=How the Body is Built.=--In a building such as the body it is well to
begin with the _unit_--the building unit. In a house this is a brick or
a stone; in all living structures, animal and vegetable, it is a _cell_.
All living structures, whether animal or vegetable, are built up of
cells (which we shall consider in due course), and these cells are
grouped together for different purposes to form different tissues. The
_tissues_ are the different materials of which the body is made. There
are eight principal tissues in the body: _bone_, _gristle_, _muscle_,
_nerve_, _skin_, _fat_, _fiber_, and _connecting tissue_.
[Illustration: =ORGANS OF CHEST CAVITY IN RELATION TO STOMACH=
=THE BRONCHIAL TUBES=
=ORGANS INVOLVED IN FIRST STAGES OF DIGESTION=]
[Illustration: =DIAGRAMS DISCLOSING HEART AND CONNECTIONS, RIBS AND
LUNGS=]
(1) The _Osseous_, or bone tissue, is the framework of the body. This
material is found, of course, in every part of the body and forms the
skeleton.
(2) The _Cartilaginous_, or gristle, forms the joints of the body. This
tissue covers the ends of the bones to form the joints; it unites the
ribs with the breastbone; it forms the rings of the windpipe and the lid
of the larynx at the back of the tongue; the lower part of the nose, the
upper eyelid, and the ear.
(3) The _Muscular_, or muscle, forms the machinery of the body. This
tissue covers all the bones with flesh, which is muscle, and is the
chief part of a number of machines by which every movement is performed.
It is also an important tissue in the wall of the abdomen and the floor
of the chest.
(4) The _Nervous_, or nerve tissue, is the moving power of the body. It
is the chief constituent of the brain and the spinal cord, inside the
backbone or spine. It also forms the nerves, which run like white
threads from the brain to all the muscles, and give them power to move.
(5) The _Epithelial_, or skin, forms the outer covering of the body.
This tissue is the skin that covers the body outside, and lines it as
mucous membrane inside, and also forms the teeth and nails.
(6) _The Adipose_, or fat, forms the under covering of the body. This
tissue is the inner protective sheathing and padding of the body,
beneath the skin, and round the internal organs. It consists of drops of
oil, enclosed in separate cells.
(7) The _Fibrous_, or fiber or sinew, is the tissue that forms the cords
and bands of the body. This tissue makes the strong tendons that fasten
the muscles to the bones, and forms the covering or sheath of the bone
itself, and the various organs.
(8) The _Connective_, or cementing tissue, joins all the parts and cells
of the body together. This substance is found everywhere, all over the
body, and is like the mortar in a house, fastening all the bricks
together. It is a sort of network of cells and long fibers.
[Illustration: =DIAGRAM ILLUSTRATING THE ALIMENTARY CANAL=]
=Special Systems.=--These eight tissues are combined together into
various groups of _organs_ or _systems_ for special purposes. These
groups are six in number, and include: the _circulatory_, _respiratory_,
_digestive_, _excretory_ or _secretory_, _locomotor_, and _nervous
systems_. There is also the _reproductive_ system, which has to do with
the propagation of the race, and involves many important and vital
questions.
We may divide these six into three groups:
There are two in the chest:
(1) The _Circulatory system_ is that by which the blood or liquid food
is distributed throughout the body to all the tiny cells. This system
includes the _heart_ or force-pump, and the _arteries_, _capillaries_,
and _veins_ or the three kinds of pipes through which the blood travels.
(2) The _Respiratory_ system is that by which we breathe, and by which
the body is fed with oxygen, which gives the blood its bright red color.
This system includes the nostrils and mouth, the windpipe and the lungs.
Then there are two in the abdomen, or stomach:
(3) The _Digestive_ system, by which all the food is made into liquid
and changed so as to nourish the body and pass into the blood. This
system includes the mouth, gullet, stomach, liver, pancreas, intestines,
and other organs.
(4) The _Secretory_, or excretory, system (for they are best grouped as
one) manufactures the various fluids of the body, such as bile, urine,
sweat, saliva, gastric juice, etc. It consists of various glands or
secretory organs in different parts of the body, such as those in the
skin, the kidneys, the lymphatic glands, the spleen, etc. It also gets
rid of the refuse of the body.
Lastly, there are two in the head and limbs:
(5) The _Locomotor_ system, by which all movement is effected. This
includes the bones, joints, and muscles.
(6) The _Nervous_ system, by which all the body is controlled, directed,
and regulated. This system includes the brain, spinal cord, and the
special senses, such as the ear, the eye, and all the nerves.
=The Human Chest, or Thorax.=--In it, the blood is purified and
circulated. The _thorax_ is closed above and below: above, by the neck,
through which the windpipe enters it in front, conveying air to the
lungs; and by the _gullet_ behind, conveying food to the stomach. Below,
the floor, dividing it from the abdomen beneath, is formed by a very
large muscle stretching right across the body, called the _Diaphragm_,
or partition wall; also called the _Midriff_. The thorax is walled in at
the sides by the ribs, and behind by the backbone in which is the other
tube that contains the spinal cord. The thorax contains the two organs
of _respiration_ and _circulation_.
The _lungs_ are the organs of respiration. They are like two sponges
filling the right and left halves of the chest. Wherever you can feel a
rib there is part of the lung underneath. Each of these lungs is
contained in a bag, like a skin, that separates it from the ribs, and is
called the _pleura_ (from _pleuron_ = a _rib_), but the lung is not
_inside_ the bag.
The _outer_ layer of the pleura is fixed to the side of the chest, the
_inner_ layer to the lung, and the two layers move on each other like a
joint when we breathe.
The lungs are full of small air-cells with minute tubes leading from
them. These gradually increase in size as they join together, till at
last they unite in one large tube, or bronchus, for each lung. These two
bronchi join together, and form the _windpipe_, or _trachea_, which
conveys the air through the larynx into the mouth.
The windpipe is kept stretched widely open by a series of elastic rings
of gristle. Behind the windpipe is the gullet, leading to the stomach.
[Illustration: =PERICARDIUM OF THE HEART=
=LEFT AURICLE AND LEFT VENTRICLE=
The heart, the main pump of the circulatory system, rests on the
diaphragm between the two lungs. The heart is enclosed in a smooth,
moist membrane or sac, the pericardium, which allows it to dilate and
contract without friction against the adjoining parts. There are four
cavities in the heart, the right and left auricle, and the right and
left ventricle. The auricles, which are thinner walled, collect blood
from the veins, while the thicker and stronger walled ventricles force
the blood into the arteries. The left auricle pumps the purified blood
into the left ventricle, the valve between the auricle and ventricle
opening to allow this passage. When the left ventricle is full the valve
between its chamber and that of the auricle closes, the ventricle itself
contracts down, and the blood is pumped out through the aorta to supply
all the tissues of the body.
After leaving the left ventricle through the aorta the purified blood is
carried to the head, arms, trunk, and lower limbs, etc. Finally, after
being deprived of its oxygen as it passes through the tiny end-arteries,
or capillaries, of the tissues it has to nourish, it is collected in the
veins and is emptied into the right auricle. Passing from the right
auricle to the right ventricle, this impure blood, which is of a dull
purplish color, is pumped into the lungs, where it is deprived of its
waste gases and once more takes up a fresh supply of oxygen. Bright
scarlet in color again, it now is collected and carried to the left
auricle by the pulmonary veins. From the auricle it passes through the
mitral valve to the left ventricle, whence it is once more pumped out
through the aorta to supply the tissues.]
[Illustration: =RESPIRATORY SYSTEM OR AIR PASSAGES OF THE BODY=
Left: larynx from behind. Middle: cross-section of the pharynx. Right:
section through larynx.]
[Illustration: =VIEWS OF THE LARYNX, SHOWING HOW THE AIR REACHES THE
LUNGS=
The organs of respiration are the nose, throat, larynx, windpipe or
trachea, and the two lungs. On the outer walls of the nasal cavities are
three shelves known as the turbinated bones, the surfaces of which
contain blood-vessels to heat the air as it passes through the nose. The
mucus which constantly forms on the lining membrane of the nose and the
little hairs in the nostrils, act as screens, preventing dust being
breathed into the lungs. The pharynx is the cavity behind the nose,
mouth and larynx. The larynx forms a prominence in the throat known as
the “Adam’s Apple.” It contains the vocal cords, the vibrations of
which, as air from the lungs passes through them, give rise to voice
sounds. The epiglottis is a cartilaginous curtain above the larynx which
blocks up its entrance when food is being swallowed. The trachea or
windpipe is a continuation of the larynx. Shortly after entering the
chest it divides into two main branches, the right and left branches,
which lead to all parts of the lungs. The lungs, two spongy, air-filled
organs, take up most of the space in the chest-box or thorax. The
smallest end-branches of the bronchial tubes open into numerous tiny
sacs known as the air vesicles, in the walls of which the end-branches
of the capillaries ramify. Here the impure gases in the blood escape
through the vessel walls into the air vesicles, while the oxygen
breathed into the lungs is taken up the same way by the blood in the
vessels.]
[Illustration: =HOW THE HUMAN BODY IS CONTROLLED BY THE BRAIN=
=CORD WITH DURA MATER=
=THE ARRANGEMENT OF THE DURA MATER=
The nervous system consists of (1) the brain; (2) the spinal cord; (3)
the nerves which run off from these structures; and (4) the sympathetic
system. The chief mass of the brain is known as the cerebrum, or
fore-brain, the small mass at the lower part being termed the
cerebellum, or little brain. From the brain, which is contained within
the bony skull, twelve pairs of cranial nerves proceed. The most
important of these are the first or nerve of smell, the second (sight),
eighth (hearing), and twelfth (taste). The fifth, one of the most
important nerves of sensation, has three main branches running to the
orbit and forehead, the jaws and teeth, and the skin of the face. Six of
the twelve pairs of cranial nerves govern the movements of different
parts (motor nerves), others have to do with the special sense organs,
taste, smell, hearing, and sight (sensory nerves), and others are a
combination of motor and sensory nerves. The spinal cord is a
continuation of the brain, and is contained in the hollow canal running
through the vertebræ of the spine. From it thirty-one pairs of nerves
originate. The nerves which run to the arm are collected in a network
called the brachial plexus. In the same way the great nerves to the leg
come together in the lumbar plexus. The sympathetic nervous system
consists of a main nerve trunk running downward along the spine from the
skull to the coccyx. This sympathetic system communicates indirectly
with the brain and spinal cord, and also with all the great arteries and
other important structures in the abdomen.
The dura mater is the strong external cranial membrane which adheres to
the skull and also penetrates into the cavities of the brain, dividing
it into partially separate compartments. These dividing portions of the
dura mater may be seen at A, A, in the diagram above. B marks the
various venous blood sinuses of the brain, which receive blood from
veins in the different parts of the brain, and, merging into one large
sinus (seen at lower right of diagram), afterwards become the jugular
vein. C is the great cerebral vein. The Roman numerals mark the great
cranial nerves.]
We take air into the lungs to pass thence into the blood, and thus be
carried to all the cells of the body to enable them to live and breathe.
=The Heart.=--The _heart_ is at the lower part of the chest, between the
two lungs. It is a _fleshy_ or muscular organ, about the size of the
fist--flat above, and pointed below like a sugar-loaf. It lies in a
slanting direction behind the breastbone--the broad part, or the _base_,
of the heart being upwards and partly to the _right_ of the breast-bone;
the point, or apex of the heart, being downwards and to the _left_,
where it can often be seen beating against the chestwall.
The heart is hollow, and acts like a pump, forcing the blood all over
the body through the great vessel that leaves the heart at the upper
part. The heart, like the lungs, is enclosed in a double layer of folded
bag, called the _pericardium_, because it is round the heart.
The _gullet_ runs right down the back of the thorax, and passes out
through the diaphragm, which forms the floor, into the abdomen.
The _abdomen_ forms the lower half of the trunk, and is often called the
_stomach_. It is full of organs belonging to the _digestive system_ and
_secretory system_, by which the fuel or food is rendered fit for use in
the blood and the body.
The _walls_ of the abdomen are not protected by ribs like the thorax,
but are all formed of flesh or muscle. The principal organs they contain
are the _stomach_, the _liver_, the _pancreas_, or _sweetbread_, the
_spleen_ or _milt_, the _kidneys_, the _intestines_, and the _bladder_.
=The Human Brain.=--The _head_ and _spine_ contain the principal nervous
systems of the body and four organs of special sense--_sight_,
_hearing_, _smelling_, and _tasting_.
The _brain_, which fills the head, consists of two parts: the
_Cerebrum_, or greater brain, and the _Cerebellum_, or lesser brain,
placed behind and below the larger one. From this brain, nerves run to
every muscle of the body, enabling them to move the limbs and body as
the mind directs; and another set of nerves run from every part of the
body and skin to the brain, enabling the mind to know and feel all that
goes on.
The brain is connected with the spinal cord by a flat band of brain
matter, that lies on the inside of the occipital bone, called the
_Medulla Oblongata_, or the Oblong Marrow. The _spinal cord_ runs
through a large hole in the occipital bone and right down the open tube
formed by the spinal vertebræ, to the bottom of the backbone, and, all
along its course, nerves leave it and enter it, as in the brain.
The _organ of sight_ consists of the _two eyes_, which receive every
image that we see, and transmit it to the brain. The _organ of hearing_
consists of the _two ears_, by which we receive all the waves of sound
that we hear, and transmit them to the brain. The _organ of smell_ is in
the upper part of the _nose_; the _organ of taste_ at the hinder part of
the _tongue_.
The _organ of the voice_ is contained in the _larynx_ in the neck, which
joins the head to the body. Just under the chin in front of the neck you
can feel what is called the _Adam’s Apple_, which is the front of the
larynx, or voice-box, by which the air coming out of the lungs is formed
into sounds.
The _sounds_ are formed into words by the _mouth_, _tongue_, and
_teeth_.
[Illustration: =PERMANENT TEETH AND THEIR NAMES=
UPPER JAW: 1, 2, incisors; 3, canine; 4, 5, premolars; 6, 7, 8, molars.