|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 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 sidetwelve
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.
|1. 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
(Fig. 90623, B) among the nerve cells
and fibers which they aid in supporting. Other cells give off fibers
which branch repeatedly (Fig. 90623, A).
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.
Fig. 90623 Neuroglia
cells of brain shown by Golgis method. A. Cell with
branched processes. B. Spider cell with unbranched processes.
|Nerve Cells (Fig. 90624).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 (Fig. 90624, E). (2) Bipolar
cells, also found in the spinal ganglia (Fig.
625), 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 (Figs. 626, 627,
628). 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.
Fig. 90624 Various
forms of nerve cells. A. Pyramidal cell. B. Small
multipolar cell, in which the axon quickly divides into numerous
branches. C. Small fusiform cell. D and E.
Ganglion cells (E shows T-shaped division of axon). ax.
Axon. c. Capsule.
Fig. 90625 Bipolar nerve
cell from the spinal ganglion of the pike. (After Kölliker.)
Fig. 90626 Motor
nerve cell from ventral horn of medulla spinalis of rabbit. The
angular and spindle-shaped Nissl bodies are well shown. (After Nissl.)
| 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 Nissls granules (Fig.
626). They extend into the dendritic processes but not into the
axis-cylinder; the small clear area at the point of exit of the axon
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.
Fig. 90627 Pyramidal
cell from the cerebral cortex of a mouse. (After Ramón y Cajal.)
Fig. 90628 Cell
of Purkinje from the cerebellum. Golgi method. (Cajal.) a.
Axon. b. Collateral. c and d. Dendrons.
| In addition to the protoplasmic network described above,
each nerve cell may be shown to have delicate neurofibrils running
through its substance (Fig. 90629); 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 kindsviz., 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 (Fig. 90630).
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 (Fig. 90633)
Fig. 90629 Nerve
cells of kitten, showing neurofibrils. (Cajal.) a. Axon.
b. Cyton. c. Nucleus. d. Neurofibrils.
| 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 (Fig.
632), 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.
Fig. 90630 Medullated
nerve fibers. X 350.
Fig. 90631 Diagram
of longitudinal sections of medullated nerve fibers. Osmic acid.
Fig. 90632 Transverse
sections of medullated nerve fibers. Osmic acid.
Fig. 90633 Diagram
of medullated nerve fibers stained with osmic acid. X 425. (Schäfer.)
R. Nodes of Ranvier. a. Neurolemma. c. Nucleus.
| The medullary sheath, or white matter of Schwann
(Fig. 90631), is regarded as being a fatty
matter in a fluid state, which insulates and protects the essential
part of the nervethe 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 (Figs.
631 and 633). 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, Ranviers crosses, on the
axis-cylinder. There may also be seen transverse lines beyond the
nodes termed Frommanns lines (Fig.
634); 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 (Fig.
631); 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.
Fig. 90634 Medullated
nerve fibers stained with silver nitrate.
Fig. 90635 A small
nervous branch from the sympathetic of a mammal. a. Two medullated
nerve fibers among a number of gray nerve fibers, b.
| 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) (Fig.
635). 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
|Structure of the Peripheral Nerves and Ganglia.The
cerebrospinal nerves consist of numerous nerve fibers collected
together and enclosed in membranous sheaths (Fig.
636). 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
| 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 Marshall
believes to be sensory, and which he has termed nervi nervorum,
are considered by him to have an important bearing upon certain neuralgic
| The nerve fibers, so far as is at present known, do
not coalesce, but pursue an uninterrupted course from the center to
the periphery. In separating a nerve, however, into its component
funiculi, it may be seen that these do not pursue a perfectly insulated
course, but occasionally join at a very acute angle with other funiculi
proceeding in the same direction; from this, branches are given off,
to joint again in like manner with other funiculi. It must be distinctly
understood, however, that in these communications the individual nerve
fibers do not coalesce, but merely pass into the sheath of the adjacent
nerve, become intermixed with its nerve fibers, and again pass on
to intermingle with the nerve fibers in some adjoining funiculus.
| Nerves, in their course, subdivide into branches, and
these frequently communicate with branches of a neighboring nerve.
The communications which thus take place form what is called a plexus.
Sometimes a plexus is formed by the primary branches of the trunks
of the nervesas the cervical, brachial, lumbar, and sacral plexusesand
occasionally by the terminal funiculi, as in the plexuses formed at
the periphery of the body. In the formation of a plexus, the component
divide, then join, and again subdivide in such a complex manner that
the individual funiculi become interlaced most intricatey; so
that each branch leaving a plexus may contain filaments from all the
primary nervous trunks which form the plexus. In the formation also
of smaller plexuses at the periphery of the body there is a free interchange
of the funiculi and primitive fibers. In each case, however, the individual
fibers remain separate and distinct.
| 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.
Fig. 90636 Transverse
section of human tibial nerve.
| 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.
| 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 singlethat
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 (Fig. 90637). 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.
Fig. 90637 Muscular
fibers of Lacerta viridis with the terminations of nerves.
a. Seen in profile. P, P. The nerve end-plates. S,
S. The base of the plate, consisting of a granular mass with
nuclei. b. The same as seen in looking at a perfectly fresh
fiber, the nervous ends being probably still excitable. (The forms
of the variously divided plate can hardly be represented in a woodcut
by sufficiently delicate and pale contours to reproduce correctly
what is seen in nature.) c. The same as seen two hours after
death from poisoning by curare.
Fig. 90638 Transverse
section of spinal ganglion of rabbit. A. Ganglion. X 30.
a. Large clear nerve cell. b. Small deeply staining
nerve cell. c. Nuclei of capsule. X 250. The lines in the
center point to the corresponding cells in the ganglion.
| In structure all ganglia are essentially similar, consisting
of the same structural elementsviz., 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 (Fig. 638) 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 (Fig. 90639) 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 (Fig. 90638) 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 (Fig. 639) 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.
Fig. 90639 Transverse
section of sympathetic ganglion of cat. A. Ganglion. X 50.
a. A nerve cell. X 250.
|Neuron Theory.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
| 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.