THE NERVOUS SYSTEM.
725. In the preceding chapters, we have seen how various and complex
are many of the motions necessary to maintain the life of an animal
whose organization is superior to all others. We have noted the
wonderful mechanism of the muscular system, in producing the varied
movements of the body, the different processes by which the food is
converted into chyle and mixed with the blood, and the circulation of
this fluid to every organ and tissue of the system, that each may
select from it the very principles which it requires for its growth.
726. Lymphatic absorption commences as soon as nutrition is completed,
and conveys the useless, worn-out particles of different tissues back
into the circulating fluid; while the respiratory organs and secretory
glands perform the work of preparing the waste products to be
eliminated from the body. Each of these processes effects a single
object, and is performed in a regular manner.
727. "They must succeed each other in proper order in propelling every
particle to its proper destination, or life would be sacrificed almost
at the moment of its commencement. There is, therefore, a mutual
dependence of all portions of the machinery of organic life upon each
other, and a necessity for some medium of communication from one organ
to another, by which they may convey mutual information of their
several conditions, if we may be permitted to employ a figurative
expression. Were there no such medium, how would the stomach notify
the heart that additional exertion on its part is required, because
the stomach is busy in digesting food?
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725. What has been noted in the preceding chapters? 726. Show the
manner in which the several processes are performed. 727. How must
they succeed each other?
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728. "When we are exerting the muscular system for a long time in some
laborious employment, how else are our members to inform the stomach
that they are too much occupied with their duties to spare the blood
necessary in digestion; that it is requisite that the appetite
should decline; and that digestion should cease for the time, even if
the stomach should be oppressed with its contents? When we are
thinking, how else are the blood-vessels to be told that an unusual
supply of their contents is wanting in the head? or when the whole
frame is weary with exertion, how, without some regular line of
intelligence between all the organs, is the brain to be instructed
that circumstances require that it should go to sleep? To supply the
necessary medium of communication, Providence has furnished all the
animals that possess distinct organs, with a peculiar apparatus
called the _Nervous System_."
ANATOMY OF THE NERVOUS SYSTEM.
729. The NERVOUS SYSTEM consists of the _Cer'e-bro-spi'nal Cen'tre_,
and of numerous rounded and flattened white cords, called _nerves_,
which are connected at one extremity with the cerebro-spinal centre,
and at the other, distributed to all the textures of the body. The
sympathetic nerve is an exception to this description; for, instead of
one, it has many small centres, which are called _gan'gli-a_, and
which communicate very freely with the cerebro-spinal centre, and with
its nerves.
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728. What is the medium of communication from one organ to another?
729-754. _Give the anatomy of the brain and cranial nerves._ 729. Of
what does the nervous system consist? What constitutes an exception to
this?
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730. The CEREBRO-SPINAL CENTRE consists of two portions: The _brain_,
and the _spinal cord_. For convenience of description, the nervous
system may be divided into the _Brain_, _Cranial Nerves_, _Spinal
Cord_, _Spinal Nerves_, and the _Sympathetic Nerve_.
731. The term BRAIN designates those parts of the nervous system,
exclusive of the nerves themselves, which are contained within the
cranium, or skull-bones; they are the _Cer'e-brum_, _Cer-e-bel'lum_,
and _Me-dul'la Ob-lon-ga'ta_. These are invested and protected by the
membranes of the brain, which are called the _Du'ra Ma'ter_,
_A-rach'noid_, and _Pi'a Ma'ter_.
[Illustration: Fig. 120. 1, 1, The scalp turned down. 2, 2, 2, The cut
edge of the bones of the skull. 3, The external strong membrane of the
brain (dura mater,) suspended by a hook. 4, The left hemisphere of the
brain, showing its convolutions.]
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730. Of what does the cerebro-spinal centre consist? How is the
nervous system divided? 731. What does the term brain designate? Name
them. How are they protected? Describe fig. 120.
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732. The CEREBRUM IS divided into two hemispheres, by a cleft, or
fissure. Into this cleft dips a portion of the dura mater, called the
_falx cer'e-bri_, from its resembling a sickle. The apparent design of
this membrane is to relieve the one side from the pressure of the
other, when the head is reclining to either side. Upon the superior
surface of the cerebrum are seen undulating windings, called
_con-vo-lu'tions_. Upon its inferior, or lower surface, each
hemisphere admits of a division into three lobes--the anterior,
middle, and posterior. (Fig. 122, 123)
[Illustration: Fig. 121 A section of the skull-bones and cerebrum. 1, 1,
The skull. 2, 2, the dura mater 3, 3, The cineritious portion of the
cerebrum. 4, 4, The medullary portion. The dark points indicate the
position of divided blood-vessels. 5, 5, The lateral ventricles.]
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732. How is the cerebrum divided? What is the use of the falx cerebri?
What is seen upon the superior surface of the brain? Its inferior?
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733. When the upper part of the hemispheres is removed horizontally
with a sharp knife, a centre of white substance is brought to view.
This is surrounded by a border of gray, which follows the depressions
of the convolutions, and presents a zigzag outline. The divided
surface will be seen studded with numerous small red points, which are
produced by the escape of blood from the division of the minute
arteries and veins. The gray border is called the cortical, or
_cineritious_ portion, while the white central portion is called the
_medullary_. The two hemispheres are connected by a dense layer of
transverse fibres, called _cor'pus cal-lo'sum_.
734. In the interior of the brain there are several cavities, two of
which are of considerable size, and are called the lateral ventricles.
They extend from the anterior to the posterior part of the brain, and
wind their way into other parts of the cerebral organ.
_Observation._ In the disease called "dropsy of the brain,"
(hydrocephalus internus,) the serum, or water, is usually deposited in
these ventricles. This is effused from the many small blood-vessels of
the membrane in these cavities.
735. The brain is of a pulpy character, quite soft in infancy and
childhood; but it gradually becomes more and more consistent, and in
middle age it assumes the form of determinate structure and
arrangement. It is more abundantly supplied with blood than any organ
of the system. No lymphatics have been detected, but it is to be
presumed that they exist in this organ.
736. The CEREBELLUM is about seven times smaller than the cerebrum.
Like that organ, it is composed of white and gray matter, but the
gray constitutes the larger portion. Its surface is formed of parallel
plates separated by fissures. The white matter is so arranged, that
when cut vertically, the appearance of the trunk and branches of a
tree (_ar'bor vi'tae_) is presented. It is situated under the posterior
lobe of the cerebrum, from which it is separated by a process of the
dura mater, called the _ten-to'ri-um_.
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733. Describe the appearance of the brain when a horizontal section
has been made. What is the gray border often called? What connects the
hemispheres? 734. Describe the ventricles of the brain. In the disease
called "dropsy of the brain," where is the water deposited? 735. What
is the character of the brain in childhood? In adults? 736. How does
the cerebellum compare in size with the cerebrum?
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[Illustration: Fig. 122. The under surface, or base, of the brain and
origin of the cranial nerves. 1, 1, The anterior lobes of the cerebrum.
2, 2, The middle lobes. 3, 3, The posterior lobes, almost concealed by
the cerebellum. 4, 4, The cerebellum. 7, 7, The longitudinal fissure that
divides the brain into two hemispheres. 8, The first pair of nerves. 9,
9, The second pair of nerves. 10, The decussation, or crossing, of its
fibres. 13, 13, The third pair of nerves. 14, The pons varolii. 15, 15,
The fourth pair of nerves. 16, 16, The fifth pair of nerves. 17, The
sixth pair of nerves. 18, 18, The seventh and eighth pair of nerves. 19,
The medulla oblongata, with the crossing of some of its fibres exhibited.
20, The ninth pair of nerves. 21, The tenth pair of nerves, 22, The
eleventh pair of nerves. 23, The twelfth pair of nerves.]
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Describe this portion of the brain. Explain fig. 122.
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737. The MEDULLA OBLONGATA, or that portion of the spinal cord which
is within the skull, consists of three pairs of bodies, (_cor'pus
py-ram-i-da'le_, _res-ti-for'me_, and _ol-i-va're_,) united in a
single bulb.
[Illustration: Fig. 123. The base of the skull and the openings through
which the cranial nerves pass. 1, 1, The first pair of nerves. 2, 2, The
cribriform plate of the ethmoid bone through which this nerve passes. 3,
3, The second pair of nerves. 4, 4, The optic foramen in the sphenoid
bone; through which passes the second pair of nerves. 5, 5, The
sphenoidal fissure. 6, 6, The third pair of nerves. 7, 7, The fifth pair
of nerves. 8, 8, The ophthalmic branch of the fifth nerve. The third, the
ophthalmic branch of the fifth and the sixth nerve pass from the brain
through the sphenoidal fissure to the eye. 9, 9, The superior maxillary
branch of the fifth nerve. 10, 10, The foramen rotundum, (round opening,)
through which the nerve 9, 9, passes to the upper jaw. 11, 11, The
inferior maxillary branch of the fifth pair. 12, 12, The foramen ovale,
(oval opening,) through which the nerve 11, 11, passes to the lower jaw.
13, 13, The sixth pair of nerves. 14, 14, The seventh and eighth pair of
nerves. 15, 15, The opening in the temporal bone, through which the
seventh and eighth nerves pass to the face and ear. 16, 16, The ninth
pair of nerves. 17, The tenth pair of nerves. 18, 18, The eleventh pair
of nerves. 19, 19, The foramen lacerum (rough opening.) The ninth, tenth,
and eleventh nerves pass from the brain through this opening. 20, The
spinal cord. 21, The foramen spinalis, through which the spinal cord
passes. 22, 22, The position of the anterior lobe of the brain. 23, 23,
The middle lobe. 24, 24, The posterior lobe. 25, 25, A section of the
skull-bones.]
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737. Describe the medulla oblongata. Explain fig. 123.
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738. The DURA MATER is a firm, fibrous membrane, which is exposed on
the removal of a section of the skull-bones. This lines the interior
of the skull and spinal column, and likewise sends processes inward,
for the support and protection of the different parts of the brain. It
also sends processes externally, which form the sheaths for the
nerves, as they quit the skull and spinal column. The dura mater is
supplied with arteries and nerves.
[Illustration: Fig. 124. A vertical section of the cerebrum, cerebellum,
and medulla oblongata, showing the relation of the cranial nerves at
their origin. 1, The cerebrum. 2, The cerebellum, with its arbor vitae
represented. 3, The medulla oblongata. 4, The spinal cord. 5, The corpus
callosum. 6, The first pair of nerves. 7, The second pair. 8, The eye. 9,
The third pair of nerves. 10, The fourth pair. 11, The fifth pair. 12,
The sixth pair. 13, The seventh pair. 14, The eighth pair. 15, The ninth
pair. 16, The tenth pair. 10, The eleventh pair. 18, The twelfth pair.
20, Spinal nerves. 21, The tentorium.]
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738. Describe the dura mater. What is its use? Explain fig. 124.
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739. The ARACHNOID, so called from its extreme tenuity, is the serous
membrane of the brain and spinal cord, and is, like other serous
membranes, a closed sac. It envelops these organs, and is reflected
upon the inner surface of the dura mater, giving to that membrane its
serous investment.
740. The PIA MATER is a vascular membrane, composed of innumerable
vessels, held together by cellular membrane. It invests the whole
surface of the brain, and dips into its convolutions. The pia mater is
the nutrient membrane of the brain, and receives its blood from the
carotid and vertebral arteries. Its nerves are minute branches of the
sympathetic, which accompany the branches of the arteries.
741. The CRANIAL NERVES, that connect with the brain, are arranged in
twelve pairs. They are called: 1st. The _Ol-fact'o-ry_. 2d. The
_Op'tic_. 3d. The _Mo-to'res Oc-u-lo'rum_. 4th. The _Pa-thet'i-cus_.
5th. The _Tri-fa'cial_. 6th. The _Ab-du-cen'tes_. 7th. The _Por'ti-o
Du'ra_. 8th. The _Por'ti-o Mol'lis_. 9th. The _Glos'so-pha-ryn'gi-al_.
10th. The _Pneu-mo-gas'tric_. 11th. The _Spi'nal Ac'ces-so-ry_. 12th.
The _Hy'po-glos'sal_.
742. The OLFACTORY NERVE (first pair) passes from the cavity of the
skull through many small openings in a plate of the _eth'moid_ bone.
(This plate is called _crib'ri-form_, from its resemblance to a
sieve.) This nerve ramifies upon the membrane that lines the nasal
passages. It is the softest nerve of the body. (Fig. 136.)
743. The OPTIC NERVE (second pair) passes from the interior of the
cranium, through an opening in the base of the skull, (_fo-ra'men
op'ti-cum_,) to the cavity for the eye. It pierces the coats of the
eye, and expands in the retina.
744. The MOTORES OCULORUM (third pair) pass from the brain, through
an opening of the _sphe'noid_ bone, (_sphe-noid'al fis'sure_,) to the
muscles of the eye.
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739. Describe the arachnoid membrane. 740. What is said respecting the
pia mater? 741. How many pairs of cranial nerves? Name them. 742.
Describe the olfactory nerve. 743. The optic nerve. 744. Describe the
motores oculorum.
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745. The PATHETICUS (fourth pair) passes from the brain, through the
sphenoidal fissure, to the superior oblique muscle of the eye.
[Illustration: Fig. 125. The distribution of the third, fourth, and sixth
pairs of nerves, to the muscles of the eye. 1, The ball of the eye and
rectus externus muscle. 2, The upper jaw. 3, The third pair, distributed
to all the muscles of the eye, except the superior oblique, and external
rectus. 4, The fourth pair passes to the superior oblique muscle. 6, The
sixth pair, is distributed to the external rectus muscle.]
746. The TRIFACIAL NERVE (fifth pair) is analogous to the spinal
nerves in its origin by two roots, from the anterior and posterior
columns of the spinal cord. It has a ganglion, like the spinal nerves
upon its posterior root. For these reasons, it ranges with the spinal
nerves, and is considered the cranial spinal nerve. This nerve divides
into three branches:--The _oph-thal'mic_, superior _max'il-la-ry_, and
inferior _max'il-la-ry_.
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745. The patheticus. What does fig. 125 represent? 746. What is the
trifacial nerve sometimes called? Why is it classed with the cranial
spinal nerves? Give the names of its branches.
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747. The ophthalmic nerve passes from the cranial cavity through the
sphenoidal fissure. It sends branches to the forehead, eye, and nose.
The superior maxillary nerve passes through an opening in the base of
the skull, (_foramen ro-tund'dum_,) and sends branches to the eye, the
teeth of the upper jaw, and the muscles of the face. The inferior
maxillary nerve escapes from the cranial cavity through an opening
called _foramen o-va'le_. It sends branches to the muscles of the
lower jaw, the ear, the tongue, and the teeth of the lower jaw.
[Illustration: Fig. 126. The distribution of the fifth pair of nerves. 1,
The orbit for the eye. 2, The upper jaw. 3, The tongue. 4, The lower jaw.
5, The fifth pair of nerves. 6, The first branch of this nerve, that
passes to the eye. 9, 10, 11, 12, 13, 14, Divisions of this branch. 7,
The second branch of the fifth pair of nerves is distributed to the teeth
of the upper jaw. 15, 16, 17, 18, 19, 20, Divisions of this branch. 8,
The third branch of the fifth pair, that passes to the tongue and teeth
of the lower jaw. 23. The division of this branch that passes to the
tongue, called the _gust'a-to-ry_. 24. The division that is distributed
to the teeth of the lower jaw.]
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747. Where do the filaments of the ophthalmic branch ramify? The
superior maxillary? The inferior maxillary? Explain fig. 126.
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748. The ABDUCENTES (sixth pair) passes through the opening by which
the carotid artery enters the cranial cavity. It is the smallest of
the cerebral nerves, and is appropriated to the external straight
muscle of the eye.
749. The PORTIO MOLLIS (seventh pair) enters the hard portion of the
_tem'po-ral_ bone at the internal auditory opening, and is distributed
upon the internal ear. (Fig. 147, 148.)
[Illustration: Fig. 127. A representation of the distribution of the
eighth pair of nerves with some branches of the fifth. 1, 2, 3, 5, 7, 9,
Are branches of the eighth pair. They are distributed over the face in a
radiated manner, which constitutes the pes anserinus, (foot of a goose.)
The nerves 4, 6, 8, are branches of the fifth pair. 10, 11, 12, 13, 14,
15, 16, Are branches of nerves from the upper part of the spinal cord,
(cervical.)]
750. The FACIAL NERVE (eighth pair) passes from the skull through an
opening situated below the ear, (_mas'toid foramen_.) It is
distributed over the face, supplying the muscles with nervous
filaments.
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748. What is said of the abducentes, or sixth pair of nerves? 749. Of
the portio mollis? Explain fig. 127. 750. Of the facial nerve?
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751. The GLOSSO-PHARYNGEAL NERVE (ninth pair) passes from the brain,
through an opening with the jugular vein, (_foramen lac'e-rum_.) It is
distributed to the mucous membrane of the tongue and throat, and also
to the mucous glands of the mouth.
752. the PNEUMOGASTRIC NERVE (tenth pair) escapes from the brain
through the foramen lacerum. It sends branches to the larynx, pharynx,
oesophagus, lungs, spleen, pancreas, liver, stomach, and intestines.
(Fig. 132.)
753. The SPINAL ACCESSORY NERVE (eleventh pair) has its origin in the
respiratory tract of the spinal cord. It connects with the ninth and
tenth pairs of nerves, and is distributed to the muscles about the
neck.
754. The HYPO-GLOSSAL NERVE (twelfth pair) passes from the brain,
through a small opening, (_con'dy-loid foramen_.) It ramifies upon the
muscles of the tongue, and is its motor nerve.
_Observation._ The cranial nerves, with the exception of the
olfactory, optic, and auditory, connect with each other by means of
filaments. They also send connecting nervous filaments to the upper
spinal nerves, (cervical,) and the sympathetic nerve.
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751. Describe the glosso-pharyngeal nerve. 752. The pneumogastric
nerve. 753. The spinal accessory nerve. 754. The hypo-glossal nerve.
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