THE NERVOUS SYSTEM.
The nervous system, which regulates all the vital processes of the
body, physical and chemical, and which is situated partly in the head
and partly in the trunk, may well form the connecting link between the
description of the head and that of the trunk. It has two divisions,
the cerebro-spinal system and the sympathetic system. The former
consists of the cerebrum or brain proper, the cerebellum or little
brain, the pons Varolii, the medulla oblongata, the spinal cord, and
the cranial and spinal nerves; the latter of a series of ganglia or
aggregations of nerve centers. The brain, which includes the cerebrum,
cerebellum, pons, and medulla, occupies the cranium and the spinal cord
is contained within the bony framework of the spinal column. In the
male the brain weighs about 49 ounces and in the female 44, while in an
idiot it seldom weighs more than 23 ounces.
The =cerebrum= or brain proper has two parts or hemispheres, roughly
oval in shape, each of which has five lobes separated by fissures, the
frontal, parietal, occipital, and temporo-sphenoidal lobes, and the
central lobe or _island of Reil_ at the base of the brain. The chief
fissures are the _longitudinal fissure_, the _fissure of Sylvius_
at the base of the brain, and the _fissure of Rolando_ between the
frontal and parietal lobes. There are also five serous cavities
called _ventricles_, the two lateral and the third, fourth, and fifth
ventricles, of which the first two, one in either hemisphere, are the
most important. Around these cavities is the _brain substance_, which
is made up of two tissues, the white and the gray, the latter forming
the outer part of the brain to the depth of perhaps half an inch, and
the white matter forming the rest. The outer or gray part is called
the _cortex_ and is largely made up of nerve cells. It might be called
the active part of the brain. The white part consists largely of nerve
fibers which are given off from the nerve cells and are carried down
into the spinal cord.
The surface of the brain is convoluted, the ridges being separated by
deep furrows or sulci, by which means a great extent of gray matter
is secured. The furrows contain fluid from the subarachnoid spaces
and vary in number and depth according to intelligence. While the
convolutions are not uniform in all brains, the principal ones are
constant.
Both the brain and the spinal cord are covered by three membranes, the
dura mater, the arachnoid, and the pia mater. The _dura mater_ is dense
and fibrous and lines the interior of the skull, being firmly adherent
to it at many points. In fact, it constitutes the internal periosteum
of the cranial bones. The _arachnoid_ is a delicate serous membrane,
with two layers, lubricated to prevent friction, which divides the
space between the dura mater and the pia mater, bridging over the
convolutions and enclosing the subdural and subarachnoid spaces which
are connected with lymphatics and contain a serous secretion, the
_cerebro-spinal fluid_. This fluid forms an elastic water cushion, on
which the brain rests, and prevents concussion. The _pia mater_ is
vascular, containing blood-vessels, lymphatics, and nerves, and is
closely attached to the surface of the brain, dipping down into all the
sulci.
At the base or under surface of the brain are some very important
structures. The _olfactory bulbs_ lie beneath the frontal lobe and
projecting back is the _olfactory tract_, through which the olfactory
nerves come from the brain. Back of the olfactory tract is the _optic
commissure_ where the optic nerves coming from the brain cross each
other. And back of the commissure again is the _optic tract_, where
the optic nerves emerge from the brain. At the base of the brain are
also the exits of the twelve cranial nerves.
[Illustration: FIG. 27.--Base of brain. (Leidy.) 1, 2, 3, cerebrum;
4 and 5, longitudinal fissure; 6, fissure of Sylvius; 7, anterior
perforated spaces; 8, infundibulum; 9, corpora albicantia; 10,
posterior perforated space; 11, crura cerebri; 12, pons Varolii; 13,
junction of spinal cord and medulla oblongata; 14, anterior pyramid;
14ˣ, decussation of anterior pyramid; 15, olivary body; 16, restiform
body; 17, cerebellum; 19, crura cerebelli; 21, olfactory sulcus; 22,
olfactory tract; 23, olfactory bulbs; 24, optic commissure; 25, motor
oculi nerve; 26, patheticus nerve; 27, trigeminus nerve; 28, abducens
nerve; 29, facial nerve; 30, auditory nerve; 31, glosso-pharyngeal
nerve; 32, pneumogastric nerve; 33, spinal accessory nerve; 34,
hypoglossal nerve.]
Upon entering the brain the _arteries_ run a tortuous course, the
tortuosity breaking the force of the blood stream in the small vessels
where congestion would be with difficulty relieved. The basilar artery,
which is formed by the juncture of the two vertebrals, divides into
the two posterior cerebrals, each of which joins one of the anterior
cerebrals by a posterior communicating artery. The two anterior
cerebrals also are joined by an anterior communicating artery, thus
completing the circle. The circle thus formed at the base of the brain
is called the _circle of Willis_ and provides for a good supply of
blood in event of an accident to any vessel. The blood is returned to
the general circulation through the cerebral veins and sinuses formed
by the separation of the dura mater into two layers.
The =cerebellum= is about one-seventh the size of the cerebrum and
weighs about 5 ounces. It lies in the lower occipital fossæ of the
skull and is oblong in shape and divided into two lateral hemispheres
by a transverse fissure. It is made up of both white and gray matter,
of which the former predominates, the gray being external as in the
cerebrum. The cells are about the same as in the cortex and its surface
is traversed by queer furrows. Of its function little is known but it
probably plays a most important part in the coördination of the nervous
and muscular acts by which the movements of the body are carried on.
At the back of the cerebrum and below the cerebellum is the =pons
Varolii=, which forms a connecting link with the medulla oblongata or
bulging part of the cord. It is made up essentially of white matter or
nerve fibers, though there is a small amount of gray matter in which
are found the nuclei of some of the cranial nerves.
In the =medulla oblongata=, which is about 1 inch long and extends from
the pons Varolii to the upper border of the atlas or first cervical
vertebra, the gray matter is not necessarily external to the white but
is found in patches in the white. The gray matter here corresponds more
or less to that of the spinal cord and the white matter is continuous
with that of the cord. From the medulla arise the fifth to twelfth
cranial nerves and the vasomotor nerves. The cardiac nerve has its
center here and here too are the centers of respiration, phonation,
deglutition, mastication, and expression. In the medulla the nerves
that arise in the cerebrum cross over from one side of the body to
the other on the crossed pyramidal tracts. The importance of this
crossing of the nerve fibers is seen in apoplexy, when a blood-vessel
is ruptured in the brain and hemorrhage causes pressure, generally on
the motor tract. Paralysis of the nerves and of the muscles to which
they go results. The paralysis is generally of one side of the body,
the opposite side from that on which the injury occurred. The seat
of injury in the brain or cord can frequently be determined by the
situation and extent of the paralysis.
=Spinal Cord.=--Extending down from the medulla through the spinal
column is the cord. Its length from the foramen magnum, where it
begins, down through the vertebræ to the lower border of the first
lumbar vertebra, where it ends in a very fine thread-like process with
no special function, called the _filum terminale_, is 17 to 18 inches.
Just before it ends a number of nerves are given off in a tail-like
expansion known as the _cauda equina_ or horse’s tail. It is not
uniform throughout its length but presents two enlargements, a cervical
enlargement in the lower cervical region, and a lumbar enlargement in
the lower dorsal region, where the nerves are given off to the arms and
legs respectively. The membranes are the same as those of the brain
and are continuous with them, but here the dura mater is not attached
to the bony walls enclosing it. For the cord does not fit closely
into the canal but is as it were suspended in it. The _subarachnoid
space_ communicates with the ventricles of the brain by the _foramen of
Majendie_ and is filled with _cerebro-spinal fluid_ for the protection
of the cord. In cerebro-spinal meningitis or spotted fever this fluid
is infected and for diagnosis lumbar puncture is performed.
[Illustration: FIG. 28.--Different views of a portion of the spinal
cord from the cervical region, with the roots of the nerves. In _A_ the
anterior surface of the specimen is shown, the anterior nerve root of
its right side being divided; in _B_ a view of the right side is given;
in _C_ the upper surface is shown; in _D_ the nerve roots and ganglion
are shown from below: 1, the anterior median fissure; 2, posterior
median fissure; 3, anterior lateral depression, over which the anterior
nerve roots are seen to spread; 4, posterior lateral groove, into
which the posterior roots are seen to sink; 5, anterior roots passing
the ganglion; 5´, in _A_, the anterior root divided; 6, the posterior
roots, the fibers of which pass into the ganglion, 6; 7, the united or
compound nerve; 7´, the posterior primary branch seen in _A_ and _D_
to be derived in part from the anterior and in part from the posterior
root. (Allen Thomson.)]
If a cross-section of the cord is made, it is found to have a pretty
definite structure. It is roughly circular and is divided by certain
fissures, of which the most important are the anterior and posterior
median, the latter being rather a dividing line or septum. By them
it is divided into halves connected by a small band in the middle
called the _commissure_. The white matter is exterior to the gray
and is divided by it into four columns, which again are divided into
tracts according to certain groups of nerves that travel through
them. The most important tract is the direct pyramidal tract in the
anterior column. The gray matter is arranged in the form of a letter H
practically, consisting of two lateral halves, more or less crescentic
in outline, connected by a narrow band, the _gray commissure_. Each
half is divided into two _horns_, the anterior, toward the front of the
cord, and the posterior, toward the back, the former being generally
much thicker and heavier than the latter. The structure of the gray
and of the white matter is essentially the same as in the brain,
but the proportion varies in different parts of the cord, the white
predominating in the cervical region and the gray being much better
developed in the lumbar region, where the nerve cells for control of
the lower extremities occur. The gray is least well developed in the
dorsal region. Through the center of the cord runs a small hole or
canal filled with cerebro-spinal fluid, the _central canal of the cord_.
[Illustration: FIG. 29.--Functional areas of the cerebral cortex, left
hemisphere. (A. A. Stevens.)]
The brain is the seat of intelligence and will, the center of all
voluntary action. Molecular change in some part of the cerebral
substance is the indispensable accompaniment of every phenomenon of
consciousness. Indeed, the brain is never in a state of complete
repose, there being dreams even during sleep. The brain is not
sensitive to injury in the sense of pain. It can be lacerated without
much pain.
Various centers exist in the brain, of which the most important perhaps
is the _motor center_. The _visual center_ is in the occipital lobe,
the _auditory center_ in the temporal lobe, the _speech center_ in the
third left frontal convolution. Thus the impulses of the senses have
been located, though the function of many parts, the so called _silent
areas_, are still in obscurity.
The _motor center_, that is, the center for motion of the skeletal
muscles, is situated about the fissure of Rolando and is divided into
three parts, one for the legs, one for the face, and one for the arms,
the one for the legs being uppermost and the others below in the order
mentioned. Fibers from these cells extend down through the brain and
cord to the muscles, the fibers being collected into well-recognized
bundles and the whole known as the _motor tract_. There may be one long
fiber from a cell in the brain down through most of the cord or there
may be a succession of shorter fibers that are not actually connected
but are in close contact with each other. In the upper pons the fibers
for the face cross to the opposite side, while the rest keep on down
through the medulla, and as they emerge from the medulla they too cross
to the other side and keep on down in the crossed pyramidal tract.
A few fibers do not cross but come down the direct pyramidal tract,
which, however, disappears part way down. The _crossed pyramidal tract_
is the true motor tract and in it the fibers are continually sending
branches to the cells in the gray matter, where they connect with the
anterior horn.
The anatomy of the _sensory tract_ is not so well understood. By it
impulses are sent to the brain by the peripheral organs, practically
the surface of the body. The sensory fibers connect with the sensory
cells in the posterior horn, from which fibers are sent to the brain,
practically the reverse of motor action. There are three chief sensory
tracts, which are supposed to transmit different sensations, one pain,
one muscular sensations, and the third sensations of touch. All these
tracts, of which the chief is the _direct cerebellar tract_, in passing
up the cord pass to the opposite side at different levels and then go
on to the cortex of the brain.
The action of the nerves is similar to reflex action, only that an
effort of will is needed to send an impulse from the brain. It is by
the help of the brain along this line that an infinity of artificial
reflexes or habits is acquired, for which volition is needed in the
beginning but which are later done unconsciously. Herein lie the
possibilities of all education.
The brain and spinal cord work together, the cord acting as a medium
between the brain, in which all the higher psychical processes, such as
will, thought, etc., originate, and the muscular apparatus. The cord,
however, has some action entirely independent of the brain, as is seen
in _reflex action_. This action is entirely involuntary, so that the
cord is sometimes spoken of as the seat of involuntary action, commonly
called reflex action. All unconscious acts are reflex acts, as when the
hand is drawn away from a hot iron. If an impulse is sent along one
of the sensory fibers, it enters the cord through the posterior horn,
where its nerve cell is found. Then, through some connection between
the nerve cell of the sensory fiber and that of the motor fiber the
impulse is transmitted to the motor cell and another impulse is sent
out of the cord along the motor fiber of the nerve to the muscle.
One of the commonest reflexes is the _knee-jerk_. Reflex action is
important because the reflexes are interfered with, delayed, destroyed,
or increased in different diseases. The time normally required for a
reflex act is very brief, that for the knee-jerk being about three
one-hundredths of a second.
The nerves of the head, known as the _cranial nerves_, arise from the
brain, while the rest of the body is supplied by the spinal nerves,
which come off at intervals from the spinal cord. The cranial nerves
consist of twelve pairs: (1) The olfactory or nerve of smell, (2) the
optic or nerve of sight, (3) the motor oculi, (4) the patheticus, which
controls the eye, (5) the trigeminus or trifacial, a nerve of general
sensation, motion, and taste, (6) the abducens, a motor nerve, (7) the
facial nerve of the face, ear, palate, and tongue, (8) the auditory or
nerve of hearing, (9) the glosso-pharyngeal, nerve of sensation and
taste, (10) the pneumogastric or vagus, which is both motor and sensory
and governs respiration, the heart, and the stomach, (11) the spinal
accessory, to the muscles of the soft palate, and (12) the hypoglossal,
the motor nerve to the tongue.
The _spinal nerves_ also are arranged in pairs: Eight cervical pairs,
twelve dorsal or thoracic, five lumbar, five sacral, and one coccygeal,
these titles denoting their point of origin near the vertebra of the
same name. Each of these nerves arises by two roots, an anterior motor
root from the anterior horn of gray matter and a posterior sensory root
from the posterior horn, the latter having a ganglion upon it. After
emerging from the cord the two roots unite to form the nerve, that the
nerve may contain both motor and sensory fibers. The motor fibers are
called efferent because they carry impulses _from_ the cord, while the
sensory are called afferent because they carry impulses back _to_ the
cord. After leaving the cord the nerves unite to form plexuses, which
again divide into various nerve trunks and are distributed to the
muscles.
The first _cervical nerves_ pass out of the spinal column above the
first cervical vertebra and the other cervical nerves below that
and the succeeding vertebræ, while the other spinal nerves emerge
each below the corresponding vertebra, as the first dorsal below
the first dorsal vertebra, etc. After emerging they break up into a
large anterior division and a small posterior division, the posterior
branches supplying the spine and the dorsal muscles and skin, the
anterior the rest of the trunk and the limbs. The _cervical plexus_ is
formed by the anterior divisions of the first four cervical nerves, the
_brachial plexus_ by the last four cervical and the first dorsal or
thoracic nerves, the _lumbar plexus_ by the four upper lumbar, and the
_sacral plexus_ by the last lumbar and the four upper sacral nerves.
The only important branch of any of the four upper cervical nerves,
which in general supply the neck and shoulders, is the _phrenic_, which
is distributed to the pericardium, the pleuræ, and the under surface of
the diaphragm.
The _brachial plexus_, as its name implies, supplies the arms and has
a number of important branches, as the circumflex to the shoulder, the
musculo-cutaneous to the upper arm, the elbow-joint, and the outer
surface of the forearm, the internal cutaneous to the inner side of
the arm, the median to the pronators and flexors and the fingers on
the radial side, and the ulnar to the elbow and wrist-joint. The
musculo-spiral runs down the spiral groove to the external condyle of
the humerus or upper arm bone, where it divides into the radial and the
posterior interosseous, the former going to the thumb and two adjacent
fingers and the latter to the wrist-joint and the muscles on the back
of the forearm. Sometimes, in fracture of the humerus the callus thrown
out pinches the musculo-spiral and causes pain.
The dorsal or _thoracic nerves_ supply the back with their posterior
divisions and their anterior divisions are the _intercostal nerves_.
The _lumbar nerves_ supply the abdomen, pelvis, and thigh, the chief
branches being the ilio-hypogastric to the abdomen and gluteal region,
the ilio-inguinal to the inguinal region and scrotum, the external
cutaneous and genito-crural to the thigh, and the obturator to the
thigh and the hip and knee-joints. The anterior crural descends beneath
Poupart’s ligament and divides into an anterior and a posterior
division which supply the thigh muscles, its branches going to the
pelvis.
[Illustration: FIG. 30.--Diagrammatic view of the sympathetic cord
of the right side, showing its connections with the principal
cerebro-spinal nerves and the main preaortic plexuses. (Reduced from
Quain’s anatomy.)]
The _sacral plexus_ supplies the organs of the pelvis, the thigh, and
the leg. Its chief branches are the great sciatic, the largest nerve in
the body, and the small sciatic, which go to the buttocks and thigh.
The great sciatic runs down the back of the thigh and divides at the
lower third of the thigh into the internal and external popliteal
nerves, the former of which passes along the back of the thigh to the
knee, where it becomes the posterior tibial, which in turn divides
at the ankle into the internal and external plantar. The external
popliteal descends along the outer side of the popliteal space and
divides an inch below the head of the fibula into the anterior tibial,
which supplies the flexors and skin of the ankle-joint, and the
musculo-cutaneous, which sends branches to the skin of the lower leg
and the dorsum of the foot.
=The Sympathetic System.=--Joined to the cerebro-spinal system by
intervening cords is the sympathetic system. This is made up of two
series of ganglia, one on either side of the spinal column, connected
by longitudinal bands and extending from the base of the skull to the
coccyx. They do not form an independent nervous system, each ganglion,
which seems to resemble the motor cells of the spinal cord, being
connected by motor and sensory fibers with the cerebral system.
The _sympathetic nerves_ are mostly gray, non-medullated fibers and
are distributed to viscera, secreting glands, and blood-vessels, whose
movements are involuntary and feelings obtuse. They form networks upon
the heart and other viscera and send branches to the cranium to the
organs of special sense. There are three main plexuses: The _solar
plexus_ behind the stomach, which supplies the abdominal viscera; the
_hypogastric plexus_ in front of the prominence of the sacrum, whose
nerves go to the pelvic organs; and the _cardiac plexus_ behind the
aortic arch for the thoracic viscera.
Over these nerves one has no control. A blow in the region between the
costal cartilages and below the sternum is a solar plexus blow and is
very upsetting.
The sympathetic system serves to maintain vitality in all the important
portions of the system and one of its important functions is to keep up
communication between one part and another, so that when any organ is
affected the others will act accordingly and help out to the best of
their ability.