be conductive and receptive for nerve impulses.



334 THE NERVE SYSTEM

Small buds are seen at times along the course of
dendrites. They are called gemmules.

Axone, or Axis-cylinder. This arises from the
body of the neurone, or nerve cell, as a cone-shaped
process, and is seen as a very delicate fiber. In
structure it differs from the dendrites. Each axone
is uniform in diameter; and consists of fine fibrillse,
embedded in a clear protoplasmic substance (neuro-
plasm). Axones may be very short or as much
as a meter in length. As a rule, only one axone
is given off from a cell, and this form is termed mon-
axic neurones ; however, more than one is present; as
two axones, they are termed diaxonic neurones; and
several axones, polyaxonic neurones.

Axones in certain portions of the nerve system
(brain and spinal cord) possess fine branches called
collaterals; they have the same structure as the axone
from which they arise. Some axones, as Golgi cells,
break up into branches after leaving the cell body,
called dendraxones. The minute endings of the axis-
cylinders and collaterals, which spread out like the
branches of a tree, are termed telodendria.

The axone is the functional element of the nerve
system which acts as the distributive or emissive
conductor of nerve impulses.

Nerve Fibers. Nerve fibers are simply continua-
tions of the axis-cylinder or axorie given off from the
cell body of a neurone, with their surrounding invest-
ments, the myelin and neurilemma. They are classi-
fied into two varieties, according to whether or not
the axis-cylinder possesses a medullary or myelin
sheath, viz., medullated or myelinic nerve fibers and
non-medullated or amyelinic nerve fibers.

Medullated Nerve-fibers. These possess three distinct
minute anatomical portions, when subjected to stain-
ing methods and examined under the microscope,
namely: An external investing sheath, the neurilemma;



THE STRUCTURE OF THE NERVE SYSTEM 335

an intervening semifluid substance, the medulla or
myelin; and an internal dark thread, the axis-cylinder
or axone.

The neurilemma is a delicate, transparent membrane
investing the myelin and axone, and occurs wherever
the meylin sheath is absent. It possesses a nucleus,
which may be seen between the nodes of Ranvier along
the course of the nerve. It acts as a protective mem-
brane to the nerve-fiber.*

The medulla, myelin, or white substance of Schwann
lies between the neurilemma and invests the axone.
This is the substance of the nerve fiber, the presence
of which imparts to the nerve tissue its white appear-
ance, and gives rise to the term white fibers in speaking
of nerve tissue, to differentiate them from gray fibers,
the latter having no myelin sheath.

Along the course of nerve fibers may be seen a
diminution or shrinkage/ in its caliber, due to an
absence of the myelin Aeath, permitting the neuri-
lemma to be in direct Imposition to the axis-cylinder.
^TKese narrowed parts are called the nodes of Ranvier,
named after their discoverer. The portion of the
nerve fiber between these interruptions is termed the
internodal segment. Axones give off their collateral
branches at the nodes of Ranvier.

Non-medullated or Amyelinic Nerve Fibers. These are
devoid of a myelin salath, or the white substance of
Schwann, thus printing a gray appearance, and
giving origin to Kfye term gray fibers. Some non-
medullated nerve fibers possess only an axis-cylinder,
or axone. These are found in the central ganglia.
Others possess a neurilemma investing the single
axis-cylinder, and are the main variety of nerve fiber
constituting the sympathetic system. Non-medullated
nerve fibers are not so abundant throughout the
nerve system as the medullated variety.

Nerves. Nerves are round, flattened bundles of
axones, held firmly together by investing connective



FIG. 118




Scheme of central motor neurone. (I type of Golgi.) The motor cell
body, together with all its protoplasmic processes, its axis-cylinder process,
collaterals, and end ramifications represent parts of a single cell or neurone,
a.h., axone-hillock devoid of Nissl bodies and showing fibrillation, c, cyto-
plasm showing Nissl bodies arid lighter ground substance. n' t nucleolus.
(Barker.)



THE STRUCTURE OF THE NERVE SYSTEM 337

tissue. The latter contains the bloodvessels and
lymphatics. Nerves are the connecting trunks which
bring the brain and spinal cord in relation with the
structures throughout the body muscles, skin, glands,
etc.

Structure of Nerves. Examined under the micro-
scope, each nerve shows on cross-section a collection
of nerve fibers, or axones, arranged in bundles. The
entire group being invested by a layer of connective
tissue called the epineurium; however, each separate
bundle of fibers within the nerve is enclosed in its
own connective-tissue sheath the perineurium, and
the numbers of axis-cylinders, or axones, within the
bundles are again enclosed in a delicate connective-
tissue net- work called the endoneurium, or sheath of
Henle.

Nerves as they pass from the brain or spinal cord
are surrounded by the epineurium, and divide and
subdivide as they give off branches to other nerves, and
pass to their terminations. As each branch is given
off the same sheath of connective tissue that enclosed
the parent nerve accompanies it. This arrangement
continues as the branches become smaller and smaller,
until they reach their terminal point of distribution,
where only a single nerve fiber remains, enclosed by a
transparent membrane, the endoneurium or sheath
of Henle. However, near the ultimate termination
of a nerve the single nerve fiber may continue to give
off branches, each one consisting of the axis-cylinder
and myelin sheath.

The multitude of nerve cells or neurones entering
into the formation of the nerve system are supported
in a non-neural or inactive set of cells as regards nerve
activity, called the supporting tissue elements of the
nerve system. These consist of two kinds : (1) neuroglia,
and (2) connective-tissue trabeculse derived from the pia
mater, or the bloodvessel channels, Neuroglia consist
22



338



THE NERVE SYSTEM



FlG - 119




Neuralgia cells of brain shown
by Golgi's method. A. Cell with
branched processes. B. Spider-cell

\vilh unhfitnclicd processes. (After
Andriezen.) (From Schfifer's 7?.\-.sv//-
tials of I



of cells glia cells and glia
fibers. There are two varie-
ties of the latter ependy-
mal cells and astrocytes.
The supporting tissue ele-
ments of the brain, etc.,
do not possess the power
of developing or conveying
nerve impulses; they are
spoken of as non-neural.

The Origin of Nerves.
Efferent nerves, those
which conduct impulses
to the periphery, muscles,
glands, bloodvessels, etc.,
in response'to stimuli from
the brain and spinal cord,
originate in the nerve cells
of the gray substance of
these structures, and the
axones are prolonged to
form the nerve fibers.
Nerves emerge from the
brain and spinal cord as
single rounded cords. They
may possess only a single
root of origin or two roots
widely apart from each
other, yet each of the two
roots may be different in
function, as seen in the
spinal nerves, the anterior
root being motor or effer-
ent, the posterior sensor or
afferent. The point at
which a single nerve root
leaves the brain or spinal
cord is called the super-
ficial origin of a nerve;



THE STRUCTURE OF THE NERVE SYSTEM 339

however, a tracing of the axones of these nerves for
a distance into the gray substance of the brain or
spinal cord, where they originate, will end in the
nerve centre, which is termed the deep origin of a
nerve.

Endings of Nerves. It must be remembered that
the course or appearance of a nerve has nothing to do
with its function, for from all external or microscopic
examination an efferent nerve cannot be differenti-
ated from an afferent nerve. Nerves end in several
ways, which vary in different situations.

The Efferent or Centrifugal Nerves. This variety
is motor and conveys nerve impulses away from the
brain and spinal cord. Upon reaching their final
ending, these lose both the neurilemma and myelin
sheath. The axis-cylinder divides and gives off
branches (collaterals) which join with other axones.
These axis-cylinders come in direct contact with the
tissue cells and are termed end arborizations, or
telodendria, also end-organs, terminal organs, or end-
tufts.

In muscles of the skeleton the axones of the nerves
lose their neurilemma and myelin sheath; at the
point they join the muscle fiber, and after giving off
branches within the sarcolemma, appear to lie in a
mass of sarcoplasm and nuclei which forms the
motor-plate.

In the muscles of the viscera (involuntary) the
nerve fibers are non-medullated, and belong to the
sympathetic system or other neurones. The axones
divide and subdivide to form plexuses which invest
the muscle-cell bundles. Other branches are given
off from the latter which finally come in intimate
relation with each cell, upon the surface of which
they are seen as granular masses.

In the glands the nerve fibers are derived from the
sympathetic and other neurones; the axones reach



340 THE NERVE SYSTEM

the acini of the glands, upon the outer surface of
the acini; they form plexuses which pierce the acini ,
wall, and give off minute branches to the gland
cells.

The Afferent or Centripetal Nerves. These end
as the former, but as this variety of nerve is sensor,
and conveys sensations to the brain and spinal cord,
the end-tufts are in intimate relation with specialized
end-organs, which are essential to the appreciation
of the complex sensations, viz., of sight, the retina
in the eye; taste, the taste-buds in the tongue; smell,
the olfactory cells in the nasal mucous membrane;
hearing, the organ of Corti in the ear.

In the skin and mucous membranes the axones
are in intimate contact with various end-organs which
give rise to the numerous conscious sensations of
touch, heat, thirst, hunger, and muscle sense, etc.
The following are the chief names of the various
end-organs :

1. Free endings in the skin.

2. Tactile cells of Merkel.

3. Tactile corpuscles in the papilla of the true skin.

4. Tactile corpuscles of Meissner.

5. Pacinian corpuscles found attached to the nerves
of the hands, feet, intercostal nerves, and nerves in *
other situations.

6. End-buds of Krause in the conjunctiva, clitoris
(female), penis (male), etc.

The Nerve Plexus. It consists of a number of
nerves communicating with other nerves, by means
of collateral branches. Some plexuses are formed by
direct branches being given off from the spinal cord, as
the brachial, lumbar, sacral plexuses; others are formed
by the terminal filaments of nerve fibers, as the
plexuses in the skin, etc., still others may contain
sympathetic nerve fibers derived from the sympathetic
system, in addition to the sensor and motor nerves.



THE STRUCTURE OF THE NERVE SYSTEM 341

I i.i. 120




Showing some varieties of peripheral terminations of afferent neurones
(or "peripheral nerve beginnings"): A. Terminal fibrillse in epithelium (after
Retzius). B. Tactile corpuscle (Meissner's, after Dogiel). C. Bulboid
corpuscle (Krause's, after Dogiel). D. Lamellated corpuscle (Pacini's, after
Dogiel, Sala, and others). E. Genital nerve corpuscle from human glaus
penis (after Dogiel). a. Axone. t. Telodendria



342 THE NERVE SYSTEM

In other words, the various nerve fibers, in forming
a plexus, maintains the same function which the
nerve possesses from its origin, as motor, sensor, or
sympathetic.

Ganglia. Aside from the arrangement of neurones
or nerve cells in the cerebrospinal system, there is
another collection of these neurones into small groups,
connected with each other and the nerves of the
brain and spinal cord called ganglia. Some ganglia
are large enough to be seen by the naked eye, others
are so small that they can scarcely be detected, unless
examined by a lens or microscope. Ganglia compose
the sympathetic system. They contain nerve cells
with dendrites and axones, the greater number of
the latter being non-medullated, and are surrounded
by a connective-tissue capsule.

Ganglia are found on the dorsal or posterior root
of the spinal cord, on the sensor root of the fifth nerve,
on the facial and auditory nerves; and on the vagus
and glossopharyngeal, along either side of the spinal
column, where they form the gangliated cord of
the sympathetic. Ganglia are receptive to impulses
from nerves and other ganglia, and have the prop-
erty of conducting impulses to other ganglia and
nerves.

Classification of Nerves. Nerves are pathways of
communication between the brain and spinal cord,
and the structures throughout the body which are
dependent upon the nerve system for their develop-
ment, growth, repair, and actions, and they require
the stimuli from the brain to excite into physiologic
activity the cells of muscles, glands, skin, mucous
membranes, organs of the thorax, abdomen, etc.

There are two sets of nerves concerned in all nerve
action or reflex. One which transmits impulses from
the brain and spinal cord to the structure whose
activities are to be increased or retarded; others



THE STRUCTURE OF THE NERVE SYSTEM 343

transmit impulses from the peripheral surfaces and
organs of the body to the brain and spinal cord, which
create conscious sensations or stimulate other reflex
activity. The former are termed efferent or centrifugal
(mostly motor nerves), the latter afferent or centripetal
(mostly sensor nerves).

Physiology of Nerves. Nerves possess the function
of developing and conducting nerve impulses from
the nerve centres in the brain and spinal cord to the
periphery of the body, and at the same time to trans-
mit nerve impulses from the periphery to the centres
in the brain and spinal cord. As long as a nerve
is capable of these qualities it is termed excitable
or irritable, or possessed with irritability or excita-
bility.

XERVE STIMULI. Xerves must receive some form
of external stimulation before they will develop or
convey nerve impulses, as they do not possess the
property of spontaneously developing and sending
out nerve impulses. A stimulus to motor nerves
(efferent), which excite it to activity, arise as a result
of some molecular disturbance within the nerve cells,
that acts upon the nerve fibers in connection with
them. In the case of sensor nerves (afferent) the
stimulus arises in the end-organs, which convey the
nerve impulse to the sensor nerve fibers in connection
with them.

Xerves react to stimulation according to their
habitual function and distribution. If we stick our
finger with a pin, the sensation of pain is felt, due to
the fact that a sensor nerve has conveyed the nerve
impulse, started in the end-organs in the skin, to the
conscious centres in the brain ; stimulation of the end-
organs in rods and cones in the retina of the eye give
rise to the sensation of light; stimulation of a motor
nerve is followed by the contraction of a muscle
which it innervates. Xerve function is supposed to



344 THE NERVE SYSTEM

depend upon the peculiarities inherent in the central
and peripheral end-organs, regardless of its con-
struction and the character of the stimuli (Brubaker's
Physiology) .

Special Stimuli. These comprise the group which
act upon the nerves of special sense and give rise to
conscious sensations, through the highly specialized
end-organs, which transfer the nerve impulse to the
filaments of the nerves in relation with them.

The afferent nerves (sensor) convey the impulse to
the higher conscious centres, in response to the special
stimuli, as follows: (1) Light or etheral vibra-
tions act upon the end-organs of the optic nerve
in the retina (sight and light); (2) sounds act upon
the end-organs of the auditory nerve (hearing) in the
ear; heat or vibrations of the air act upon the end-
organs in the skin; (4) chemic agents act upon the
end-organs of the olfactory (smell) and gustatory
(taste) nerves of the nose and tongue respectively.

The efferent nerves (motor) convey impulses to the
muscles, glands, etc., in response to stimuli which
are supposed to arise as a result of a molecular dis-
turbance in the central nerve cell, a combination of
physical and chemic processes attended by the libera-
tion of energy, which passes from molecule to molecule.
The passage of the nerve impulse is accompanied by
changes of electric tension.

Thus, to sum up, all nerve impulses have their
origin in the nerve cells or neurones, and these millions
of associated neurones are the basis of all nerve activity.
A theory has been created to simplify the understand-
ing of nerve cell activity. It is called the neurone theory
of Waldeyer, who explains it, based on the works of
Golgi, Cajal, Forel, and others, as follows: (1) Each
neurone is a distinct and separate entity; (2) the
collaterals and other terminals of the neurones form
no connections among themselves; (3) neurones are



THE CENTRAL NERVE SYSTEM



345



associated, and impulses conveyed, by contact or
contiguity of the axonic terminals of one axone with
the cell body or dendrites of another neurone.



THE CENTRAL NERVE SYSTEM

The central nerve system or cerebrospinal axis
consists of the (1) brain (encephalon) and its cranial
nerves and associated ganglia; (2) the spinal cord and
its spinal nerves and associated ganglia.

FIG. 121



ARTERIA
VERTEBRALIS




LIGAMENTUM
DENTICULATUM



Ventral view of medulla oblongata and upper part of spinal cord. Dura
and arachnoid cut along median line and folded aside. A and B are fairly
constant velar folds of the arachnoid. (After Key and Retzius.)

The Spinal Cord. The spinal cord is the portion
of the nerve svstem which is connected with the brain



346



THE NERVE SYSTEM



above and the periphery of the body by thirty-one
pairs of nerves. It is lodged in the spinal canal,
ensheathed by the membranes, dura mater, arachnoid,
and pia mater, and commences above at the atlas



FIG. 122




SULCO-MARGINAL TRACT VENTRAL VESTI BU LOSP1 N AL TRACT
FROM SUP. QUADRIG. FROM FASTIGIUM (LOWCnthal's)



FIG. 123




VENTRAL VtSTIBULOSPINAL TRACT



THE CENTRAL NERVE SYSTEM



347



FIG. 124




WHITE VENTRAL COMMISSURE

FIGS. 122, 123, and 124. Sections of the spinal cord at the level of the
sixth cervical, sixth thoracic, and third lumbar segments, the conducting
tracts being indicated on the right side of each section: C. Comma tract
of Schultze. //. Olivospinal tract of Helweg. M. Marginal tract of Spitzka-
Lissauer. O. Oval field of Flechsig.



or margins of the foramen magnum and extends to
the lower border of the first lumbar vertebra below,
from which point it is continued as a narrow thread
of gray substance, the filium terminalis. The spinal
cord is 18 to 20 inches in length, and weighs one
ounce/

The columns of the spinal cord are divided into three
chief columns or funiculi. The ventral, dorsal, and
lateral.

The columns are simply connecting pathways for
the transmission of nerve impulses from the brain
centres to spinal centres, and contain efferent, afferent,
and association fibers. The course and connecting
pathways of these nerve fibers within the white
substance is seen to consist of numbers of medullated



348 THE NERVE SYSTEM

nerve fibers, without possessing any neurilemma.
They run in a vertical direction, and with their sup-
porting frame-work of neuroglia and connective tissue
the latter is derived from the pia mater and blood-
vessels are grouped into bundles of axones, termed
columns or tracts, which are marked off by fissures
that can be seen with the naked eye on the surface
of the cord.

The Structure of the Spinal Cord. If a cross-section
of the spinal cord be examined, it presents a central
gray substance and a surrounding white substance,
the former consisting of bodies of nerve cells and
their non-medullated axones; the latter contain the
medullated axones arranged in columns. The neuroglia
pervades both the white and gray substance, and is
the supporting tissue frame-work for the nerve cells,
their dendrites, and axones.

THE GRAY SUBSTANCE OF THE CORD. It is arranged
within the spinal cord in the form of two crescents
joined in the centre, or a figure resembling the letter H.
The gray substance on either side extends nearly to the
surface of the cord, surrounded by the white matter,
the posterior projections are called the dorsal or
posterior horn, and the anterior the ventral or anterior
horn; the two halves of the gray substance are con-
nected by a bridge of gray substance termed the
commissure. The latter presents in its centre a
narrow canal (neural) which extends the entire length
of the cord. It is lined by cylindric epithelial cells
and surrounded by a gelatinous material.

The anterior horn of the gray substance is broader
than the posterior, and is completely surrounded by
white substance. The posterior horn is narrower and
approaches nearer to the surface of the cord than the
anterior horn does, and is enclosed by a gelatinous
substance called the substantia gelatinosa. In the
lower cervical and thoracic portions of the cord the



THE CENTRAL NERVE SYSTEM 349

gray matter is expanded into a projection called the
lateral horn. This is seen on both sides. Microscopic
examination of the gray substance will show that it
is practically an aggregation of neurones nerve cells
with their dendrites and non-medullated axones,
lymphatics, bloodvessels, all supported in a frame-
work of non-neural tissue the neuroglia.

Classification of nerve cells within the gray sub-
stance as regards their function: They are divided
into intrinsic, efferent, and afferent.

The intrinsic cells are simply associative or connec-
tive in character, their processes enteV the white
substance horizontally, and give off branches which
ascend -and descend, reen'tering the gray substance
at different levels, where their axones again associate
with the dendrites of other intrinsic cells.

The efferent motor cells is the term given to the
cells found in the anterior horns, whicn are sub-
stations for the reception from the brain and other
neurones, of motor impulses which they in turn
conduct through their efferent axones, to the periphery,
and promote activity in the muscles, glands, viscera,
bloodvessels, as well as influence the growth, develop-
ment, and metabolism (trophic) of the tissues.

Afferent cells (sensor) is the name given to the
cells of the posterior horn, which receive from the
afferent nerves impulses and conduct the same by
their afferent axones, to the cortex of the brain, giving
rise to conscious sensations, as heat, pain, sensation
of touch, etc.

The Spinal Nerves. There are thirty -one pairs,
divided according to the portion of the cord they
arise from, as follows:



Cervical pairs 8

Thoracic pairs ,.

Lumbar pairs . - v A a^-

Sacral pairs . . . _ . ""

Coccygeal pairs

Total



; 1*



350 THE NERVE SYSTEM

All spinal nerves leave the spinal canal between the
vertebra (inter vertebral foramen). If one examines
the spinal cord after removal, the spinal nerves will
be seen to consist of two roots by which they arise
from the sides near the anterior and posterior aspects
of the cord. The two roots are named anterior or
ventral and posterior or dorsal. The anterior and
posterior roots join to form a single nerve trunk just
before they leave the spinal canal. The dorsal root
presents an enlargement near the point at which it
joins the anterior root a small grayish body called
a ganglion. The roots do not leave the cord as a
single rounded nerve, but are formed by the joining
of four to six large nerve fibers.

FIG. 125



DORSAL RO



INAL ROOT GANG



IL NERVE




VENTRAL FISSURE

Showing origin of two pairs of spinal nerves (schematic). (Gray.)

The posterior root is sensor or afferent, and contains
bundles of axones which convey impulses from the
end-organs in the skin, mucous membranes, etc.,
after they have been received and conveyed by axones



THE CENTRAL NERVE SYSTEM 351

and dendrites of the ganglia on its root, to the groups
of cells in the dorsal horn of the spinal cord, and as
we have mentioned before, these impulses ascend and
descend by means of collateral branches, axones, and
dendrites, communicating with other centres in the
cord at different levels and ultimately reach the
sensor areas in the brain. The anterior root is motor
or efferent, and contains axones which transmit
impulses from the groups of cells or centres in the