PHYSIOLOGICAL ANATOMY.
THE NERVOUS SYSTEM.
Hitherto, we have only considered the anatomy and functions of the
organs employed in Digestion, Absorption, Circulation, Respiration,
Secretion and Excretion. We have found the vital process of nutrition to
be, in all its essential features, a result of physical and chemical
forces; in each instance we have presupposed the existence and activity
of the nerves. There is not an inch of bodily tissue into which their
delicate filaments do not penetrate, and form a multitude of conductors,
over which are sent the impulses of motion and sensation.
[Illustration: Fig. 54.
The Nervous System.]
Two elements, _nerve-fibers_ and _ganglionic corpuscles_, enter into the
composition of nervous tissue. Ordinary nerve-fibers in the living
subject, or when fresh, are cylindrical-shaped filaments of a clear, but
somewhat oily appearance. But soon after death the matter contained in
the fiber coagulates, and then the fiber is seen to consist of an
extremely delicate, structureless, outer membrane, which forms a tube
through the center of which runs the _axis-cylinder_. Interposed between
the axis-cylinder and this tube, there is a fluid, containing a
considerable quantity of fatty matter, from which is deposited a highly
refracting substance which lines the tube. There are two sets of
nerve-fibers, those which transmit sensory impulses, called _afferent_
or _sensory_ nerves, and those which transmit motor impulses, called
_efferent_ or _motor_ nerves. The fibers when collected in bundles are
termed nerve trunks. All the larger nerve-fibers lie side by side in the
nerve-trunks, and are bound together by delicate connective tissue,
enclosed in a sheath of the same material, termed the _neurilemma_. The
nerve-fibers in the trunks of the nerves remain perfectly distinct and
disconnected from one another, and seldom, or never, divide throughout
their entire length. However, where the nerves enter the nerve-centers,
and near their outer terminations, the nerve-fibres often divide into
branches, or at least gradually diminish in size, until, finally, the
axis-cylinder, and the sheath with its fluid contents, are no longer
distinguishable. The investing membrane is continuous from the origin to
the termination of the nerve-trunk.
[Illustration: Fig. 55.
Division of a
nerve, showing a
portion of a nervous
trunk (_a_)
and separation of
its filaments (_b, c, d, e_.)]
In the brain and spinal cord the nerve-fibers often terminate in minute
masses of a gray or ash-colored granular substance, termed _ganglia_, or
_ganglionic corpuscles_.
The ganglia are cellular corpuscles of irregular form, and possess
fibrous appendages, which serve to connect them with one another. These
ganglia form the cortical covering of the brain, and are also found in
the interior of the spinal cord. According to Kölliker, the larger of
these nerve-cells measure only 1/200 of an inch in diameter. The brain
is chiefly composed of nervous ganglia.
Nerves are classified with reference to their origin, as
_cerebral_--those originating in the brain, and _spinal_--those
originating in the spinal cord.
There are two sets of nerves and nerve-centers, which are intimately
connected, but which can be more conveniently studied apart. These are
the _cerebro-spinal_ system, consisting of the cerebro-spinal axis, and
the cerebral and spinal nerves; and the _sympathetic_ system, consisting
of the chain of sympathetic ganglia, the nerves which they give off, and
the nervous trunks which connect them with one another and with the
cerebro-spinal nerves.
THE CEREBRO-SPINAL SYSTEM.
THE CEREBRO-SPINAL AXIS consists of the brain and spinal cord. It lies
in the cavities of the cranium and the spinal column. These cavities are
lined with a very tough fibrous membrane, termed the _dura mater_, which
serves as the periosteum of the bones which enter into the formation of
these parts. The surface of the brain and spinal cord is closely
invested with an extremely vascular, areolar tissue, called the _pia
mater_. The numerous blood-vessels which supply these organs traverse
the pia mater for some distance, and, where they pass into the substance
of the brain or spinal cord, the fibrous tissue of this membrane
accompanies them to a greater or less depth. The inner surface of the
dura mater and the outer surface of the pia mater are covered with an
extremely thin, serous membrane, which is termed the _arachnoid_
membrane. Thus, one layer of the arachnoid envelopes the brain and
spinal cord, and the other lines the dura mater. As the layers become
continuous with each other at different points, the arachnoid, like the
pericardium, forms a shut sac, and, like other serous membranes, it
secretes a fluid, known as the _arachnoid fluid_. The space between the
internal and the external layers of the arachnoid membrane of the brain
is much smaller than that enclosed by the corresponding layers of the
arachnoid membrane of the spinal column.
[Illustration: Fig. 56.
Cross-section of spinal cord.]
THE SPINAL CORD is a column of soft, grayish-white substance, extending
from the top of the spinal canal, where it is continuous with the brain,
to about an inch below the small of the back, where it tapers off into a
filament. From this nerve are distributed fibers and filaments to the
muscles and integument of at least nine-tenths of the body.
The spinal cord is divided in front through the middle nearly as far as
its center, by a deep fissure, called the _anterior fissure_, and
behind, in a similar manner, by the posterior _fissure_. Each of these
fissures is lined with the pia mater, which also supports the
blood-vessels which supply the spinal cord with blood. Consequently, the
substance of the two halves of the cord is only connected by a narrow
isthmus, or bridge, perforated by a minute tube, which is termed the
_central canal_ of the spinal cord.
Each half of the spinal cord is divided lengthwise into three nearly
equal parts, which are termed the anterior, lateral, and posterior
columns, by the lines which join together two parallel series of bundles
of nervous filaments, which compose the roots of the spinal nerves. The
roots of those nerves, which are found along that line nearest the
posterior surface of the cord, are termed the posterior roots; those
which spring from the other line are known as the anterior roots.
Several of these anterior and posterior roots, situated at about the
same height on opposite sides of the spinal cord, converge and combine
into what are called the _anterior_ and _posterior bundles_; then two
bundles, anterior and posterior, unite and form the trunk of a spinal
nerve.
The nerve trunks make their way out of the spinal canal through
apertures between the vertebra, called the _inter-vertebral foramina_
and then divide into numerous branches, their ramifications extending
principally to the muscles and the skin. There are thirty-one pairs of
spinal nerves, eight of which are termed cervical, twelve dorsal, five
lumbar, and six sacral, with reference to that part of the cord from
which they originate.
When the cord is divided into transverse sections, it is found that each
half is composed of two kinds of matter, a white substance on the
outside, and a grayish substance in the interior. The _gray matter_, as
it is termed, lies in the form of an irregular crescent, with one end
considerably larger than the other, and having the concave side turned
outwards. The ends of the crescent are termed the _horns_, or _cornua_,
the one pointing forward being called the _anterior cornu_, the other
one the _posterior cornu_. The convex sides of these cornua approach
each other and are united by the bridge, which contains the central
canal.
There is a marked difference in the structure of the gray and the white
matter. The white matter is composed entirely of nerve fibers, held
together by a framework of connective tissue. The gray matter contains a
great number of ganglionic corpuscles, or nerve-cells, in addition to
the nerve-fibers.
When the nerve-trunks are irritated in any manner, whether by pinching,
burning, or the application of electricity, all the muscles which are
supplied with branches from this nerve-trunk immediately contract, and
pain is experienced, the severity of which depends upon the degree of
the irritation; and the pain is attributed to that portion of the body
to which the filaments of the nerve-trunk are distributed. Thus, persons
who have lost limbs often complain in cold weather of an uneasiness or
pain, which they locate in the fingers or toes of the limb which has
been amputated, and which is caused by the cold producing an irritation
of the nerve-trunk, the filaments, or fibers of which, supplied the
fingers or toes of the lost member.
On the other hand, if the anterior bundle of nerve-fibers given off from
the spinal cord is irritated in precisely the same way, only half of
these effects is produced. All the muscles which are supplied with
fibers from that trunk contract, but no pain is experienced. Conversely,
if the posterior bundle of nerve-fibers is irritated, none of the
muscles to which the filaments of the nerve are distributed contract,
but pain is felt throughout the entire region to which these filaments
are extended. It is evident, from these facts, that the fibers composing
the posterior bundles of nerve-roots only transmit sensory impulses, and
the filaments composing the anterior nerve-roots only transmit motor
impulses; accordingly, they are termed respectively the _sensory_ and
the _motor_ nerve-roots. This is illustrated by the fact that when the
posterior root of a spinal nerve is divided, all sensation in the parts
to which the filaments of that nerve are distributed is lost, but the
power of voluntary movement of the muscles remains. On the other hand,
if the anterior roots are severed, the power of voluntary motion of the
muscles is lost, but sensation remains.
It appears from these experiments, that, when a nerve is irritated, a
change in the arrangement of its molecules takes place, which is
transmitted along the nerve-fibers. But, if the nerve-trunks are
divided, or compressed tightly at any point between the portion
irritated, and the muscle or nerve-centre, the effect ceases
immediately, in a manner similar to that in which a message is stopped
by the cutting of a telegraph wire. When the nerves distributed to a
limb are subjected to a pressure sufficient to destroy the molecular
continuity of their filaments, it "goes to sleep," as we term it. The
power of transmitting sensory and motor impulses is lost, and only
returns gradually, as the molecular continuity is restored.
From what has been said, it is plain that a sensory nerve is one which
conveys a sensory impulse from the peripheral or outer part of a nerve
to the spinal cord or brain, and which is, therefore, termed _afferent_;
and that a motor nerve is one which transmits an impulse from the nerve
centre, or is _efferent_. So difference in structure, or in chemical or
physical composition, can be discerned between the afferent and the
_efferent_ nerves. A certain period of time is required for the
transmission of all impulses. The speed with which an impulse travels
has been found to be comparatively slow, being even less than that of
sound, which is 1,120 feet per second.
The experiments heretofore related have been confined solely to the
nerves. We may now proceed to the consideration of what takes place when
the spinal cord is operated upon in a similar way. If the cord be
divided with a knife or other instrument, all parts of the body supplied
with nerves given off below the division will become paralyzed and
insensible, while all parts of the body supplied with nerves from the
spinal cord _above_ the division will retain their sensibility and power
of motion. If, however, only the posterior half of the spinal cord is
divided, or destroyed, there is loss of sensation alone; and, if the
anterior portion is cut in two, and the continuity of the posterior part
is left undisturbed, there is loss of voluntary motion of the lower
limbs, but sensation remains.
REFLEX ACTION OF THE SPINAL CORD. In relation to the brain, the spinal
cord is a great mixed motor and sensory nerve, but, in addition to this,
it is also a distinct nervous centre, in which originate and terminate
all those involuntary impulses which exert so potent an influence in the
preservation and economy of the body. That peculiar power of the cord by
which it is enabled to convert sensory into motor impulse is that which
distinguishes it, as a central organ, from a nerve, and is called
_reflex action_.
The gray matter, and not the white, is the part of the cord which
possesses this power. This reflex action is a special function of the
spinal cord, and serves as a monitor to, and regulator of the organs of
nutrition and circulation, by placing them, ordinarily, beyond the
control of conscious volition.
[Illustration: Fig. 57.]
If the foot of a decapitated frog is irritated, there is an instant
contraction of the corresponding limb; if the irritation is intense the
other limb also contracts. These motions indicate the existence, in some
part of the spinal cord, of a distinct nerve-centre, capable of
converting and reflecting impulses. It has been found by experiment,
that the same movements will take place if the irritation be applied to
any portion of the body to which the spinal nerves are distributed, thus
giving undoubted evidence that the spinal cord in its entirety is
capable of causing these reflections. Fig. 57 represents the course of
the nervous impulses. The sensory impulse passes upward along the
posterior root, _a_, until it reaches the imbedded gray matter, _b_, of
the cord, by which it is reflected, as a motor impulse, downward along
the anterior root, _c_, to the muscles whence the sensation was
received. This is the reflex action of the spinal cord. There is no
consciousness or sensation connected with this action, and the removal
of the brain and the sympathetic system does not diminish its activity.
Even after death it continues for some time, longer in cold-blooded than
in warm-blooded animals, on account of the difference in temperature,
thus showing this property of the spinal cord. By disease, or the use of
certain poisons, this activity may be greatly augmented, as is
frequently observed in the human subject. A sudden contact with a
different atmosphere may induce these movements. The contraction of the
muscles, or cramp, often experienced by all persons, in stepping into a
cold bath, or emerging from the cozy sitting-room into a chilly December
temperature, are familiar illustrations of reflex movements. It has been
demonstrated that the irritability of the nerves may be impaired or
destroyed, while that of the muscles to which they are distributed
remains unchanged; and that the motor and sensory classes of filaments
may be paralyzed independently of each other.
The reflex actions of the spinal cord have been admirably summed up by
Dr. Dalton, as exerting a general, protective influence over the body,
presiding over the involuntary action of the limbs and trunk, regulating
the action of the sphincters, rectum, and bladder, and, at the same
time, exercising an indirect influence upon the nutritive changes in all
parts of the body to which the spinal filaments are distributed.
THE BRAIN. The brain is a complex organ, which is divided into the
_medulla oblongata_, the _cerebellum_, and the _cerebrum_.
The _medulla oblongata_ is situated just above the spinal cord, and is
continuous with it below, and the brain above. It has distinct functions
which are employed in the preservation and continuance of life. It has
been termed the "vital knot," owing to the fact that the brain may be
removed and the cord injured and still the heart and lungs will continue
to perform their functions, until the medulla oblongata is destroyed.
The arrangement of the white and gray matter of the medulla oblongata is
similar to that of the spinal cord; that is to say, the white matter is
external and the gray internal; whereas in the cerebellum and cerebrum
this order is reversed. The fibres of the spinal cord, before entering
this portion of the brain, decussate, those from the right side crossing
to the left, and those from the left crossing to the right side. By some
authors this crossing of the sensory and motor filaments has been
supposed to take place near the medulla oblongata. Dr. Brown-Sequard
shows, however, that it takes place at every part of the spinal cord.
The medulla oblongata is traversed by a longitudinal fissure, continuous
with that of the spinal cord. Each of the lateral columns thus formed
are subdivided into sections, termed respectively the _Corpora
Pyramidalia_, the _Corpora Olivaria_, the _Corpora Restiformia_ and the
_Posterior Pyramids_.
The _Corpora Pyramidalia_ (see 1, 1, Fig. 58) are two small medullary
eminences or cords, situated at the posterior surface of the medulla
oblongata; approaching the Pons Varolii these become larger and rounded.
The _Corpora Olivaria_ (3, 3, Fig. 58) are two elliptical prominences,
placed exterior to the corpora pyramidalia. By some physiologists these
bodies are considered as the nuclei, or vital points, of the medulla
oblongata. Being closely connected with the nerves of special sensation,
Dr. Solly supposed that they presided over the movements of the larynx.
[Illustration: Fig. 58.]
[Illustration: Fig. 59.]
The _Corpora Restiformia_ (5, 5, Fig. 59) are lateral and posterior
rounded projections of whitish medulla, which pass upward to the
cerebellum and form the _crura cerebelli_, so called because they
resemble a leg. The filaments of the pneumogastric nerve originate in
the ganglia of these parts.
The _Posterior Pyramids_ are much smaller than the other columns of the
medulla oblongata. They are situated (4, 4, Fig. 59) upon the margin of
the posterior fissures in contact with each other.
The functions of the medulla oblongata, which begin with the earliest
manifestations of life, are of an instinctive character. If the
cerebellum and cerebrum of a dove be removed, the bird will make no
effort to procure food, but if a crumb of bread be placed in its bill,
it is swallowed naturally and without any special effort. So also in
respiration the lungs continue to act after the intercostal muscles are
paralyzed; if the diaphragm loses its power, suffocation is the result,
but there is still a convulsive movement of the lungs for sometime,
indicating the continued action of the medulla oblongata.
The _Cerebellum_, or little brain, is situated in the posterior chamber
of the skull, beneath the _tentorium_, a tent-like process of the dura
mater which separates it from the cerebrum. It is convex, with a
transverse diameter of between three and one-half and four inches, and
is little more than two inches in thickness. It is divided on its upper
and lower surfaces into two lateral hemispheres, by the superior and
inferior vermiform processes, and behind by deep notches. The cerebellum
is composed of gray and white matter, the former being darker than that
of the cerebrum. From the beautiful arrangement of tissue, this organ
has been termed the _arbor vitae_.
The _peduncles of the cerebellum_, the means by which it communicates
with the other portions of the brain, are divided into three pairs,
designated as the _superior_, _middle_ and _inferior_. The first pass
upward and forward until they are blended with the tubercles of the
_corpora quadrigemina_. The second are the _crura cerebelli_, which
unite in two large _fasciculi_, or pyramids, and are finally lost in the
_pons varolii_. The inferior peduncles are the corpora restiformia,
previously described, and consist of both sensory and motor filaments.
Some physiologists suppose that the cerebellum is the source of that
harmony or associative power which co-ordinates all voluntary movements,
and effects that delicate adjustment of cause to effect, displayed in
muscular action. This fact may be proved by removing the cerebellum of a
bird and observing the results, which are an uncertainty in all its
movements, and difficulty in standing, walking, or flying, the bird
being unable to direct its course. In the animal kingdom we find an
apparent correspondence between the size of the cerebellum and the
variety and extent of the movements of the animal. Instances are cited,
however, in which no such proportion exists, and so the matter is open
to controversy. The general function of the cerebellum, therefore,
cannot be explained, but the latest experiments in physiological and
anatomical science seem to favor the theory that it is in some way
connected with the harmony of the movements. This co-ordination, by
which the adjustment of voluntary motion is supposed to be effected, is
not in reality a _faculty_ having its seat in the brain substance, but
is the harmonious action of many forces through the cerebellum.
The _Cerebrum_ occupies five times the space of all the other portions
of the brain together. It is of an ovoid form, and becomes larger as it
approaches the posterior region of the skull. A longitudinal fissure
covered by the dura mater separates the cerebrum into two hemispheres,
which are connected at the base of the fissure, by a broad medullary
band, termed the _corpus callosum_. Each hemisphere is subdivided into
three lobes. The anterior gives form to the forehead, the middle rests
in the cavity at the base of the skull, and the posterior lobe is
supported by the tentorium, by which it is separated from the cerebellum
beneath. One of the most prominent characteristics of the cerebrum is
its many and varied _convolutions_ These do not correspond in all
brains, nor even on the opposite sides of the same brain, yet there are
certain features of similarity in all; accordingly, anatomists enumerate
four _orders of convolutions_. The first order begins at the _substantia
perforata_ and passes upward and around the corpus callosum toward the
posterior margin of that body, thence descends to the base of the brain,
and terminates near its origin. The second order originates from the
first, and subdivides into two convolutions, one of which composes the
exterior margin and superior part of the corresponding hemisphere, while
the other forms the circumference of the _fissure of Sylvius_. The third
order, from six to eight in number, is found in the interior portion of
the brain, and inosculates between the first and second orders. The
fourth is found on the outer surface of the hemisphere, in the space
between the sub-orders of the second clasp. A peculiar fact relating to
these convolutions is observed by all anatomists: mental development is
always accompanied by an increasing dissimilarity between their
proportional size.
The cerebral hemispheres may be injured or lacerated without any pain to
the patient. The effect seems to be one of stupefaction without
sensation or volition. A well-developed brain is a very good indication
of intelligence and mental activity. That the cerebrum is the seat of
the reasoning powers, and all the higher intellectual functions, is
proved by three facts. (1.) If this portion of the brain is removed, it
is followed by the loss of intelligence. (2.) If the human cerebrum is
injured, there is an impairment of the intellectual powers. (3.) In the
animal kingdom, as a rule, intelligence corresponds to the size of the
cerebrum. This general law of development is modified by differences in
the cerebral texture. Men possessing comparatively small brains may have
a vast range of thought and acute reasoning powers. Anatomists have
found these peculiarities to depend upon the quantity of gray matter
which enters into the composition of the brain.
In the cerebro-spinal system there are three different kinds of reflex
actions. (1.) Those of the spinal cord and medulla oblongata are
performed without any consciousness or sensation on the part of the
subject. (2.) The second class embraces those of the tuber annulare,
where the perception gives rise to motion without the interference of
the intellectual faculties. These are denominated purely _instinctive_
reflex actions, and include all those operations of animals which seem
to display intelligent forethought; thus, the beaver builds his
habitation over the water, but not a single apartment is different from
the beaver homestead of a thousand years ago; there is no improvement,
no retrogression. Trains of thought have been termed a third class of
reflex actions. It is evident that the power of reasoning is, in a
degree, possessed by some of the lower-animals: for instance, a tribe of
monkeys on a foraging expedition will station guards at different parts
of the field, to warn the plunderers of the approach of danger. A cry
from the sentinel, and general confusion is followed by retreat. Reason
only attains its highest development in man, in whom it passes the
bounds of ordinary existence, and, with the magic wand of love, reaches
outward into the vast unknown, lifting him above corporeal being, into
an atmosphere of spiritual and divine Truth.
[Illustration: Fig. 60.
Section of the brain and an ideal
view of the pneumogastric nerve
on one side, with its branches, _a_.
Vertical section of the cerebrum.
_b_. Section of the cerebellum, _c_.
Corpus callosum. _d_. Lower section
of medulla oblongata. Above
_d_, origin of the pneumogastric
nerve. 1. Pharyngeal branch. 2.
Superior laryngeal. 5. Branches
to the lungs. 4. Branches to the
liver. 6. Branches to the stomach.]
THE CRANIAL NERVES. From the brain, nerves are given off in pairs, which
succeed one another from in front backwards to the number of twelve. The
_first_ pair, the _olfactory_ nerves, are the nerves of the sense of
smell. The _second_ pair are the _optic_, or the nerves of the sense of
sight. The _third_ pair are called the _motores oculi_, the movers of
the eye, from the fact that they are distributed to all the muscles of
the eye with the exception of two. The _fourth_ pair and the _sixth_
pair each supply one of the muscles of the eye, on each side, the fourth
extending to the superior oblique muscle, and the sixth to the external
rectus muscle. The nerves of the _fifth_ pair are very large; they are
each composed of two bundles of filaments, one motor and the other
sensory, and have, besides, an additional resemblance to a spinal nerve
by having a ganglion on each of their sensory roots, and, from the fact
that they have three chief divisions, are often called the _trigeminal_,
or _trifacial_, nerves. They are nerves of special sense, of sensation,
and of motion. They are the sensitive nerves which supply the cranium
and face, the motor nerves of the muscles of mastication, the
_buccinator_ and the _masseter_, and their third branches, often called
the _gustatory_, are distributed to the front portion of the tongue, and
are two of the nerves of the special sense of taste. The _seventh_ pair,
called also the _facial_ nerves, are the motor nerves of the muscles of
the face, and are also distributed to a few other muscles; the _eighth_
pair, termed the auditory nerves, are the nerves of the special sense of
hearing. As the _seventh_ and _eighth_ pairs of nerves emerge from the
cavity of the skull together, they are frequently classed by anatomists
as one, divided into the _facial_, or _portio dura_, as it is sometimes
called, and the _auditory_, or _portio mollis_. The _ninth_ pair, called
the _glosso-pharyngeal,_ are mixed nerves, supplying motor filaments to
the _pharyngeal muscles_ and filaments of the special sense of taste to
the back portion of the tongue. The _tenth_ pair, called the
_pneumogastric_, or _par vagum_, are very important nerves, and are
distributed to the larynx, the lungs, the heart, the stomach, and the
liver, as shown in Fig. 60. This pair and the next are the only cerebral
nerves which are distributed to parts of the body distant from the head.
The _eleventh_ pair, also called _spinal accessory_, arise from the
sides of the spinal marrow, between the anterior and posterior roots of
the dorsal nerves, and run up to the medulla oblongata, and leave the
cranium by the same aperture as the pneumogastric and glosso-pharyngeal
nerves. They supply certain muscles of the neck, and are purely motor.
As the glosso-pharyngeal, pneumogastric, and spinal accessory nerves
leave the cranium together, they are by some anatomists counted as the
_eighth_ pair. The _twelfth_ pair, known as the _hypoglossal,_ are
distributed to the tongue, and are the motor nerves of that organ.
THE GREAT SYMPATHETIC.
A double chain of nervous ganglia extends from the superior to the
inferior parts of the body, at the sides and in front of the spinal
column, and is termed, collectively, the system of the _great
sympathetic_. These ganglia are intimately connected by nervous
filaments, and communicate with the cerebro-spinal system by means of
the motor and sensory filaments which penetrate the sympathetic. The
nerves of this system are distributed to those organs over which
conscious volition has no direct control.
[Illustration: Fig. 61.
Course and distribution of the great Sympathetic Nerve]
Four of the sympathetic centers, situated in the front and lower
portions of the head, are designated as the _ophthalmic,
spheno-palatine, submaxillary_ and _otic ganglia_. The first of these,
as its name indicates, is distributed to the eye, penetrates the
_sclerotic membrane_ (the white, opaque portion of the eyeball, with its
transparent covering), and influences the contraction and dilation of
the iris. The second division is situated in the angle formed by the
sphenoid and maxillary bone, or just below the ear. It sends motor and
sensory filaments to the palate, and _velum palati_. Its filaments
penetrate the carotid plexus, are joined by others from the motor roots
of the facial nerve and the sensory fibres of the superior maxillary.
The third division is located on the submaxillary gland. Its filaments
are distributed to the sides of the tongue, the sublingual, and
submaxillary glands. The otic ganglion is placed below the base of the
skull, and also connects with the _carotid plexus_. Its filaments of
distribution supply the internal muscles of the _malleus_, the largest
bones of the _tympanum_, the membranous linings of the tympanum and the
_eustachian tube._ Three ganglia, usually designated as the _superior,
middle_, and _inferior_, connect with the cervical and spinal nerves.
Their interlacing filaments are distributed to the muscular walls of the
larynx, pharynx, trachea, and esophagus, and also penetrate the _thyroid
gland_. The use of this gland is not accurately known. It is composed of
a soft, brown tissue, and consists of lobules contained in lobes of
larger size. It forms a spongy covering for the greater portion of the
larynx, and the first section of the trachea. That it is an important
organ, is evident from the fact that it receives four large arteries,
and filaments from two pairs of nerves.
The sympathetic ganglia of the chest correspond in number with the
terminations of the ribs, over which they are situated. Each ganglion
receives two filaments from the intercostal nerve, situated above it,
thus forming a double connection. The thoracic ganglia supply with motor
fibres that portion of the aorta which is above the diaphragm, the
esophagus, and the lungs.
In the abdomen the sympathetic centers are situated upon the _coeliac_
artery, and are termed, collectively, the _semilunar coeliac ganglion_.
Numerous inosculating branches radiate from this center and are called,
from the method of their distribution, the _solar plexus_. From this,
also, originate other plexi which are distributed to the stomach, liver,
kidneys, intestines, spleen, pancreas, supra-renal glands, and to the
organs of generation. Four other pairs of abdominal ganglia connected
with, the lumbar branches are united by filaments to form the semilunar
ganglion.
The sympathetic ganglia of the pelvis consist of five pairs, which are
situated upon the surface of the sacrum. At the extremity of the spinal
column this system terminates in a single knot, designated as the
_ganglion impar_.
Owing to the position of the sympathetic ganglia, deeply imbedded in the
tissues of the chest and abdomen, it is exceedingly difficult to subject
them to any satisfactory experiments. A few isolated facts form the
basis of all our knowledge concerning their functions. They give off
both motor and sensory filaments. The contraction of the _iris_ is one
of the most familiar examples of the action of the sympathetic system.
In the reflex actions of the nerves of special sense, the sensation is
transmitted through the cerebro-spinal system, and the motor impulse is
sent to the deep-seated muscles by the sympathetic system. Physiologists
enumerate three kinds of reflex actions, which are either purely
sympathetic, or partially influenced by the cerebro-spinal system. Dr.
Dalton describes them as follows:
_First_.--"Reflex actions taking place from the internal organs, through
the sympathetic and cerebro-spinal systems, to the voluntary muscles and
sensitive surfaces.--The convulsions of young children are often owing
to the irritation of undigested food in the intestinal canal. Attacks of
indigestion are also known to produce temporary amaurosis [blindness],
double vision, strabismus, and even hemiplegia. Nausea, and a diminished
or capricious appetite, are often prominent symptoms of early pregnancy,
induced by the peculiar condition of the uterine mucous membrane."
_Second_.--"Reflex actions taking place from the sensitive surfaces,
through the cerebro-spinal and sympathetic systems to the involuntary
muscles and secreting organs.--Imprudent exposure of the integument to
cold and wet, will often bring on a diarrhea. Mental and moral
impressions, conveyed through the special senses, will affect the
motions of the heart, and disturb the processes of digestion and
secretion. Terror, or an absorbing interest of any kind, will produce a
dilatation of the pupil, and communicate in this way a peculiarly wild
and unusual expression to the eye. Disagreeable sights or odors, or even
unpleasant occurrences, are capable of hastening or arresting the
menstrual discharge, or of inducing premature delivery."
_Third_.--"Reflex actions taking place through the sympathetic system
from one part of the body to another.--The contact of food with the
mucous membrane of the small intestine excites a peristaltic movement in
the muscular coat. The mutual action of the digestive, urinary, and
internal generative organs upon each other takes place entirely through
the medium of the sympathetic ganglia and their nerves. The variation of
the capillary circulation in different abdominal viscera, corresponding
with the state of activity or repose of their associated organs, are to
be referred to a similar nervous influence. These phenomena are not
accompanied by any consciousness on the part of the individual, nor by
any apparent intervention of the cerebro-spinal system."
* * * * *