ANATOMY OF THE NERVOUS SYSTEM, CONTINUED.
755. The spinal column contains the spinal cord, the roots of the
spinal nerves, and the membranes of the cord.
756. The SPINAL CORD extends from the medulla oblongata to the second
lumbar vertebra, where it terminates in a rounded point. It presents a
difference of diameter in different parts of its extent, and exhibits
three enlargements. The uppermost of these is the medulla oblongata.
There is no distinct demarkation between this enlargement and the
spinal cord. The next corresponds with the origin of the nerves
distributed to the upper extremities; the third enlargement is
situated near the termination of the cord, and corresponds with the
attachment of the nerves which are intended for the supply of the
lower extremities.
757. An anterior and posterior fissure divides the spinal cord into
two lateral cords. These are united by a thin layer of white
substance. The lateral cords are each divided by furrows into three
distinct sets of fibres, or columns; namely the _anterior_, _lateral_,
and _posterior_ columns. The anterior are the motor columns; the
posterior are the columns of sensation; the lateral columns are
divided in their function between motion and sensation. They contain
the fasciculus described, by Sir Charles Bell, as the respiratory
tract.
[Illustration: Fig. 128. A section of the brain and spinal column. 1, The
cerebrum. 2, The cerebellum. 3, The medulla oblongata. 4, 4, The spinal
cord in its canal.]
[Illustration: Fig. 129. Anterior view of the brain and spinal cord. 1,
1, The two hemispheres of the cerebrum. 3, 3, The cerebellum. 4, The
olfactory nerve. 5, The optic nerve. 7, The third pair of nerves. 8, The
pons varolii. 9, The fourth pair of nerves. 10, The lower portion of the
medulla oblongata. 11, 11, The spinal cord. 12, 12, Spinal nerves. 13,
13, The brachial plexus. 14, 14, The lumbar and sacral plexus.]
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755-767. _Give the anatomy of the spinal cord, spinal nerves, and the
sympathetic nerve._ 755. What does the spinal column contain? 756.
Give the extent of the spinal cord. How many enlargements has this
cord? What is said of each enlargement? 757. Into how many parts is
the spinal cord divided? Give the function of these columns.
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758. The SPINAL NERVES, that connect with the spinal cord, are
arranged in thirty-one pairs, each arising by two roots; an anterior,
or _motor_ root, and a posterior, or _sensitive_ root. Each nerve,
when minutely examined, is found to consist of an aggregate of very
delicate filaments, enclosed in a common cellular envelope.
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758. How many pairs of nerves issue from the spinal cord? Explain fig.
128. Fig. 129.
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759. The anterior roots arise from a narrow white line upon the
anterior columns of the spinal cord. The posterior roots arise from a
narrow gray band formed by the internal gray substance of the cord.
They are larger, and the filaments of origin more numerous than those
of the anterior roots. A ganglion is found upon each of the posterior
roots in the openings between the bones of the spinal column through
which the nerve passes.
[Illustration: Fig. 130. A section of the spinal cord, surrounded by its
sheath. B, A spinal nerve, formed by the union of the motor root (C) and
the sensitive root (D.) At D, the ganglion upon this root is seen.]
760. After the formation of the ganglion, the two roots unite, and
constitute a spinal nerve, which passes through the opening between
the vertebrae on the sides of the spinal column. The nerves divide and
subdivide, until their minute filaments ramify on the tissues of the
different organs.
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759. Give the origin of the anterior roots. Of the posterior roots. In
what respect do the posterior roots differ from the anterior? 760.
When do the two roots unite, and where do they pass?
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761. The _spinal nerves_ are divided into--
Cervical, 8 pairs,
Dorsal, 12 "
Lumbar, 5 "
Sacral, 6 "
762. The four lower cervical and upper dorsal pass into each other and
then separate to reunite. This is called the _brach'i-al plex'us_.
From this plexus six nerves proceed, which ramify upon the muscles and
skin of the upper extremities.
763. The last dorsal and the five lumbar nerves form a plexus called
the lumbar, similar to that of the cervical. Six nerves pass from this
plexus, which ramify upon the muscles and skin of the lower
extremities.
764. The last lumbar and the four upper sacral unite to form the
sacral plexus. From this plexus five nerves proceed, that are
distributed to the muscles and skin of the hip and lower extremities.
765. The SYMPATHETIC NERVE[19] consists of a series of _Gan'gli-a_, or
knots, extending each side of the spinal column, forming a chain its
whole length. It communicates with both the cranial and spinal nerves.
With the exception of the neck, there is a ganglion for each
intervertebral space. These ganglia are composed of a mixture of
cineritious and medullary matter, and are supposed to be productive of
peculiar nervous power.
[19] The structure of this nerve is very complicated, and different
physiologists ascribe to it various functions. The character of
its diseases are not well understood.
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761. Give the division of the spinal nerves. 762. What nerves
constitute the brachial plexus? How many nerves pass from this plexus?
763. How many nerves from the lumbar plexus, and where do they ramify?
764. How is the sacral plexus formed? 765. Of what does the
sympathetic nerve consist? How is the sympathetic nerve distributed?
What exception? Of what are the ganglia composed?
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[Illustration: Fig. 131. A beautiful representation of the sympathetic
ganglia and their connection with other nerves. It is from the grand
engraving of Manec, reduced in size. A, A, A, The semilunar ganglion and
solar plexus, situated below the diaphragm and behind the stomach. This
ganglion is situated in the region (pit of the stomach) where a blow
gives severe suffering. D, D, D, The thoracic ganglia, ten or eleven in
number. E, E, The external and internal branches of the thoracic ganglia.
G, H, The right and left coronary plexus, situated upon the heart. I, N,
Q, The inferior, middle, and superior cervical ganglia. 1, The renal
plexus of nerves that surrounds the kidneys. 2, The lumbar ganglion. 3,
Their internal branches. 4, Their external branches. 5, The aortic plexus
of nerves that lies upon the aorta. The other letters and figures
represent nerves that connect important organs and nerves with the
sympathetic ganglia.]
766. The GANGLIA may be considered as distinct centres, giving off
branches in four directions; namely, the superior, or ascending, to
communicate with the ganglion above; the inferior, or descending, to
communicate with the ganglion below; the external, to communicate with
the spinal nerves; and the internal, to communicate with the
sympathetic filaments. It is generally admitted that the nerves that
pass from the ganglia are larger than those that entered them; as if
they imparted to the nerve some additional power.
767. The branches of distribution accompany the arteries which supply
the different organs, and form communications around them, which are
called plexuses, and take the name of the artery with which they are
associated. Thus we have the mesenteric plexus, hepatic plexus,
splenic plexus, &c. All the internal organs of the head, neck, and
trunk, are supplied with branches from the sympathetic, and some of
them exclusively; for this reason, it is considered a nerve of organic
life.
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What is the design of fig, 131? 766. How may the ganglia be
considered? 767. What is said of the branches of the sympathetic
nerve?
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