Physiology is a science, because it embodies a collection of general
principles and ascertained truths relating to a particular subject,
and is called a natural science because these truths are founded on
observation. The word “physiology” is derived from two Greek words
meaning a discourse about nature; but it is used in a restricted
sense, and is the science of the functions of the different parts of
any living body. Thus we have animal and vegetable physiology, while
the former is divided into human and comparative. The first of these
divisions relates only to man, while the other reviews the entire
animal kingdom.
Our object is to teach some of the simple truths of human physiology;
such as may be intelligible without any extended knowledge of other
sciences. It must be remembered, however, that a more thorough
and complete study of physiology can not be undertaken without a
considerable acquaintance with such sciences as mechanics, hydraulics,
optics, etc., without which the action of the muscles, the circulation
of fluids, and vision can not be properly and fully comprehended.
Whenever a piece of mechanism, designed for some particular use,
is brought under our notice, and we wish to understand its manner
of working, we naturally inquire about its structure; for without
some knowledge of how its component parts are put together, and by
what means it is put in motion, we can not hope to understand how it
performs the part which we see it do. Such is the case, for example,
with a watch or steam engine; their parts must be carefully studied in
order that their workings may be fully understood.
Hence, it will be observed from what has been said, that it is
impossible to study the uses of various parts of the body without some
knowledge of anatomy, this being that branch of knowledge which treats
of structure.
In order that anatomy may be studied the organs must be dissected.
The words “anatomy” and “dissection” have the same literal meaning, the
former being derived from the Greek language, and the latter from the
Latin, meaning to cut apart or separate. But anatomy is employed to
signify the science of structure in living bodies, and dissection is
used to denote the unravelling or laying bare the parts of the body, by
means of which anatomy is studied.
The words “organ,” “organization,” and “organize,” are so convenient
and necessary that we must know what they mean. The word “organ”
signifies some part or parts of the body that have a particular use or
function: thus, muscles are organs of motion, and nerves of sensation;
the eye is the organ of sight, and the heart and blood vessels are the
organs for circulating the blood. Now, any structure is said to be
organized, or to have organization, that possesses the properties which
distinguish a living body from one that never had life. Therefore we
speak of the organic world as distinguished from the inorganic; the
former includes plants and animals, the latter minerals, etc.
The nature of that mysterious principle which we call life is unknown
to scientists. Yet we may know and understand many of those things
which are believed by physiologists concerning life. Certain of them
believe that life is a phenomenon that follows organization; or, in
other words, that organization is the cause, life the result; while
others contend that organization is the result of life. In the former
life is produced by changes that take place in matter, under the
influence of those forces of nature called heat, light, electricity,
and chemism; by the latter theory all these forces are present and
act under the influence of a potent force called life. This will be
our belief: Life is a distinct endowment, capable of propagation, and
superior to all other forces by which it is attended. Let us see by
what means living bodies are distinguished from unorganized. Living
bodies increase in size; so do minerals; the former by the addition
of material throughout the tissue, the latter by outside additions.
Organized bodies have a limited existence. All are subject to constant
change, and to final dissolution. They all spring from a parent, and
only originate in this method. The opposite characteristics belong to
unorganized matter.
It has been the business of chemistry to determine the ingredients
of the earth and atmosphere, by resolving them into what are called
elements. By the word element we mean that which can not be resolved
into any simpler form. For example, take a piece of chalk; by chemical
action it can be divided into a gas called carbonic acid, and a solid
called lime; therefore, it is known that chalk is a compound body. If
we take a piece of iron or gold there is no process known by which we
can resolve it into simpler form, and these are therefore elements.
There are over sixty elements and many of them are found in the human
body. All these elements are derived from unorganized matter, and the
special conditions under which they are formed in organisms is due to
the principle of life. The predominant elements that make up the human
body are carbon, hydrogen, oxygen, and nitrogen, while iron, potassium,
sodium, phosphorous, calcium, exist in smaller quantity. These
different elements are variously combined to form compounds, of which
water is the most abundant, for it forms more than two-thirds of the
entire weight of the body: water is a compound of hydrogen and oxygen.
The principal organic compounds are albuminous, of which the white of
the egg is a typical example; the gelatinous, or jelly-like compounds,
including cartilage, oleaginous, or fatty compounds, and saccharine,
or sugary compounds, such as starch. The first two contain nitrogen,
hydrogen, oxygen, and carbon, while fat and starch are rich in carbon,
but lack nitrogen. In tracing back the development of living matter to
its simplest form, it is found to exist as specks without any definite
shape or structure, or as granules of extreme minuteness, perhaps the
ten thousandth part of an inch in diameter.
This elemental living matter is called protoplasm. Its simplest form is
termed a cell, and the word is applied to little bodies varying much in
form. Thus some cells are really little bags filled with fluid. Such
are those in which the fat is deposited; others are disk like, others
lengthened, while some so-called cells are simply masses of jelly.
Out of these organic compounds all the softer tissue of the body is
formed, such as fibrous, muscular, cellular, and adipose tissue.
_Fibrous tissue_ consists of fine threads, arranged in various ways, to
adapt it to some special use; some of these threads are elastic, others
are wholly inelastic. These tissues of mixed character form what is
called connective tissue, which is found throughout the body.
Fat is deposited in little cells situated in the connective tissue.
_Muscular tissue_, which forms the great bulk of the body, is easily
recognized by its ruddy color, and it constitutes what is known as
“flesh.” It is fibrous, and if these fibers be examined under the
microscope there will appear, in those under the influence of the will,
transverse markings, while the muscles not under the will-power lack
these markings.
_Nervous tissue_ is that which superintends all the actions of the
living body. It is accumulated in nerve centers of which the principal
is the brain. It consists of minute cells in these nerve centers, and
of delicate tubes filled with nervous matter throughout the organs of
the body. Such tubes are called nerves.
_The skeleton_ is the frame-work of the body, and serves to support all
the softer organs as well as to protect them. It is composed of bones
which serve as attachments for the muscles.
_Bone_ is the firm tissue of the body, and to a certain degree is hard
and brittle. If bone be burned its brittleness is very much increased,
but if it be placed in acid it loses its brittle properties. Thus we
see that bone is made of two entirely different materials. The one
called “animal matter” is easily burned out, while the other called
“mineral matter” resists the action of heat, but is quickly dissolved
by acid. The mineral matter is a compound of lime. This it will be seen
gives to the bones their rigidity. Bones are of various shapes and
sizes, adapted to the work which they have to perform.
The most remarkable part of the skeleton is the spinal column, commonly
called the back-bone. It is made up of twenty-six bones, or spines,
which are united together in a marvelous manner, combining strength and
freedom of motion. Each spine has a central aperture which communicates
with that of the adjoining spines, thus forming a long canal in which
the spinal cord lies. This cord connects the brain with the various
parts of the body. There is a soft cushion of cartilage between each
spine, which adds much to the elasticity of the back-bone, protecting
the brain from shocks. This protection is further accomplished by
the curvature of the spine. These bones are not placed directly over
each other, but are so arranged as to give three curves to the spinal
column. Along the column there are openings, at each joint, through
which the nerves come to supply fibers to the different organs of the
body.
_The spine rests on the pelvis_, which is a large double bone,
basin-shaped, that sustains the abdominal organs. The pelvis, in turn,
rests on the thigh bones.
_The thigh_ bone, “femur,” is the largest single bone in the body; it
is joined to the pelvis by a ball and socket joint, an arrangement
which permits its motion in any direction. The femur is supported by
the bones of the leg. These are two in number in each leg, the tibia
and fibula. The former is much the larger, and the latter is often
called the “splint bone.”
The bones of the arm corresponding to those of the leg are the humerus,
the radius, and the ulna. The humerus is the bone of the upper arm,
and is large and strong; it articulates with the body in so perfect a
manner that the great variety of motions required to be performed is
easily and gracefully accomplished. This bone is fastened by a ball
and socket joint, but is not, like the thigh bone, firmly fixed to an
immovable bone. It is attached to a broad bone called the scapula, or
blade bone; this is fastened to the body by muscles which give it great
freedom of motion. The bones of the fore arm are the radius and ulna:
these are nearly of the same size; in this they differ from the leg
bones.
_The wrist_, composed of eight bones, forms the support for the
tendons, blood vessels, and nerves which pass to the hand. To these are
attached the metacarpal bones which support the phalanges, or bones
of the fingers. The arrangement of the bones of the foot is, in many
respects, like that of the hand. However, the bones of the foot form an
arch which acts like a spring when the weight of the body falls on it,
thus helping to ward off shocks and jars.
_The skull_ is a large bony case in which the brain is lodged, and by
which it is protected. It is composed of several pieces firmly united
by a very irregular line of union. The attachments of the skull are the
jaw bones, nasal bones, and the cheek bones.
_Joints_ have been provided wherever parts of the skeleton require
motion; they are of several kinds, chief among which are the ball and
socket, hinge, and flat joints. The hip has been mentioned as a perfect
example of the ball and socket joint, while the knee presents an
example of the hinge.
_Muscles_ are necessary to all motion, and are found everywhere
throughout the body. They are the active agents, which, under the
control of the nerve centers, do all the work necessary to carrying
on the functions of the organs. They are endowed with a remarkable
property called contraction. How this is accomplished is not fully
understood, but when stimulated by nerve force or electricity they
shorten and widen. This brings their ends nearer together; also the
parts to which they are attached. They are always attached to movable
parts; their attachment is a tendon at one end, and a fan-like
distribution of their fibers at the other. Tendons are inelastic
fibrous tissue, very strong, and consequently much smaller than the
muscle. They are to muscles what tugs are to horses. Muscles are either
flexors or extensors; the former bend the joints, the latter straighten
them.
_The skin_ forms an exterior covering for the body, and is employed for
other and very important functions. Its appendages are the hair and
nails. Some of the uses of the skin are these: It is highly sensitive
and capable of distinguishing various sensations; it has the property
of excreting, of secreting, and of absorbing. It is an organized
texture containing blood vessels, glands and nerves. It is composed of
several layers; the outside, called the epidermis, is not supplied with
blood vessels, and is produced by the under layers.
_The glands_ of the skin are first, those which secrete the
perspiration. This is poured out to keep the temperature of the body
even, also to rid the circulation of many injurious substances. Then
there are glands which secrete an oily matter which aids in preserving
the softness and pliability of the skin. These glands are called
sebaceous glands. Hair is said to be an appendage of the skin because
its texture is essentially similar to that of the cuticle. Nails, also,
are but modifications of the skin. The fact that the epidermis is being
constantly cast off and renewed, and that matter is both secreted and
excreted continually, renders frequent washing of the skin necessary to
health.
_Digestion_—In animals the arrangements which exist for converting
aliment into blood are more or less complicated according to the
requirements of each class; and this conversion is called assimilation,
a term which includes digestion and those changes which take place in
the blood and tissues, by which new material is added to them.
The preparation of the food for assimilation by the tissues is
accomplished in a long tube called the alimentary canal. This canal
is made up of various parts having different functions and different
construction. These parts we will briefly describe. They are the mouth,
pharynx, æsophagus, stomach, small and large intestine.
As solid food requires to be broken up or ground before it passes
into the stomach, the mouth is provided with teeth firmly implanted
in the jaws, while the lower jaw is moved by strong muscles in two
directions, one vertical, the other lateral. Man is supplied with two
sets of teeth; the first adapted to the jaws of childhood; the second
larger, which replace the former, are designed to last through life.
The rudiments of each set are found in the jaws before birth. During
the grinding or mastication of the food it is moistened and softened by
a fluid called saliva. This also acts chemically upon it, changing the
starch into sugar. The food is carried from the mouth to the stomach in
a long tube called the æsophagus, by means of the muscular contraction
of this tube.
_The stomach_ is a flask-shaped organ consisting of a double wall of
tissue, the outer one being muscular, the inner one vascular. This
latter membrane has a large supply of blood vessels, which convey the
blood out of which the gastric juice is manufactured and secreted by
the little glands of this membrane. The gastric juice is a fluid which
contains an active ingredient called pepsin. This, aided by the acid
which this fluid contains, effects a remarkable change on the albumen
of the food, making it a liquid. From the stomach the food passes into
the small intestine, where it receives from a small tube the pancreatic
juice which changes the fat into an emulsion.
_The intestine_ is a tube, about twenty-five feet in length, which,
like the stomach, has a double wall. Its inner coat contains multitudes
of little projections called “villi.” These contain blood vessels which
absorb and carry off the liquified food. It also secretes a fluid
called intestinal juice, which acts upon the unchanged starch, making
it into sugar. Its muscular wall by a continuous contraction produces a
motion known as the “peristaltic motion,” which carries the food onward
in its course. The intestine also receives the bile, a fluid produced
by the liver.
_The liver_ is the largest gland in the body. It receives the portal
vein which conveys the blood from the intestines. This vein, after
dividing and sub-dividing, thus bringing blood into communication with
all parts of this organ, is again collected into a main trunk which
passes on toward the heart. During its passage through the liver the
bile is eliminated from the blood by the little cells of which this
gland is composed. From the liver the bile is carried toward the
intestine, into which it is poured to assist in digesting the food, and
to be itself changed.
Besides the little blood vessels in the _villi_ of the intestines there
is another set of vessels called the lacteals, which aid in taking up
the digested food and pouring it into the circulation; also, throughout
the body a set of similar vessels collect the waste material and pour
it into the great veins, returning to the heart, in order that it may
be renovated or cast off; these are the lymphatics.
_The kidneys_ are great excretory organs, and are similar in shape to
those of a sheep, but are somewhat larger. They are glands, and excrete
urea, as well as other salts and waste materials, all of which are
highly poisonous if not removed from the blood.
_Blood_ is the life-giving fluid of the body: it is the source from
which all tissues are built, and it is the workman that carries the
waste material away from the tissues. In order to accomplish its work
it must circulate, and this requires a separate set of organs. First,
the impure blood must be carried back from the different parts of the
body; then it must be distributed again. It must also be made to flow
onward in a continuous current. Blood is a viscid fluid, of a red
color, containing over seventy per cent. water, with solid matter. Its
color differs on each side of the heart. When it returns from the body
it is blue, but when it leaves it is red. Under the microscope it is
seen to contain minute globules, or disk-like cells; to these the blood
owes its color. It possesses the remarkable property of spontaneous
coagulation when drawn from the veins. It is forced on by the heart.
_The heart_ is a muscle and consists of four cavities; two called
auricles having weak walls, and two ventricles with strong muscular
walls. The blood returning from the body is poured into the right
auricle, thence into the right ventricle; from this cavity it is forced
through the lungs and returns again to the heart, being poured into the
left auricle, which empties into the left ventricle. This ventricle
forces the blood throughout the body. The blood received by the heart
from the body is impure, and is sent to the lungs, where it gives up
part of its impurities and receives oxygen from the air.
_Arteries_ are those vessels that distribute the blood, while the
veins collect it and return it to the heart, thus all streams of blood
leaving the heart are conveyed in arteries, and those pouring into it
are carried in veins.
_The lungs_ are the organs that purify the blood, and in order that
this be thoroughly done, the blood is distributed throughout their
substance in minute capillary vessels. The lungs themselves are
vascular; being made up of a multitude of air cells, their surface
is greatly increased; hence their power of absorption. The diaphragm
is a muscular partition lying below the lungs. It is dome-shaped, and
when its fibers are shortened it enlarges the cavity in which the lungs
are situated. This creates a partial vacuum, causing the air to rush
into the lungs. The blood absorbs the oxygen from the air and gives up
carbonic acid gas. When the muscles of the diaphragm are relaxed the
elastic force of the air cells in the lungs expels the remaining gases
from the lungs. The diaphragm is assisted by the action of muscles
situated between the ribs; these lift the ribs and enlarge the cavity
of the thorax. The lungs also act as a reservoir for the air used in
the production of vocal sounds. They communicate with the atmosphere
by means of a tube called the trachea; this terminates in the pharynx,
with which the nostrils also communicate, thus completing a passage to
the outside air.
_The voice_ is produced by the modifications which the teeth, tongue,
lips and throat make upon the sound produced by the vocal cords. The
vocal cords are stretched across the upper end of the trachea, which is
called the larynx. The air from the lungs is forced past them, setting
them in vibration, thus producing sound.
_The nervous system_ consists first of all of the brain. This is
composed of nervous matter and constitutes the mass contained in the
skull. It is divided into three parts: the cerebrum, the cerebellum,
and the medulla oblongata. These three parts seem to preside over
different functions. Thus, the cerebrum is the seat of the faculties of
the mind, while the cerebellum presides over the muscular actions. The
brain is connected with distant parts of the body by means of fibers
which are distributed to all parts. These fibers are of two kinds,
one set carrying the impressions to the brain, the other carrying the
stimulus from the brain to the organs. The former are called sensitive,
the latter motor nerves. A deep fissure separates the brain into two
lateral halves, and these parts are connected with opposite sides of
the body. Besides these nerves, which originate in the brain and pass
through the spinal cord, there are twelve pairs that pass directly
through the skull to the organs which they supply. These are called
cranial nerves, and are distributed to the eyes, the nose, the ears,
the larynx, the lungs, the face, stomach, etc. Ten of these pairs of
nerves originate in the medulla oblongata. Of the functions presided
over by these nerves may be mentioned those performed by the tongue,
(taste), eyes, ears, and nose.
Many of these nerves are not sensitive, in the ordinary use of the
word. Thus, the retina of the eye is the expansion of the optic nerve,
and, while it is sensitive to light, it is not to ordinary impressions,
such as material contact. Also, the nerve of the ear is only sensitive
to the vibrations of fluids. We see by light reflected from objects.
This light passes through a set of lenses, and by means of these an
image is formed on the retina, which impression is carried to the
brain. Just how all this is accomplished is not known. The nerve of the
ear floats in a fluid called lymph. This fluid receives the vibrations
of bodies through the air, through the membranes and chain of bones,
and thus the nerves receive and transmit them to the brain, which act
constitutes hearing.
Volumes could not tell all that one single fiber of muscle contains
that is instructive, much less the entire functions, constructions,
and mysteries of a single organ of special sense. And to perform all
the allotted functions every part must be in the best repair. This
constitutes health. Health is maintained by cleanliness, by repose,
by muscular activity, by moderate eating, by plenty of fresh air, by
a contented disposition, and a clear and active mind. Watch over your
body with a jealous care, for all your future depends upon its good
condition.