. Meninges of the brain. Dural sinuses. Circulation of
spinocerebral fluid. Blood vessels of the brain and spinal cord
The
brain is enclosed by in 3 meninges: dura
mater, arachnoid and pia mater.
Dura mater is the periosteum for internal surface of the skull bones and there
is no space between dura mater and bones. There is subarachnoid space between choroid and pia mater, which is filled
in by cerebrospinal fluid. This space has expansions, which are called
‘cisterns’:
·
cerebellomedullary cistern;
·
chiasmatic cistern;
·
interpeduncular cistern;
·
cistern of the lateral fossa;
·
pontocerebellar cistern;
·
superior
cistern.
Diagram showing the positions of the three principal
subarachnoid cisternæ.
Dura
mater has some processes extend into the cavity of the scull and separating the
brain parts:
·
Falx
cerebri occupies a longitudinal fissura cerebri and separates
the brain to right and left hemispheres;
·
Falx
cerebelli lies in longitudinal cerebellum furrow and separates it
into right and left cerebellum hemispheres;
·
Diaphragma
sellae closes hypophysial fossa, separating a hypophysis
from diencephalon.
·
Tentorium
cerebelli occupies a tranverse fissura cerebri and separates
the cerebellum from the occipital lobe of the telencephalon.
·
Dura mater forms a trigeminal cavity on anterior surface of
the temporal pyramide.
The
processes of dura mater approaching sulcuses on the skull bones slit and fasten
to sulcus edges, forming the venous
sinuses where venous blood flows:
Diagrammatic representation of a section across the top of the skull, showing
the membranes of the brain,
·
superior sagittal sinus;
·
inferior sagittal sinus;
·
straight [rectus] sinus;
·
occipital sinus;
·
inferior petrosal sinus;
·
superior petrosal sinus;
·
transverse sinus;
·
cavernous sinus;
·
intercavernous sinus;
·
sphenoperietal sinus;
·
sygmoid
sinus.
These
sinuses flow together in confluence sinuum. Venous blood from all sinuses
passes into sygmoid sinus, and then - into internal jugular vein. The arachnoid
forms the Pachioni’s arachnoid granulation. They extend from venous
sinuses in internal surface skull vault bone and form foveoli granulares.
Arachnoid granulations provide a cerebrospinal fluid flow from subarachnoid
space into venous blood. Emissary veins and diploic veins communicate the
sinuses of dura mater with the superficial head veins.
The brain and medulla spinalis are enclosed within three membranes.
These are named from without inward: the dura mater, the arachnoid,
and the pia mater.
The Dura Mater
The dura mater is a thick and dense inelastic
membrane. The portion which encloses the brain differs in several essential particulars
from that which surrounds the medulla spinalis, and
therefore it is necessary to describe them separately; but at the same time it
must be distinctly understood that the two form one complete membrane, and are
continuous with each other at the foramen magnum.
The Cranial Dura Mater (dura mater encephali; dura
of the brain) lines the interior of the skull, and serves the twofold
purpose of an internal periosteum to the bones, and a membrane for the
protection of the brain. It is composed of two layers, an inner or meningeal
and an outer or endosteal, closely connected together, except in certain
situations, where, as already described (page 654), they separate to form
sinuses for the passage of venous blood. Its outer surface is rough and fibrillated,
and adheres closely to the inner surfaces of the bones, the adhesions being
most marked opposite the sutures and at the base of the skull its inner surface
is smooth and lined by a layer of endothelium. It sends inward four processes
which divide the cavity of the skull into a series of freely communicating
compartments, for the lodgement and protection of the different parts of the
brain; and it is prolonged to the outer surface of the skull, through the
various foramina which exist at the base, and thus becomes continuous with the
pericranium; its fibrous layer forms sheaths for the nerves which pass through
these apertures. Around the margin of the foramen magnum it is closely adherent to the bone, and is continuous with the spinal dura
mater.
Dura
mater and its processes exposed by removing part of the right half of the skull
and the brain.
Processes.—The processes of the
cranial dura mater, which projects into the cavity of the skull, are formed by
reduplications of the inner or meningeal layer of the membrane, and are four in
number: the falx cerebri, the tentorium cerebelli, the falx
cerebelli, and the diaphragma sellæ.
The falx cerebri (765),
so named from its sickle-like form, is a strong, arched process which descends
vertically in the longitudinal fissure between the cerebral hemispheres. It is
narrow in front, where it is attached to the crista galli of the ethmoid; and
broad behind, where it is connected with the upper surface of the tentorium
cerebelli. Its upper margin is convex, and attached to
the inner surface of the skull in the middle line, as far back as the internal
occipital protuberance; it contains the superior sagittal sinus. Its lower
margin is free and concave, and contains the inferior sagittal sinus.
The tentorium cerebelli (766)
is an arched lamina, elevated in the middle, and
inclining downward toward the circumference. It covers the superior surface of
the cerebellum, and supports the occipital lobes of the brain. Its anterior
border is free and concave, and bounds a large oval opening, the incisura
tentorii, for the transmission of the cerebral peduncles. It is attached,
behind, by its convex border, to the transverse ridges upon the inner surface
of the occipital bone, and there encloses the transverse sinuses; in front, to
the superior angle of the petrous part of the temporal bone on either side,
enclosing the superior petrosal sinuses. At the apex of the petrous part of the
temporal bone the free and attached borders meet, and, crossing one another,
are continued forward to be fixed to the anterior and posterior clinoid
processes respectively. To the middle line of its upper surface the posterior
border of the falx cerebri is attached, the straight sinus being placed at
their line of junction.
Tentorium cerebelli seen from above.
The falx cerebelli is a small triangular
process of dura mater, received into the posterior cerebellar notch. Its base
is attached, above, to the under and back part of the tentorium; its posterior
margin, to the lower division of the vertical crest on the inner surface of the
occipital bone. As it descends, it sometimes divides into two smaller folds,
which are lost on the sides of the foramen magnum.
The diaphragma sellæ is a small circular
horizontal fold, which roofs in the sella turcica and almost completely covers
the hypophysis; a small central opening transmits the infundibulum.
Structure.—The cranial dura mater
consists of white fibrous tissue and elastic fibers arranged in flattened
laminæ which are imperfectly separated by lacunar spaces and bloodvessels
into two layers, endosteal and meningeal. The endosteal layer
is the internal periosteum for the cranial bones, and contains the bloodvessels
for their supply. At the margin of the foramen magnum it is continuous with the
periosteum lining the vertebral canal. The meningeal or supporting
layer is lined on its inner surface by a layer of nucleated flattened
mesothelium, similar to that found on serous membranes.
The arteries of the dura mater are very numerous.
Those in the anterior fossa are the anterior meningeal branches of the anterior
and posterior ethmoidal and internal carotid, and a branch from the middle
meningeal. Those in the middle fossa are the middle and accessory meningeal of
the internal maxillary; a branch from the ascending pharyngeal, which enters
the skull through the foramen lacerum; branches from the internal carotid, and
a recurrent branch from the lacrimal. Those in the posterior fossa are
meningeal branches from the occipital, one entering the skull through the
jugular foramen, and another through the mastoid foramen; the posterior
meningeal from the vertebral; occasional meningeal branches from the
ascending pharyngeal, entering the skull through the jugular foramen and
hypoglossal canal; and a branch from the middle meningeal.
The veins returning the blood from the cranial dura
mater anastomose with the diploic veins and end in the various sinuses. Many of
the meningeal veins do not open directly into the sinuses, but indirectly
through a series of ampullæ, termed venous lacunæ. These are
found on either side of the superior sagittal sinus, especially near its middle
portion, and are often invaginated by arachnoid granulations; they also exist
near the transverse and straight sinuses. They communicate with the underlying
cerebral veins, and also with the diploic and emissary veins.
The nerves of the cranial dura mater are filaments
from the semilunar ganglion, from the ophthalmic, maxillary, mandibular, vagus,
and hypoglossal nerves, and from the sympathetic.
The Spinal Dura Mater (dura mater spinalis; spinal
dura) (767)
forms a loose sheath around the medulla spinalis, and represents only the inner
or meningeal layer of the cranial dura mater; the outer or endosteal layer
ceases at the foramen magnum, its place being taken by the periosteum lining
the vertebral canal. The spinal dura mater is separated from the arachnoid by a
potential cavity, the subdural cavity; the two membranes are, in fact,
in contact with each other, except where they are separated by a minute
quantity of fluid, which serves to moisten the apposed surfaces. It is
separated from the wall of the vertebral canal by a space, the epidural
space, which contains a quantity of loose areolar tissue and a plexus of
veins; the situation of these veins between the dura mater and the periosteum
of the vertebræ corresponds therefore to that of the cranial sinuses
between the meningeal and endosteal layers of the cranial dura mater. The
spinal dura mater is attached to the circumference of the foramen magnum, and
to the second and third cervical vertebræ; it is also connected to the
posterior longitudinal ligament, especially near the lower end of the vertebral
canal, by fibrous slips. The subdural cavity ends at the lower border of the
second sacral vertebra; below this level the dura mater closely invests the
filum terminale and descends to the back of the coccyx, where it blends with
the periosteum. The sheath of dura mater is much larger than is necessary for
the accommodation of its contents, and its size is greater in the cervical and
lumbar regions than in the thoracic. On each side may be seen the double
openings which transmit the two roots of the corresponding spinal nerve, the
dura mater being continued in the form of tubular prolongations on them as they
pass through the intervertebral foramina. These prolongations are short in the
upper part of the vertebral column, but gradually become longer below, forming
a number of tubes of fibrous membrane, which enclose the lower spinal nerves
and are contained in the vertebral canal.
The medulla spinalis and its membranes.
Structure.—The spinal dura mater
resembles in structure the meningeal or supporting layer of the cranial dura mater,
consisting of white fibrous and elastic tissue arranged in bands or
lamellæ which, for the most part, are parallel with one another and have
a longitudinal arrangement. Its internal surface is smooth and covered by a
layer of mesothelium. It is sparingly supplied with bloodvessels, and a few
nerves have been traced into it.
The Arachnoid—The
arachnoid is a delicate membrane enveloping the brain and medulla
spinalis and lying between the pia mater internally and the dura mater
externally; it is separated from the pia mater by the subarachnoid cavity,
which is filled with cerebrospinal fluid.
The Cranial Part (arachnoidea encephali) of
the arachnoid invests the brain loosely, and does not dip into the sulci
between the gyri, nor into the fissures, with the
exception of the longitudinal. On the upper surface of the brain the arachnoid
is thin and transparent; at the base it is thicker, and slightly opaque toward
the central part, where it extends across between the two temporal lobes in
front of the pons, so as to leave a considerable interval between it and the
brain.
The Spinal Part (arachnoidea spinalis) of the
arachnoid is a thin, delicate, tubular membrane loosely investing the medulla
spinalis. Above, it is continuous with the cranial arachnoid; below,
it widens out and invests the cauda equina and the nerves proceeding from it.
It is separated from the dura mater by the subdural space, but here and
there this space is traversed by isolated connective-tissue trabeculæ,
which are most numerous on the posterior surface of the medulla spinalis.
The arachnoid surrounds the cranial and spinal nerves, and
encloses them in loose sheaths as far as their points of exit from the skull
and vertebral canal.
Structure.—The
arachnoid consists of bundles of white fibrous and elastic tissue intimately
blended together. Its outer surface is covered with a layer of low cuboidal
mesothelium. The inner surface and the trabeculæ are likewise covered by
a somewhat low type of cuboidal mesothelium which in places are
flattened to a pavement type. Vessels of considerable size, but few in number,
and, according to Bochdalek, a rich plexus of nerves derived from the motor
root of the trigeminal, the facial, and the accessory nerves, are found in the
arachnoid.
The Subarachnoid Cavity (cavum subarachnoideale;
subarachnoid space) is the interval between the arachnoid and pia mater. It
is occupied by a spongy tissue consisting of trabeculæ of delicate
connective tissue, and intercommunicating channels in which the subarachnoid
fluid is contained. This cavity is small on the surface of the hemispheres of
the brain; on the summit of each gyrus the pia mater and the arachnoid are in
close contact; but in the sulci between the gyri, triangular spaces are left,
in which the subarachnoid trabecular tissue is found, for the pia mater dips
into the sulci, whereas the arachnoid bridges across them from gyrus to gyrus.
At certain parts of the base of the brain, the arachnoid is separated from the
pia mater by wide intervals, which communicate freely with each other and are
named subarachnoid cisternæ; in these the subarachnoid tissue is
less abundant.
Subarachnoid Cisternæ (cisternæ subarachnoidales)
—The cisterna cerebellomedullaris (cisterna magna) is
triangular on sagittal section, and results from the arachnoid bridging over
the interval between the medulla oblongata and the under surfaces of the
hemispheres of the cerebellum; it is continuous with the subarachnoid cavity of
the medulla spinalis at the level of the foramen magnum. The cisterna pontis
is a considerable space on the ventral aspect of the pons. It contains the
basilar artery, and is continuous behind with the subarachnoid cavity of the
medulla spinalis, and with the cisterna cerebellomedullaris; and in front of
the pons with the cisterna interpeduncularis. The cisterna interpeduncularis
(cisterna basalis) is a wide cavity where the arachnoid extends across
between the two temporal lobes. It encloses the cerebral peduncles and the structures contained in the interpeduncular fossa, and contains
the arterial circle of Willis. In front, the cisterna interpeduncularis extends
forward across the optic chiasma, forming the cisterna chiasmatis, and
on to the upper surface of the corpus callosum, for the arachnoid stretches
across from one cerebral hemisphere to the other immediately beneath the free
border of the falx cerebri, and thus leaves a space in which the anterior
cerebral arteries are contained. The cisterna fossæ cerebri lateralis
is formed in front of either temporal lobe by the arachnoid bridging across the
lateral fissure. This cavity contains the middle cerebral artery. The cisterna
venæ magnæ cerebri occupies the interval between the splenium
of the corpus callosum and the superior surface of the cerebellum; it extends
between the layers of the tela chorioidea of the third ventricle and contains
the great cerebral vein.
The
subarachnoid cavity communicates with the general ventricular cavity of the
brain by three openings; one, the foramen of Majendie, is in the middle
line at the inferior part of the roof of the fourth ventricle; the other two
are at the extremities of the lateral recesses of that ventricle, behind the
upper roots of the glossopharyngeal nerves and are known as the foramina of
Luschka. It is still somewhat uncertain whether these foramina are actual
openings or merely modified areas of the inferior velum which permit the
passage of the cerebrospinal fluid from the ventricle into the subarachnoid
spaces as through a permeable membrane.
Structure.—An arachnoidal villus represents an invasion of
the dura by the arachnoid membrane, the latter penetrates the dura in such a
manner that the arachnoid mesothelial cells come to lie directly beneath the
vascular endothelium of the great dural sinuses. It consists of the following
parts: (1) In the interior is a core of subarachnoid
tissue, continuous with the meshwork of the general subarachnoid tissue through
a narrow pedicle, by which the villus is attached to the arachnoid. (2) Around
this tissue is a layer of arachnoid membrane, limiting and enclosing the
subarachnoid tissue. (3) Outside this is the thinned wall of the lacuna, which
is separated from the arachnoid by a potential space which corresponds to and
is continuous with the subdural cavity. (4) And finally, if the villus projects
into the sagittal sinus, it will be covered by the greatly thinned wall of the
sinus which may consist merely of endothelium. It will be seen, therefore, that
fluid injected into the subarachnoid cavity will find its way into these villi,
and it has been found experimentally that it passes from the villi into the
venous sinuses into which they project.
Diagrammatic transverse section of the medulla spinalis and its
membranes.
The
cerebrospinal fluid, 129 for the most part elaborated by the
choroid plexuses, is poured into the cerebral ventricles which are lined by
smooth ependyma. That portion of the fluid formed in the lateral ventricles
escapes by the foramen of Monro into the third ventricle and thence by the
aqueduct into the fourth ventricle. Likewise an ascending current of fluid
apparently occurs in the central canal of the spinal cord; this, representing a
possible product of the ependyma, may be added to the intraventricular supply.
From the fourth ventricle the fluid is poured into the subarachnoid spaces
through the medial foramen of Majendie and the two lateral foramina of Luschka.
There is no evidence that functional communications between the cerebral
ventricles and the subarachnoid spaces exist in any region except from the
fourth ventricle.
The internal carotid and vertebral arteries. Right side.
Artery of the Pterygoid Canal.
4.
The hypophyseal branches are one or two minute vessels supplying the
hypophysis.
5.
The semilunar branches are small vessels to the semilunar ganglion.
The ophthalmic artery and its branches.
The Medial
Palpebral Arteries (aa. palpebrales mediales; internal palpebral
arteries), two in number, superior and inferior, arise
from the ophthalmic, opposite the pulley of the Obliquus superior; they leave
the orbit to encircle the eyelids near their free margins, forming a superior
and an inferior arch, which lie between the Orbicularis oculi and the tarsi.
The superior palpebral anastomoses, at the lateral angle of the orbit,
with the zygomaticoörbital branch of the temporal artery and with the
upper of the two lateral palpebral branches from the lacrimal artery; the inferior
palpebral anastomoses, at the lateral angle of the orbit, with the lower of
the two lateral palpebral branches from the lacrimal and with the transverse
facial artery, and, at the medial part of the lid, with a branch from the
angular artery. From this last anastomoses a branch
passes to the nasolacrimal duct, ramifying in its mucous membrane, as far as
the inferior meatus of the nasal cavity.
Branches.—In its course the anterior cerebral artery
gives off the following branches:
Branches.—The branches of this vessel are the:
Outer surface of cerebral hemisphere, showing areas supplied by cerebral
arteries.
Medial surface of cerebral hemisphere, showing areas supplied by cerebral
arteries.
Diagram of the arterial circulation at the base of the brain. A.L. Antero-lateral. A.M.
Antero-medial. P.L. Postero-lateral. P.M. Posteromedial ganglionic branches.
Prepared
by
Boymystruk
I.I.