General anatomy of the
peripheral nervous system
THE NERVOUS SYSTEM is the most complicated
and highly organized of the various systems which make up the human body. It is
the mechanism concerned with the correlation and integration of various bodily processes
and the reactions and adjustments of the organism to its environment. In
addition the cerebral cortex is concerned with conscious life. It may be
divided into two parts, central and peripheral.
The central nervous system consists of the
encephalon or brain, contained within the cranium, and the medulla
spinalis or spinal cord lodged in the vertebral canal; the two
portions are continuous with one another at the level of the upper border of
the atlas vertebra.
The nerve cell and its processes collectively
constitute what is termed a neuron, and Waldeyer formulated the theory
that the nervous system is built up of numerous neurons, “anatomically and
genetically independent of one another.” According to this theory (neuron
theory) the processes of one neuron only come into contact, and are never
in direct continuity, with those of other neurons; while impulses are
transmitted from one nerve cell to another through these points of contact, the
synapses.
The synapse or synaptic membrane
seems to allow nervous impulses to pass in one direction only, namely, from the
terminals of the axis-cylinder to the dendrons. This theory is based on the
following facts, viz.: (1) embryonic nerve cells or neuroblasts are entirely
distinct from one another; (2) when nervous tissues are stained by the Golgi
method no continuity is seen even between neighboring neurons; and (3) when
degenerative changes occur in nervous tissue, either as the result of disease
or experiment, they never spread from one neuron to another, but are limited to
the individual neurons, or groups of neurons, primarily affected. It must,
however, be added that within the past few years the validity of the neuron
theory has been called in question by certain eminent histologists, who
maintain that by the employment of more delicate histological methods, minute
fibrils can be followed from one nerve cell into another. Their existence,
however, in the living is open to question. Mott and Marinesco made careful
examinations of living cells, using even the ultramicroscope and agree that
neither Nissl bodies nor neurofibrils are present in the living state.
For the present we may look upon the neurons as
the units or structural elements of the nervous system. All the neurons are
present at birth which are present in the adult, their division ceases before
birth; they are not all functionally active at birth, but gradually assume
functional activity. There is no indication of any regeneration after the
destruction of the cell-body of any individual neuron.
The peripheral nervous system consists of
a series of nerves by which the central nervous system is connected with the
various tissues of the body. For descriptive purposes these nerves may be
arranged in two groups, cerebrospinal and sympathetic, the
arrangement, however, being an arbitrary one, since the two groups are
intimately connected and closely intermingled. Both the cerebrospinal and
sympathetic nerves have nuclei of origin (the somatic efferent and sympathetic
efferent) as well as nuclei of termination (somatic afferent and sympathetic
afferent) in the central nervous system. The cerebrospinal nerves are
forty-three in number on either side—twelve cranial, attached to the
brain, and thirty-one spinal, to the medulla spinalis. They are
associated with the functions of the special and general senses and with the
voluntary movements of the body. The sympathetic nerves transmit the impulses
which regulate the movements of the viscera, determine the caliber of the
bloodvessels, and control the phenomena of secretion. In relation with them are
two rows of central ganglia, situated one on either side of the middle
line in front of the vertebral column; these ganglia are intimately connected
with the medulla spinalis and the spinal nerves, and are also joined to each
other by vertical strands of nerve fibers so as to constitute a pair of knotted
cords, the sympathetic trunks, which reach from the base of the skull to
the coccyx. The sympathetic nerves issuing from the ganglia form three great
prevertebral plexuses which supply the thoracic, abdominal, and pelvic viscera;
in relation to the walls of these viscera intricate nerve plexuses and numerous
peripheral ganglia are found.
Structure of the Nervous
System
The nervous tissues are composed of nerve
cells and their various processes, together with a supporting tissue called
neuroglia, which, however, is found only in the brain and medulla
spinalis. Certain long processes of the nerve cells are of special importance,
and it is convenient to consider them apart from the cells; they are known as nerve
fibers.
To the naked eye a difference is obvious between
certain portions of the brain and medulla spinalis, viz., the gray substance
and the white substance. The gray substance is largely composed of nerve
cells, while the white substance contains only their long processes, the nerve
fibers. It is in the former that nervous impressions are received, stored, and
transformed into efferent impulses, and by the latter that they are conducted.
Hence the gray substance forms the essential constituent of all the ganglionic
centers, both those in the isolated ganglia and those aggregated in the brain
and medulla spinalis; while the white substance forms the bulk of the
commissural portions of the nerve centers and the peripheral nerves.
Neuroglia.—Neuroglia,
the peculiar ground substance in which are imbedded the true nervous
constituents of the brain and medulla spinalis, consists of cells and fibers.
Some of the cells are stellate in shape, with ill-defined cell body, and their
fine processes become neuroglia fibers, which extend radially and
unbranched among the nerve cells and
fibers which they aid in supporting. Other cells give off fibers which branch
repeatedly. Some of the fibers start from the epithelial cells lining the
ventricles of the brain and central canal of the medulla spinalis, and pass
through the nervous tissue, branching repeatedly to end in slight enlargements
on the pia mater. Thus, neuroglia is evidently a connective tissue in function
but is not so in development; it is ectodermal in origin, whereas all
connective tissues are mesodermal.
Nerve Cells
are largely aggregated in the gray substance of the brain and medulla spinalis,
but smaller collections of these cells also form the swellings, called ganglia,
seen on many nerves. These latter are found chiefly upon the spinal and cranial
nerve roots and in connection with the sympathetic nerves. The nerve
cells vary in shape and size, and have one or more processes. They may be divided
for purposes of description into three groups, according to the number of
processes which they possess: (1) Unipolar cells, which are found in the
spinal ganglia; the single process, after a short course, divides in a T-shaped
manner. (2) Bipolar cells, also found in the spinal ganglia when the
cells are in an embryonic condition. They are best demonstrated in the spinal
ganglia of fish. Sometimes the processes come off from opposite poles of the
cell, and the cell then assumes a spindle shape; in other cells both processes
emerge at the same point. In some cases where two fibers are apparently
connected with a cell, one of the fibers is really derived from an adjoining
nerve cell and is passing to end in a ramification around the ganglion cell,
or, again, it may be coiled spirally around the nerve process which is issuing
from the cell. (3) Multipolar cells, which are pyramidal or stellate in
shape, and characterized by their large size and by the numerous processes
which issue from them. The processes are of two kinds: one of them is termed
the axis-cylinder process or axon because it becomes the
axis-cylinder of a nerve fiber. The others are termed the protoplasmic
processes or dendrons; they begin to divide and subdivide soon after
they emerge from the cell, and finally end in minute twigs and become lost
among the other elements of the nervous tissue. The body of
the nerve cell, known as the cyton, consists of a finely fibrillated
protoplasmic material, of a reddish or yellowishbrown color, which occasionally
presents patches of a deeper tint, caused by the aggregation of pigment
granules at one side of the nucleus, as in the substantia nigra and locus
cæruleus of the brain. The protoplasm also contains peculiar angular granules,
which stain deeply with basic dyes, such as methylene blue; these are known as Nissl’s
granules. They extend into the dendritic processes but not into the
axis-cylinder; the small clear area at the point of exit of the axon in some
cell types is termed the cone of origin. These granules disappear (chromatolysis)
during fatigue or after prolonged stimulation of the nerve fibers connected
with the cells. They are supposed to represent a store of nervous energy, and
in various mental diseases are deficient or absent. The nucleus is, as a rule,
a large, well-defined, spherical body, often presenting an intranuclear
network, and containing a well-marked nucleolus. In addition
to the protoplasmic network described above, each nerve cell may be shown to
have delicate neurofibrils running through its substance;
these fibrils are continuous with the fibrils of the axon, and are believed to
convey nerve impulses. Golgi has also described an extracellular network, which
is probably a supporting structure.
Nerve Fibers.—Nerve
fibers are found universally in the peripheral nerves and in the white
substance of the brain and medulla spinalis. They are of two kinds—viz., medullated
or white fibers, and non-medullated or gray fibers.
The medullated fibers form the white part
of the brain and medulla spinalis, and also the greater part of every cranial
and spinal nerve, and give to these structures their opaque, white aspect. When
perfectly fresh they appear to be homogeneous; but soon after removal from the
body each fiber presents, when examined by transmitted light, a double outline
or contour, as if consisting of two parts . The
central portion is named the axis-cylinder; around this is a sheath of
fatty material, staining black with osmic acid, named the white substance of
Schwann or medullary sheath, which gives to the fiber its double
contour, and the whole is enclosed in a delicate membrane, the neurolemma,
primitive sheath, or nucleated sheath of Schwann.
The axis-cylinder is the essential part of
the nerve fiber, and is always present; the medullary sheath and the neurolemma
are occasionally absent, expecially at the origin and termination of the nerve
fiber. The axis-cylinder undergoes no interruption from its origin in the nerve
center to its peripheral termination, and must be regarded as a direct
prolongation of a nerve cell. It constitutes about one-half or one-third of the
nerve fiber, being greater in proportion in the fibers of the central organs
than in those of the nerves. It is quite transparent, and is therefore
indistinguishable in a perfectly fresh and natural state of the nerve. It is
made up of exceedingly fine fibrils, which stain darkly with gold chloride and
at its termination may be seen to break up into these fibrillæ. The fibrillæ
have been termed the primitive fibrillæ of Schultze. The axis-cylinder
is said by some to be enveloped in a special reticular sheath, which separates
it from the medullary sheath, and is composed of a substance called neurokeratin.
The more common opinion is that this network or reticulum is contained in the
white matter of Schwann, and by some it is believed to be produced by the
action of the reagents employed to show it.
The medullary sheath, or white matter
of Schwann is regarded as being a fatty matter in a fluid state, which
insulates and protects the essential part of the nerve—the axis-cylinder. It
varies in thickness, in some forming a layer of extreme thinness, so as to be
scarcely distinguishable, in others forming about one-half the nerve fiber. The
variation in diameter of the nerve fibers (from 2 to 16μ)
depends mainly upon the amount of the white substance, though the axis cylinder
also varies within certain limits. The medullary sheath undergoes interruptions
in its continuity at regular intervals, giving to the fiber the appearance of
constriction at these points: these are known as the nodes of Ranvier.
The portion of nerve fiber between two nodes is called an internodal
segment. The neurolemma or primitive sheath is not interrupted at the
nodes, but passes over them as a continuous membrane. If the fiber be treated
with silver nitrate the reagent penetrates the neurolemma at the nodes, and on
exposure to light reduction takes place, giving rise to the appearance of black
crosses, Ranvier’s crosses, on the axis-cylinder. There may also be seen
transverse lines beyond the nodes termed Frommann’s lines the
significance of these is not understood. In addition to these interruptions
oblique clefts may be seen in the medullary sheath, subdividing it into
irregular portions, which are termed medullary segments, or segments
of Lantermann, there is reason to believe that these clefts are
artificially produced in the preparation of the specimens. Medullated nerve
fibers, when examined in the fresh condition, frequently present a beaded or
varicose appearance: this is due to manipulation and pressure causing the oily
matter to collect into drops, and in consequence of the extreme delicacy of the
primitive sheath, even slight pressure will cause the transudation of the fatty
matter, which collects as drops of oil outside the membrane.
The neurolemma or primitive sheath
presents the appearance of a delicate, structureless membrane. Here and there
beneath it, and situated in depressions in the white matter of Schwann, are
nuclei surrounded by a small amount of protoplasm. The nuclei are oval and
somewhat flattened, and bear a definite relation to the nodes of Ranvier, one
nucleus generally lying in the center of each internode. The primitive sheath
is not present in all medullated nerve fibers, being absent in those fibers
which are found in the brain and medulla spinalis.
Wallerian Degeneration.—When
nerve fibers are cut across, the central ends of the fibers degenerate as far
as the first node of Ranvier; but the peripheral ends degenerate simultaneously
throughout their whole length. The axons break up into fragments and become
surrounded by drops of fatty substance which are formed from the breaking down
of the medullary sheath. The nuclei of the primitive sheath proliferate, and
finally absorption of the axons and fatty substance occurs. If the cut ends of
the nerve be sutured together regeneration of the nerve fibers takes place by
the downgrowth of axons from the central end of the nerve. At one time it was
believed that the regeneration was peripheral in origin, but this has been
disproved, the proliferated nuclei in the peripheral portions taking part
merely in the formation of the so-called scaffolding along which the new axons
pass.
Non-medullated Fibers.—Most
of the fibers of the sympathetic system, and some of the cerebrospinal, consist
of the gray or gelatinous nerve fibers (fibers of Remak). Each of these consists of an axis-cylinder to which
nuclei are applied at intervals. These nuclei are believed to be in connection
with a delicate sheath corresponding with the neurolemma of the medullated
nerve fiber. In external appearance the non-medullated nerve fibers are
semitransparent and gray or yellowish gray. The individual fibers vary in size,
generally averaging about half the size of the medullated fibers.
Structure of the Peripheral Nerves
and Ganglia.—The cerebrospinal nerves consist of
numerous nerve fibers collected together and enclosed in membranous sheaths. A
small bundle of fibers, enclosed in a tubular sheath, is called a funiculus;
if the nerve is of small size, it may consist only of a single funiculus; but
if large, the funiculi are collected together into larger bundles or fasciculi,
which are bound together in a common membranous investment. In structure the
common membranous investment, or sheath of the whole nerve (epineurium),
as well as the septa given off from it to separate the fasciculi, consist of
connective tissue, composed of white and yellow elastic fibers, the latter
existing in great abundance. The tubular sheath of the funiculi (perineurium)
is a fine, smooth, transparent membrane, which may be easily separated, in the
form of a tube, from the fibers it encloses; in structure it is made up of
connective tissue, which has a distinctly lamellar arrangement. The nerve
fibers are held together and supported within the funiculus by delicate
connective tissue, called the endoneurium. It is continuous with septa
which pass inward from the innermost layer of the perineurium, and shows a
ground substance in which are imbedded fine bundles of fibrous connective
tissue running for the most part longitudinally. It serves to support capillary
vessels, arranged so as to form a net-work with elongated meshes. The
cerebrospinal nerves consist almost exclusively of medullated nerve fibers,
only a very small proportion of non-medullated being present.
The bloodvessels supplying a nerve end in a
minute capillary plexus, the vessels composing which pierce the perineurium,
and run, for the most part, parallel with the fibers; they are connected
together by short, transverse vessels, forming narrow, oblong meshes, similar
to the capillary system of muscle. Fine non-medullated nerve fibers, vasomotor
fibers, accompany these capillary vessels, and break up into elementary
fibrils, which form a network around the vessels. Horsley has demonstrated
certain medullated fibers running in the epineurium and terminating in small spheroidal
tactile corpuscles or end bulbs of Krause. These nerve fibers, which
It is probable that through this interchange of
fibers, every branch passing off from a plexus has a more extensive connection
with the spinal cord than if it had proceeded to its distribution without
forming connections with other nerves. Consequently the parts supplied by these
nerves have more extended relations with the nervous centers; by this means,
also, groups of muscles may be associated for combined action.
The sympathetic nerves are constructed in
the same manner as the cerebrospinal nerves, but consist mainly of
non-medullated fibers, collected in funiculi and enclosed in sheaths of
connective tissue. There is, however, in these nerves a certain admixture of
medullated fibers. The number of the latter varies in different nerves, and may
be estimated by the color of the nerve. Those branches of the sympathetic,
which present a well-marked gray color, are composed chiefly of non-medullated
nerve fibers, intermixed with a few medullated fibers; while those of a white
color contain many of the latter fibers, and few of the former.
In structure all ganglia are essentially similar,
consisting of the same structural elements—viz., nerve cells and nerve fibers.
Each nerve cell has a nucleated sheath which is continuous with the neurolemma
of the nerve fiber with which the cell is connected. The nerve cells in the
ganglia of the spinal nerves are pyriform in shape, and have each a single
process. A short distance from the cell and while still within the ganglion
this process divides in a T-shaped manner, one limb of the cross-bar turning
into the medulla spinalis, the other limb passing outward to the periphery. In the
sympathetic ganglia the nerve cells are multipolar and each has one
axis-cylinder process and several dendrons; the axon emerges from the ganglion
as a non-medullated nerve fiber. Similar cells are found in the ganglia
connected with the trigeminal nerve, and these ganglia are therefore regarded
as the cranial portions of the sympathetic system. The sympathetic nervous
system includes those portions of the nervous mechanism in which a medullated
nerve fiber from the central system passes to a ganglion, sympathetic or
peripheral, from which fibers, usually non-medullated, are distributed to such
structures, e. g., bloodvessels, as are not under voluntary control. The
spinal and sympathetic ganglia differ somewhat in the size and disposition of
the cells and in the number of nerve fibers entering and leaving them. In the
spinal ganglia the nerve cells are much larger and for the most part collected
in groups near the periphery, while the fibers, which are mostly medullated,
traverse the central portion of the ganglion; whereas in the sympathetic
ganglia the cells are smaller and distributed in irregular groups throughout
the whole ganglion; the fibers also are irregularly scattered; some of the
entering ones are medullated, while many of those leaving the ganglion are
non-medullated.
Fasciculi, tracts
or fiber systems are groups of axons having homologous origin and
homologous distribution (as regards their collaterals, subdivisions and
terminals) and are often named in accordance with their origin and termination,
the name of the nucleus or the location of the cell body from which the axon or
fiber arises preceding that of the nucleus or location of its termination. A
given topographical area seldom represents a pure tract, as in most cases
fibers of different systems are mixed.
The cerebrospinal and sympathetic nerve fibers
convey various impressions. The sensory nerves, called also centripetal
or afferent nerves, transmit to the nervous centers impressions made
upon the peripheral extremities of the nerves, and in this way the mind,
through the medium of the brain, becomes conscious of external objects. The centrifugal
or efferent nerves transmit impressions from the nervous centers to the
parts to which the nerves are distributed, these impressions either exciting
muscular contraction or influencing the processes of nutrition, growth, and
secretion.
Origins and Terminations of Nerves.—By
the expression “the terminations of nerve fibers” is signified their
connections with the nerve centers and with the parts they supply. The former
are sometimes called their origins or central terminations; the
latter their peripheral terminations.
Origins of Nerves.—The
origin in some cases is single—that is to say, the whole nerve emerges from the
nervous center by a single root; in other instances the nerve arises by two or
more roots which come off from different parts of the nerve center, sometimes
widely apart from each other, and it often happens, when a nerve arises in this
way by two roots, that the functions of these two roots are different; as, for
example, in the spinal nerves, each of which arises by two roots, the anterior
of which is motor, and the posterior sensory. The point where the nerve root or
roots emerge from the surface of the nervous center is named the superficial
or apparent origin, but the fibers of the nerve can be traced for a
certain distance into the substance of the nervous center to some portion of
the gray matter, which constitutes the deep or real origin of the
nerve. The centrifugal or efferent nerve fibers originate in the nerve cells of
the gray substance, the axis-cylinder processes of these cells being prolonged
to form the fibers. In the case of the centripetal or afferent nerves the
fibers grow inward either from nerve cells in the organs of special sense, e.
g., the retina, or from nerve cells in the ganglia. Having entered the
nerve center they branch and send their ultimate twigs among the cells,
without, however, uniting with them.
Peripheral Terminations of Nerves.—Nerve
fibers terminate peripherally in various ways, and these may be conveniently
studied in the sensory and motor nerves respectively. The terminations of the
sensory nerves are dealt with in the section on Sense Organs.
Motor nerves can
be traced into either unstriped or striped muscular fibers. In the unstriped
or involuntary muscles the nerves are derived from the sympathetic, and
are composed mainly of non-medullated fibers. Near their terminations they
divide into numerous branches, which communicate and form intimate plexuses. At
the junction of the branches small triangular nuclear bodies (ganglion cells)
are situated. From these plexuses minute branches are given off which divide
and break up into the ultimate fibrillæ of which the nerves are composed. These
fibrillæ course between the involuntary muscle cells, and, according to
Elischer, terminate on the surfaces of the cells, opposite the nuclei, in
minute swellings
In the striped or voluntary muscle
the nerves supplying the muscular fibers are derived from the cerebrospinal
nerves, and are composed mainly of medullated fibers. The nerve, after entering
the sheath of the muscle, breaks up into fibers or bundles of fibers, which
form plexuses, and gradually divide until, as a rule, a single nerve fiber
enters a single muscular fiber. Sometimes, however, if the muscular fiber be
long, more than one nerve fiber enters it. Within the muscular fiber the nerve
terminates in a special expansion, called by Kühne, who first accurately described
it, a motor end-plate. The nerve fiber, on approaching the muscular
fiber, suddenly loses its medullary sheath, the neurolemma becomes continuous
with the sarcolemma of the muscle, and only the axis-cylinder enters the
muscular fiber. There it at once spreads out, ramifying like the roots of a
tree, immediately beneath the sarcolemma, and becomes imbedded in a layer of
granular matter, containing a number of clear, oblong nuclei, the whole
constituting an end-plate from which the contractile wave of the muscular fiber
is said to start.
Ganglia are small
aggregations of nerve cells. They are found on the posterior roots of the
spinal nerves; on the sensory roots of the trigeminal, facial,
glossopharyngeal, and vagus nerves, and on the acoustic nerves. They are also
found in connection with the sympathetic nerves. On section they are seen to
consist of a reddish-gray substance, traversed by numerous white nerve fibers;
they vary considerably in form and size; the largest are found in the cavity of
the abdomen; the smallest, not visible to the naked eye, exist in considerable
numbers upon the nerves distributed to the different viscera. Each ganglion is
invested by a smooth and firm, closely adhering, membranous envelope,
consisting of dense areolar tissue; this sheath is continuous with the
perineurium of the nerves, and sends numerous processes into the interior to
support the bloodvessels supplying the substance of the ganglion.
The 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
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.
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.