951. _What are nerves?_
The nerves are branches of the _brain_ and the _spinal cord_; they are
distributed in great numbers to all the active and sensitive parts of
the body.
952. _What is the spinal cord?_
The spinal cord is a long and large cord of nervous matter,
which extends from the brain through a continuous tube formed by
corresponding hollows in the bones of the back. It serves as a nervous
trunk _for the distribution of nerves_, just as the aorta distributes
branches of blood-vessels.
953. _Why is the spinal cord placed in the grooves formed by the
back-bone?_
Being a very vital part of the system, and from the delicacy of
its structure liable to injuries, it is set in the back-bone for
_protection_; and so great is its security that it is only by force of
an unusual kind that it can be injured.
[Verse: "A sound heart is the life of the flesh: but envy is the
rottenness of the bones."--PROVERBS XIV.]
954. _How can branches proceed from it, if it is so securely encased in
bone?_
Because in the bones, on each side of the spinal cord, there are
_smaller grooves_ for the transmission of the nervous branches.
955. _Of what does the nervous system consist?_
Of the _brain_, the _spinal cord_, and the branches which are called
_nerves_.
[Illustration: Fig. 59.--SHOWING THE DISTRIBUTION OF NERVES AND VEINS,
AND ILLUSTRATING THE MANNER IN WHICH THEY PASS THROUGH THE FLESH TO
REACH THE PARTS TO WHICH THEIR FUNCTIONS BELONG.]
A. B. _Veins_ of the fore-arm.
B. Canal formed in the muscle, through which a _trunk-vein_
emerges.
C. Canal formed in the muscle, through which a large _nerve_
emerges.
D. Canal through which a _vein_ enters to communicate with the
deep muscles of the arm.
956. _What is the constitution of a nerve?_
It consists of a thin membrane, or sheath, surrounding _a greyish oily
matter_, which forms the nervous marrow. In the centre of this marrow
is usually found _a small fibre_, which is supposed to be the essential
part of the nerve; and most nerves consist of a number of these sheaths
enclosing fibres running in parallel directions.
957. _What is the nervous fluid?_
The term _nervous fluid_ is used to express our ideas of the mode by
which the brain and spinal cord influence the remote parts: just as we
say the _electric fluid_, without knowing that such a fluid exists. It
is the most convenient form of expression.
958. _How many classes of nerves are there?_
There are:--
1. The nerves of _motion_.
2. The nerves of _sensation_.
3. The nerves of _special sense_.
4. The nerves of _sympathy_.
[Verse: "Having many things to write unto you, I would not write
with paper and ink; but I trust to come unto you, and speak face to
face, that our joy may be full."--II JOHN.]
959. _What are the nerves of motion?_
The _nerves of motion_ are those which, in obedience to the will,
_stimulate the muscles to act_, and apportion the amount of stimulation
they convey to the degree of exertion required.
[Illustration: Fig. 60.--MUSCLES OF THE HEAD AND FACE, WITH NERVES
DISTRIBUTED THERETO.]
A A A. The _facial nerve_ emerging from underneath the ear, and
distributing branches to the cheeks, temple, forehead, &c. This
nerve excites the muscles of the face, and is chiefly instrumental
in producing the expressions of the countenance under the changing
emotions of the mind.
B B B. _Muscles_ by which various motions are imparted to the
head, face, mouth, &c., under the _stimulus of the nerves_.
960. _What are the nerves of sensation?_
The _nerves of sensation_ are those which _impart a consciousness to
the brain_ that its commands to the nerves of motion have been obeyed,
and how far they have been fulfilled.
[Verse: "Oh that men would praise the Lord for his goodness, and for
his wonderful works to the children of men."--PSALM CVII.]
Let us perform a simple experiment, which will more clearly
illustrate the phenomena of _motion_ and of _sensation_, which we
are now describing, than a great deal of writing upon the subject.
You hold in your hand this book: close it, and set it upon the
table; lay your hands passively upon your lap, and then _will_
your hand, to take up the book, which is the same as to say,
_command_ your hand to take up the book. What occurs? The hand,
immediately obeying your desire, stretches forward to the book,
and takes hold of it. How do you know that you have hold of it?
You _see_ that you have: but were your eyes closed, you would be
equally aware that the hand had reached the book, and fulfilled
your wishes. It is by the nerves of _sensation_ that you are made
aware that the hand has fulfilled your instructions.
Consider what took place in the simple action. In the first
instance, a desire arose in your mind to take up the book. The
_brain_ is the organ of the mind; and having branches either
proceeding from itself, or from the spinal cord, to every part of
the body--branches that traverse like telegraphic wires throughout
every part of the system,--it transmitted instructions along the
nerves that proceed to the muscles of the arm and hand, directing
them to take up the book. This was done instantly; and as soon
as it was done you became conscious that your will had been
obeyed--because _the nerves sent back a sensation to the brain
acquainting it that the book had been taken up_, and that at the
moment of the dispatch it was in the firm hold of the hand.
In all the varied motions of the body this double action of the
nerves takes place. It is obvious that without an _outward_
impulse from the brain, upon which the desire of the mind first
made an impression, no motion of the muscles of the arm and the
hand could have taken place; and it is also obvious that without
an _inward_ impulse from the nerves to the brain you would not
have known that the muscles had fulfilled your instructions. The
hand might have dropped by the side of the book, or have gone too
far, or not far enough, and you would not have been aware of the
result, but for an inward communication through the nerves.
We are not now speaking of the nerves which endow us with the
sense of _feeling_, because they are regarded as separate and
distinct from those nerves that produce in us consciousness
of muscular response. When we walk, rise, or sit, we are made
conscious, without any special feeling being exerted, that
the muscles have placed the limb, or the body, in the desired
position, that it is set down safely and firmly, and that we may
repose upon it securely without further attention. We refer the
impressions made by the book upon the nerves of the hand, and
which enable us to tell whether it feels hot or cold, whether
its surface is rough or smooth, and so on, to the special sense
of _feeling_. The consciousness of muscular action is a separate
and distinct function; and it is generally believed that the same
nerves that convey the command of the will outward, bringing back
the intimation that the will has been obeyed, but that _different
fibres_ of the nerves convey the _outward_ and the _inward_
impulses. A single nerve may therefore be likened to a _double
wire_ connected with the electric telegraph: one transmitting
despatches in one direction, and the other in the opposite
direction.
961. _What are the nerves of special sense?_
The nerves of special sense are those through which we _hear_, _see_,
_feel_, _smell_, and _taste_.
[Verse: "For the Lord seeth not as man seeth; for man looketh on the
outward appearance, but the Lord looketh on the heart."--SAMUEL XVI.]
962. _What are the nerves of sympathy?_
The nerves of sympathy, or the system of _sympathetic nerves_, are
those which are distributed to the _internal organs_, and which are
independent of the will. They regulate the motions of the heart, the
lungs, the stomach, &c., and stimulate the organs of secretion, so that
those organs _work in harmony with each other_.
As the internal organs are all more or less dependent upon each other,
and unite their functions for similar ends, it is obvious that there
should prevail among them a _mutual consciousness_ of their state.
Otherwise, when the stomach had formed chyme, the liver might have no
bile ready to fulfil its office; the absorbents might be in a state of
rest at the moment when nutrition was set before them; and the heart
might beat slowly, while the lungs were in active exertion to obtain
additional blood to support an active exercise. The sympathetic system
of nerves therefore _regulates and harmonises these internal functions_.