1. General structure of the nervous system. Spinal cord (external structure). Membranes
of the spinal cord. Spinal cord (internal structure)
2. general review of brain.
3. Membranes of braine. sinuses of
dura matter.
Lesson 16
Theme 1.
General structure of the nervous system. Spinal cord (external structure). Membranes
of the spinal cord.
Spinal
cord (internal structure)
Nervous system may be divided
into two parts, central and peripheral, also somatic and autonomic.
The central nervous system consists
of the brain [encephalon], contained within the cranium, and the
medulla spinalis
or spinal cord, lodged
in the vertebral canal. The peripheral
nervous system consists of a series of cranial and spinal nerves. Autonomic
nervous system has a parasympathetic and sympathetic parts.
Spinal
cord is located in vertebral canal and extends inferiorly
from the position of the foramen magnum of the occipital bone to the level of
the first-second lumbar vertebra. The terminal portion of the spinal cord is
called the conus medullaris
and filum terminale
extends inferiorly from it to the level second coccyx vertebrae. The cervical enlargement is
located at the level
of 6th cervical vertebrae, and the lumbosacral enlargement
at the level of 12th thoracic vertebrae. The
Diagrams of the medulla spinalis.
The
medulla spinalis or spinal cord forms the elongated, nearly cylindrical,
part of the central nervous system which occupies the upper two-thirds of the
vertebral canal. Its average length in the male is about
The
position of the medulla spinalis varies with the movements of the vertebral
column, its lower extremity being drawn slightly upward when the column is
flexed. It also varies at different periods of life; up to the third month of
fetal life the medulla spinalis is as long as the vertebral canal, but from
this stage onward the vertebral column elongates more rapidly than the medulla spinalis,
so that by the end of the fifth month the medulla spinalis terminates at the
base of the sacrum, and at birth about the third lumbar vertebra.
Cauda equina and filum terminale seen from behind. The dura mater has
been opened and spread out, and the arachnoid has been removed.
The
medulla spinalis does not fill the part of the vertebral canal in which it
lies; it is ensheathed by three protective membranes, separated from each other
by two concentric spaces. The three membranes are named from without inward,
the dura mater, the arachnoid, and the pia mater. The dura
mater is a strong, fibrous membrane which forms a wide, tubular sheath;
this sheath extends below the termination of the medulla spinalis and ends in a
pointed cul-de-sac at the level of the lower border of the second sacral vertebra.
The dura mater is separated from the wall of the vertebral canal by the epidural
cavity, which contains a quantity of loose areolar tissue and a plexus of
veins; between the dura mater and the subjacent arachnoid is a capillary
interval, the subdural cavity, which contains a small quantity of fluid,
probably of the nature of lymph. The arachnoid is a thin, transparent
sheath, separated from the pia mater by a comparatively wide interval, the subarachnoid
cavity, which is filled with cerebrospinal fluid. The pia mater
closely invests the medulla spinalis and sends delicate septa into its
substance; a narrow band, the ligamentum denticulatum, extends along
each of its lateral surfaces and is attached by a series of pointed processes
to the inner surface of the dura mater.
Sagittal section of vertebral canal to show the lower end of the medulla
spinalis and the filum terminale. Li, Lv. First and fifth lumbar
vertebræ. Sii. Second sacral vertebra. 1. Dura mater. 2. Lower
part of tube of dura mater. 3. Lower extremity of medulla spinalis. 4.
Intradural, and 5, Extradural portions of filum terminale. 6. Attachment of
filum terminale to first segment of coccyx.
Thirty-one
pairs of spinal nerves spring from the medulla spinalis, each nerve having an
anterior or ventral, and a posterior or dorsal root, the latter being
distinguished by the presence of an oval swelling, the spinal ganglion,
which contains numerous nerve cells. Each root consists of several bundles of
nerve fibers, and at its attachment extends for some distance along the side of
the medulla spinalis. The pairs of spinal nerves are grouped as follows:
cervical 8, thoracic 12, lumbar 5, sacral 5, coccygeal 1, and, for convenience
of description, the medulla spinalis is divided into cervical, thoracic, lumbar
and sacral regions, corresponding with the attachments of the different groups
of nerves.
Enlargements.—The medulla spinalis is not quite cylindrical, being
slightly flattened from before backward; it also presents two swellings or
enlargements, an upper or cervical, and a lower or lumbar.
The
cervical enlargement is the more pronounced, and corresponds with the
attachments of the large nerves which supply the upper limbs. It extends from
about the third cervical to the second thoracic vertebra, its maximum
circumference (about
The
lumbar enlargement gives attachment to the nerves which supply the lower
limbs. It commences about the level of the ninth thoracic vertebra, and reaches
its maximum circumference, of about
The
Anterior Median Fissure (fissura mediana anterior) has an average
depth of about
The Posterior Median Sulcus (sulcus
medianus posterior) is very shallow; from it a septum of neuroglia reaches
rather more than half-way into the substance of the medulla spinalis; this
septum varies in depth from 4 to
On either side of the posterior median sulcus,
and at a short distance from it, the posterior nerve roots are attached along a
vertical furrow named the posterolateral sulcus. The portion of the
medulla spinalis which lies between this and the posterior median sulcus is named
the posterior funiculus. In the cervical and upper thoracic regions this
funiculus presents a longitudinal furrow, the postero-intermediate sulcus;
this marks the position of a septum which extends into the posterior funiculus
and subdivides it into two fasciculi—a medial, named the fasciculus gracilis
(tract of Goll); and a lateral, the fasciculus cuneatus (tract
of Burdach). The portion of the medulla spinalis
which lies in front of the posterolateral sulcus is termed the antero-lateral
region. The anterior nerve roots, unlike the posterior, are not attached in
linear series, and their position of exit is not marked by a sulcus. They arise
by separate bundles which spring from the anterior column of gray substance
and, passing forward through the white substance, emerge over an area of some
slight width. The most lateral of these bundles is generally taken as a
dividing line which separates the antero-lateral region into two parts, viz.,
an anterior funiculus, between the anterior median fissure and the most
lateral of the anterior nerve roots; and a lateral funiculus, between
the exit of these roots and the postero-lateral sulcus. In the upper part of
the cervical region a series of nerve roots passes outward through the lateral
funiculus of the medulla spinalis; these unite to form the spinal portion of
the accessory nerve, which runs upward and enters the cranial cavity through
the foramen magnum.
Gray Substance (substantia grisea
centralis).—The gray substance consists of two symmetrical
portions, one in each half of the medulla spinalis: these are joined across the
middle line by a transverse commissure of gray substance, through which runs a
minute canal, the central canal, just visible to the naked eye. In a
transverse section each half of the gray substance is shaped like a comma or
crescent, the concavity of which is directed laterally; and these, together
with the intervening gray commissure, present the appearance of the letter H.
An imaginary coronal plane through the central canal serves to divide each
crescent into an anterior or ventral, and a posterior or dorsal
column.
The Anterior Column (columna anterior;
anterior cornu), directed forward, is broad and of a rounded or
quadrangular shape. Its posterior part is termed the base, and its anterior
part the head, but these are not differentiated from each other by any
well-defined constriction. It is separated from the surface of the medulla
spinalis by a layer of white substance which is traversed by the bundles of the
anterior nerve roots. In the thoracic region, the postero-lateral part of the
anterior column projects lateralward as a triangular field, which is named the lateral
column (columna lateralis; lateral cornu).
The Posterior Column (columna
posterior; posterior cornu) is long and slender, and is directed backward
and lateralward: it reaches almost as far as the posterolateral sulcus, from
which it is separated by a thin layer of white substance, the tract of
Lissauer. It consists of a base, directly continuous with the base
of the anterior horn, and a neck or slightly constricted portion, which
is succeeded by an oval or fusiform area, termed the head, of which the apex
approaches the posterolateral sulcus. The apex is capped by a V-shaped or
crescentic mass of translucent, gelatinous neuroglia, termed the substantia
gelatinosa of Rolando, which contains both neuroglia cells, and small nerve
cells. Between the anterior and posterior columns the gray substance extends as
a series of processes into the lateral funiculus, to form a net-work called the
formatio reticularis.
The anterior
median fissure (fissura mediana
anterior) and right and left anterolateral sulcuses located on anterior
surface of the spinal cord. Posterior
median sulcus (sulcus medianus
posterior) and also right
and
left posterolateral sulcus located
on back surface of the spinal cord.
Each
spinal nerve is attached to the medulla spinalis by two roots, an anterior or ventral (motor and sympathetic fibbers), and a posterior or dorsal, the being characterized by the
presence of a ganglion, the spinal
ganglion (with sensory pseudounipolar cells).
A spinal nerve with its anterior and posterior roots.
After
emerging from the intervertebral foramen, each spinal nerve gives off a small meningeal branch which supplies the
dura mater, and an anterior or ventral,
posterior or dorsal divisions,
also white and gray communicating branches for nearest sympathetic ganglion (in
thoracic-lumbar part).
levels.
The
medulla spinalis is ensheathed by three protective membranes, separated from
each other by two concentric spaces:
1. The Dura mater
2. The Arachnoid
3. The Pia mater
The dura mater is separated from the
wall of the vertebral canal
by the epidural cavity, which contains a quantity of fat tissue and a
plexus of veins. Between the dura mater and the subjacent arachnoid is a
capillary interval, the subdural cavity,
which contains a small quantity of fluid, probably of the nature of lymph. The arachnoid is a thin, transparent
sheath, separated from the pia mater by a comparatively wide interval, the subarachnoid cavity, which is filled
with cerebrospinal fluid. The pia mater
closely invests the medulla spinalis and sends delicate septa into its
substance; a narrow band, the ligamentum
denticulatum, extends along each of its lateral surfaces and is attached by
a series of pointed processes to the inner surface of the dura mater.
|
Structure |
Location/Description |
Notes |
|
arachnoid
mater |
intermediate one of the three layers of meninges |
|
|
denticulate ligament |
a lateral extension of pia mater from the spinal
cord |
denticulate ligament attaches to the dura mater to
anchor the spinal cord; it forms a scalloped free border; there are 2 (one on
each side) |
|
dura mater |
outermost of the meningeal layers covering the brain
and spinal cord |
"tough
mother"; it is the most durable of the meninges and provides support and
protection for the brain and spinal cord; two types are described which
differ in structure: cranial and spinal |
|
dura mater, spinal |
outermost covering of the spinal cord, it forms the
dural sac containing the spinal cord within vertebral canal |
dural sac ends at S2, coccygeal ligament (filum
terminale externum) continues inferiorly to attach to coccyx |
|
epidural
fat |
loose connective tissue within the epidural space |
|
|
epidural
space |
the space external to the sac of spinal dura mater
within the vertebral canal |
the epidural space contains epidural fat and the
internal vertebral plexus of veins which is valveless (clinically relevant) |
|
filum terminale internum |
thread-like extension of the pia mater from the
conus medullaris of the spinal cord |
filum terminale internum is best seen between
vertebral levels L2 and S2; it becomes enclosed within the filum terminale
externum |
|
filum terminale externum |
thread-like extension of the dura mater below the
end of the dural sac at S2 |
it attaches to the coccyx; also known as the
coccygeal ligament |
|
meninges |
three layers of connective tissue covering the brain
and spinal cord; dura mater, arachnoid mater, and pia mater |
meninges provide protection and nourishment of the
brain, brainstem and spinal cord |
|
pia mater |
delicate membrane that lies on surface of the brain
and spinal cord |
"delicate mother", it is the most delicate
of the meninges; this layer faithfully follows all surface contours of the
brain and spinal cord; pia mater has 2 specializations: denticulate ligament
and filum terminale internum |
|
subdural
space |
the space between the dura mater and the arachnoid
mater |
this is a potential space only; the pressure of CSF
in the subarachnoid space pushes arachnoid against dura |
|
subarachnoid
space |
the space between the arachnoid and the pia mater |
subarachnoid space contains cerebrospinal fluid and
spider web-like filaments |
|
cauda equina |
dorsal and ventral roots of all spinal nerves
inferior to L1 |
lies within the lumbar cistern |
|
conus medullaris |
cone-shaped inferior end of the spinal cord; located
at vertebral level L1 |
at birth, the conus medullaris is at the level of
L2/L3 |
|
cervical enlargement |
vertebral level C4 through T1 |
created by the rootlets of spinal nerves C5-T1 that
form the brachial plexus |
|
lumbrosacral enlargement |
vertebral level T11 through L1 |
Created by the rootlets of spinal nerves L1-S4 that
form the lumbosacral plexus |
Spinal cord consists of the gray substance that is surrounded by the white substance. Posterior median sulcus passes to the gray
substance and divides by the white substance into two parts. Anterior median
fissure does not pass to the gray substance so it is white commisura that
connect anterior symmetrical regions of the white matter.
The gray
substance is largely composed of nervous cell bodies. The gray substance
consists of anterior, posterior and lateral columns (only in thoracic-lumbar
portion) also central intermediate
zone
round central canal. In transverse section columns are looking as horns, so
they differ anterior, posterior and lateral horns.
Transverse section of the medulla spinalis in the mid-thoracic region.
Anterior
horns contain motor cells that arranged in 5 motor nuclei (nucleus
anterior [medial and lateral], central nucleus and posterior [medial and
lateral] nuclei). Their axons form anterior
roots that pass with spinal nerves to the skeletal muscles of the trunk and
limbs. Posterior horns contain intermediate cells. They receive impulse from
sensory cells and carry them to the another cell. Intermediate cells form the spongious
zone, gelatinous substance, proper nucleus and thoracic
nucleus (Clarc-Steiling column).
Lateral
columns in thoracic-lumbar portion contain autonomic cells that form lateral intermediate nucleus. In
intermediate central part medial
intermediate nucleus carry intermediate cells.
White
substance contains only neuron long processes (axons), the nerve fibbers which
form ascending (sensory, afferent) and descending (motor, efferent) pathways.
White substance divided by sulcuses and is arranged in three funiculi:
anterior, lateral, and posterior. Both anterior funiculi are communicated
each other by white commisura.
Fasciculi
in the Posterior Funiculus comprises
the fasciculus gracilis [tract of Goll] (lies next the posterior
median septum) and the fasciculus
cuneatus [tract of Burdach]
(laterally). They conduct impulses of
conscious muscle sense.
Pathways
in the Anterior Funiculus (descending):
anterior corticospinal [pyramidal] tract, tectospinal tract, reticulospinal
tract, olivospinal tract, and vestibulospinal tract. Ascending
pathway - anterior spinothalamic tract.
Diagram of
the principal fasciculi of the spinal cord.
Ascending pathways in the Lateral Funiculus:
· dorsal spinocerebellar tract
(tract of Flechsig)
· ventral spinocerebellar tract (tract of Gowers)
· lateral spinothalamic
· tract
Medially they can
find descending pathways:
· corticospinal
[pyramidal] tract
· rubrospinal
tract (of Monakow)
Portion
of the spinal cord that carries two pairs of ventral and dorsal roots or one
pair of spinal nerves called segment.
There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal segments.
Roots which exit from the spinal cord lower then second lumbar segment form
together with the filum termanale cauda
equina.
Gray
Substance (substantia grisea centralis).—The gray substance
consists of two symmetrical portions, one in each half of the medulla spinalis:
these are joined across the middle line by a transverse commissure of gray
substance, through which runs a minute canal, the central canal, just
visible to the naked eye. In a transverse section each half of the gray
substance is shaped like a comma or crescent, the concavity of which is
directed laterally; and these, together with the intervening gray commissure,
present the appearance of the letter H. An imaginary coronal plane through the
central canal serves to divide each crescent into an anterior or ventral,
and a posterior or dorsal column.
The
Anterior Column (columna anterior; anterior cornu), directed
forward, is broad and of a rounded or quadrangular shape. Its posterior part is
termed the base, and its anterior part the head, but these are not
differentiated from each other by any well-defined constriction. It is
separated from the surface of the medulla spinalis by a layer of white
substance which is traversed by the bundles of the anterior nerve roots. In the
thoracic region, the postero-lateral part of the anterior column projects
lateralward as a triangular field, which is named the lateral column (columna
lateralis; lateral cornu).
The
Posterior Column (columna posterior; posterior cornu) is long and
slender, and is directed backward and lateralward: it reaches almost as far as
the posterolateral sulcus, from which it is separated by a thin layer of white
substance, the tract of Lissauer. It consists of a base, directly
continuous with the base of the anterior horn, and a neck or slightly
constricted portion, which is succeeded by an oval or fusiform area, termed the
head, of which the apex approaches the posterolateral sulcus. The
apex is capped by a V-shaped or crescentic mass of translucent, gelatinous
neuroglia, termed the substantia gelatinosa of Rolando, which contains
both neuroglia cells, and small nerve cells. Between the anterior and posterior
columns the gray substance extends as a series of processes into the lateral
funiculus, to form a net-work called the formatio reticularis.
The
quantity of gray substance, as well as the form which it presents on transverse
section, varies markedly at different levels. In the thoracic region it is
small, not only in amount but relatively to the surrounding white substance. In
the cervical and lumbar enlargements it is greatly increased: in the latter,
and especially in the conus medullaris, its proportion to the white substance is
greatest (665).
In the cervical region its posterior column is comparatively narrow, while its
anterior is broad and expanded; in the thoracic region, both columns are
attenuated, and the lateral column is evident; in the lumbar enlargement, both
are expanded; while in the conus medullaris the gray substance assumes the form
of two oval masses, one in each half of the cord, connected together by a broad
gray commissure.
The
Throughout
the cervical and thoracic regions the central canal is situated in the anterior
third of the medulla spinalis; in the lumbar enlargement it is near the middle,
and in the conus medullaris it approaches the posterior surface. It is filled
with cerebrospinal fluid, and lined by ciliated, columnar epithelium, outside
of which is an encircling band of gelatinous substance, the substantia
gelatinosa centralis. This gelatinous substance consists mainly of
neuroglia, but contains a few nerve cells and fibers; it is traversed by processes
from the deep ends of the columnar ciliated cells which line the central canal .
Structure
of the Gray Substance.—The gray substance consists of
numerous nerve cells and nerve fibers held together by neuroglia. Throughout the
greater part of the gray substance the neuroglia presents the appearance of a
sponge-like network, but around the central canal and on the apices of the
posterior columns it consists of the gelatinous substance already referred to.
The nerve cells are multipolar, and vary greatly in size and shape. They
consist of (1) motor cells of large size, which are situated in the anterior
horn, and are especially numerous in the cervical and lumbar enlargements; the
axons of most of these cells pass out to form the anterior nerve roots, but before leaving the white substance they
frequently give off collaterals, which reënter and ramify in the gray
substance. 113 (2) Cells of small or medium size, whose axons pass into the white
matter, where some pursue an ascending, and others a descending course, but
most of them divide in a T-shape manner into descending and ascending processes.
They give off collaterals which enter and ramify in the gray substance, and the
terminations of the axons behave in a similar manner. The lengths of these
axons vary greatly: some are short and pass only between adjoining spinal
segments, while others are longer and connect more distant segments. These
cells and their processes constitute a series of association or intersegmental neurons (668), which link together the different parts of the medulla spinalis. The
axons of most of these cells are confined to that side of the medulla spinalis
in which the nerve cells are situated, but some cross to the opposite side
through the anterior commissure, and are termed crossed commissural fibers.
Some of these latter end directly in the gray substance, while others enter the
white substance, and ascend or descend in it for varying distances, before
finally terminating in the gray substance. (3) Cells of the type II of Golgi,
limited for the most part to the posterior column, are found also in the
substantia gelatinosa of Rolando; their axons are short and entirely confined
to the gray substance, in which they break up into numerous fine filaments.
Most of the nerve cells are arranged in longitudinal columns, and appear as
groups on transverse section (Figs. 669, 670, 671).
Transverse sections of
the medulla spinalis at different
Nerve
Cells in the Anterior Column.—The nerve cells in the anterior
column are arranged in columns of varying length. The longest occupies the
medial part of the anterior column, and is named the antero-medial column:
it is well marked in C4, C5, again from C8 to L4, it disappears in L5 and S1
but is well marked in S2, S3 and S4 (Bruce). 114
Behind it is a dorso-medial column of small cells, which is not
represented in L5, S1, S2 nor below S4. Its axons
probably pass into the dorsal rami of the spinal nerves to supply the dorsal
musculature of the spinal column. In the cervical and lumbar enlargements,
where the anterior column is expanded in a lateral direction, the following
additional columns are present, viz.: (a) antero-lateral, which
consists of two groups, one in C4, C5, C6 the other in C6, C7, C8 in the
cervical enlargement and of a group from L2 to S2 in the lumbo-sacral
enlargement; (b) postero-lateral, in the lower five cervical, lower
four lumbar, and upper three sacral segments; (c) post-postero-lateral,
in the last cervical, first thoracic, and upper three sacral segments; and (d)
a central, in the lower four lumbar and upper two sacral segments. These
cell groups are evidently related to the nerve roots of the brachial and sacral
plexuses and supply fibers to the muscles of the arm and leg. Throughout the
base of the anterior column are scattered solitary cells, the axons of some of
which form crossed commissural fibers, while others constitute the motor fibers
of the posterior nerve roots. (See footnote, page 755.)
Nerve
Cells in the Lateral Column.—These form a column which is best
marked where the lateral gray column is differentiated, viz., in the thoracic
region; 115
but it can be traced throughout the entire length of the medulla spinalis in
the form of groups of small cells which are situated in the anterior part of
the formatio reticularis. In the upper part of the cervical region and lower part
of the medulla oblongata as well as in the third and fourth sacral segments
this column is again differentiated. In the medulla it is known as the lateral
nucleus. The cells of this column are fusiform or star-shaped, and of a
medium size: the axons of some of them pass into the anterior nerve roots, by
which they are carried to the sympathetic nerves: they constitute the white
rami and are sympathetic or visceral efferent fibers; they are also known as preganglionic
fibers of the sympathetic system; the axons of others pass into the
anterior and lateral funiculi, where they become longitudinal.
Nerve
Cells in the Posterior Column.—1. The dorsal nucleus (nucleus
dorsalis; column of Clarke) occupies the medial part of the base of the
posterior column, and appears on the transverse section as a well-defined oval
area. It begins below at the level of the second or third lumbar nerve, and
reaches its maximum size opposite the twelfth thoracic nerve. Above the level
of the ninth thoracic nerve its size diminishes, and the column ends opposite
the last cervical or first thoracic nerve. It is represented, however, in the
other regions by scattered cells, which become aggregated to form a cervical
nucleus opposite the third cervical nerve, and a sacral nucleus in
the middle and lower part of the sacral region. Its cells are of medium size,
and of an oval or pyriform shape; their axons pass into the peripheral part of
the lateral funiculus of the same side, and there ascend, probably in dorsal
spinocerebellar (direct cerebellar) fasciculus. 2. The nerve
cells in the substantia gelatinosa of Rolando are arranged in three zones:
a posterior or marginal, of large angular or fusiform cells; an intermediate,
of small fusiform cells; and an anterior, of star-shaped cells. The axons of
these cells pass into the lateral and posterior funiculi, and there assume a
vertical course. In the anterior zone some Golgi cells are found whose short
axons ramify in the gray substance. 3. Solitary cells of varying form
and size are scattered throughout the posterior column. Some of these are
grouped to form the posterior basal column in the base of the posterior
column, lateral to the dorsal nucleus; the posterior basal column is
well-marked in the gorilla (Waldeyer), but is ill-defined in man. The axons of
its cells pass partly to the posterior and lateral funiculi of the same side,
and partly through the anterior white commissure to the lateral funiculus of
the opposite side. Golgi cells, type II, located in this region send axons to the
lateral and ventral columns.
A
few star-shaped or fusiform nerve cells of varying size are found in the
substantia gelatinosa centralis. Their axons pass into the lateral funiculus of
the same, or of the opposite side.
The
nerve fibers in the gray substance form a dense interlacement of minute fibrils
among the nerve cells. This interlacement is formed partly of axons which pass
from the cells in the gray substance to enter the white funiculi or nerve
roots; partly of the axons of Golgi’s cells which ramify only in the gray
substance; and partly of collaterals from the nerve fibers in the white
funiculi which, as already stated, enter the gray substance and ramify within
it.
White
Substance (substantia alba).—The white
substance of the medulla spinalis consists of medullated nerve fibers imbedded
in a spongelike net-work of neuroglia, and is arranged in three funiculi:
anterior, lateral, and posterior. The anterior funiculus lies between the
anterior median fissure and the most lateral of the anterior nerve roots: the
lateral funiculus between these nerve roots and the postero-lateral sulcus; and
the posterior funiculus between the postero-lateral and the posterior median
sulci (672).
The fibers vary greatly in thickness, the smallest being found in the
fasciculus gracilis, the tract of Lissauer, and inner part of the
lateral funiculus; while the largest are situated in the anterior funiculus,
and in the peripheral part of the lateral funiculus. Some of the nerve fibers
assume a more or less transverse direction, as for example those which cross
from side to side in the anterior white commissure, but the majority pursue a
longitudinal course and are divisible into (1) those connecting the medulla
spinalis with the brain and conveying impulses to or from the latter, and (2)
those which are confined to the medulla spinalis and link together its
different segments, i. e., intersegmental or association fibers.
Nerve
Fasciculi.—The longitudinal fibers are grouped into more or less
definite bundles or fasciculi. These are not recognizable from each other in
the normal state, and their existence has been determined by the following
methods: (1) A. Waller discovered that if a bundle of nerve fibers be cut, the portions
of the fibers which are separated from their cells rapidly degenerate and
become atrophied, while the cells and the parts of the fibers connected with
them undergo little alteration. 116
This is known as Wallerian degeneration. Similarly, if a group of nerve
cells be destroyed, the fibers arising from them undergo degeneration. Thus, if
the motor cells of the cerebral cortex be destroyed, or if the fibers arising
from these cells be severed, a descending degeneration from the seat of
injury takes place in the fibers. In the same manner, if a spinal ganglion be
destroyed, or the fibers which pass from it into the medulla spinalis be cut,
an ascending degeneration will extend along these fibers. (2)
Pathological changes, especially in man, have given important information by
causing ascending and descending degenerations. (3) By tracing the development
of the nervous system, it has been observed that at first the nerve fibers are
merely naked axis-cylinders, and that they do not all acquire their medullary
sheaths at the same time; hence the fibers can be grouped into different
bundles according to the dates at which they receive their medullary sheaths.
(4) Various methods of staining nervous tissue are of great value in tracing
the course and mode of termination of the axis-cylinder processes.
Fasciculi
in the Anterior Funiculus.—Descending Fasciculi.—The
anterior cerebrospinal (fasciculus cerebrospinalis anterior; direct
pyramidal tract), which is usually small, but varies inversely in size with
the lateral cerebrospinal fasciculus. It lies close to the anterior median
fissure, and is present only in the upper part of the medulla spinalis;
gradually diminishing in size as it descends, it ends about the middle of the
thoracic region. It consists of descending fibers which arise from cells in the
motor area of the cerebral hemisphere of the same side, and which, as they run
downward in the medulla spinalis, cross in succession through the anterior
white commissure to the opposite side, where they end, either directly or
indirectly, by arborizing around the motor cells in the anterior column. A few
of its fibers are said to pass to the lateral column of the same side and to
the gray matter at the base of the posterior column. They conduct voluntary
motor impulses from the precentral gyrus to the motor centers of the cord.
The
vestibulospinal fasciculus, situated chiefly in the marginal part of the
funiculus and mainly derived from the cells of Deiters’ nucleus, of the same
and the opposite side, i. e., the chief terminal nucleus of the
vestibular nerve. Fibers are also contributed to this fasciculus from scattered
cells of the articular formation of the medulla oblongata, the pons and the
mid-brain (tegmentum). The other terminal nuclei of the vestibular nerve also
contribute fibers. In the brain stem these fibers form part of the median
longitudinal bundle. The fasciculus can be traced to the sacral region. Its
terminals and collaterals end either directly or indirectly among the motor
cells of the anterior column. This fasciculus is probably concerned with
equilibratory reflexes.
The
tectospinal fasciculus, situated partly in the anterior and partly in
the lateral funiculus, is mainly derived from the opposite superior colliculus
of the mid-brain. The fibers from the superior colliculus cross the median
raphé in the fountain decussation of Meynert and descend as the ventral
longitudinal bundle in the reticular formation of the brain-stem. Its
collaterals and terminals end either directly or indirectly among the motor
cells of the anterior column of the same side. Since the superior colliculus is
an important visual reflex center, the tectospinal fasciculus is probably
concerned with visual reflexes.
Ascending
Fasciculi.—The ventral spinothalamic fasciculus, situated in the
marginal part of the funiculus and intermingled more or less with the
vestibulo-spinal fasciculus, is derived from cells in the posterior column or
intermediate gray matter of the opposite side. Their axons cross in the
anterior commissure. This is a somewhat doubtful fasciculus and its fibers are
supposed to end in the thalamus and to conduct certain of the touch impulses.
The
remaining fibers of the anterior funiculus constitute what is termed the anterior
proper fasciculus (fasciculus anterior proprius; anterior basis bundle).
It consists of (a) longitudinal intersegmental fibers which arise from
cells in the gray substance, more especially from those of the medial group of
the anterior column, and, after a longer or shorter course, reënter the
gray substance; (b) fibers which cross in the anterior white commissure
from the gray substance of the opposite side.
Fasciculi
in the Lateral Funiculus.—1. Descending Fasciculi.—(a)
The lateral cerebrospinal fasciculus (fasciculus cerebrospinalis
lateralis; crossed pyramidal tract) extends throughout the entire length of
the medulla spinalis, and on transverse section appears as an oval area in
front of the posterior column and medial to the cerebellospinal. Its fibers
arise from cells in the motor area of the cerebral hemisphere of the opposite
side. They pass downward in company with those of the anterior cerebrospinal
fasciculus through the same side of the brain as that from which they
originate, but they cross to the opposite side in the medulla oblongata and
descend in the lateral funiculus of the medulla spinalis.
It
is probable 117
that the fibers of the anterior and lateral
cerebrospinal fasciculi are not related in this direct manner with the cells of
the anterior column, but terminate by arborizing around the cells at the base
of the posterior column and the cells of Clarke’s column, which in turn link
them to the motor cells in the anterior column, usually of several segments of
the cord. In consequence of these interposed neurons the fibers of the
cerebrospinal fasciculi correspond not to individual muscles, but to associated
groups of muscles.
The
anterior and lateral cerebrospinal fasciculi constitute the motor fasciculi of
the medulla spinalis and have their origins in the motor cells of the cerebral
cortex. They descend through the internal capsule of the cerebrum, traverse the
cerebral peduncles and pons and enter the pyramid of the medulla oblongata. In
the lower part of the latter about two-thirds of them cross the middle line and
run downward in the lateral funiculus as the lateral cerebrospinal fasciculus,
while the remaining fibers do not cross the middle line, but are continued into
the same side of the medulla spinalis, where they form the anterior
cerebrospinal fasciculus. The fibers of the latter, however, cross the middle
line in the anterior white commissure, and thus all the motor fibers from one
side of the brain ultimately reach the opposite side of the medulla spinalis.
The proportion of fibers which cross in the medulla oblongata is not a constant
one, and thus the anterior and lateral cerebrospinal fasciculi vary inversely
in size. Sometimes the former is absent, and in such cases it may be presumed
that the decussation of the motor fibers in the medulla oblongata has been
complete. The fibers of these two fasciculi do not acquire their medullary
sheaths until after birth. In some animals the motor fibers are situated in the
posterior funiculus.
(b)
The rubrospinal fasciculus (Monakow) (prepyramidal tract), lies
on the ventral aspect of the lateral cerebrospinal fasciculus and on transverse
section appears as a somewhat triangular area. Its fibers descend from the
mid-brain, where they have their origin in the red nucleus of the tegmentum of
the opposite side. Its terminals and collaterals end either directly or
indirectly in relation with the motor cells of the anterior column. The
rubrospinal fasciculus is supposed to be concerned with cerebellar reflexes
since fibers which pass from the cerebellum through the superior peduncle send
many collaterals and terminals to the red nucleus.
(c)
The olivospinal fasciculus (Helweg) arises in the vicinity of the
inferior olivary nucleus in the medulla oblongata, and is seen only in the
cervical region of the medulla spinalis, where it forms a small triangular area
at the periphery, close to the most lateral of the anterior nerve roots. Its
exact origin and its mode of ending have not yet been definitely made out.
2.
Ascending Fasciculi.—(a) The dorsal spinocerebellar fasciculus (fasciculus
cerebellospinalis; direct cerebellar tract of Flechsig) is situated at the
periphery of the posterior part of the lateral funiculus, and on transverse
section appears as a flattened band reaching as far forward as a line drawn
transversely through the central canal. Medially, it is in contact with the
lateral cerebrospinal fasciculus, behind, with the fasciculus of Lissauer. It
begins about the level of the second or third lumbar nerve, and increasing in
size as it ascends, passes to the vermis of the cerebellum through the inferior
peduncle. Its fibers are generally regarded as being formed by the axons of the
cells of the dorsal nucleus (Clarke’s column); they receive their
medullary sheaths about the sixth or seventh month of fetal life. Its fibers
are supposed to conduct impulses of unconscious muscle sense.
The
superficial antero-lateral fasciculus (tract of Gowers) consists
of four fasciculi, the ventral spinocerebellar, the lateral spinothalamic, the
spinotectal and the ventral spinothalamic.
(b)
The ventral spinocerebellar fasciculus (Gowers) skirts the
periphery of the lateral funiculus in front of the dorsal spinocerebellar
fasciculus. In transverse section it is shaped somewhat like a comma, the
expanded end of which lies in front of the dorsal spinocerebellar fasciculus
while the tail reaches forward into the anterior funiculus. Its fibers come
from the same but mostly from the opposite side of the medulla spinalis and
cross both in the anterior white commissure and in the gray commissure; they
are probably derived from the cells of the dorsal nucleus and from other cells
of the posterior column and the intermediate portion of the gray matter. The
ventral spinocerebellar fasciculus begins about the level of the third pair of
lumbar nerves, and can be followed into the medulla oblongata and pons almost
to the level of the inferior colliculus where it crosses over the superior
peduncle and then passes backward along its medial border to reach the vermis
of the cerebellum. In the pons it lies along the lateral edge of the lateral
lemniscus. Some of its fibers join the dorsal spinocerebellar fasciculus at the
level of the inferior peduncle and pass with them into the cerebellum. Other
fibers are said to continue upward in the dorso-lateral part of the tegmentum
of the mid-brain probably as far as the thalamus.
(c)
The lateral spinothalamic fasciculus is supposed to come from cells in
the dorsal column and the intermediate gray matter whose axons cross in the
anterior commissure to the opposite lateral funiculus where they pass upward on
the medial side of the ventral spinocerebellar fasciculus; on reaching the
medulla oblongata they continue in the formatio reticularis near the median
fillet and probably terminate in the ventro-lateral region of the thalamus. It
is supposed to conduct impulses of pain and temperature. The lateral and
ventral spinothalamic fasciculi are sometimes termed the secondary sensory
fasciculus or spinal lemniscus.
(d)
The spinotectal fasciculus is supposed to arise in the dorsal column and
terminate in the (inferior ?) and superior colliculi. It is situated ventral to
the lateral spinothalamic fasciculus, but its fibers are more or less
intermingled with it. It is also known as the spino-quadrigeminal system of
Mott. In the brain-stem the fibers run lateral from the inferior olive,
ventro-lateral from the superior olive, then ventro-medial from the spinal
tract of the trigeminal; the fibers come to lie in the medial portion of the
lateral lemniscus.
(e)
The fasciculus of Lissauer is a small strand situated in relation to the
tip of the posterior column close to the entrance of the posterior nerve roots.
It consists of fine fibers which do not receive their medullary sheaths until
toward the close of fetal life. It is usually regarded as being formed by some
of the fibers of the posterior nerve roots, which ascend for a short distance
in the tract and then enter the posterior column, but since its fibers are
myelinated later than those of the posterior nerve roots, and do not undergo
degeneration in locomotor ataxia, they are probably intersegmental in
character.
In
addition the fasciculus or tract of Lissauer contains great numbers of
fine non-medullated fibers derived mostly from the dorsal roots but partly
endogenous in origin. These fibers are intimately related to the substantia
gelatinosa which is probably the terminal nucleus. The non-medullated fibers
ascend or descend for short distances not exceeding one or two segments, but
most of them enter the substantia gelatinosa at or near the level of their
origin. Ransom 118
suggests that these non-medullated fibers and the substantia gelatinosa are
concerned with the reflexes associated with pain impulses.
(f)
The lateral proper fasciculus (fasciculus lateralis proprius; lateral
basis bundle) constitutes the remainder of the lateral column, and is
continuous in front with the anterior proper fasciculus. It consists chiefly of
intersegmental fibers which arise from cells in the gray substance, and, after
a longer or shorter course, reënter the gray substance and ramify in it.
Some of its fibers are, however, continued upward into the brain under the name
of the medial longitudinal fasciculus.
Fasciculi
in the Posterior Funiculus.—This funiculus comprises two main
fasciculi, viz., the fasciculus gracilis, and the fasciculus
cuneatus. These are separated from each other in the cervical and upper
thoracic regions by the postero-intermediate septum, and consist mainly of
ascending fibers derived from the posterior nerve roots.
The
fasciculus gracilis (tract of Goll) is wedge-shaped on transverse
section, and lies next the posterior median septum, its base being at the
surface of the medulla spinalis, and its apex directed toward the posterior
gray commissure. It increases in size from below upward, and consists of long
thin fibers which are derived from the posterior nerve roots, and ascend as far
as the medulla oblongata, where they end in the nucleus gracilis.
The
fasciculus cuneatus (tract of Burdach) is triangular on
transverse section, and lies between the fasciculus gracilis and the posterior
column, its base corresponding with the surface of the medulla spinalis. Its
fibers, larger than those of the fasciculus gracilis, are mostly derived from
the same source, viz., the posterior nerve roots. Some ascend for only a short
distance in the tract, and, entering the gray matter, come into close
relationship with the cells of the dorsal nucleus; while others can be traced
as far as the medulla oblongata, where they end in the gracile and cuneate
nuclei.
The
fasciculus gracilis and fasciculus cuneatus conduct (1) impulses of conscious
muscle sense, neurons of the second order from the nucleus gracilis and nucleus
cuneatus, pass in the median lemniscus to the thalamus and neurons of the third
order from the thalamus to the cerebral cortex; (2) impulses of unconscious
muscle sense, via neurons of the second order from the nucleus gracilis
and nucleus cuneatus pass in the internal and external arcuate fibers of the
medulla oblongata to the inferior peduncle and through it to the cerebellum;
(3) impulses of tactile discrimination, via neurons of the second order
from the nucleus cuneatus and nucleus gracilis pass in the median lemniscus to
the thalamus, neurons of the third order pass from the thalamus to the cortex.
The
Posterior Proper Fasciculus (posterior ground bundle; posterior basis
bundle) arises from cells in the posterior column; their axons bifurcate
into ascending and descending branches which occupy the ventral part of the
funiculus close to the gray column. They are intersegmental and run for varying
distances sending off collaterals and terminals to the gray matter.
Some
descending fibers occupy different parts at different levels. In the cervical
and upper thoracic regions they appear as a comma-shaped fasciculus in
the lateral part of the fasciculus cuneatus, the blunt end of the comma being
directed toward the posterior gray commissure; in the lower thoracic region
they form a dorsal peripheral band on the posterior surface of the
funiculus; in the lumbar region, they are situated by the side of the posterior
median septum, and appear on section as a semi-elliptical bundle, which,
together with the corresponding bundle of the opposite side, forms the oval
area of Flechsig; while in the conus medullaris they assume the form of a triangular
strand in the postero-medial part of the fasciculus gracilis. These
descending fibers are mainly intersegmental in character and derived from cells
in the posterior column, but some consist of the descending branches of the posterior
nerve roots. The comma-shaped fasciculus was supposed to belong to the second
category, but against this view is the fact that it does not undergo descending
degeneration when the posterior nerve roots are destroyed.
Roots
of the Spinal Nerves.—As already stated, each spinal nerve
possesses two roots, an anterior and a posterior, which are
attached to the surface of the medulla spinalis opposite the corresponding
column of gray substance (675);
their fibers become medullated about the fifth month
of fetal life.
The
Anterior Nerve Root (radix anterior) consists of efferent fibers,
which are the axons of the nerve cells in the ventral part of the anterior and
lateral columns. A short distance from their origins, these axons are invested
by medullary sheaths and, passing forward, emerge in two or three irregular
rows over an area which measures about
The
Posterior Root (radix posterior) comprises some six or eight
fasciculi, attached in linear series along the postero-lateral sulcus. It
consists of afferent fibers which arise from the nerve cells in a spinal
ganglion. Each ganglion cell gives off a single fiber which divides in a
T-shaped manner into two processes, medial and lateral. The lateral processes extend
to the sensory end-organs of the skin, muscles, tendons, joints, etc. (somatic
receptors), and to the sensory end-organs of the viscera (visceral
receptors). The medial processes of the ganglion cells grow into the
medulla spinalis as the posterior roots of the spinal nerves.
The
posterior nerve root enters the medulla spinalis in three chief bundles,
medial, intermediate, and lateral. The medial strand passes directly
into the fasciculus cuneatus: it consists of coarse fibers, which acquire their
medullary sheaths about the fifth month of intrauterine life; the intermediate
strand consists of coarse fibers, which enter the gelatinous substance of
Rolando; the lateral is composed of fine fibers, which assume a
longitudinal direction in the tract of Lissauer, and do not acquire their
medullary sheaths until after birth. In addition to these medullated fibers
there are great numbers of non-medullated fibers which enter with the lateral
bundle. They are more numerous than the myelinated fibers. They arise from the
small cells of the spinal ganglia by T-shaped axons similar to the myelinated.
They are distributed with the peripheral nerves chiefly to the skin, only a few
are found in the nerves to the muscles. 119
Having
entered the medulla spinalis, all the fibers of the posterior nerve roots
divide into ascending and descending branches, and these in their turn give off
collaterals which enter the gray substance (676).
The descending fibers are short, and soon enter the gray substance. The
ascending fibers are grouped into long, short, and intermediate: the long
fibers ascend in the fasciculus cuneatus and fasciculus gracilis as far as the
medulla oblongata, where they end by arborizing around the cells of the cuneate
and gracile nuclei; the short fibers run upward for a distance of only 5 or
The
long fibers of the posterior nerve roots pursue an oblique course upward, being
situated at first in the lateral part of the fasciculus cuneatus: higher up,
they occupy the middle of this fasciculus, having been displaced by the
accession of other entering fibers; while still higher, they ascend in the
fasciculus gracilis. The upper cervical fibers do not reach this fasciculus,
but are entirely confined to the fasciculus cuneatus. The localization of these
fibers is very precise: the sacral nerves lie in the medial part of the
fasciculus gracilis and near its periphery, the lumbar nerves lateral to them,
the thoracic nerves still more laterally; while the cervical nerves are
confined to the fasciculus cuneatus (673).
Theme 2. General review of the brain.
The Brain [encephalon,
cerebrum] during the fourth week develops from three primary cerebral vesicles:
prosencephalon, mesencephalon and rhomboencephalon.
Prosencephalon forms two secondary cerebral vesicles: telencephalon and diencephalon.
Rhomboencephalon gives origin for medulla oblongata and metencephalon.
Mesencephalon separates from rhomboencephalon by isthmus.
Metencephalon developes into pons and cerebellum. Midbrain comprises tectum
and pedunculi cerebri. Diencephalon
contains thalamus and hypothalamus. Telencephalon (forebrain)
gives origin for rhinencephalon, basal nuclei of gray matter, pallium of
hemispheres, corpus callosum and fornix.
Scheme showing the connections of the several parts of the brain.
The brain can be divided
into two cerebral hemispheres, brain stem and cerebellum. A hemisphere has a
base, dorsolateral and medial surfaces. 12 pairs of the cranial nerves exit
from the encephalon base (see table below).
On
the brain base students should find:
·
medulla oblongata
·
pons
·
medii cerebellar pedunculi
·
cerebral pedunculi,
interpeduncular fossa lies between the two peduncles. Its floor
is perforated by large numbers of blood vessels, the posterior perforated substance
·
2 mammillary bodies are
located forward from interpeduncular fossa
·
tuber cinereum
with infundibulum that carries the hypophysis
·
optic tracts
which join each other and form optic chiasm
·
optic nerve
·
olfactory triangles with
anterior perforated substance
·
olfactory tracts
·
olfactory bulbs
Cranial
Nerves:
|
Num-ber |
Name |
Position |
Foramen |
Function |
|
I |
Olfactory |
smell |
||
|
II |
Optic |
optic
chiasm |
vision |
|
|
III |
Oculomotor |
interpeduncular
fossa |
superior
orbital fissure |
eye muscles |
|
IV |
Trochlear |
superior
medullary velum |
superior
orbital fissure |
eye muscles |
|
V |
Trigeminal |
between the
pons and medii cerebellar pedunculi |
(1) Ophthalmic:
superior orbital fissure; (2) Maxillary:
foramen rotundum; (3) Mandibular:
foramen ovale |
face
sensation & mastication |
|
VI |
Abducent |
between the
pons and pyramids |
eye muscles |
|
|
VII |
Facial |
cerebellopontine
angle |
internal
acoustic meatus, stylomastoid foramen |
face
muscles; salivary & lacrimal glands |
|
VIII |
Vestibulo-cochlear |
cerebellopontine
angle |
internal
acoustic meatus |
hearing
& balance |
|
IX |
Glosso-pharyngeal |
dorsal
lateral sulcus of medulla oblongata |
jugular
foramen |
pharynx;
tongue & parotid gland |
|
X |
Vagus |
dorsal
lateral sulcus of medulla oblongata lower X pair |
jugular
foramen |
pharynx,
larynx & viscera |
|
XI |
Accessory |
dorsal
lateral sulcus of medulla oblongata lower XI pair |
jugular
foramen |
neck
muscles (Trapezius/sternocleidomastoid) |
|
XII |
Hypoglossal |
between
olive and pyramid |
hypoglossal
canal |
tongue
& neck muscles |
Brain
or Encephalon. General Considerations and Divisions.—The
brain, is contained within the cranium, and constitutes the upper, greatly expanded
part of the central nervous system. In its early embryonic condition it
consists of three hollow vesicles, termed the hind-brain or rhombencephalon,
the mid-brain or mesencephalon, and the fore-brain or prosencephalon;
and the parts derived from each of these can be recognized in the adult.
Base of brain.
Thus in the process of development the wall of
the hind-brain undergoes modification to form the medulla oblongata, the pons,
and cerebellum, while its cavity is expanded to form the fourth ventricle. The
mid-brain forms only a small part of the adult brain; its cavity becomes the cerebral
aqueduct (aqueduct of Sylvius), which serves as a tubular
communication between the third and fourth ventricles; while its walls are
thickened to form the corpora quadrigemina and cerebral peduncles. The
fore-brain undergoes great modification: its anterior part or telencephalon
expands laterally in the form of two hollow vesicles, the cavities of which
become the lateral ventricles, while the surrounding walls form the cerebral
hemispheres and their commissures; the cavity of the posterior part or diencephalon
forms the greater part of the third ventricle, and from its walls are developed
most of the structures which bound that cavity.
Schematic
representation of the chief ganglionic categories (I to V).
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.
Lateral
surface of left cerebral hemisphere, viewed from above.
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.
Theme 3. Membranes of the brain. sinuses of dura
mater.
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.
Prepared
by
Reminetskyy
B.Y.