anterior horn of the spinal cord, which are further
conveyed by means of the efferent axones or nerve
fibers to the periphery, where they stimulate muscles,
glands, viscera, etc., into activity.
In other words, motor impulses arise in the cells of
the anterior horn of the spinal cord and the motor area
in the brain, and are conveyed toward the periphery
of the body (muscles, glands, etc.), by means of
neurones and their efferent processes. Sensor impulses
arise within the structures of the skin, mucous mem-
brane, etc., and are conveyed to the sensor group of
cells in the dorsal horn of the cord, and are trans-
mitted to the sensor and conscious areas in the
brain by aggregations of neurones and their (afferent)
processes coursing through the cord and brain sub-
stance.
The functions of the spinal cord are classified under
anatomatism, reflex action, association conduction, sensor
conduction, motor conduction.
Automatism, or Automatic, Autochthonic Action.
This is a function possessed by the spinal nerve cells
whereby they influence the growth, development, and
nutrition of the numerous cells of the various tissues,
and thus maintain their normal physiologic activity.
By this expression is meant a discharge of energy
from the cells occasioned by a change in their environ-
ment, i. e.j in the chemic composition of the blood or
lymph by which they are surrounded, and independent
352 THE NERVE SYSTEM
of any excitation coming through afferent (sensor)
nerves from the periphery. If the cell activity is
continuous it causes an even and regular control over
the processes of cell nourishment, development, etc.
(trophic); muscle and vascular activity, which is
spoken of as tonus.
Reflex Actions. They are defined as the reception
of nerve impulses transmitted to the nerve cells in
the spinal cord and higher centres in the brain, by
afferent (sensor) nerves; and the response of the nerve
cells in the efferent centres to this stimulation, which
result in nerve impulses being excited in the nerve
cells, and conveyed by the efferent (motor) nerves,
which pass from these cells to the structures to be
innervated, and cause them to act, as, to muscles,
causing contraction; to gland cells, secretion; to
bloodvessels, increasing or decreasing their caliber;
and to organs, increasing or decreasing their activity.
For any reflex act to be mechanically possible there
must be present the following structures:
1. A surface to receive the stimuli: skin, mucous
membrane, sense organ.
2. An afferent nerve fiber and cell, to convey the
nerve-impulse arising as a result of the stimulus
exciting the sense organ, etc., into activity.
3. An emissive cell, from which arises an (4) efferent
nerve, distributed to a responsive organ, as muscle,
gland, bloodvessel, etc. //
In a more practical way the most sim|lfe reflex
action can be explained as follows: If a muscle is
stimulated by a strong current of electricity or pinched
by an instrument, there is developed in the terminals
of the afferent nerve a nerve impulse which is conveyed
to the nerve cells in the posterior horn of the spinal
cord, the dendrites of these cells transmit the impulse
to the dendrites of the nerve cells in the anterior
horn, where by means of a molecular jpllturbance
/*^
0*
THE CENTRAL NERVE SYSTEM 353
within the cells, energy is liberated and the nerve
impulse is conveyed by the efferent nerve to the muscle
and it contracts.
In most reflex actions there is and must be a more
complex arrangement to account for the varied move-
ments and functions of the different structures of the
body which are taking place in response to external
and internal stimuli. These complex reflexes are
due to the nerve centres in the cord communicating
by means of axones and dendrites of other cells with
other higher centres at different levels of the cord,
not only on the same but the opposite side; and a
still more complex arrangement is produced, due to
the fact that the centres in the medulla oblongata are
in connection with the spinal centres by pathways
of nerve fibers.
Thus reflex actions can be carried on without the
individual being conscious thereof, or by the presence
of the associated neurones other centres convey sensa-
tions to the brain, of which we are conscious, and the
return impulse can excite a voluntary movement.
Reflex actions are controlled by centres in the brain
(medulla) which transmit impulses to the spinal
centres which either decrease or inhibit, either increase
or accelerate their activity, thus regulating the recep-
tion of and response to nerve impulses by these
centres which exert a controlling influence based on
the needs of the physiologic functions of the human
body.
Association Conduction. The spinal cord is divided
into segments which have a controlling influence
over the physiologic functions of certain parts of the
body, as the arm, leg, etc. These segments to properly
work together and receive impulses from other reflex
centres at different levels in the cord, which control
movements in response to stimuli from other portions
of the body, are held in communication by pathways
23
354 THE NERVE SYSTEM
of nerve axones, termed association fibers, or this
phenomenon is spoken of as association conduction.
Semor Conduction. This term is used in speaking
of the pathways of afferent or sensor nerve fibers in
the tracts of the spinal cord, which convey sensa-
tions of pain, external temperature, thirst, etc., from
the skin, mucous membranes, etc., to the centres
in the brain directly or indirectly, which are received
by nerve cells in the cortex, giving rise to conscious
sensations. These afferent pathways are not thoroughly
understood, but the main one is called the crossed
pyramidal tract.
Motor Conduction. This is the term used in speak-
ing of the pathways of efferent or motor nerve fibers
in the tracts of the spinal cord which convey motor
impulses from the cells in the cortex of the brain to
centres in the spinal cord, that transmit nerve impulses
to the muscles, glands, etc., and promote their activity.
The main motor pathway is called the direct pyramidal
tract.
THE ANATOMY AND PHYSIOLOGY OF THE
BRAIN
The Brain (Encephalon). The encephalon or brain
is that part of the cerebrospinal system which, with
its membranes, is contained in the cranium. It is
composed of the cerebrum, cerebellum, pons Varolii,
and medulla oblongata.
The Membranes of the Brain. These are the dura,
the pia, and arachnoid.
The dura mater is similar in structure to the dura of
the cord, but differs from it in being closely attached
to the cranial bones, forming, in fact, their inner peri-
osteum. It is continuous with that of the cord at the
foramen magnum, and with the external periosteum of
ANATOMY AND PHYSIOLOGY OF THE BRAIN 355
the cranial bones by means of its prolongations into
the many foramina. It sends in various processes to
support and separate the different parts of the brain,
and its layers separate to form the cranial sinuses
(venous). In the vicinity of the superior longitudinal
sinus are to be found, on its outer surface, several
glandule Pacchionii. They may also be seen on its
inner surface and within the sinus, as well as on the
pia mater.
The processes of the dura include the falces cerebri
et cerebelli and the teritorium cerebelli.
The arachnoid is a similar membrane to that of the
cord, and is separated, as in the cord, by the subarach-
noid fluid from the pia. In front it leaves a space
between it and the pia mater, viz., along the pons
and interpeduncular region, the anterior subarach-
noid space; and behind, between the medulla and
and the cerebellum, is a second interval called the
posterior subarachnoidean space. Both are connected
with the ventricles of the brain by the foramen of
Majendie in the pia mater covering the fourth ven-
tricle.
The pia mater is a very vascular delicate membrane
which dips into the sulci and forms the various
choroid plexuses and also the velum of the third
ventricle. The vessels of the brain run in the pia
mater before entering the brain.
The brain, for purposes of description, includes the
cerebrum, cerebellum, medulla oblongata, and pons
varolii.
Medulla Oblongata. The medulla oblongata is a
pyramidal body, f to 1 inch long, along its ventral
surface, and f inch thick. Its larger extremity is
continuous with the pons above; its smaller extremity,
directed downward and backward, blends with the
spinal cord below. The anterior surface lies on the
basilar groove of the occipital bone.
356 THE NERVE SYSTEM
In front and behind it is marked by the continuation
of the anterior and posterior median fissures of the
cord, the former, with its process of pia mater, ending
in a cul-de-sac just below the pons, the foramen cecum.
The posterior expands into the fourth ventricle.
Each lateral half of the medulla is divided into
areas.
THE AREAS OF THE MEDULLA OBLONGATA. These
are: (1) Ventral area; containing the pyramid. (2)
Lateral area; containing the lateral tract olive. (3)
Dorsal area; containing the funiculus gracilis, funiculus
cuneatus, funiculus lateralis, and tuberculum cinereum.
The restiform body succeeds the gracile and cuneate
nuclei in the dorsolateral part of the medulla oblongata.
Its fibers converge from various sources and ultimately
enter the cerebellum as its inferior peduncle. (Gray.)
The Decussation of the Pyramids. It is a term
applied to the interlacing bundles seen on the ventral
aspect of the medulla, at the junction of the medulla
and the spinal cord. Ninety per cent, of the fibers
cross the median line in this decussation to continue
as the crossed pyramidal tract.
STRUCTURE OF THE MEDULLA OBLONGATA. Gray
and white matter are constituents of the medulla;
the former is, in the internal part, continuous with
the gray substance of the cord, while the white
substance is external.
The Gray substance of the medulla, examined under
the microscope, presents numerous groups of nerve
cells similar in arrangement to the cells in the spinal
cord, but they are not so regular, due to the changed
course of the fibers of the white substance. These
nerve cells are supported by neuroglia and connective
tissue. The nerve cells give off axones which ascend
to the brain and descend to the cord, conveying nerve-
impulses to the brain (sensor or conscious), and
transmitting other impulses from the brain to the
ANATOMY AND PHYSIOLOGY OF THE BRAIN 357
cord (motor or volitional), and others give off axones
which form portions of the cranial nerves.
The white substance is composed of bundles of nerve
fibers supported by a frame-work of neuroglia and
connective tissue. These columns formed of bundles
of nerve fibers are the connecting or conducting
pathways coursing from the brain to the spinal cord,
for the transmission of nerve impulses between the
brain and the periphery.
The Pons Varolii. The pons is a white mass on the
anterior aspect of the brain stem placed between the
medulla oblongata and the crura cerebri. It is convex
from side to side, containing mostly transverse and
longitudinal fibers. The transverse fibers are collected
into rounded bundles, to continue as the middle
peduncles into the white substance of the correspond-
ing cerebellar hemispheres. The middle peduncles are
commissural paths consisting of axones coursing in
opposite directions connecting the nuclei with the
cerebellum; then some axones pass into the opposite
middle peduncle, forming uninterrupted commissural
systems; again, a few fibers communicate with nuclei
in the brain stem, notably the oculomotor, trochlear,
and abducent cranial nerves.
The Structure of the Pons Varolii. It consists of a
central gray and white mass of nerve tissue, supported
by neuroglia and connective tissue. Microscopically,
bundles of nerve fibers and nerve cells can be seen
in groups, the former coursing in a longitudinal and
transverse direction, as continuations of the pathways
of nerve fibers from the cord and medulla below,
and the cerebrum and cerebellum above. The trans-
verse bundles of nerve fibers in the pons convey
impulses from the corresponding and opposite hemi-
spheres of the brain. The nerve cells in the pons give
off axones which form some of the cranial nerves.
Functions of the Medulla Oblongata and Pons
Varolii. The medulla and pons contain tracts of
358 THE NKHYK SYSTEM
nerve fibers which convey impulses from the brain
and cerebellum to the spinal cord. The anterior
portion of the medulla and pons contain pathways
for the transmission of volitional efferent nerve
impulses from the higher centres in the brain to the
spinal cord; the posterior portion contains pathways
for the conduction of afferent nerve impulses from
the spinal cord to the brain. The medulla and pons
contain groups of nerve cells and nerve fibers, called
nerve centres, which are in connection with and can
be influenced reflexly by other nerve impulses received
from associated groups of nerve fibers.
The Cerebellum. The cerebellum is the largest portion
of the hind-brain. It lies in the posterior fossa of the
skull separated from the occipital lobes of the cere-
brum by the tentorium cerebelli. It is behind the
pons and medulla oblongata, connected with the
former through the middle peduncles, and partly
embracing the latter; and connected with the restiform
body (medulla) by means of the inferior peduncles;
the superior peduncles contain fibers which pass from
the cerebellum to the tegmentum of the rnidbrain in
front.
The cerebellum is divided into a medial segment,
the vermis or worm; two lateral hemispheres; a ventral
and dorsal notch; and a superior and inferior surface;
and is subdivided into lobes and fissures.
The arbor vitse is the name given to the arrangement
of the white substance of the cerebellum, seen on a
median section. The cerebellum weighs 5.8 ounces in
the male; and 5.4 ounces in the female. The propor-
tion between the cerebellum and cerebrum is 1 to 7.5
in the adult; 1 to 8.5 among eminent men; 1 to 20 in
the newborn. (Gray.)
The Structure of the Cerebellum. Examined on a
cross-section, the cerebellum consists of gray and
white matter. The gray matter is external with the
ANATOMY AND I'llYMOLOGY OF THE BRAIN 359
white matter in the centre. The gray substance
consists of masses of nerve cells, their axones, and
dendrites. The cells are arranged in layers. The white
matter consists of nerve fibers which pass in different
directions and connect various portions of the cere-
bellum with one another. Nerve fibers are grouped
in bundles and connect cerebellum with the cerebrum,
pons varolii, medulla oblongata, and spinal cord.
The Function of the Cerebellum. It is the centre
for maintaining the equilibrium of the body, by
sending out nerve impulses, which cause a combined
action of groups of muscles that enable the body to
stand erect without swaying, and assist in the various
and complex movements seen in walking, dancing,
running, etc. The centres in the cerebellum are
reflexly influenced by nerve impulses arising in the
end-organs of the skin, retina of the eye, tactile (touch)
sense, and the labyrinth of the ear. These impulses
are transmitted to the cerebellum by afferent nerve
fibers and they stimulate the centres to activity and
the nerve impulses are conveyed by efferent nerves,
though the pons, medulla, and spinal cord and nerves
to the general muscle system.
The Cerebrum. The cerebrum is the largest part of
the brain, and consists of two lateral halves or hemi-
spheres, separated by the great longitudinal fissure
and connected to each other by a great commissure,
the corpus callosum. The latter constitutes a great
system of association nerve fibers for the bilateral
coordination of corresponding parts of the nerve cells
in the cortex. The hemispheres are subdivided into
lobes, and the latter present over their entire surfaces
convoluted eminences, the gyri or convolutions, sepa-
rated by depressions, the sulci or fissures. (See Figs.
126 and 127.)
The cerebrum, as a whole, is convex from before
backward and from side to side, narrower in front
360
THE NERVE SYSTEM
FIG. 126
Fissures and gyres of the lateral surface of the left hemicerebrum. (Gray )
FIG. 127
G. = GYRE
F = FISSURE
Fissures and gyres of the mesal surface of the left hemicerebrum. (Gray.)
ANATOMY AND PHYSIOLOGY OF THE BRAIN 361
than behind. Its inferior surface is flattened and
overlaps the midbrain and cerebellum, from which it
is separated by the tentorium cerebelli. The outer
surface, including the fissures, is composed of gray
matter, the cortical substance, while the interior is
of white matter.
LOBES OF THE CEREBRUM. The lobes are the
frontal, the parietal, the occipital, the temporal, and
the central lobe or Island of Reil.
The Frontal Lobe. The lateral surface is separated
behind from the parietal lobe by the central fissure or
the fissure of Rolando, and below, from the temporal
lobe by the Sylvian fissure, in part, and rests on the
orbital plate of the frontal bone.
The Parietal Lobe. The lateral surface is bounded
in front by the central fissure, below by the Sylvian
fissure, above by the back part of the internal border;
it is only partially separated from the occipital lobe
by the occipital fissure, merging gradually into the
temporal lobe.
The Occipital Lobe. The lateral surface is bounded
anteriorly by the occipital fissure, which partially
separates it from the parietal lobe, also the para-
occipital and exoccipital fissures are seen extending
into the lobe.
The Temporal Lobe. The lateral surface is bounded
by the basisylvian and Sylvian fissures and by the
ventrolateral border; posteriorly, it merges into the
adjacent parietal and occipital lobes.
The Island of Reil (central lobe or insula). This is
seen after separating the lips of the sylvian cleft,
after raising the frontal lobe; it is overlapped by the
opercula; the latter removed, the island of Reil is
seen as a tetrahedral-shaped mass with its apex
directed forward and upward.
The Rhinencephalon or Olfactory Lobe. This con-
stitutes the central olfactory structures, as distin-
362
7 7/A' NERVE ,STX77','.W
from the rest of the fore-brain. It comprises:
(1) Peripheral parts; (2) central or cortical portions;
the former is divided into pre- and postolfactory
portions.
THE VENTRICLES OF THE BRAIN. The ventricles
of the brain are narrow cavities enclosed with the
substance of the cord. They are filled with cerebro-
FIG. 128
FORAMEN OF MONRO
DDLC COMMISSURE
CHOROID PLEXUS OF
THIRD VENTRICLE
TAENIA THAI
Mesal aspect of a brain sectioned in the median sagittal plane.
spinal fluid and communicate with one another by
means of narrow canals and foramen. The ventricles
are called the lateral, third, and fourth.
The lateral ventricles are serous cavities, have a
thin lining membrane covered by a layer of epithelium
cells (ependyma), which secretes a serous fluid. They
are contained one in each hemisphere, separated by
ANATOMY AND PHYSIOLOGY OF THE BRAIN :
the septum lueidum, and each is divided into a body
and three eoruna, an anterior, posterior, and middle.
The foramen of Monro connects them with the third
ventricle.
The third ventricle is derived from the primitive
fore-brain vesicle, except that portion which also
enters into the formation of the lateral ventricles. It
is a narrow space between the two thalami and hypo-
thalamic gray, limited in front by the terma, behind
continuous with the aqueduct of Sylvius, and laterally
is continuous with the lateral ventricles through the
foramen of Monro.
The fourth ventricle is an irregularly pyramidal-
shaped cavity, with a lozenge-shaped base, and ridge-
like apex; found between the medulla oblongata and
the back part of the pons varolii in front and the
cerebellum behind. It is divided into a roof and a
floor. Below the fourth ventricle is continuous with the
small central canal of the cord and post-oblongata
(in part) ; above it communicates with the third
ventricle by means of the aqueduct of Sylvius. The
fourth ventricle has an opening through the tela
choroidea, which permits of communication with
the subarachnoid space; it is called the foramen of
Majendie.
THE STRUCTURE OF THE CEREBRUM. It consists
of masses of gray and white substance. The gray
being outside, makes up the cortex. The gray sub-
stance is composed of layers of nerve cells and nerve
fibers, with their axones and dendrites embedded in
a net-work of neuroglia. The nerve fibers may be
amyelinic or myelinic. Their direction may be either
transverse or vertical.
The white substance of the cerebrum is composed
of myelinic nerve fibers interwoven into an intricate
series of pathways, which are classified into (1) asso-
ciation fibers, which connect neighboring or distant
364 THE NERVE SYSTEM
portions with the same half of the cerebrum; (2)
commissural fibers which pass between the two
halves of the cerebrum and connect similar areas
w r ithin each; they cross in the middle line of the
brain and form commissures; (3) projecting fibers,
which connect the cerebrum with lower nerve centres
in the brain and spinal cord, and other fibers that
connect the lower centres with the brain.
The Weight of the Brain. The average weight of
the human brain in the adult male is 1400 grams
(49.5 ounces); in the female, 1250 grams (44 ounces);
in the newborn, 400 grams (14.1 ounces).
THE FUNCTIONS OF THE CEREBRUM. The functions
of the cerebrum have been discovered as a result of
a study of the anatomic development of the brain,
and examination of brains of various animals, human
beings, etc., and a study of diseases or injured brains,
certain parts of which have been rendered inactive
by the destruction of nerve cells and their processes;
such a destruction of tissue has been manifested in
different parts of the body by an interference or loss
of the function of the extremity, etc., to which the
nerve leading from or to the diseased region in the
brain is distributed. Thus it has been determined
that certain areas of the brain contain nerve cells
and nerve fibers which control definite functions of
the body, and are grouped into definite areas, irregu-
larly marked off by fissures, and correspond to the
convolutions seen on the surface of the cerebrum.
These areas are spoken of as the cortical localization
of function.
Cortical Localization of Function} Motor Area.
Comprises the precentral gyre and parts of the frontal
gyres adjacent thereto, together with the paracentral,
and the adjacent portion of the superfrontal gyre on
its inner aspect. This area comprises the centres for
1 See Figs. 129 and 130, page 365.
ANATOMY AND PHYSIOLOGY OF THE BRAIN 365
the muscle control of the following parts of the body
located as follows:
FIG. 129
Lateral view of left cerebral hemisphere, showing localization of functions
(Gray.)
FIG. 130
Mesal view of left cerebral hemisphere, showing localization of functions.
366 THE NERVE SYSTEM
Lower Limbs. Back part of precentral and para-
central fissures.
Trunk. Toward the front part, both on the under
and in the back superf rental fissure.
Upper Limbs. Midportion of precentral fissure.
Facial. Front part of precentral fissure.
Tongue, Larynx, Muscles of Mastication, /V/a /////.<.
Frontal opercular part.
Movements of Head and Eye. Medifrontal fissure,
adjacent to precentral fissure.
Owing to a decussation of the pyramidal (motor)
tracts in their course to the primary motor centres,
the motor centres in one cerebral hemisphere control
the movements of the opposite side of the body.
Sensor Areas. Tactile and temperature impressions.
Postcentral fissure, in corresponding order with the
neighboring precentral motor area; the postcentral
(sensor) and precentral (motor) fissures are so closely
associated in the highest category of the reflex arc
system represented in the cerebral cortex, that they
are included under the term of somesthetic or senso-
motor area devoted to the registration of cutaneous
impressions, impressions from the muscles, tendons,
and joints; in short, the sense of movement.
Stereognostic Sense Area (concrete perception of the
form and solidity of objects). Parietal fissure and
its extension in the precuneus on the inner aspect.
Auditory Area. Middle third of supertemporal, and
adjacent transtemporal fissures in the Sylvian cleft.
Visual Area. Calcarine fissure and cuneus as a whole.