Spinal Cord or 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 45 cm., in the female from 42 to 43 cm., while its
weight amounts to about 30 gms. It extends from the level of the upper
border of the atlas to that of the lower border of the first, or upper
border of the second, lumbar vertebra. Above, it is continuous with
the brain; below, it ends in a conical extremity, the conus medullaris,
from the apex of which a delicate filament, the filum terminale,
descends as far as the first segment of the coccyx (Fig. 90661).

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.

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.

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.

Although no trace of transverse segmentation is visible
on the surface of the medulla spinalis, it is convenient to regard
it as being built up of a series of superimposed spinal segments
or neuromeres, each of which has a length equivalent to the
extent of attachment of a pair of spinal nerves. Since the extent
of attachment of the successive pairs of nerves varies in different
parts, it follows that the spinal segments are of varying lengths;
thus, in the cervical region they average about 13 mm., in the mid-thoracic
region about 26 mm., while in the lumbar and sacral regions they diminish
rapidly from about 15 mm. at the level of the first pair of lumbar
nerves to about 4 mm. opposite the attachments of the lower sacral
nerves.

FIG. 90661 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. (Testut.)

As a consequence of the relative inequality in the rates
of growth of the medulla spinalis and vertebral column, the nerve
roots, which in the early embryo passed transversely outward to reach
their respective intervertebral foramina, become more and more oblique
in direction from above downward, so that the lumbar and sacral nerves
descend almost vertically to reach their points of exit. From the
appearance these nerves present at their attachment to the medulla
spinalis and from their great length they are collectively termed
the cauda equina (Fig. 90662).

The filum terminale is a delicate filament, about
20 cm. in length, prolonged downward from the apex of the conus medullaris.
It consists of two parts, an upper and a lower. The upper part, or
filum terminale internum, measures about 15 cm. in length and
reaches as far as the lower border of the second sacral vertebra.
It is contained within the tubular sheath of dura mater, and is surrounded
by the nerves forming the cauda equina, from which it can be readily
recognized by its bluish-white color. The lower part, or filum
terminale externum, is closely invested

by, and is adherent to, the dura mater; it extends downward from the
apex of the tubular sheath and is attached to the back of the first
segment of the coccyx. The filum terminale consists mainly of fibrous
tissue, continuous above with that of the pia mater. Adhering to its
outer surface, however, are a few strands of nerve fibers which probably
represent rudimentary second and third coccygeal nerves; further,
the central canal of the medulla spinalis extends downward into it
for 5 or 6 cm.

FIG. 90662 Cauda
equina and filum terminale seen from behind. The dura mater has
been opened and spread out, and the arachnoid has been removed.

FIG. 90663 Diagrams
of the medulla spinalis.

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 (Fig. 90663).

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 38 mm.) being
on a level with the attachment of the sixth pair of cervical nerves.

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 33 mm., opposite the last thoracic vertebra, below which
it tapers rapidly into the conus medullaris.

Fissures and Sulci (Fig. 90664).An
anterior median fissure and a posterior median sulcus incompletely
divide the medulla spinalis into two symmetrical parts, which are
joined across the middle line by a commissural band of nervous matter.

FIG. 90664 Transverse
section of the medulla spinalis in the mid-thoracic region.

The Anterior Median Fissure (fissura mediana
anterior) has an average depth of about 3 mm., but this is increased
in the lower part of the medulla spinalis. It contains a double fold
of pia mater, and its floor is formed by a transverse band of white
substance, the anterior white commissure, which is perforated
by bloodvessels on their way to or from the central part of the medulla
spinalis.

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 6 mm., but diminishes considerably
in the lower part of the medulla spinalis.

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 fasciculia medial, named the fasciculus gracilis
(tract of Goll); and a lateral, the fasciculus cuneatus
(tract of Burdach)

(Fig. 90672). 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.

FIG. 90665 Curves
showing the sectional area at different levels of the cord. The
ordinates show the area in sq. mm. (Donaldson and Davis.)

The Internal Structure of the Medulla Spinalis.On examining
a transverse section of the medulla spinalis (Fig. 90664) it is seen to consist of gray and white nervous substance,
the former being enclosed within the latter.

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 (Fig. 90665). 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 Central Canal (canalis centralis)
runs throughout the entire length of the medulla spinalis. The portion
of gray substance in front of the canal is named the anterior gray
commissure; that behind it, the posterior gray commissure.
The former is thin, and is in contact anteriorly with the anterior
white commissure: it contains a couple of longitudinal veins, one
on either side of the middle line. The posterior gray commissure reaches
from the central canal to the posterior median septum, and is thinnest
in the thoracic region, and thickest in the conus medullaris. The
central canal is continued upward through the lower part of the medulla
oblongata, and opens into the fourth ventricle of the brain; below,
it reaches for a short distance into the filum terminale. In the lower
part of the conus medullaris it exhibits a fusiform dilatation, the
terminal ventricle; this has a vertical measurement of from
8 to 10 mm., is triangular on cross-section with its base directed
forward, and tends to undergo obliteration after the age of forty
years.

FIG. 90666 Transverse
sections of the medulla spinalis at different levels.

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 (Fig. 90667).

FIG. 90667 Section
of central canal of medulla spinalis, showing ependymal and neuroglial
cells. (v. Lenhossek.)

FIG. 90668 Cells
of medulla spinalis. Diagram showing in longitudinal section the
intersegmental neurons of the medulla spinalis. The gray and white
parts correspond respectively to the gray and white substance of
the medulla spinalis. (Poirier.)

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
(Fig. 90668), 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).

FIG. 90669 Diagram
showing a few of the connections of afferent (sensory) fibers of
the posterior root with the efferent fibers from the ventral column
and with the various long ascending fasciculi.

FIG. 90670 Diagram
showing possible connection of long descending fibers from higher
centers with the motor cells of the ventral column through association
fibers.

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.)

FIG. 90671 Transverse
sections of the medulla spinalis at different levels to show the
arrangement of the principal cell columns.

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
Golgis 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 (Fig. 90672). 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.

FIG. 90672 Diagram
of the principal fasciculi of the spinal cord.

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 Clarkes 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
(Clarkes 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.

FIG. 90673 Formation
of the fasciculus gracilis. Medulla spinalis viewed from behind.
To the left, the fasciculus gracilis is shaded. To the right, the
drawing shows that the fasciculus gracilis is formed by the long
fibers of the posterior roots, and that in this tract the sacral
nerves lie next the median plane, the lumbar to their lateral side,
and the thoracic still more laterally. (Poirier.)

FIG. 90674 Descending
fibers in the posterior funiculi, shown at different levels. A.
In the conus medullaris. B. In the lumbar region. C.
In the lower thoracic region. D. In the upper thoracic region.
(After Testut.)

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 (Fig. 90675); their fibers become medullated about the fifth month of
fetal life.

FIG. 90675 A spinal
nerve with its anterior and posterior roots.

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 3 mm. in width.

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 (Fig. 90676). 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 6 mm. and enter the gray substance; while
the intermediate fibers, after a somewhat longer course, have a similar
destination. All fibers entering the gray substance end by arborizing
around its nerve cells or the dendrites of cells, those of intermediate
length being especially associated with the cells of the dorsal nucleus.

FIG. 90676 Posterior
roots entering medulla spinalis and dividing into ascending and
descending branches. (Van Gehuchten.) a. Stem fiber. b,
b. Ascending and descending limbs of bifurcation. c.
Collateral arising from stem fiber.

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 (Fig. 90673).

Note 113. Lenhossek and Cajal found that in the chick embryo the axons of a few of these nerve cells passed backward through the posterior column, and emerged as the motor fibers of the posterior nerve roots. These fibers are said to control the peristaltic movements of the intestine. Their presence, in man, has not yet been determined. [back]

Note 114. Topographical Atlas of the Spinal Cord, 1901. [back]

Note 115. According to Bruce and Pirie (B. M> J., November 17, 1906) this column extends from the middle of the eighth cervical segment to the lower part of the second lumbar or the upper part of the third lumbar segment. [back]

Note 116. Somewhat later a change, termed chromatolysis, takes place in the nerve cells, and consists of a breaking down and an ultimate disappearance of the Nissl bodies. Further, the body of the cell is swollen, the nucleus displaced toward the periphery, and the part of the axon still attached to the altered cell is diminished in size and somewhat atrophied. Under favorable conditions the cell is capable of reassuming its normal appearance, and its axon may grow again. [back]

Note 117. Schäfer, Proc. Physiolog. Soc., 1899. [back]

Note 118. Ransom, Am. Jour. Anat., 1914; Brain, 1915. [back]

Note 119. Ransom, Brain, 1915, 38. [back]

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