A B C D E F G H I J K L M N O P R S T U V W X Z

Hygienic Physiology

J >> Joel Dorman Steele >> Hygienic Physiology




FIG. 50.

[Illustration: _The Nervous System._ A, _cerebrum_; B,
_cerebellum._]

The BRAIN is the seat of the mind. [Footnote: In proportion to the rest
of the nervous matter in the body, it is larger in man than in any of the
lower animals. It is the function which the brain performs that
distinguishes man from all other animals, and it is by the action of his
brain that he becomes a conscious, intelligent, and responsible being. The
brain is the seat of that knowledge which we express when we say _I_.
I know it, I feel it, I saw it, are expressions of our individual
consciousness, the seat of which is the brain. It is when the brain is at
rest in sleep that there is least consciousness. The brain may be put
under the influence of poisons, such as alcohol and chloroform, and then
the body is without consciousness. From these and other facts the brain is
regarded as the seat of _consciousness_.--LANKESTER.] Its average
weight is about fifty ounces. [Footnote: Cuvier's brain weighed 64 1/2
ounces; Webster's, 53 1/2 ounces; James Fisk's, 58 ounces; Ruloff's, 59
ounces; an idiot's, 19 ounces. See Table in FLINT'S _Nervous
System_.] It is egg-shaped, and, soft and yielding, fills closely the
cavity of the skull. It reposes securely on a water bed, being surrounded
by a double membrane _(arachnoid)_, delicate as a spider's web, which
forms a closed sac filled, like the spaces in the brain itself, with a
liquid resembling water. Within this, and closely investing the brain, is
a fine tissue (_pia mater_), with a mesh of blood vessels which dips
down into the hollows, and bathes them so copiously that it uses one fifth
of the entire circulation of the body. Around the whole is wrapped a tough
membrane (_dura mater_), which lines the bony box of the skull, and
separates the various parts of the organ by strong partitions. The brain
consists of two parts--the _cerebrum_, and the _cerebellum_.

The CEREBRUM fills the front and upper part of the skull, and comprises
about seven eighths of the entire weight of the brain. As animals rise in
the scale of life, this higher part makes its appearance. It is a mass of
white fibers, with cells of gray matter sprinkled on the outside, or
lodged here and there in ganglia. It is so curiously wrinkled and folded
as strikingly to resemble the meat of an English walnut. This structure
gives a large surface for the gray matter,--sometimes as much as six
hundred and seventy square inches. The convolutions are not noticeable in
an infant, but increase with the growth of the mind, their depth and
intricacy being characteristic of high mental power.

FIG. 51.

[Illustration: _Surface of the Cerebrum._]

The cerebrum is divided into two hemispheres, connected beneath by fibers
of white matter. Thus we have two brains, [Footnote: This doubleness has
given rise to some curious speculations. In the case of the hand, eye,
etc, we know that the sensation is made more sure. Thus we can see with
one eye, but not so well as with both. It is perhaps the same with the
brain. We may sometimes carry on a train of thought, "build an air castle"
with one half of our brain, while the other half looks on and watches the
operation; or, we may read and at the same time think of something else.
So in delirium, a patient often imagines himself two persons, thus showing
a want of harmony between the two halves.--DRAPER, _Human
Physiology_, p. 320.] as well as two hands and two eyes. This provides
us with a surplus of brains, as it were, which can be drawn upon in an
emergency. A large part of one hemisphere has been destroyed without
particularly injuring the mental powers, [Footnote: A pointed iron bar,
three and a half feet long and one inch and a quarter in diameter, was
driven by the premature blasting of a rock completely through the side of
the head of a man who was present. It entered below the temple, and made
its exit at the top of the forehead, just about the middle line. The man
was at first stunned, and lay in a delirious, semistupefied state for
about three weeks. At the end of sixteen months, however, he was in
perfect health, with wounds healed and mental and bodily functions
unimpaired, except that sight was lost in the eye of the injured side.--
DALTON. It is noticeable, however, that the man became changed in
disposition, fickle, impatient of restraint, and profane, which he was not
before. He died epileptic, nearly thirteen years after the injury. The
tamping iron and the skull are preserved in the Warren Anatomical Museum,
Boston.]--just as a person has been blind in one eye for a long time
without having discovered his loss. The cerebrum is the center of
intelligence and thought. [Footnote: In man, the cerebrum presents an
immense preponderance in weight over other portions of the brain; in some
of the lower animals, the cerebrum is even less in weight than the
cerebellum. Another interesting point is the development of cerebral
convolutions in certain animals, by which the relative amount of gray
matter is increased. In fishes, reptiles, and birds, the surface of the
hemispheres is smooth; but, in many mammalia, especially in those
remarkable for intelligence, the cerebrum presents a greater or less
number of convolutions, as it does in the human subject.--FLINT. The
average weight of the human brain in proportion to the entire body is
about 1 to 36. The average of mammalia is 1 to 186; of birds, 1 to 212; of
reptiles, 1 to 1,321; and of fishes, 1 to 5,668. There are some animals in
which the weight of the brain bears a higher proportion to the body than
it does in man; thus in the blue-headed tit, the proportion is as 1 to 12;
in the goldfinch, as 1 to 24; and in the field mouse, as 1 to 31. "It does
not hence follow, however, that the _cerebrum_ is larger in
proportion; in fact, it is probably not nearly so large; for in birds and
rodent animals the sensory ganglia form a very considerable portion of the
entire brain. M. Baillarger has shown that the _surface_ and the
_bulk_ of the cerebral hemispheres are so far from bearing any
constant proportion to each other in different animals that,
notwithstanding the depth of the convolutions in the human cerebrum, its
bulk is two and a half times as great in proportion to its surface as it
is in the rabbit, the surface of whose cerebrum is smooth. The _size_
of the cerebrum, considered alone, is not, however, a fair test of its
intellectual power. This depends upon the quantity of _vesicular
matter_ which it contains, as evinced not only by superficial area, but
by the number and depth of the convolutions and by the thickness of the
cortical layer."--CARPENTER.] Persons in whom it is seriously injured or
diseased often become unable to converse intelligently, both from
inability to remember words and from loss of power to articulate them.

THE CEREBELLUM lies below the cerebrum, and in the back part of the head
(Fig. 50). It is about the size of a small fist. Its structure is similar
to that of the brain proper, but instead of convolutions it has parallel
ridges, which, letting the gray matter down deeply into the white matter
within, give it a peculiar appearance, called the _arbor vitæ_, or
tree of life (Fig. 55). This part of the brain is the center for the
control of the voluntary muscles, [Footnote: The exact nature of the
functions of the cerebellum is one of those problems concerning which
there is no unanimity of opinion amongst physiologists. It may be
premised, however, that the knowledge we at present possess does enable us
to come to one very important conclusion with respect to the functions of
the cerebellum,--it enables us to say that this organ has no independent
function either in the province of mind or in the province of motility.
And we may perhaps safely affirm still further, that the cerebellum is
much more intimately concerned with the production of bodily movements
than with the evolution of mental phenomena. The anatomical distinctness
of the cerebellum from the larger brain and other parts of the nervous
system is more apparent than real....That there is an habitual community
of action between the cerebellum and the spinal cord is, I believe,
doubted by none, and the fact that an intimate functional relationship
exists between the cerebrum and the cerebellum is shown by the
circumstance that atrophy of one cerebral hemisphere entails a
corresponding atrophy of the opposite half of the cerebellum. The
subordinate or supplementary nature of the cerebellar function, however,
in this latter relation seems equally well shown by the fact that atrophy
of one side of the cerebellum (when it occurs as the primary event) does
not entail any appreciable wasting in the opposite half of the cerebrum.
What other conclusion can be drawn? If the cutting off of certain cerebral
stimuli leads to a wasting of the opposite half of the cerebellum, this
would seem to show that each half of the cerebellum is naturally called
into activity in response to, or conjointly with, the opposite cerebral
hemisphere. Whilst conversely, if atrophy of one half of the cerebellum
does not entail a relative diminution in the opposite cerebral hemisphere,
this would go to show that the cerebral hemispheres do not act in response
to cerebellar stimuli, since their nutrition does not suffer when such
stimuli are certainly absent. The action of the cerebrum is therefore
shown to be primary, whilst that of the cerebellum is secondary or
subordinate in the performance of those functions in which they are both
concerned.--H. CHARLTON BASTIAN, _Paralysis from Brain Disease_.]
particularly those of locomotion. Persons in whom it is injured or
diseased walk with tottering and uncertain movements as if intoxicated,
and can not perform any orderly work.

THE SPINAL CORD occupies the cavity of the backbone. It is protected by
the same membranes as the brain, but, unlike it, the white matter is on
the outside, and the gray matter is within. Deep fissures separate it into
halves (Fig. 50), which are, however, joined by a bridge of the same
substance. Just as it starts from the brain, there is an expansion called
the _medulla oblongata_ (Fig. 55).

THE NERVES are glistening, silvery threads, composed, like the spinal
cord, of white matter without and gray within. They ramify to all parts of
the body. Often they are very near each other, yet are perfectly distinct,
each conveying its own impression. [Footnote: Press two fingers together,
and, closing the eyes, let some one pass the point of a pin lightly from
one to the other; you will be able to tell which is touched, yet if the
nerves came in contact with each other anywhere in their long route to the
brain, you could not thus distinguish.] Those which carry the orders of
the mind to the different organs are called the _motory_ nerves;
while those which bring back impressions which they receive are styled
_sensory_ nerves. If the sensory nerve leading to any part be cut,
all sensation in that spot will be lost, while motion will remain; if the
motory nerve be cut, all motion will be destroyed, while sensation will
exist as before.

TRANSFER OF PAIN.--Strictly speaking, pain is not in any organ, but in the
mind, since only that can feel. When any nerve brings news to the brain of
an injury, the mind refers the pain to the end of the nerve. A familiar
illustration is seen in the "funny bone" behind the elbow. Here the nerve
(_ulnar_) gives sensation to the third and fourth fingers, in which,
if this bone be struck, the pain will seem to be. Long after a limb has
been amputated, pain will be felt in it, as if it still formed a part of
the body--any injury in the stump being referred to the point to which the
nerve formerly led. [Footnote: Only about five per cent. of those who
suffer amputation lose the feeling of the part taken away. There is
something tragical, almost ghastly, in the idea of a spirit limb haunting
a man through his life, and betraying him in unguarded moments into some
effort, the failure of which suddenly reminds him of his loss. A gallant
fellow, who had left an arm at Shiloh, once, when riding, attempted to use
his lost hand to grasp the reins while with the other he struck his horse.
A terrible fall was the result of his mistake. When the current of a
battery is applied to the nerves of an arm stump, the irritation is
carried to the brain, and referred to all the regions of the lost limb. On
one occasion a man's shoulder was thus electrized three inches above the
point where the limb was cut off. For two years he had ceased to be
conscious of his limb. As the electric current passed through, the man,
who had been profoundly ignorant of its possible effects, started up,
crying, "Oh, the hand! the hand!" and tried to seize it with the living
grasp of the sound fingers. No resurrection of the dead could have been
more startling.--DR. MITCHELL _on "Phantom Limbs" in Lippincott's
Magazine_.]

The nerves are divided into three general classes--the _spinal_, the
_cranial_, and the _sympathetic_.

FIG. 54.

[Illustration: P, _posterior root of a spinal nerve;_ G,
_ganglion;_ A, _anterior root;_ S, _spinal nerve. The white
portions of the figure represent the white fibers; and the dark, the
gray._]

THE SPINAL NERVES, of which there are thirty-one pairs, issue from the
spinal cord through apertures provided for them in the backbone. Each
nerve arises by two roots; the anterior is the motory, and the posterior
the sensory one. The posterior alone connects directly with the gray
matter of the cord, and has a small ganglion of gray matter of its own at
a little distance from its origin. These roots soon unite, _i. e_.,
are bound up in one sheath, though they preserve their special functions.
When the posterior root of a nerve is cut, the animal loses the power of
feeling, and when the anterior root is cut, that of motion.

THE CRANIAL NERVES, twelve pairs in number, spring from the lower part of
the brain and the medulla oblongata.

1. The _olfactory_, or first pair of nerves, ramify through the
nostrils, and are the nerves of smell.

2. The _optic_, or second pair of nerves, pass to the eyeballs, and
are the nerves of vision.

3, 4, 6. The _motores oculi_ (eye movers) are three pairs of nerves
used to move the eyes.

5. The _trifacial_, or fifth pair of nerves, divide each into three
branches--hence the name--the first to the upper part of the face, eyes,
and nose; the second to the upper jaw and teeth; the third to the lower
jaw and the mouth, where it forms the nerve of taste. These nerves are
implicated when we have the toothache or neuralgia.

7. The _facial_, or seventh pair of nerves, are distributed over the
face, and give it expression. [Footnote: If it is palsied, on one side
there will be a blank, while the other side will laugh or cry, and the
whole face will look funny indeed. There were some cruel people in the
middle ages who used to cut the nerve and deform children's faces in this
way, for the purpose of making money of them at shows. When this nerve was
wrongly supposed to be the seat of neuralgia, or tic douloureux, it was
often cut by surgeons. The patient suffered many dangers, and no relief of
pain was gained.--MAPOTHER.]

FIG. 55.

[Illustration: _The Brain and the origin of the twelve pairs of Cranial
Nerves._ F, E, _the cerebrum;_ D, _the cerebellum, showing the
arbor vitæ;_ G, _the eye;_ H, _the medulla oblongata;_ A,
_the spinal cord;_ C and B, _the first two pairs of spinal
nerves._]

8. The _auditory_, or eighth pair of nerves, go to the ears, and are
the nerves of hearing.

9. The _glos-so-pha-ryn'-ge-al_, or ninth pair of nerves, are
distributed over the mucous membrane of the pharynx, tonsils, etc.

10. The _pneu-mo-gas'-tric_, or tenth pair of nerves, preside over
the larynx, lungs, liver, stomach, and one branch extends to the heart.
This is the only nerve which goes so far from the head.

11. The _accessory_, or eleventh pair of nerves, rise from the spinal
cord, run up to the medulla oblongata, and thence leave the skull at the
same opening with the ninth and tenth pairs. They regulate the vocal
movements of the larynx.

12. The _hyp-o-glos'-sal_, or twelfth pair of nerves, give motion to
the tongue.

FIG. 56.

[Illustration: _Spinal Nerve, Sympathetic Cord, and the Network of
Sympathetic Nerves around the Internal Organs_. K, _aorta;_ A,
_œophagus;_ B, _diaphragm;_ C, _stomach._]

THE SYMPATHETIC SYSTEM contains the nerves of organic life. It consists of
a double chain of ganglia on either side of the backbone, extending into
the chest and abdomen. From, these, delicate nerves, generally soft and of
a grayish color, run to the organs on which life depends--the heart,
lungs, stomach, etc.--to the blood vessels, and to the spinal and cranial
nerves over the body. Thus the entire system is bound together with cords
of sympathy, so that, "if one member suffers, all the members suffer with
it."

Here lies the secret of the control exercised by the brain over all the
vital operations. Every organ responds to its changing moods, especially
those of respiration, circulation, digestion, and secretion,--processes
intimately linked with this system, and controlled by it. (See p. 330.)

CROSSING OF CORDS.--Each half of the body is presided over, not by its own
half of the brain, but that of the opposite side. The motory nerves, as
they descend from the brain, in the medulla oblongata, cross each other to
the opposite side of the spinal cord. So the motor nerves of the right
side of the body are connected with the left side of the brain, and
_vice versa_. Thus a derangement in one half of the brain may
paralyze the opposite half of the body. The nerves going to the face do
not thus cross, and therefore the face may be motionless on one side, and
the limbs on the other. Each of the sensory fibers of the spinal nerves
crosses over to the opposite side of the spinal cord, and so ascends to
the brain; an injury to the spinal cord may, therefore, cause a loss of
motion in one leg and of feeling in the other.

REFLEX ACTION.--Since the gray matter generates the nervous force, a
ganglion is capable of receiving an impression, and of sending back or
_reflecting_ it so as to excite the muscles to action. This is done
without the consciousness of the mind. [Footnote: Instances of an
unconscious working of the mind are abundant. An illustration, often
quoted, is given, as follows, by Dr. Abercrombie, in his _Intellectual
Powers_:

"A lawyer had been excessively perplexed about a very complicated
question. An opinion was required from him, but the question was one of
such difficulty that he felt very uncertain how he should render it. The
decision had to be given at a certain time, and he awoke in the morning of
that day with a feeling of great distress. He said to his wife, 'I had a
dream, and the whole thing was clearly arranged before my mind, and I
would give anything to recover the train of thought.' His wife said to
him, 'Go and look on your table.' She had seen him get up in the night and
go to his table and sit down and write. He did so, and found there the
opinion which he had been most earnestly endeavoring to recover, lying in
his own handwriting. There was no doubt about it whatever."

In this case the action of the brain was clearly automatic, _i. e._,
reflex. The lawyer had worried his brain by his anxiety, and thus
prevented his mind from doing its best. But it had received an impulse in
a certain direction, and when left to itself, worked out the result. (See
Appendix for other illustrations.)] Thus we wink involuntarily at a flash
of light or a threatened blow. [Footnote: A very eminent chemist a few
years ago was making an experiment upon some extremely explosive compound
which he had discovered. He had a small quantity of this compound in a
bottle, and was holding it up to the light, looking at it intently; and
whether it was a shake of the bottle or the warmth of his hand, I do not
know, but it exploded in his hand, and the bottle was shivered into a
million of minute fragments, which were driven in every direction. His
first impression was that they had penetrated his eyes, but to his intense
relief he found presently that they had only struck the outside of his
eyelids. You may conceive how infinitesimally short the interval was
between the explosion of the bottle and the particles reaching his eyes;
and yet in that interval the impression had been made upon his sight, the
mandate of the reflex action, so to speak, had gone forth, the muscles of
his eyelids had been called into action, and he had closed his eyelids
before the particles had reached them, and in this manner his eyes were
saved. You see what a wonderful proof this is of the way in which the
automatic action of our nervous apparatus enters into the sustenance of
our lives, and the protection of our most important organs from injury.--
DR. CARPENTER.] We start at a sudden sound. We jump back from a precipice
before the mind has time to reason upon the danger. The spinal cord
conducts certain impressions to the brain, but responds to others without
troubling that organ. [Footnote: There is a story told of a man, who,
having injured his spinal cord, had lost feeling and motion in his lower
extremities. Dr. John Hunter experimented upon him. Tickling his feet, he
asked him if he felt it; the man, pointing to his limbs, which were
kicking vigorously about, answered, "No, but you see my legs do."
Illustrations of this independent action of the spinal cord are common in
animals. A headless wasp will ply its sting energetically. A fowl, after
its head is cut off, will flap its wings and jump about as if in pain,
although, of course, all sensation has ceased. "A water beetle, having had
its head removed, remained motionless as long as it rested on a dry
surface, but when cast into water, it executed the usual swimming motions
with great energy and rapidity, striking all its comrades to one side by
its violence, and persisting in these for more than half an hour."] The
medulla oblongata carries on the process of respiration. The great
sympathetic system binds together all the organs of the body.

USES OF REFLEX ACTION.--We breathe eighteen times every minute; we stand
erect without a consciousness of effort; [Footnote: In this way we account
for the perilous feats performed by the somnambulist. He is not conscious,
as his operations are not directed by the cerebrum, but by the other
nervous centers. Were he to attempt their repetition when awake, the
emotion of fear might render it impossible.] we walk, eat, digest, and at
the same time carry on a train of thought. Our brain is thus emancipated
from the petty detail of life. If we were obliged to attend to every
breath, every pulsation of the heart, every wink of the eye, our time
would be wasted in keeping alive. Mere standing would require our entire
attention. Besides, an act which at first demands all our thought soon
requires less, and at last becomes mechanical, [Footnote: "As every one
knows," says Huxley, "it takes a soldier a long time to learn his drill--
for instance, to put himself into the attitude of 'attention' at the
instant the word of command is heard. But, after a time, the sound of the
word gives rise to the act, whether the soldier be thinking of it or not.
There is a story, which is credible enough, though it may not be true, of
a practical joker, who, seeing a discharged veteran carrying home his
dinner, suddenly called out 'Attention!' whereupon the man instantly
brought his hands down and lost his mutton and potatoes in the gutter. The
drill had been thorough, and its effects had become embodied in the man's
nervous structure."] as we say, _i. e._, reflex. Thus we play a
familiar tune upon an instrument and carry on a conversation at the same
time. All the possibilities of an education and the power of forming
habits are based upon this principle. No act we perform ends with itself.
It leaves behind it in the nervous centers a tendency to do the same thing
again. Our physical being thus conspires to fix upon us the habits of a
good or an evil life. Our very thoughts are written in our muscles, so
that the expression of our face and even our features grow into harmony
with the life we live.

BRAIN EXERCISE.--The nervous system demands its life and activity. The
mind grows by what it feeds on. One who reads mainly light literature, who
lolls on the sofa or worries through the platitudes of an idle or
fashionable life, decays mentally; his system loses tone, and physical
weakness follows mental poverty. On the other hand, an excessive use of
the mind withdraws force from the body, whose weakness, reacting on the
brain, produces gradual decay and serious diseases. (See p. 331.)

The brain grows by the growth of the body. The body grows through good
food, fresh air, and work and rest in suitable proportion. For the full
development and perfect use of a strong mind, a strong body is essential.
Hence, in seeking to expand and store the intellect, we should be equally
thoughtful of the growth and health of the body.

Copyright (c) 2007. famouswriterz.com. All rights reserved.