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PHYSIOLOGY AND HYGIENE.
No. 35— The Nervous System.
n^HE general function of the nervous sys
?*- tern is to govern and control the whole
body and; mind, . so that all parts.' work
-harmoniously and in unisom with one an
other. .
The nervous system consists of- three
parts: Brain, spinal cord, and . nerves.
These like all other parts of the body are
built' up of cells, and though these cells
are highly specialised, ? they live. Jn the
same way as do all other cells, i.e., they
absorb oxygen'-and ioodj- -build- up new
tissue; and excrete waste, and their health
depends on the circulation and quality of
the blood, which again depends largely on
good food and fresh, air. Hence these
factors largely' influence mental power and
state of mind. The chief difference be
tween nerve-cells and other cells is the
elongation of the protoplasm of the former
into many short branching fibres, and one
longer fibre. The short ones are called
dendrites, and though they do not actually
fuse with those irom neighbouring cells,
they lie in very close contact with them
and probably serve to transmit nervous
energy from nearby cell ? to nearby cell,
to some extent. , The single, longer fibre
is called a nerve-fibre or axon, and it is
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. A diagram of a nerve ceil.
. A diagram of a nerve ceil.
covered with a sheath of white fat which
insulates it from other fibres and it com
municates with distant parts. It has a
few lateral branches, and both it and its
.branches end in tiny bunches of fibrils in
the muscles or some sense organ. What
we call a nerve is a bundle of such fibres,
held together by ' connective tissue and
looks like a piece of white string. A nerve
may contain both sensory and motor fibres,,
distinct and separate from one another
but bound up together (to form the nerve..
Each nerve fibre is necessarily connected'
with a nerve cell and if the fibre is' sepa-
rated from the cell, the fibre dies. No
impulse can travel along such a fibre and
that nerve .becomes paralysed.
Some nerve^fibres are very, long — others
are very short. Nerve-cells are grey iii
colour. In the brain, the, outer part is
grey 'in colour, because it is built up of
cells, and the inner '-part white.v because
built of fibres, running from cell to cell,
or from cells to spinal cord, etc. In the
spinal cord the colours, are reversed, be
cause- the cells are inside and the fibres
outside. '
Nervous Impulses.
Nerve cells may be divided into two
classes; J3erisory. cells, which feel pr^ re-
ceive impressions such as. heat, cqld, pain,
sight, hearing, taste, smell, etc., and motor
cells, which send out impressions to the
working organs: Those fibres which carry
impressions to the receiving cells are called'
sensory fibres; those which carry impulses
from the .cells to the working organs are
motor '-fibres, an,d are so called because
many of , them cause movement- of the
muscles. ? ? , ?? -.?-.. ? ? .
Nerve cells are: not scattered uniformly
in nervous tissue, but are gathered ' into
groups. A group of nerve cells is called
a ganglion. One or more ganglia having
a single function, such- as to 'control the.
muscles 'of breathing, form,. what is called
a ' nerve centre. The brain consists of
a number of nerve centres with their con
nective fibres. The nerve fibres from
nearly all over the body lead to cells situ
ated in a large cord in the spinal column
called the spinal cord.
There are two functions of the cord;
reflex action and transmission of impulses'
from the body to the brain. Reflex action
is action .that takes place without the aid
of the will. The spinal cord acts as a
distributing' centre for nerve fibres chiefly
to the parts of the body .below the head*
It thus acts as a means of conduction of
impulses between the brain and other parts
of the body. The thirty-one nerves com
ing off each side of the spinal cord con
tain both sensory and motor fibres. Fried
rich Goltz showed that a frog without a
brain but with a spinal cord, will, if ir
ritated, hop, swim, jump out of water,
and croak, but will otherwise sit like a
dummy, and although s.urrounded' with
food will die of starvation; because it has
no brain it' has become devoid- of will,
intellect, and memory. The spinal cord
enables .ihe 'frog when' stimulated or ir-'
ritated to carry out these various actions,
e.g., swimming, jumping, etc., by .reflex
action.
Reflex action never begins in the cord,'
but at the outer end of a sensory fibre,
usually located 'in the skin. The impression
goes to the cord along a sensory fibre. It
is received in a sensory cell and ' trans
ferred to a motor cell which sends back
an impulse along a motor fibre .to. a
muscle; the muscle contracts and .the action
is complete. There are always two nerve
cells concerned in every reflex, action.
Consciousness and Will.
Usually not all of the force of the im
pulse is transferred to the motor celL The
sensory cell by means of another of its
.many branches may transfer part of the
impulse to a cell which sends it to the
brain. Hence a reflex act is not neces
sarily an unconscious one. If you unin
tentionally put the hand on a' hot stove
pipe, you may be conscious of the pain
and the involuntary , jerking away of
the hand at the same time. Reflex ac
tion takes place during sleep as readily as
when we are awake. This may be proved
by tickling the foot of a sleeping person.
Sometimes the will may inhibit or pre
vent a reflex act. Many reflex acts occur
in spite, of the effort of the' will to pre
vent them. You cannot always keep from
closing your eyes when some one strikes
at you, even if from the other side of a
plate glass window, and you ? know there
is, no danger. Reflex acts are quicker
than' conscious acts. An eighth of a second
is about the time required for .a 'person
to press an electric button after seeing
a signal; a reflex act may occur in a
shorter time.
.The centre called the vital knot, i.e.,
the centre for breathing, ; is not said ' to
regulate the ? breathing reflexly but auto
matically. Reflex acts start in the skin:
Automatic acts start in the interior of the
body. The condition of .the blood regulates
the breathing automatically. If too much
carbon dioxide accumulates in the blood,
this excites , the . vital knot which sends
out stronger impulses to the respiratory
muscles. Deeper breathing follows, which
purifies the blood, and the breathing i
then quiet until an excess of carbon diox
ide occurs again.
The Sympathetic System.
The sympathetic system is that portion
of the nervous system which controls the
viscera or internal organs. The nerves' that
go to the viscera branch off from the
spinal ? nerves not - far from the - spinal
column and enter a row of ganglia on each
side of the spine. Each ganglion is con
nected by nerves to the one above and
below it.
Why are these nerves called the sym
pathetic system? Thoee nerves after leav
ing the double chain of ganglia/ form
many intricate networks of ganglia and
fibres. Each network is called' a plexus.
The largest of the plexuses is just back
of the stomach, and is called. the solar
plexus. A blow- upon the stomach may
paralyse this plexus and cause sudden
death. The plexuses and fibres connect
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A diagram illustrating reflex action.
A diagram illustrating reflex action.
the viscera so perfectly that one organ
cannot suffer without the others suffering
or sympathising with it. An overloaded
stomach causes the heart to beat faster
and send it more blood; a loss of appe
tite usually accompanies illness and' al
lows the stomach to rest.' This sympathy
is necessary for if one organ is diseased,
the. others do not continue to work and
tax the strength of the ailing organ: '
- The nerves from the central nervous
system supply the organs under the power
of the will such as the muscles which we can
move if we wish; these muscles are called
voluntary muscles. The nerves connected
with the sympathetic nervous system sup
ply organs which are not under the power
of the will, such as the muscle of the
blood vessels and of the wall of the ali
mentary canal; these muscles are. called
involuntary muscles, because we cannot
make them contract by will. It is clear
then' that although some of the .organs
are under the control of the- will, others
ire not. ' ' '
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