part of that great human organization known as society. For men to work
together, to be mutually helpful one to another, requires the ability to
exchange ideas and this in turn requires some means of communication.(115)
One means of communication is found in certain movements of the
atmosphere, known as _sound waves_. In the exchange of ideas by this means
there are employed two of the most interesting divisions of the body--the
larynx and the ear. The first is an instrument for the production of sound
waves; the second is the sense organ which enables the sound waves to act
as stimuli to the nervous system.
*Nature of Sound Waves.*--If some sonorous body, as a bell, be struck, it
is given a quivering, or vibratory, motion. This is not confined to the
bell, but is imparted to the air and other substances with which the bell
comes in contact. These take up the movements and pass them to objects
more remote, and they in turn give them to others, until a very
considerable distance is reached. Such progressive vibrations are known as
waves, and, since they act as stimuli to the organs of hearing, they are
called _sound waves_. Sound waves _always originate in vibrating
bodies_.(116) They are transmitted chiefly _by the air_, which, because of
its lightness, elasticity, and abundance, readily takes up the vibrations
and spreads them in all directions (Fig. 148).
While these vibratory movements of the atmosphere are correctly classified
as waves, they bear little resemblance to the waves on water. Instead of
being made of crests and troughs, as are the water waves, the sound waves
consist of alternating successions of slightly condensed and rarefied
layers of air. Then, while the general movement of the water waves is that
of ever widening circles _over a surface_, the sound waves spread as
enlarging spherical shells _through_ the air. In sound waves, as in all
other waves, however, it is only the form of the wave that moves forward.
The individual particles of air that make up the wave simply vibrate back
and forth.
[Fig. 148]
Fig. 148--Diagram illustrating the spreading of sound waves through air.
*How Sound Waves act as Stimuli.*--Any sound wave represents a small but
definite amount of energy, this being a part of the original force that
acted on the vibrating body to set it in motion. The hammer, for instance,
in striking a bell imparts to it a measurable quantity of energy, which
the bell in turn imparts to the air. This energy is in the sound waves and
is communicated to the bodies against which they strike.(117) Though the
force exerted by most sound waves is, indeed, very slight, it is
sufficient to enable them to act as stimuli to the nervous system.
*How Sounds Differ.*--Three distinct effects are produced by sound waves
upon the nerves of hearing, and through them upon the mind. These are
known as _pitch, intensity_, and _quality_, and they are dependent upon
the vibrations of the sound-producing bodies.
_Pitch_, which has reference to the height, or degree of sharpness, of
tones, is determined by the rapidity of the vibrations of the vibrating
body. The more rapid the vibrations, the higher the pitch, the number of
vibrations doubling for each musical interval known as the octave.
_Intensity_ is the energy, or force, of the sound waves. This is
recognized by the strength of the sensation and is expressed by the term
_loudness_. Intensity is governed mainly by the width of the vibrations of
the vibrating body, and the width depends upon the force applied to the
body to make it vibrate.
_Quality_ is that peculiarity of sound that enables tones from different
instruments to sound differently, although they may have the same pitch
and intensity. Quality depends upon the fact that most tones are complex
in nature and result from the blending together of simple tones of
different pitch.
*Reenforcement of Sound Waves.*--The sound vibrations from small bodies are
not infrequently reenforced by surrounding conditions so that their
outgoing waves reach farther and are more effective than waves from larger
bodies. This is true of the sound waves produced by most musical
instruments and also those produced by the human larynx. Such
reenforcement is effected in two general ways--by sounding boards and by
inclosed columns of air. Stringed instruments--violin, guitar, piano,
etc.--employ sounding boards, while wind instruments, as the flute, pipe
organ, and the various kinds of horns, employ air columns for reenforcing
their vibrations. In the use of the sounding board, the vibrations are
communicated to a larger surface, and in the use of the air column the
vibrations are communicated to the inclosed air. (See Practical Work.)
*Value of Sound Waves to the Body.*--From a physiological standpoint, the
value of sound waves is not easily overestimated. In addition to the use
made of them in the communication of ideas, they serve the purpose of
protecting the body, and in the sphere of music provide one of the most
elevating forms of entertainment. Sounds from different animals, as well
as from inanimate objects, may also be the means of supplying needed
information. The existence of two kinds of sound instruments in the
body--the one for the production, the other for the detection, of sound--is
certainly suggestive of the ability of the body to adjust itself to, and
to make use of, its physical environment. Both the larynx and the ear are
constructed with special reference to the nature and properties of sound
waves.
THE LARYNX
*The Sound-producing Mechanism of the Body* consists of the following
parts:
1. Delicately arranged bodies that are easily set in vibration.
2. An arrangement for supplying the necessary force for making these
bodies vibrate.
3. Contrivances for modifying the vibrating parts so as to produce changes
in pitch and intensity.
4. Parts that reenforce the vibrations.
5. Organs by means of which the sounds are converted into the forms of
speech.
The central organ in this complex mechanism is
*The Larynx.*--The larynx forms a part of the air passages, being a short
tube at the upper end of the trachea. Mucous membrane lines the inside of
it and muscles cover most of the outer surface. The framework is made of
cartilage. At the top it is partly encircled by a small bone (the hyoid),
and its opening into the pharynx is guarded by a flexible lid, called the
_epiglottis_. The cartilage in its walls is in eight separate pieces, but
the greater portion of the structure is formed of two pieces only. These
are known as the _thyroid cartilage_ and the _cricoid cartilage_ (Fig.
149). Both can be felt in the throat--the thyroid as the projection known
as "Adam's apple," and the cricoid as a broad ring just below.
[Fig. 149]
Fig. 149--The larynx.--_A._ Outside view. _B._ Vertical section through
larynx, showing inside. 1. Thyroid cartilage. 2. Cricoid cartilage. 3.
Trachea. 4. Hyoid bone. 5. Epiglottis. 6. Vocal cord. 7. False vocal cord.
8. Lining of mucous membrane.
The _thyroid cartilage_ consists of two V-shaped pieces, one on either
side of the larynx, meeting at their points in front, and each terminating
at the back in an upward and a downward projection. Between the back
portions of the thyroid is a space equal to about one third of the
circumference of the larynx. This is occupied by the greater portion of
the _cricoid cartilage_. This cartilage has the general shape of a signet
ring and is so placed that the part corresponding to the signet fits into
the thyroid space, while the ring portion encircles the larynx just below
the thyroid. Muscles and connective tissue pass from the thyroid to the
cricoid cartilage at all places, save one on each side, where the downward
projections of the thyroid form hinge joints with the cricoid. These
joints permit of motion of either cartilage upon the other.
At the summit of the cricoid cartilage, on each side, is a small piece of
triangular shape, called the _arytenoid cartilage_. Each arytenoid is
movable on the cricoid and is connected with one end of a vocal cord.
[Fig. 150]
Fig. 150--*Vocal cords* as seen from above. _A._ In producing sound, _B._
During quiet breathing.
*The Vocal Cords* are formed by two narrow strips of tissue which,
connecting with the thyroid cartilage in front and the arytenoid
cartilages behind, lie in folds of the mucous membrane. They have the
general appearance of ridge-like projections from the sides of the larynx,
but at their edges they are sharp and smooth. The open space between the
cords is called the _glottis_. When sound is not being produced, the
glottis is open and has a triangular form, due to the spreading apart of
the arytenoid cartilages and the attached cords. But when sound is being
produced, the glottis is almost completely closed by the cords. Above the
vocal cords, and resembling them in appearance, are two other folds of
membrane, called the _false vocal cords_ (B, Fig. 149). The false cords do
not produce sound, but they aid in the closing of the glottis.
*How the Voice is Produced.*--The voice is produced through the vibrations
of the vocal cords. A special set of muscles draws the arytenoid
cartilages toward each other, thereby bringing their edges very near and
parallel to each other in the passage. At the same time other muscles act
on the thyroid and cricoid cartilages to separate them at the top and give
the cords the necessary tension. With the glottis now almost closed,
blasts of air from the lungs strike the sharp edges of the cords and set
them in vibration (Fig. 150). The vocal cords do not vibrate as strings,
like the strings of a violin, but somewhat as reeds, similar to the reeds
of a French harp or reed organ.
The location of the vocal cords in the air passages enables the lungs and
the muscles of respiration to aid in the production of the voice. It is
their function to supply the necessary force for setting the cords in
vibration. The upper air passages (mouth, nostrils, and pharynx) supply
resonance chambers for reenforcing the vibrations from the vocal cords,
thereby greatly increasing their intensity. In ordinary breathing the
vocal cords are in a relaxed condition against the sides of the larynx and
are not acted upon by the air as it enters or leaves the lungs.
*Pitch and Intensity of the Voice.*--Changes in the pitch of the voice are
caused mainly by variations in the tension of the cords, due to the
movements of the thyroid and cricoid cartilages upon each other.(118) In
the production of tones of very high pitch, the vibrating portions of the
cords are thought to be actually shortened by their margins being drawn
into contact at the back. This raises the pitch in the same manner as does
the shortening of the vibrating portion of a violin string.
The _intensity_, or loudness, of the voice is governed by the force with
which the air is expelled from the lungs. The vibrations of the cords,
however, are greatly reenforced by the peculiar structure of the upper air
passages, as stated above.
*Production of Speech.*--The sounds that form our speech or language are
produced by modifying the vibrations from the vocal cords. This is
accomplished by "mouthing" the sounds from the larynx. The distinct
sounds, or words, are usually complex in nature, being made up of two or
more elementary sounds. These are classed either as _vowels_ or
_consonants_ and are represented by the different letters of the alphabet.
The vowel sounds are made with the mouth open and are more nearly the pure
vibrations of the vocal cords. The consonants are modifications of the
vocal cord vibrations produced by the tongue, teeth, lips, and throat.
*Words and their Significance.*--In the development of language certain
ideas have become associated with certain sounds so that the hearing of
these sounds suggests the ideas. Our words, therefore, consist of so many
sound signals, each capable of arousing a definite idea in the mind. To
talk is to express ideas through these signals, and to listen is to assume
an attitude of mind such that the signals may be interpreted. In learning
a language, both the sounds of the words and their associated ideas are
mastered, this being necessary to their practical use in exchanging ideas.
From spoken language man has advanced to written language, so that the
sight of the written or printed word also arouses in the mind the
associated idea.
THE EAR
*The Ear* is the sense organ which enables sound waves to so act upon
afferent neurons as to excite impulses in them. The effect upon the mind
which these impulses produce is known as the _sensation of hearing_. In
the performance of its function the ear receives and transmits sound waves
and also concentrates them upon a suitable exposure of nerve cells. It
includes three parts--the _external ear_, the _middle ear_, and the
_internal ear_.
*External Ear.*--The external ear consists of the part on the outside of
the head called the _pinna_, or auricle, and the tube leading into the
middle ear, called the _auditory canal_ (Fig. 151). The pinna by its
peculiar shape aids to some extent the entrance of sound waves into the
auditory canal.(119) It consists chiefly of cartilage. The auditory canal
is a little more than an inch in length and one fourth of an inch in
diameter, and is closed at its inner end by a thin, but important
membrane, called
*The Membrana Tympani.*--This membrane consists of three thin layers. The
outer layer is continuous with the lining of the auditory canal; the inner
is a part of the lining of the middle ear; and the middle is a fine layer
of connective tissue. Being thin and delicately poised, the membrana
tympani is easily made to vibrate by the sound waves that enter the
auditory canal. In this way it serves as a receiver of sound waves from
the air. It also protects
[Fig. 151]
Fig. 151--*Diagram of section through the ear*, showing relations of its
various parts. (See text.)
*The Middle Ear.*--The middle ear, or tympanum,(120) consists of an
irregular cavity in the temporal bone which is lined with mucous membrane
and filled with air. It is connected with the pharynx by a slender canal
called the _Eustachian tube_. Extending across the middle ear and
connecting with the membrana tympani on one side, and with a membrane
closing a small passage to the internal ear on the other, is a tiny bridge
formed of three small bones. These bones, named in their order from the
membrana tympani, are the _malleus_, the _incus_, and the _stapes_ (Fig.
151). Where the malleus joins the membrane is a small muscle whose
contraction has the effect of tightening the membrane. The Eustachian tube
admits air freely to the middle ear, providing in this way for an equality
of atmospheric pressure on the two sides of the drum membrane. The bridge
of bones and the air in the middle ear receive vibrations from the
membrana tympani and communicate them to the membrane of the internal ear.
*Purposes of the Middle Ear. *--The middle ear serves two important
purposes. In the first place, it makes it possible for sound waves to set
the membrana tympani in vibration. This membrane could not be made to
vibrate by the more delicate of the sound waves if it were stretched over
a bone, or over some of the softer tissues, or over a liquid. Its
vibration is made possible by the presence of air on _both_ sides, and
this condition is supplied, on the inner side, by the middle ear. The
Eustachian tube, by providing for an _equality_ of pressure on the two
sides of the membrane, also aids in this purpose.
In the second place, the middle ear provides a means for _concentrating
the force of the sound waves_ as they pass from the membrana tympani to
the internal ear. This concentration is effected in the following manner:
1. The bridge of bones, being pivoted at one point to the walls of the
middle ear, forms a lever in which the malleus is the long arm, and the
incus and stapes the short arm, their ratio being about that of three to
two. This causes the incus to move through a shorter distance, but with
greater force than the end of the malleus.
2. The area of the membrana tympani is about twenty times as great as the
membrane of the internal ear which is acted upon by the stapes. The force
from the larger surface is, therefore, concentrated by the bridge of bones
upon the smaller surface. By the combination of these two devices, the
waves striking upon the membrane of the internal ear are rendered some
thirty times more effective than are the same waves entering the auditory
canal.
*The Internal Ear*, or labyrinth, occupies a series of irregular channels
in the petrous process of the temporal bone.(121) It is very complicated
in structure, and at the same time is very small. Its greatest length is
not more than three fourths of an inch and its greatest diameter not more
than one half of an inch. It is filled with a liquid which at one place is
called the _perilymph_, and at another place the _endolymph_. It is a
double organ, being made up of an outer portion which lies next to the
bone, and which surrounds an inner portion of the same general form. The
outer portion is surrounded by a membrane which serves as periosteum to
the bone and, at the same time, holds the liquid belonging to this part,
called the perilymph. The inner portion, called the _membranous
labyrinth_, consists essentially of a closed membranous sac, which is
filled with the endolymph. The auditory nerve terminates in this portion
of the internal ear. Three distinct divisions of the labyrinth have been
made out, known as the _vestibule_, the _semicircular canals_, and the
_cochlea_ (Fig. 152).
[Fig. 152]
Fig. 152--*General form, of internal ear.* The illustration represents the
structures of the internal ear surrounded by a thin layer of bone. 1.
Vestibule. 2. Cochlea. 3. Semicircular canals. 4. Fenestra ovalis. 5.
Fenestra rotunda.
*The Vestibule* forms the central portion of the internal ear and is
somewhat oval in shape. It is in communication with the middle ear through
a small opening in the bone, called the _fenestra ovalis_, at which place
it is separated from the middle ear only by a thin membrane. Sound waves
enter the liquids of the internal ear at this point, the foot of the
stapes being attached to the membrane. Six other openings lead off from
the vestibule at different places. One of these enters the cochlea. The
other five open into
*The Semicircular Canals.*--These canals, three in number, pass through the
bone in three different planes. One extends in a horizontal direction and
the other two vertically, but each plane is at right angles to the other
two. Both ends of each canal connect with the vestibule, though two of
them join by a common opening. The inner membranous labyrinth is
continuous through each canal, and is held in position by small strips of
connective tissue.
The purpose of the semicircular canals is not understood. It is known,
however, that they are not used in hearing. On the other hand, there is
evidence to the effect that they act as equilibrium sense organs, exciting
sensations necessary for balancing the body. Their removal or injury,
while having no effect upon the hearing, does interfere with the ability
to keep the body in an upright position.
[Fig. 153]
Fig. 153--Diagram showing the divisions of cochlear canal.
*The Cochlea* is the part of the internal ear directly concerned in
hearing. It consists of a coiled tube which makes two and one half turns
around a central axis and bears a close resemblance to a snail shell
(Figs. 151 and 152). It differs in plan from a snail shell, however, in
that its interior space is divided into three distinct channels, or
canals. These lie side by side and are named, from their relations to
other parts, the _scala vestibula_, the _scala tympani_, and the _scala
media_. Any vertical section of the cochlea shows all three of these
channels (Fig. 153).
*The Scala Vestibula and the Scala Tympani* appear in cross section as the
larger of the canals. The former, so named from its connection with the
vestibule, occupies the upper position in all parts of the coil. The
latter lies below at all places, and is separated from the channels above
partly by a margin of bone and partly by a membrane. It receives its name
from its termination at the tympanum, or middle ear, from which it is
separated only by a thin membrane.(122) Both the scala vestibula and the
scala tympani belong to the outer portion of the internal ear and are, for
this reason, filled with the perilymph. At their upper ends they
communicate with each other by a small opening, making by this means one
continuous canal through the cochlea. This canal passes from the vestibule
to the tympanum and, in so doing, goes entirely around
*The Scala Media.*--This division of the cochlea lies parallel to and
between the other two divisions. It is above the scala tympani and below
the scala vestibula, and is separated from each by a membrane. The scala
media belongs to the membranous portion of the internal ear and is,
therefore, filled with the endolymph. It receives the terminations of
fibers from the auditory nerve and may be regarded as the true sense organ
of hearing. The nerve fibers terminate upon the membrane known as the
_basilar membrane_, which separates it from the scala tympani. This
membrane extends the length of the cochlear canals, and is stretched
between a projecting shelf of bone on one side and the outer wall of the
cochlea on the other. It is covered with a layer of epithelial cells, some
of which have small, hair-like projections and are known as the _hair
cells_. Above the membrane, and resting partly upon it, are two rows of
rod-like bodies, called the _rods of Corti_. These, by leaning toward each
other, form a kind of tunnel beneath. They are exceedingly numerous,
numbering more than 6000, and form a continuous series along the margin of
the membrane.
[Fig. 154]
Fig. 154--*Diagram* illustrating passage of sound waves through the ear.
*How We Hear.*--The sound waves which originate in vibrating bodies are
transmitted by the air to the external ear. Passing through the auditory
canal, the waves strike against the membrana tympani, setting it into
vibration. By the bridge of bones and the air within the middle ear the
vibrations are carried to and concentrated upon the liquid in the internal
ear (Fig. 154). From here the vibrations pass through the channels of the
cochlea and set into vibration the contents of the scala media and
different portions of the basilar membrane. This serves as a stimulus to
the fibers of the auditory nerve, causing them to transmit impulses which,
on passing to the brain, produce the sensation of hearing.
Much of the peculiar structure of the cochlea is not understood. Its
minute size and its location in the temporal bone make its study extremely
difficult. The connection of the scala vestibula with the scala tympani,
and this with the middle ear, is necessary for the passage of vibrations
through the internal ear. Its liquids, being practically incompressible
and surrounded on all sides by bones, could not otherwise yield to the
movements of the stapes. (See Practical Work.) The rods of Corti are
thought to act as dampers on the basilar membrane, to prevent the
continuance of vibrations when once they are started.
*Detection of Pitch.*--The method of detecting tones of different pitch is
not understood. Several theories have been advanced with reference to its
explanation, one of the most interesting being that proposed by Helmholtz.
This theory is based on our knowledge of sympathetic vibrations. The
basilar membrane, while continuous throughout, may be regarded as made up
of many separate cords of different lengths stretched side by side. A tone
of a given pitch will set into vibration only certain of these cords,
while tones of different pitch will set others into vibration.
Another theory is that the basilar membrane responds to all kinds of
vibrations and the analysis of sound takes place in the brain.
A third view is that the filaments from the hair cells, rather than the
basilar membrane, respond to the vibrations and in turn stimulate the
terminations of the nerve fibers.
[Fig. 155]
Fig. 155--*Diagram* showing how wax may plug the auditory canal and cause
deafness.
*Hygiene of the Ear.*--The ear, being a delicate organ, is frequently
injured by careless or rough treatment. The removal of the ear wax by the
insertion of pointed instruments has been found to interfere with the
natural method of discharge and to irritate the membrane. It should never
be practiced. It is unnecessary in the healthy ear thus to cleanse the
auditory canal, as the wax is passed by a natural process to where it is
easily removed by a damp cloth. If the natural process is obstructed,
clean warm water and a soft linen cloth may be employed in cleansing the
canal, without likelihood of injury. Clean warm water may also be
introduced into the auditory canal as a harmless remedy in relieving
inflammation of the auditory canal and of the middle ear. Children's ears
are easily injured, and it goes without saying that they should never be
pulled nor boxed.
It frequently happens that a mass of wax collects in the auditory canal
and closes the passage so completely as to cause deafness (Fig. 155). This
may come about without pain and so gradually that one does not think of
seeking medical aid. Such masses are easily removed by the physician, the
hearing being then restored. Both for painful disturbances of the ear and
for the gradual loss of hearing, the physician should be consulted.
*The Hearing of School Children.*--School children not infrequently have
defective hearing and for this reason are slow to learn. The hearing is
easily tested with a watch, the normal ear being able to hear the watch
tick at a distance of at least two feet. Pupils with defective hearing
should, of course, have medical attention, and in the classroom should be
seated where they can hear to the best advantage.
*Summary.*--Sound waves constitute the external stimuli for the sensation
of hearing. They consist of progressive vibratory movements of the air
that originate in vibrating bodies. Through the larynx and the ear, sound
waves are utilized by the body in different ways, but chiefly as a means
of communication. The larynx produces sound waves which are reenforced and
modified by the air passages. The ear supplies suitable conditions for the
action of sound waves upon nerve cells. Both the ear and the larynx are
constructed with special reference to the nature and properties of sound
waves, and they illustrate the body's ability to adjust itself to, and to
make use of, its physical environment.
*Exercises.*--1. For what different purposes are sound waves employed in
the body?
2. How do sound waves originate? How are they transmitted? How do they
differ from the waves on water?
3. How are sound waves able to act as nerve stimuli?
4. Describe two methods of reenforcing sound waves. Which method is
employed in the body?
5. Name all the parts of the body that are directly or indirectly
concerned in the production of sound.
6. Describe the larynx.
7. Describe the condition of the vocal cords in speaking and in ordinary
breathing.
8. How are sounds differing in pitch and intensity produced by the larynx?
9. How is the sound produced by the vocal cords changed into speech?
10. What parts of the ear are concerned in transmitting sound waves?
11. Give the purposes of the middle ear.
12. Trace a sound wave from a bell to the basilar membrane, and trace the
impulse that it causes from there to the brain.
13. Give the purpose of the Eustachian tubes; of the rods of Corti; of the
semicircular canals.
14. Give directions for the proper care of the ear.
PRACTICAL WORK
*To illustrate the Origin of Sound.*--1. Strike a bell an easy blow and
hold some light substance, as a pith ball attached to a thread, against
the side, noting the result. 2. Sound a tuning fork by striking it against
the table. Test it for vibrations as above, or by letting the vibrating
prongs touch the surface of water. 3. Pluck a string of a guitar or
violin, and find proof that it is vibrating while giving out sound.
*To show the Transmission of Sound.*--1. Vibrate a tuning fork and press
the stem against a table or desk. The vibrations which are reenforced in
this way will be heard in all parts of the room. Now press one end of a
wooden rod, as a broom handle, against the table, and bring the stem of
the vibrating fork against the other end. The vibrations now move down the
stick to the table, from whence they are communicated to the air. Observe
that the sound waves, to reach the ear, must pass through the rod, the
table, and the air. 2. Fasten the tuning fork to a flat piece of cork by
pressing the stem into a small hole in the center. Vibrate the fork and
let the cork rest on the surface of water in a half-filled tumbler on the
table. The sound will, as before, pass to the table and then to the air.
Observe that in this case the vibrations are transmitted by a liquid, a
solid, and by the air. Compare this action with the transmission of sound
waves by different portions of the ear.
*To show Effects of Sound Waves.*--1. Place two large tuning forks of the
same pitch, and mounted on thin boxes for reenforcing their vibrations,
near each other on a table. Vibrate one of the forks for a moment and then
stop it by means of the hand. Observe that the other fork has been set in
vibration. (This experiment does not work with forks of different pitch.)
2. While holding a thin piece of paper against a comb with the open lips,
produce musical tones with the vocal cords. These will set the paper in
vibration, producing the so-called "comb music." 3. Examine the disk in a
telephone which is set in vibration by the voice. Observe that it is a
thin disk and, like the membrane of the ear, has air on both sides of it.
*To show the Reenforcement of Sound.*--1. Vibrate a tuning fork in the air,
noting the feebleness of the tone produced. Then hold the stem against a
door or the top of a table, noting the difference. 2. Hold a vibrating
tuning fork over a tall jar, or bottle, and gradually add water. If the
vessel is sufficiently tall, a depth will be reached where the air in the
vessel reenforces the sound from the fork. 3. Hold a vibrating fork over
the mouth of a small fruit jar, partly covered with a piece of cardboard.
By varying the size of the opening, a position will be found where the
sound is reenforced. If not successful at first, try bottles and jars of
different sizes.
*To illustrate the Manner of Vibration of the Liquid in the Internal
Ear.*--Tie a piece of dental rubber over the end of a glass or wooden tube
about half an inch in diameter and six inches in length. Fill the tube
entirely full of water and, without spilling, tie a piece of thin rubber
tightly over the other end. Holding the tube horizontally, press the
rubber in at one end and note that it is pushed out at the other end. Make
an imitation of a vibration with the finger against the rubber at one end
of the tube and note the effect at the other end. To what do the tube and
the rubber on the ends of the tube correspond in the internal ear?
[Fig. 156]
Fig. 156--*Simple apparatus* for demonstrating the larynx.
*To show the Plan of the Larynx.*--Cut from stiff paper four pieces of
different shapes as indicated in Fig. 156. (The piece to the left should
have a length of about six inches, the others proportionally large.) The
largest represents the thyroid cartilage, the next in size the cricoid,
and the two smallest the arytenoid cartilages. By means of pins, or
threads, connect these with each other according to the description of the
larynx on page 253. With this simple model the movements of the different
cartilages and their effect upon the vocal cords may be illustrated.
*To show the Relation of the Movements of the Vocal Organs to the
Production of Different Sounds.*--1. Lightly grasp the larynx with the
fingers while talking. Observe the changes, both in the position and shape
of the larynx, in the production of sounds of different pitch. 2. Observe
the difference in the action of the muscles of respiration in the
production of loud and faint sounds. 3. Pronounce slowly the vowels, A, E,
I, O, U, and the consonants C, F, K, M, R, S, T, and V, noting the shape
of the mouth, the position of the tongue, and the action of the lips in
each case.
*To demonstrate the Ear.*--Examine a dissectible model of the ear, locating
and naming the different parts. Trace as far as possible the path of the
sound waves and find the termination of the auditory nerve. Note also the
relative size of the parts, and calculate the number of times the model is
larger than the natural ear. _Suggestion_: The greatest diameter of the
internal ear is about three fourths of an inch.
In an extended course it is a profitable exercise to dissect the ear of a
sheep or calf, observing the auditory canal, middle ear, bridge of bones,
and the tympanic membrane with attached malleus and tensor tympanic
muscle. Pass a probe from the nasal pharynx through the Eustachian tube
into the middle ear. With bone forceps or a fine saw, split open the
petrous portion of the temporal bone and observe the cochlea and the
semicircular canals. By a careful dissection other parts of interest may
also be shown.
CHAPTER XXII - THE EYE
Sight is considered the most important of the sensations. It is the chief
means of bringing the body into proper relations with its surroundings
and, even more than the sensation of hearing, is an avenue for the
reception of ideas. The sense organs for the production of sight are the
eyes; the external stimulus is
*Light.*--Light, like sound, consists of certain vibrating movements, or
waves. They differ from sound waves, however, in form, velocity, and in
method of origin and transmission. Light waves are able to pass through a
vacuum, thus showing that they are not dependent upon air for their
transmission. They are supposed to be transmitted by what the physicist
calls ether--a highly elastic and exceedingly thin substance which fills
all space and penetrates all matter. As a rule, light waves originate in
bodies that are highly heated, being started by the vibrations of the
minute particles of matter.
Light is influenced in its movements by various conditions. In a substance
of uniform density it moves with an unchanging velocity and in a straight
line. If it enters a less dense, or rarer, substance, its velocity
increases; if one more dense, its velocity diminishes; and if it enters
either the rarer or denser substance in any direction other than
perpendicularly, it is bent out of its course, or _refracted_. If it
strikes against a body lying in its course, it may be thrown off
(_reflected_), or it may enter the body and either be passed on through
(_transmitted_) or _absorbed_ (Fig. 157). Light which is absorbed is
transformed into heat.
*Kinds of Reflection.*--Waves of light striking against the smooth surface
of a mirror are thrown off in definite directions, depending on the angle
at which they strike. (Illustrate by holding a mirror in the direct rays
of the sun.) But light waves that strike rough surfaces are reflected in
practically all directions and apparently without reference to the angle
at which they strike. (Illustrate by placing a piece of white paper in the
direct rays of the sun. It matters not from what direction it is viewed,
waves of light strike the eye.) This kind of reflection is called
_diffusion_, and it serves the important purpose of making objects
visible. The light waves passing out in all directions from objects which
have received light from the sun, or some other luminous body, enable them
to be seen.
[Fig. 157]
Fig. 157--*Diagram illustrating passage of light waves.*On the right the
light is transmitted by the glass, reflected by the mirror, refracted by
the prism, and absorbed by the black cloth. On the left the light from the
candle forms an image by passing through a small hole in a cardboard and
falling upon a screen.
*Formation of Images.*--Another principle necessary to seeing is that of
refraction. _Refraction_ means the bending, or turning, of light from a
straight course. One of the most interesting effects of refraction is the
formation of images of objects, such as may be accomplished by light from
them passing in a certain manner through convex lenses. If, for example, a
convex lens be moved back and forth between a candle and a screen in a
dimly lighted room, a position will be found where a picture of the candle
falls upon the screen. This picture, called the _image_, results from the
refraction of the candle light in passing through the lens.
[Fig. 158]
Fig. 158--*Diagram illustrating formation of images.* On the right the
image is formed by a double convex lens; on the left by the lenses of the
eye. The candle flame represents a luminous, or light-giving, body; but
light passes from the large arrow by reflection. (See text.)
In order to form an image, the light waves spreading out from the object
must be brought together, or focused. Focusing means literally the
bringing of light to a point, but it is evident in the formation of an
image that all the waves are not brought to a single point. If they were,
there would be no image. In the example of the candle given above, the
explanation is as follows:
The light from the candle comes from a great number of separate and
distinct points in the candle flame. The lens, by its peculiar shape,
bends the waves coming from any single point so that they are brought to a
corresponding point on the screen. Furthermore, the points of focused
light are made to occupy the same relative positions on the screen as the
points from which they emanate in the candle flame (Fig. 158). This is why
the area of light on the screen has the same form as the candle, or makes
an image of it. The same explanation applies if, instead of the luminous
candle, a body that simply reflects light, as a book, is used.
*The Problem of Seeing.*--What we call _seeing_ is vastly more than the
stimulation of the brain through the action of light upon afferent
neurons. It is the _perceiving _of all the different things that make up
our surroundings. If one looks toward the clear sky, he receives a
_sensation of light_, but sees no object. He may also get a sensation of
light with the eyelids closed, if he turn the eyes toward the window or
some bright light. But how different when the light from various objects
enters the eyes. There is apparently no consciousness of light, but
instead a consciousness of the size, form, color, and position of the
objects. _Seeing is perceiving objects._ Stimulation by the light waves is
only the means toward this end. The chief problem in the study of sight is
that of determining _how light waves enable us to become conscious of
objects._
*Sense Organs of Sight.*--The sense organs of sight consist mainly of the
two eyeballs. Each of these is located in a cavity of the skull bones,
called the _orbit_, where it is held in position by suitable tissues and
turned in different directions by a special set of muscles. A cup-shaped
receptacle is provided within the orbit, by layers of fat, and a smooth
surface is supplied by a double membrane that lies between the fat and the
eyeball. In front the eyeballs are provided with movable coverings, called
the _eyelids_. These are composed of dense layers of connective tissue,
covered on the outside by the skin and lined within by a sensitive
membrane, called the _conjunctiva_. At the base of the lids the
conjunctiva passes to the eyeball and forms a firmly attached covering
over its front surface. This membrane prevents the passage of foreign
materials back of the eyeball, and by its sensitiveness stimulates effort
for the removal of irritating substances from beneath the lids. The
eyelashes and the eyebrows are also a means of protecting the eyeballs.
*The Eyeball*, or globe of the eye, is a device for _focusing_ light upon
a sensitized nervous surface which it incloses and protects. In shape it
is nearly spherical, being about an inch in diameter from right to left
and nine tenths of an inch both in its vertical diameter and from front to
back. It has the appearance of having been formed by the union of two
spherical segments of different size. The smaller segment, which forms
about one sixth of the whole, is set upon the larger and forms the
projecting transparent portion in front. The walls of the eyeballs are
made up of three separate layers, or coats--an _outer coat_, a _middle
coat_, and an _inner coat_ (Fig. 159).
[Fig. 159]
Fig. 159--*Diagram of the eyeball in position.* 1. Yellow spot. 2. Blind
spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens.
7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris
containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13.
Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle
of eyelid. 16. Bone. _A._ Posterior chamber containing the vitreous humor.
_B._ Anterior chamber containing the aqueous humor.
*The Outer Coat* surrounds the entire globe of the eye and consists of two
parts--the sclerotic coat and the cornea. The _sclerotic coat_ covers the
greater portion of the larger spherical segment and is recognized in front
as "the white of the eye." It is composed mainly of fibrous connective
tissue and is dense, opaque, and tough. It preserves the form of the
eyeball and protects the portions within. It is pierced at the back by a
small opening which admits the optic nerve, and in front it becomes
changed into the peculiar tissue that makes up the cornea.
The _cornea_ forms the transparent covering over the lesser spherical
segment of the eyeball, shading into the sclerotic coat at its edges. It
has a complex structure, consisting in the main of a transparent form of
connective tissue. It serves the purpose of admitting light into the
eyeball.
*The Middle Coat* consists of three connected portions--the _choroid coat_,
the _ciliary processes_, and the _iris_. These surround the larger
spherical segment. All three parts are rich in blood vessels, containing
the blood supply to the greater portion of the eyeball.
The _choroid coat_ lies immediately beneath the sclerotic coat at all
places except a small margin toward the front of the eyeball. It is
composed chiefly of blood vessels and a delicate form of connective tissue
that holds them in place. It contains numerous pigment cells which give it
a dark appearance and serve to absorb surplus light. Near where the
sclerotic coat joins the cornea, the choroid coat separates from the outer
wall and, by folding, forms many slight projections into the interior
space. These are known as the _ciliary processes_. The effect of these
folds is to collect a large number of capillaries into a small space and
to give this part of the eyeball an extra supply of blood. Between the
ciliary processes and the sclerotic coat is a small muscle, containing
both circular and longitudinal fibers, called the _ciliary muscle_.
The _iris_ is a continuation of the choroid coat across the front of the
eyeball. It forms a dividing curtain between the two spherical segments
and gives the color to the eye. At its center is a circular opening,
called the _pupil_, which admits light to the back of the eyeball. By
varying the size of the pupil, the iris is able to regulate the amount of
light which passes through and it employs for this purpose two sets of
muscular fibers. One set of fibers forms a thin band which encircles the
pupil and serves as a sphincter to diminish the opening. Opposing this are
radiating fibers which are attached between the inner and outer margins of
the iris. By their contraction the size of the opening is increased. Both
sets of fibers act reflexively and are stimulated by variations in the
light falling upon the retina.
[Fig. 160]
Fig. 160--*Diagram showing main nervous elements in the retina.* Light
waves stimulate the rods and cones at back surface of the retina, starting
impulses which excite the ganglion cells at the front surface. Fibers from
the ganglion cells pass into the optic nerve.
*The Inner Coat, or Retina.*--This is a delicate membrane containing the
expanded termination of the optic nerve. It rests upon the choroid coat
and spreads over about two thirds of the back surface of the eyeball.
Although not more than one fiftieth of an inch in thickness, it presents a
very complex structure, essentially nervous, and is made up of several
distinct layers. Of chief importance in the outer layer are the cells
which are acted upon directly by the light and are named, from their
shape, the _rods_ and _cones_. In contact with these, but occupying a
separate layer, are the ends of small afferent nerve cells. These in turn
communicate with nerve cells in a third layer, known as the ganglion
cells, that send their fibers into the optic nerve (Fig. 160).
In the center of the retina is a slight oval depression having a faint
yellowish color, and called, on that account, the _yellow spot_. This is
the part of the retina which is most sensitive to light. Directly over the
place of entrance of the optic nerve is a small area from which the rods
and cones are absent and which, therefore, is not sensitive to light. This
is called the _blind spot_. (See Practical Work.)
*The Crystalline Lens.*--Immediately back of the iris and touching it is a
transparent, rounded body, called the crystalline lens. This is about one
fourth of an inch thick and one third of an inch through its long
diameter, and is more curved on the back than on the front surface. It is
inclosed in a thin sheath, called the _membranous capsule_, which connects
with a divided sheath from the sides of the eyeball, called the
_suspensory ligament_ (Fig. 159). Both the lens and the capsule are highly
elastic.
*Chambers and "Humors" of the Eyeball.*--The crystalline lens together with
the suspensory ligament and the ciliary processes form a partition across
the eyeball. This divides the eye space into two separate compartments,
which are filled with the so-called "humors" of the eye. The front cavity
of the eyeball, which is again divided in part by the iris, is filled with
the _aqueous_ humor. This is a clear, lymph-like liquid which contains an
occasional white corpuscle. It has a feeble motion and is slowly added to
and withdrawn from the eye. It is supplied mainly by the blood vessels in
the ciliary processes and finds a place of exit through a small lymph duct
at the edge of the cornea (Fig. 159).
The back portion of the eyeball is filled with a soft, transparent,
jelly-like substance, called the _vitreous_ humor. It is in contact with
the surface of the retina at the back and with the attachments of the lens
in front, being surrounded by a thin covering of its own, called the
_hyaloid membrane_. The aqueous and vitreous humors aid in keeping the
eyeball in shape and also in focusing.
*How we see Objects.*--To see an object at least four things must happen:
1. Light must pass from the object into the eye. Objects cannot be seen
where there is no light or where, for some reason, it is kept from
entering the eye.
2. The light from the object must be focused (made to form an image) on
the retina. In forming the image, an area of the retina is stimulated
which corresponds to _the form of the object_.
3. Impulses must pass from the retina to the brain, stimulating it to
produce the sensations.
4. The sensations must be so interpreted by the mind as to give an
impression of the object.
*Focusing Power of the Eyeball.*--The eyeball is essentially a device for
focusing light. All of its transparent portions are directly concerned in
this work, and the portions that are not transparent serve to protect and
operate these parts and hold them in place. Of chief importance are the
crystalline lens and the cornea. Both of these are lenses. The cornea with
its inclosed liquid is a plano-convex lens, while the crystalline lens is
double convex.(123) Because of the great difference in density between the
air on the outside and the aqueous humor within, the cornea is the more
powerful of the two. The crystalline lens, however, performs a special
work in focusing which is of great importance. The iris also aids in
focusing since it, through the pupil, regulates the amount of light
entering the back chamber of the eyeball and causes it to fall in the
center of the crystalline lens, the part which focuses most accurately.
[Fig. 161]
Fig. 161--*Diagram showing changes in shape of crystalline lens* to adapt
it to near and distant vision.
*Accommodation.*--A difficulty in focusing arises from the fact that the
degree of divergence of the light waves entering the eye from different
objects, varies according to their distance. Since the waves from any
given point on an object pass out in straight lines in all directions, the
waves that enter the eye from distant objects are at a different angle
from those that enter from near objects. In reality waves from distant
objects are practically parallel, while those from very near objects
diverge to a considerable degree. To adjust the eye to different distances
requires some change in the focusing parts that corresponds to the
differences in the divergence of the light. This change, called
_accommodation_, occurs in the crystalline lens.(124) In the process of
accommodation, changes occur in the shape of the crystalline lens, as
follows:
1. In looking from a distant to a near object, the lens becomes more
convex, _i.e._, rounder and thicker (Fig. 161). This change is necessary
because the greater divergence of the light from the near objects requires
a greater converging power on the part of the lens.(125)
2. In looking from near to distant objects, the lens becomes flatter and
thinner (Fig. 161). This change is necessary because the less divergent
waves from the distant objects require less converging power on the part
of the lens.
The method employed in changing the shape of the lens is difficult to
determine and different theories have been advanced to account for it. The
following, proposed by Helmholtz, is the theory most generally accepted:
The lens is held in place back of the pupil by the suspensory ligament.
This is attached at its inner margin to the membranous capsule, and at its
outer margin to the sides of the eyeball, and entirely surrounds the lens.
It is drawn perfectly tight so that the sides of the eyeball exert a
continuous tension, or pull, on the membranous capsule, which, in its
turn, exerts pressure on the sides of the lens, tending to flatten it.
This arrangement brings the elastic force of the eyeball into opposition
to the elastic force of the lens. The ciliary muscle plays between these
opposing forces in the following manner:
_To thicken the lens_, the ciliary muscle contracts, pulling forward the
suspensory ligament and releasing its tension on the membranous capsule.
This enables the lens to thicken on account of its own elastic force. _To
flatten the lens_, the ciliary muscle relaxes, the elastic force of the
eyeball resumes its tension on the suspensory ligament, and the membranous
capsule resumes its pressure on the sides of the lens. This pressure,
overcoming the elastic force of the lens, flattens it.
*Movements of the Eyeballs.*--In order that the light may enter the
eyeballs to the best advantage, they must be moved in various directions.
These movements are brought about through the action of six small muscles
attached to each eyeball. Four of these, named, from their positions, the
superior, inferior, internal, and external recti muscles, are attached at
one end to the sides of the eyeball and at the other end to the back of
the orbit (Fig. 162). These, in the order named, turn the eyes upward,
downward, inward, and outward. The other two, the superior and inferior
oblique muscles, aid in certain movements of the recti muscles and, in
addition, serve to rotate the eyes slightly. The movements of the eyeballs
are similar to those of ball and socket joints.
[Fig. 162]
Fig. 162--*Exterior muscles of eyeball.*
*Binocular Vision.*--In addition to directing the eyeballs so that light
may enter them to the best advantage from different objects, the muscles
also enable two eyes to be used as one. Whenever the eyes are directed
toward the same object, an image of this object is formed on the retina of
each. Double vision is prevented only by having the images fall on
corresponding places in the two eyes. This is accomplished by the muscles.
In each act of seeing, it becomes the task of the superior and inferior
recti muscles to keep the eyes in the same plane, and of the external and
internal recti muscles to give just the right amount of convergence. If
slight pressure is exerted against one of the eyes, the action of the
muscles is interfered with and, as a consequence, one sees double. The
advantages of two eyes over one in seeing lie in the greater distinctness
and broader range of vision and in the greater correctness of judgments of
distance.
*Visual Sensations.*--The visual sensations include those of _color_ and
those of a _general sensibility to light_. Proof of the existence of these
types of sensation is found in color blindness, a defect which renders the
individual unable to distinguish certain colors when he is still able to
see objects. Color sensations are the results of light waves of different
lengths acting on the retina. While the method by which waves of one
length produce one kind of sensation and those of another length a
different sensation is not understood, the cones appear to be the portions
of the retina acted on to produce the color. On the other hand, the rods
are sensitive to all wave lengths and give general sensibility to light.
*Visual Perceptions.*--"Seeing" is very largely the mental interpretation
of the primary sensations and the conditions under which they occur. For
example, our ability to see objects in their natural positions when their
images are inverted on the retina is explained by the fact that we are not
conscious of the retinal image, but of the mind's interpretation of it
through experience. Experience has also taught us to locate objects in the
direction toward which it is necessary to turn the eyes in order to see
them. In other words, we see objects in the direction from which the light
enters the eyes. That the object is not always in that direction is shown
by the image in the mirror. The apparent size and form of objects are
inferences, and they are based in part upon the size and form of the area
of the retina stimulated. We judge of distance by the effort required to
converge the eyes upon the objects, by the amount of divergence of the
waves entering the pupil, and also by the apparent size of the object.
*The Lachrymal Apparatus.*--Seeing requires that the light penetrate to the
retina. For this reason all the structures in front of the retina are
transparent. One of these structures, the cornea, on account of its
exposure to the air, is liable to become dry, like the skin, and to lose
its transparency. To preserve the transparency of the cornea, and also to
lubricate the eyelids and aid in the removal of foreign bodies, a
secretion, called _tears_, is constantly supplied.
[Fig. 163]
Fig. 163--*Diagram of irrigating system of the eye.* After wetting the
eyeball the tears may also moisten the air entering the lungs.
The lachrymal, or tear, glands are situated at the upper and outer margins
of the orbits. They have the general structure of the salivary glands and
discharge their liquid by small ducts beneath the upper lids. From here
the tears spread over the surfaces of the eyeballs and find their way in
each eye to two small canals whose openings may be seen on the edges of
the lids near the inner corner (Fig. 163). These canals unite to form the
_nasal duct_, which conveys the tears to the nasal cavity on the same side
of the nose. When by evaporation the eyeball becomes too dry, the lids
close reflexively and spread a fresh layer of tears over the surface. Any
excess is passed into the nostrils, where it aids in moistening the air
entering the lungs.
HYGIENE OF THE EYE
*Defects in Focusing.*--The delicacy and complexity of the sense organs of
sight render them liable to a number of imperfections, or defects, the
most frequent and important being those of focusing. Such defects not only
result in the imperfect vision of objects, but they throw an extra strain
upon the nervous system and may render the process of seeing exceedingly
painful.
A normal eye is able, when relaxed, to focus light accurately from objects
which are twenty feet or more away and to accommodate itself to objects as
near as five inches. An eye is said to be _myopic_, or _short-sighted_,
when it is unable to focus light waves from distant objects, but can only
distinguish the objects which are near at hand. In such an eye the ball is
too long for the converging power of the lenses, and the image is formed
in front of the retina (_C_, Fig. 164).
[Fig. 164]
Fig. 164--*Diagrams illustrating long-sightedness and short-sightedness*,
and method of remedying these defects by lenses. _A._ Normal eye. _B._
Long-sighted eye. _C._ Short-sighted eye.
A _long-sighted_, or _hypermetropic_, eye is one which can focus light
from distant objects, but not from near objects. In such an eye the ball
is too short for the converging power of the lenses and the image tends to
form back of the retina (_B_, Fig. 164). These defects in focusing are
remedied by wearing glasses with lenses so shaped as to counteract them.
Short-sightedness is corrected by concave lenses and long-sightedness by
convex lenses, as shown in diagrams above.
_Astigmatism_ is another defect in the focusing power of the eye. In
astigmatism the parts of the eye fail to form the image in the same plane,
so that all portions of the object do not appear equally distinct. Certain
parts of it are indistinct, or blurred. The cause is found in some
difference in curvature of the surfaces of the cornea or crystalline lens.
It is corrected by lenses so ground as to correct the particular defects
present in a given eye.
Whenever defects in focusing are present, particularly in astigmatism,
extra work is thrown on the ciliary muscle as well as the muscles that
move the eyeballs. The result is frequently to induce a condition, known
as _muscle weakness_, which renders it difficult to use the eyes. Even
after the defect in focusing has been remedied, the muscles recover slowly
and must be used with care. For this reason glasses should be fitted by a
competent oculist(126) as soon as a defect is known to exist. When one is
unduly nervous, or suffers from headache, the eyes should be examined for
defects in focusing (page 326).
*Eye Strain and Disease.*--The extra work thrown upon the nervous system
through seeing with defective eyes, especially in reading and other close
work, is now recognized as an important cause of disease. Through the tax
made upon the nervous system by the eyes, there may be left an
insufficient amount of nervous energy for the proper running of the vital
processes. As a result there is a decline of the health. Ample proof that
eye strain interferes with the vital processes and causes ill health, is
found in the improvements that result when, by means of glasses, this is
relieved.
*The Eyes of School Children.*--School children often suffer from defects
of vision which render close work burdensome, and cause headache, general
nervousness, and disease. Furthermore, the visual defects may be unknown
both to themselves and to their parents. Pupils showing indications of
eye-strain should be examined by an oculist, and fitted with glasses
should defects be discovered.(127) The precaution, adopted by many
schools, of having the eyes of all children examined by a competent
physician employed for the purpose, is most excellent and worthy of
imitation.
*Reading Glasses.*--Many people whose eyes are weak, because slightly
defective, find great relief in the use of special glasses for reading and
other close work. By using such glasses they may postpone the time when
they are compelled to wear glasses constantly. It is in the close work
that the extra strain comes upon the eyes, and if this is relieved, one
can much better withstand the work of distant vision. The reading glasses
should be fitted by a competent oculist, and used only for the purpose for
which they are intended.
*General Precautions in the Use of the Eyes.*--If proper care is exercised
in the use of the eyes, many of their common ailments and defects may be
avoided. Any one, whether his eyes are weak or strong, will do well to
observe the following precautions:
1. Never read in light that is very intense or very dim. 2. When the eyes
hurt from reading, stop using them. 3. Never hold a book so that the
smooth page reflects light into the eyes. The best way is to sit or stand
so that the light passes over the shoulder to the book. 4. Never study by
a lamp that is not shaded. 5. Practice cleanliness in the care of the
eyes. Avoid rubbing the eyes with the fingers unless sure the fingers are
clean.
If the eyes are weak, use them less and avoid, if possible, reading by
artificial light. Weak eyes are sometimes benefited by bathing them in
warm water, or with water containing enough salt to make them smart
slightly. Boracic acid dissolved in water (40 grains to 4 ounces of
distilled water) is also highly recommended as a wash for weak eyes.
[Fig. 165]
Fig. 165--*Method of procedure in lifting the eyelid* (Pyle).
*Removal of Foreign Bodies from the Eyes.*--Foreign bodies embedded in the
eyeball should be removed by the oculist or physician. Small particles of
dust or cinder may be removed without the aid of the physician, by
exercising proper care. First let the tears, if possible, wash the
offending substance to the corner of the eye, or edge of the lid, where it
can be removed with a soft cloth. If it sticks to the ball or the under
surface of the lid, it will be necessary to find where it is located, and
then dislodge it from its position. Begin by examining the lower lid. Pull
it down sufficiently to expose the inner surface, and, if the foreign
substance be there, wipe it off with the hem of a clean handkerchief. If
it is not under the lower lid, it will be necessary to fold back the upper
lid. "The patient is told to look down, the edge of the lid and the lashes
are seized with the forefinger and thumb of the right hand (Fig. 165), and
the lid is drawn at first downward and forward away from the globe; then
upward and backward over the point of the thumb or forefinger of the left
hand, which is held stationary on the lid, and acts as a fulcrum."(128)
The foreign body is now removed in the same manner as from the lower lid.
A large lens may be used to good advantage in finding the irritating
substance.
*Strong Chemicals in the Eyes.*--Students in the laboratory frequently,
through accident, get strong chemicals, as acids and bases, in the eyes.
The first thing to do in such cases is quickly and thoroughly to _flood
the eyes with water_. Any of the chemical which remains may then be
counteracted by the proper reagent, care being taken to use a very dilute
solution. To counteract an acid, use sodium bicarbonate (cooking soda),
and for bases use a very dilute solution of acetic acid (vinegar). To
guard against getting the counteractive agent too strong for the inflamed
eye, it should first be tried on an eye that has not been injured.
*Summary.*--The nervous impulses that cause the sensation of sight are
started by light waves falling upon a sensitized nervous surface, called
the retina. By means of refractive agents, forming a part of the eyeball
in front of the retina, light from different objects is focused and made
to form images of the objects upon the surface. In this way the light is
made to stimulate a portion of the retina corresponding to the form of the
object. This, _the image method of stimulation_, enables the mind to
recognize objects and to locate them in their various positions. While the
greater portion of the eyeball is concerned in the focusing of light, the
crystalline lens, operated by the ciliary muscle, serves as the special
instrument of accommodation. Muscles attached to the eyeballs turn them in
different directions, and so adjust them with reference to each other that
double vision is avoided.
*Exercises.*--1. Under what conditions are light waves reflected,
refracted, and absorbed?
2. Why does the body not need a light-producing apparatus, corresponding
to the larynx in the production of sound?
3. How is the light from a candle made to form an image?
4. What different things must happen in order that one may see an object?
5. Make a sectional drawing of the eyeball, locating and naming all the
parts.
6. Of what parts are the outer, middle, and inner coats of the eyeball
made up?
7. What portions of the eyeball reflect light? What absorb light? What
transmit light? What refract light?
8. Show how the iris, the crystalline lens, the retina, the ciliary
muscle, and the cornea aid in seeing.
9. Trace a wave of light from a visible object to the retina.
10. Why does not the inverted image on the retina cause us to see objects
upside down?
11. What change occurs in the shape of the crystalline lens when we look
from distant to near objects? From near to distant objects? Why are these
changes necessary? How are they brought about?
12. How does the method of adjustment, or accommodation, of the eyeball
differ from that of a telescope or a photographer's camera?
13. With two eyes how are we kept from seeing double?
14. What different purposes are served by the tears. Trace them from the
lachrymal glands to the nostrils.
15. Show how the proper lenses remedy short- and long-sightedness.
16. Describe the conjunctiva and give its functions. Why should it be so
sensitive?
17. How may eye strain cause disease in parts of the body remote from the
eyes?
18. How does "image stimulation" differ from light stimulation in general?
PRACTICAL WORK
*To illustrate Simple Properties of Light.*--1. Heat an iron or platinum
wire in a clear gas flame. Observe that when a high temperature is reached
it gives out light or becomes luminous.
2. Cover one hand with a white and the other with a black piece of cloth,
and hold both for a short time in the direct rays of the sun. Note and
account for the difference in temperature which is felt.
3. Stand a book or a block of wood by the side of an empty pan in the
sunlight, so that the end of the shadow falls on the bottom of the pan.
Mark the place where the shadow terminates and fill the pan with water.
Account for the shadow's becoming shorter.
4. Place a coin in the center of an empty pan and let the members of the
class stand where the coin is barely out of sight over the edges of the
pan. Fill the pan with water and account for the coin's coming into view.
Show by a drawing how light, in passing from the water into the air, is so
bent as to enter the eye.
5. With a convex lens, in a darkened room, focus the light from a candle
flame so that it falls on a white screen and forms an image of the candle.
Observe that the image is inverted. In a well-lighted room focus the light
from a window upon a white screen. Show that, as the distance from the
window to the screen is changed, the position of the lens must also be
changed. (Accommodation.)
6. Hold a piece of cardboard, about eight inches square and having a
smooth, round hole an eighth of an inch in diameter in the center, in
front of a lighted candle in a darkened room. Back of the opening place a
muslin or paper screen (Fig. 157). Observe that a dim image is formed.
Account for the fact that it is inverted. Hold a lens between the
cardboard and the screen so that the light passes through it also. The
image should now appear smaller and more distinct.
[Fig. 166]
Fig. 166--*Diagram* for proving presence of the blind spot.
*To prove the Presence of the Blind Spot.*--Close the left eye and with the
right gaze steadily at the spot on the left side of this page (Fig. 166).
Then starting with the book a foot or more from the face, move it slowly
toward the eye. A place will be found where the spot on the right entirely
disappears. On bringing it nearer, however, it is again seen. As the book
is moved forward or backward, the position of the image of this spot
changes on the retina. When the spot cannot be seen, it is because the
image falls on the blind spot.
*Dissection of the Eyeball.*--Procure from the butcher two or three
eyeballs obtained from cattle. After separating the fat, connective
tissue, and muscle, place them in a shallow vessel and cover with water.
Insert the blade of a pair of sharp scissors at the junction of the
sclerotic rotic coat with the cornea and cut from this point nearly around
the entire circumference of the eyeball, passing near the optic nerve.
Spread open in the water and identify the different parts from the
description in the text. Open the second eyeball in water by cutting away
the cornea. Examine the parts in front of the lens.
[Fig. 167]
Fig. 167--*Model* for demonstrating the eyeball.
*To illustrate Accommodation.*--Paste together the ends of a strip of stiff
writing paper (two by five inches) making a ring a little less than three
inches in diameter. This is to represent the crystalline lens. Now paste a
piece of thin paper (two by seven inches) upon a second strip of the same
size, leaving an open place in the middle for the insertion of the paper
lens. A flexible piece of cardboard (three by twelve inches) is now bent
into the form of a half circle and to its ends are fastened the strips of
paper containing the ring. Make a small hole in each of the four corners
of the bent cardboard. Through these holes pass two loops of thread, or
fine string, in opposite directions, letting the ends hang loose from the
cardboard.
When everything is in position, the tension from the cardboard flattens
the paper lens, while pulling the strings releases this tension and
permits the lens to become more rounded. With this simple device the
changes in the curvature of the lens for near and distant vision are
easily shown.
CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL
"To cure was the voice of the Past: to prevent is the divine
whispering of To-day."
As stated in the introduction to our study, the fundamental law of hygiene
is the law of harmony: _Habits of living must harmonize with the plan of
the body._ Having acquainted ourselves with the plan of the body, we may
now review briefly those conditions that help or hinder its various
activities. The hygiene already presented in connection with the study of
the various organs may be condensed into general rules, or laws, as
follows:
1. Of exercise: Exercise daily the important groups of muscles.
2. Of form: Preserve the natural form of the body.
3. Of energy: Observe regular periods of rest and exercise and avoid
exhaustion.
4. Of nutriment: Eat moderately of a well-cooked and well-balanced diet
and drink freely of pure water.
5. Of respiration: Breathe freely and deeply of pure air and spend a part
of each day out of doors.
6. Of nervous poise: Suppress wasteful and useless forms of nervous
activity, avoid nervous strain, and practice cheerfulness.
7. Of cleanliness: Keep the body and its immediate surroundings clean.
8. Of restraint: Abstain from the unnecessary use of drugs as well as from
the practice of any form of activity known to be harmful to the body.
9. Of elimination: Observe all the conditions that favor the regular
discharge of waste materials from the body.
Obedience to these laws is of vast importance in the proper management of
the body. They should, indeed, be so thoroughly impressed upon the mind as
to become fixed habits. There are, however, other conditions that relate
to this problem, and it is to these that we now turn. These conditions
have reference more specifically to
*The Prevention of Disease.*--While the average length of life is not far
from thirty-five years, the length of time which the average individual is
capable of living is, according to some of the lowest estimates, not less
than seventy years. This difference is due to disease. People do not, as a
rule, die on account of the wearing out of the body as seen in extreme old
age, but on account of the various ills to which flesh is heir. It is true
that many people meet death by accident and not a few are killed in wars,
but these numbers are small in comparison with those that die of bodily
disorders. The prevention of disease is the greatest of all human
problems. Though the fighting of disease is left largely to the physician,
much is to be gained through a more general knowledge of its causes and
the methods of its prevention.
*Causes of Disease.*--Disease, which is some _derangement of the vital
functions_, may be due to a variety of causes. Some of these causes, such
as hereditary defects, are remote and beyond the control of the
individual. Others are the result of negligence in the observance of
well-recognized hygienic laws. Others still are of the nature of
influences, such as climate, the house in which one lives, or one's method
of gaining a livelihood, that produce changes in the body, imperceptible
at the time, but, in the long run, laying the foundations of disease. And
last, and most potent, are the minute living organisms, called microbes or
germs, that find their way into the body. Although there are two general
kinds of germs, known as _bacteria_ (one-celled plants) and _protozoa_
(one-celled animals), most of our germ diseases are caused by bacteria.
*Effects of Germs.*--While there are many kinds of germs that have no ill
effect upon the body and others that are thought to aid it in its work,
there are many well-known varieties that produce effects decidedly
harmful. They gain an entrance through the lungs, food canal, or skin,
and, living upon the fluids and tissues, multiply with great rapidity
until they permeate the entire body. Not only do they destroy the
protoplasm, but they form waste products, called _toxins_, which act as
poisons. Diseases caused by germs are known as infectious, or contagious,
diseases.(129) The list is a long one and includes smallpox, measles,
diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe,
malaria, yellow fever, and others of common occurrence. In addition to the
diseases that are well pronounced, it is probable that germs are
responsible also for certain bodily ailments of a milder character.(130)
*Avoidance of Germ Diseases.*--The problem of preventing diseases caused by
germs is an exceedingly difficult one and no solution for all diseases has
yet been found. One's chances of avoiding such diseases, however, may be
greatly enhanced:
1. By strengthening the body through hygienic living so that it offers
greater resistance to the invasions of germs.
2. By living as far as possible under conditions that are unfavorable to
germ life.
3. By understanding the agencies through which disease germs are spread
from person to person.
*Conditions Favorable and Unfavorable for Germs.*--Conditions favorable for
germ life are supplied by animal and vegetable matter, moisture, and a
moderate degree of warmth. Hence disease germs may be kept alive in damp
cellars and places of filth. Even living rooms that are poorly lighted or
ventilated may harbor them. Water may, if it contain a small per cent of
organic matter, support such dangerous germs as those of typhoid fever.
Fresh air, sunlight, dryness, cleanliness, and a high temperature, on the
other hand, are destructive of germs. The germs in impure water, as
already noted (page 165), are destroyed by boiling.
*How Germs are Spread.*--Some of the more common methods by which the germs
of disease are spread, and by so doing find new victims, are as follows:
1. _By Means of Foods._--Foods, on account of the locality in which they
are produced or the method of gathering or of handling-them, may become
contaminated with germs, which are then transported with the foods to the
consumer.
2. _By Means of Dust._--Material containing germs, _e.g._, discharges from
the throat and lungs, will on drying form dust. This is lifted with other
fine particles by the air and may be carried quite a distance. The dust
from public halls and other places where people congregate is the kind
most likely to contain disease germs. Dust should be breathed as little as
possible and only through the nostrils. Where one is compelled, as in
sweeping, to breathe dust-laden air for some time, he should inhale
through a moistened sponge, or cloth, tied in front of the nostrils.
3. _By Means of Domestic Pets and Different Kinds of Household
Vermin._--Germs sticking to the bodies of small animals are carried about
and may be easily communicated to people. By this means, rats, mice,
bedbugs, etc., where such exist, are frequently the means of spreading
disease; and particularly dangerous, on this account, is the common house
fly. Feeding as it does on filth of all kinds, it is easy for it to
transfer the bacteria that may stick to its body to the food which is
supplied to the table. The proper screening of houses and the destruction
of material in which flies may develop, such as the refuse from stables,
are necessary precautions.
Germs are spread also by the clothing of people, by railroad and steamship
lines, by the mails, and by the natural elements. In fact, any kind of
carrier, in or upon which germs can live, may serve as a means of
spreading those of certain kinds.
*Public Sanitation.*--The general conditions under which germs may thrive
and some of the means by which they are scattered, emphasize the practical
value of measures which have for their purpose the making of one's
surroundings more wholesome and hygienic. Such measures may be directed
both toward one's immediate surroundings--the home--and toward the
neighborhood, town, or city in which one lives. The hygienic conditions of
primary importance in every city or town are as follows:
1. An adequate public supply of pure water.
2. An efficient system of underground pipes for the removal of sewage.
3. An efficient system for removing from the streets and alleys everything
of the nature of waste.
4. Prevention, by enforcement of ordinances, of spitting upon sidewalks
and the floors of public halls and conveyances.
5. A hospital or sanitarium in which people can be cared for when sick
with infectious diseases.
In the larger cities other hygienic measures demand attention, such as
provisions for parks and playgrounds, the proper housing of the poor of
the city, and the suppression of the smoke and dust nuisances. Crowded
together as people are in the cities, the welfare of each individual
depends in a large measure upon the welfare of all. Hence the problems of
public sanitation are matters in which all are vitally concerned.
*Sanitary Conditions of the Home.*--The home, being the feeding and resting
place for the entire family, is the most important factor in one's
physical, as well as moral, environment. For this reason there is no place
where careful attention to hygienic requirements will yield better
results. Much of the danger from germs may be prevented by instituting and
maintaining proper sanitary conditions in and about the home.
One of the first requisites of the home is a suitable location for the
house. The house should be built upon ground that is well drained, and if
natural drainage be lacking, artificial drainage must be supplied. It
should not be situated nearer than a quarter of a mile to any marsh or
swamp and, if so near as that, it ought to be on the side from which the
wind usually blows. A stone foundation should be provided, and at least
eighteen inches of ventilated air space should be left between the ground
and the floor. Ample provisions must be made for pure air and sunlight in
all the rooms. The cellar, if one is desired, needs to be constructed with
special care. It should be perfectly dry and provided with windows for
light and ventilation. Adequate means must also be provided, by sewage
pipes and other methods, for the disposal of all waste. Where drainage
pipes are provided, care must be taken to prevent the entrance of sewer
gas into the house and also the passage of material from these pipes into
the water supply. The placing and connecting of sewer pipes should, of
course, be under the direction of a plumber.
*The Water Supply.*--Since water readily takes up and holds the impurities
with which it comes in contact, it should be exposed as little as possible
in the process of collecting. Where cistern water is used, care must be
taken to prevent filth from the roof (Fig. 168), water pipes, or soil from
getting into the reservoir. Water should be collected from the roof only
after it has rained long enough for the roof and pipes to have been
thoroughly cleaned. The cistern should have no leaks (Fig. 169), and the
top should be tightly closed to prevent the entrance of small animals and
rubbish.
[Fig. 168]
Fig. 168--*Contamination of cistern water* by birds nesting in the gutter
trough.
Shallow wells are to be condemned, as a rule, because of the likelihood of
surface drainage (Fig. 169), and water from springs should, for the same
reason, be used with caution. Deep wells that are kept clean usually may
be relied on to furnish water free from organic impurities, but such water
often holds in solution so much of mineral impurities as to render it
unfit for drinking. The presence in water of any considerable quantity of
the compounds of iron or calcium makes it objectionable for regular use.
[Fig. 169]
Fig. 169--*Sources of contamination of cistern and well water.*
Illustration shows liability of contamination from surface drainage and
from entrance of filth at top.
*Hygienic Housekeeping.*--However carefully a house has been constructed
from a sanitary standpoint, the constant care of an intelligent
housekeeper is required to keep it a healthful place in which to live.
Daily cleaning and airing of all living rooms are necessary, while such
places as the kitchen, the cellar, and the closets need extra
thoughtfulness and, at times, hard work. Moreover, the problem is not all
indoors. The immediate premises must be kept clean and sightly, and all
decaying vegetable and animal matter should be removed. Home sanitation
consists, not of one, but of many, problems, all more or less complex.
None of these can be slighted or turned over to a novice.
*Destruction of Infectious Material.*--At times the housekeeping has to be
directed especially toward hygienic requirements, such an occasion being
the sickness of one of the inmates with some contagious disease. Unless
special precautions are taken, the disease will spread to other members of
the household and may reach people in the neighborhood. Not only must
great care be exercised that nothing used in connection with the sick
shall serve as a carrier of disease, but germs passing from the patient
should, as far as possible, be actually destroyed. All discharges from the
body likely to contain bacteria, should be burned or treated with
disinfectants and buried deeply at a remote distance from the water supply
to the house.
After recovery all clothing, bedding, and furniture used in connection
with the sick should be disinfected or burned. The room also in which the
sick was cared for should be thoroughly disinfected and cleaned; in some
instances the woodwork ought to be repainted and the walls repapered or
calcimined. The purpose is, of course, to destroy all germs and prevent,
by this means, a recurrence of the disease.
*Fumigation.*--To destroy germs in the air or adhering to the walls of
rooms, furniture, clothing, etc., fumigation is employed. This is
accomplished by saturating the air of rooms with some vapor or gas which
will destroy the germs. Fumigation is quite generally employed in the
general cleaning after the patient leaves his room. This, to be effective,
must be thorough. Formaldehyde is considered the best disinfectant for
this purpose, and it should be evaporated with heat in the proportion of
one half pint of the 40 per cent solution to 1000 cu. ft. of space. Since
formaldehyde is inflammable and easily boils over, it has to be evaporated
with care. It should be boiled in a tall vessel (a tin or copper vessel
which holds about four times the quantity to be evaporated) over a quick
fire, the room being tightly closed (openings around windows and doors
plugged with cotton or cloth). After three or four hours the room may be
opened and thoroughly aired. Since formaldehyde is most disagreeable to
breathe, one should not attempt to occupy the room until it is free from
the gas. This will require a day or more of thorough ventilation.
*Facts Relating to the Spread of Certain Diseases.*--The problem of
preventing disease in general often resolves itself into the problem of
preventing the spread of some particular disease. It is then of vital
importance to know the special method by which the germs of this disease
leave the body of the patient and are conveyed to the bodies of others.
Some of these methods are novel in the extreme, and are not at all in
accord with prevailing notions. Particularly is this true of that disease
known as
*Malaria, or Malarial Fever.*--This disease, so common in warm climates and
also prevalent to a large extent in the temperate zones, is due to animal
germs (protozoa), which attack and destroy the red corpuscles of the
blood. These germs, it is found, pass from malarial patients to others
through the agency of a variety of mosquitoes known as _Anopheles_. In
sucking the blood of a malarial patient, the mosquito first infects her
own body.(131) In the body of the mosquito the germs undergo an essential
stage of their development, after which they are injected beneath the skin
of whomsoever the mosquito feeds upon. For the spreading of malaria, then,
two conditions are necessary: first, there must be people who have the
disease; and second, there must be in the neighborhood the special variety
of mosquito that spreads the disease. If either condition be lacking, the
disease is not spread. The malarial mosquito (_Anopheles_) may be
distinguished from the harmless variety (_Culex_) by the position which it
assumes in resting, as shown in Fig. 170.
[Fig. 170]
Fig. 170--*Mosquitoes* in resting position. (From Howard's _Mosquitoes_.)
On left the malarial mosquito (_Anopheles_); on the right the harmless
mosquito (_Culex_).
*Remedies against Mosquitoes.*--The natural method of preventing the spread
of malaria is, of course, the destruction of mosquitoes. This is
accomplished by draining pools of water where they are likely to breed,
and by covering pools of water that cannot be drained with crude petroleum
or kerosene. The kerosene, by destroying the larvae, prevents the
development of the young. In communities where such measures have been
diligently carried out, the mosquito pest has been practically eliminated.
Other methods are also under investigation, such as the stocking of
shallow bodies of water with varieties of fish that feed upon the mosquito
larvae.
[Fig. 171]
Fig. 171--*Stegomyia*, the yellow-fever mosquito (after Howard).
*Yellow Fever.*--This scourge of the tropics is, like malaria, caused by
animal germs. It is also propagated in the same manner as malaria, but by
a different variety of mosquito (_Stegomyia_, Fig. 171). The stamping out
of yellow fever in Havana, the Panama Canal Zone, and other places,
through the destruction of this variety of mosquito, affords ample proof
of the correctness of the "mosquito theory."
[Fig. 172]
Fig. 172--*Consumption germs* from the spit of one having the disease.
Highly magnified and stained. (Huber's _Consumption and Civilization_.)
*Consumption*, or tuberculosis of the lungs, spoken of as the "white
plague," was among the first diseases shown to be due to bacteria.
Consumption is now recognized as an infectious disease, though not so
readily communicated as some other diseases. Several methods are
recognized by which the germs are passed from the sick to the well, the
most important being as follows:
1. By personal contact of the sick with the well, especially in kissing.
2. By the sputum, or spit, which, if allowed to dry, is blown about as
dust and breathed into the lungs(132) (Fig. 172).
3. By means of objects (drinking cups, tableware, etc.) that have been
handled by consumptives.
4. By infectious material associated with houses or rooms in which
consumptives have lived.
These methods of spreading consumption suggest the necessity for the
greatest care, on the part of both the patient and those having him in
charge.(133) The material coughed up from the lungs and throat should be
collected on cloths or paper handkerchiefs and afterwards burned. The
house where a consumptive has lived should be disinfected, repapered or
calcimined, and thoroughly cleaned before it is again occupied. The inside
woodwork should also be repainted. The approaches to the house where the
patient may have expectorated should be disinfected and cleaned. Since the
germs are able to live in the soil, fresh lime or wood ashes should be
spread around the doorsteps and along the walks.
*Typhoid Fever*, one of our most dangerous diseases, is caused by germs
(bacteria) that enter the body through the food canal. They attack certain
glands in the walls of the small intestine, where they produce toxins that
pass with the germs to all parts of the body. Typhoid fever germs spread
from those having the disease to others, chiefly through the discharges
from the bowels and the kidneys. The germs contained in these, if not
destroyed by disinfectants, find their way into the soil, or into sewage,
where they may be picked up by water and widely distributed. Finding
suitable places, such as those containing decaying material, the germs may
rapidly increase in number, and from these sources find their way into the
bodies of new victims. They are likely, on account of manures, to get on
vegetables; on account of uncleanly methods of milking, to get into the
milk supply; and from sewerage outlets, to get into the oysters that grow
in bays and harbors near seaboard cities; but they are most frequently
introduced into the body through the drinking of impure water.
*Diphtheria*, also known as "membranous croup," is caused by germs that
attack the membranes of the throat. This most dangerous of children's
diseases is spread chiefly by discharges from the mouth and throat. These
should be collected on cloths and burned, or rendered harmless with
disinfectants. The disease may be spread also by objects brought into
contact with the mouth, such as cups, toys, pencils, etc. Children are
known to have diphtheria germs in the mouth for some time after recovering
from the disease, and should, for this reason, be kept away from other
children until pronounced safe by the physician.
The _antitoxin method_ of treating diphtheria has robbed this disease of
much of its terror, yet it not infrequently happens that the physician is
called too late to administer this remedy to the best advantage. Since
certain cases of diphtheria are likely to be mistaken for croup, the
parent frequently does not realize the serious condition of the child. A
croupy cough _that lasts through the day_, or a sore throat which shows
small white patches, are indications of diphtheria.
*Scarlet Fever, Measles, Chicken Pox, and Smallpox*, on account of the
eruptions of the skin which attend them, are classed as eruptive diseases.
As the eruptions heal, scales separate from the skin, and these are
supposed to be the chief means of spreading the germs. Attention must be
given to the destruction of these scales by burning or thoroughly
disinfecting all objects, such as clothing, bedding, etc., that may serve
as carriers of them. Those having eruptive diseases should be confined to
their rooms as long as the scales continue to separate from the body.
*Vaccination.*--The method of preventing smallpox known as vaccination,
which has been practiced since its discovery in 1796 by Jenner, has always
proved effective. In some instances the sore arm causes considerable
inconvenience, but this generally results from neglect to cleanse the arm
thoroughly before applying the virus, or from contact of the sore with the
clothing later. The virus should be applied by a physician and the wound
should be protected after the operation. If discomfort is felt when it
"takes," medical advice should be sought.
*Isolation*, or quarantining, is a most important method of combating
contagious diseases. By removing the sick from the well many outbreaks of
disease are quickly checked. Isolation of individual patients, and
sometimes of infected neighborhoods, is absolutely necessary; and while
this works a hardship to the few, it is frequently the only safeguard of
the many. The community, on the other hand, should make ample provision
for the care of the afflicted in the way of hospitals, or sanitaria, and
if it is deemed necessary to remove people from their homes, they should
not be subjected to unnecessary hardship.
Where one is sick from some contagious disease in the home and there is
liability of communicating it to the other members of the family, _room
isolation_ should be practiced. Infection cannot spread through solid
walls, and where the doors, and the cracks around the doors, are kept
completely closed and the usual precautions are observed by those
attending the patient, the other inmates of the house can be protected
from the disease.
*The Physician and His Work.*--In combating disease the services of the
physician are a prime necessity. The special knowledge which he has at his
command enables the conflict to be carried on according to scientific
requirements and vastly increases the chances for recovery. He should be
called early and his directions should be carefully followed. Everything,
however, must not be left to the physician, for recovery depends as much
upon proper nursing and feeding as upon the drugs that are administered.
Of great importance is _the saving of the energy of the patient_, and to
accomplish this visitors should, as a rule, be excluded from the sick
room.
*Precautions in Recovery from Disease.*--Many diseases, if severe, not only
leave the body in a weakened condition, but may, through the toxins which
the germs deposit, cause untold harm if the patient leaves his bed or
resumes his usual activities too soon. Especially is this true of typhoid
fever,(134) diphtheria, scarlet fever, and measles. Rheumatism and
affections of the heart, lungs, kidneys, and other bodily organs
frequently follow these diseases, as the result of slight exposure or
exertion before the body has sufficiently recovered from the effects of
the toxins. To guard against such results, certain physicians require
their patients to keep their beds for a week, or longer, after apparent
recovery from diseases like typhoid fever, diphtheria, and scarlet fever.
*Relation of Vocation to Disease.*--With a few exceptions, the pursuit of
one's vocation, or calling in life, does not supply either the quantity or
the kind of activity that is most in harmony with the plan of the body.
Especially is this true of work that requires most of the time to be spent
indoors, or which exercises but a small portion of the body. The effect of
such vocations, if not counteracted, is to weaken certain organs, thereby
disturbing the functional equilibrium of the body--a result that may be
brought about either by the overwork of particular organs or by lack of
exercise of others. Herein lies the explanation of the observed fact that
people of the same calling in life have similar diseases.
*A Special Problem for the Brain Worker.*--Farthest removed from those
forms of activity which harmonize with the plan of the body, and which
therefore are most hygienic, is that class of workers known as the
professional class, or the "brain workers." This class includes not only
the members of the learned professions--law, medicine, and the ministry--but
a vast army of business men, engineers, teachers, stenographers, office
clerks, etc., a class that is ever increasing as our civilization
advances. It is this class in particular that must give attention to those
conditions that indirectly, but profoundly, influence the bodily
well-being and must seek to obviate if possible such weaknesses as the
occupation induces.
*The Remedy* lies in two directions--that of spending sufficient time away
from one's work to allow the body to recover its normal condition, and
that of counteracting the effect of the work by special exercise or other
means. In many cases the first symptoms of weakness indicate a suitable
remedy. Thus exhaustion from overwork suggests rest and recreation. The
diverting of too much blood from other parts of the body to the brain
suggests some form of exercise which will equalize the circulation. If
feebleness of the digestive organs is being induced, some natural method
of increasing the blood supply to these organs is to be looked for. And
effects arising from lack of fresh air and sunlight are counteracted by
spending more time out of doors.
*Exercise as a Counteractive Agent.*--In counteracting tendencies to
disease and in the maintenance of the functional equilibrium of the body,
no agent has yet been discovered of greater importance than physical
exercise, when applied systematically and persistently. This may consist
of exercises that call into play all the muscles of the body, or which are
concentrated upon special parts. When general tonic effects are desired,
the exercise should be well distributed; but when counteractive or
remedial effects are wanted, it must be applied chiefly to the parts that
are weak or that have not been called into action by the regular work.
Unfortunately, health is sometimes confused with physical strength and
exercise is directed toward the stronger parts of the body with the effect
of making them still stronger. Not only is health not to be measured by
the pounds that one can lift or by some gymnastic feat that one can
perform, but the possession of great muscular power may, if the heart and
other vital organs be not proportionally strong, prove a menace to the
health. This being true, one having his health primarily in view will use
physical exercise, in part at least, as a means of building up organs that
are weak. Since the body, like a chain, can be no stronger than its
weakest part, this is clearly the logical method of fortifying it against
disease.
*Value of Work.*--Although there may exist in one's vocation certain
tendencies to disease, it must not be inferred that work in itself is
detrimental to health. Health demands activity, and those forms of
activity that provide a regular and systematic outlet for one's surplus
energy and compel the formation of correct habits of eating, sleeping, and
recreating best serve the purpose. Work furnishes activity of this kind
and serves also as a safeguard against the unhealthful and immoral habits
contracted so often from idleness. Even physical exercise which has for
its purpose the reenforcement of the body against disease may frequently
consist of useful work without diminishing its hygienic effects.
*The Mental Attitude.*--While a proper thoughtfulness and care for the body
is both desirable and necessary, it is also true that over-anxiety about,
or an unnatural attention to, the needs of the body reacts unfavorably
upon the nervous system. Observance of the laws of health, therefore,
should be natural and without special effort--a matter of habit. The
attention should never be turned with anxiety upon any organ or process,
but the mental attitude should at all times be that of _confidence in the
power of the body organization to do its work_. Fear and morbidity, which
are disturbing and paralyzing factors, should be supplanted by courage,
cheerfulness, and hopefulness.
Let it be borne in mind that hygienic living requires nothing more than
the application of the same intelligence and practical common sense to the
care of the body that the skillful mechanic applies to an efficient, but
delicate, machine. And, just as in the case of the machine, care of the
body keeps its efficiency at the maximum and lengthens the period that it
may be used. This end and aim of hygienic living is best attained by
cultivating that attitude of mind toward the body that avoids interference
in the vital processes and permits the natural appetites, sensations, and
desires to indicate very largely the body's needs.
*Attitude toward Habit-forming Drugs.*--Among the different substances
introduced into the body, either as foods or as medicines, are a number
which have the effect of developing an artificial appetite or craving
which leads to their continued use. Since the effect of such substances is
usually harmful and since they tend to engraft themselves upon communities
as social customs, they present a twofold relation to the general problem
of keeping well. The individual may be injured through the personal use
which he makes of them, or he may be injured through the effect which they
have upon relatives or friends or upon society at large. Since our social
environment is a factor in health little less important than our physical
environment, the conditions that make for their continuance should be more
generally understood.
*How Social Agencies perpetuate the Use of Habit-forming Drugs.*--When the
use of some habit-forming drug has risen to the importance of a general
custom, a number of conditions arise which tend to continue its use, even
though the fact may be quite generally known that the substance does harm.
In the first place, those who have formed the habit suffer inconvenience
and distress when deprived of its use. In the second place, a number of
people will have become interested in the production and sale of the
substance, and these will lose financially if it is discontinued. In the
third place, those of the rising generation will, from imitation or
persuasion, be constantly acquiring the habit before they are sufficiently
mature to decide what is best for them. Thus may the use of a substance
most harmful, such as the opium of the Chinese, be indefinitely
continued--a species of slavery from which the individual finds it hard to
escape.
Such is human nature and such are the forces and influences of human
society, that the freeing of a people from the bondage of some
habit-forming drug cannot be accomplished without strenuous and persistent
effort. Education, persuasion, the good example of abstainers, and legal
restrictions must be pitted against the forces that make for its
continuance. Such a struggle is now in progress in all civilized countries
relative to the use of alcoholic beverages.(135)
*How the Use of Alcohol became a Social Custom.*--The general use of
alcohol as a beverage may be accounted for by three facts. Alcohol is a
habit-forming drug; it has a stimulating effect which many have found
agreeable; and being a product of the fermentation of fruit juices and
other liquids containing sugar, it is easily obtained. Through the
operation of these causes the human family became habituated very early to
the use of alcohol. The "wine" of primitive man, however, did little harm
as compared with the alcoholic liquors of modern times. It was a weak
solution and on account of the crude methods of manufacture and storage
could only be produced in limited quantities. Perhaps the worst effect of
its early use was the establishment of a general belief in its power to
benefit, since this laid the foundation for excess in its use when the
developments of a later period made it possible.
During the eleventh century the method of making alcoholic drinks from
starch-producing substances, such as wheat, barley, and potatoes, became
quite generally known, and also the method of concentrating them by
distillation. This knowledge made possible the manufacture of alcoholic
drinks in large quantities and in considerable variety. Alcoholic
indulgence was now no longer the pastime of the few, but the privilege of
all. Its evil effects followed as a matter of course; and as these became
more and more apparent, there began the struggle to restrict the
consumption of alcohol which has continued with varying success to the
present time.
*Counts against Alcohol.*--The statements found in different parts of this
book relative to the effects of alcohol upon the body may here be
summarized as follows:--
1. Alcohol has an injurious effect upon the white corpuscles of the blood
and lessens the power of the body to resist attacks of disease (pages 35,
98).
2. Alcohol injures the heart and the blood vessels (page 56).
3. Alcohol causes diseases of the liver and kidneys and interferes with
the discharge of waste through these organs (pages 210, 212).
4. Alcohol interferes seriously with the regulation of the body
temperature (page 271).
5. Alcohol is one of the worst enemies to the nervous system (pages 326,
332-334. 336, 337).
6. Through its effect upon the nervous system and through its interference
with the production of bodily energy (page 195), alcohol greatly
diminishes the efficiency of the individual.
7. The taking of alcohol in amounts that apparently do not harm the
tissues is, nevertheless, liable to produce a habit which leads to its use
in amounts that are decidedly harmful.
*Alcohol and the Social Environment.*--Our social environment includes the
people with whom we are directly or indirectly associated. The presence in
any community of those who are immoral, inefficient, or defective, places
a burden upon those who are mentally and physically capable and renders
them liable to results which are the outgrowth of weakness or viciousness.
The fact that alcohol causes pauperism, crime, and general inefficiency,
thereby rendering the social environment less conducive to what is best in
life, is plainly evident. To realize how alcohol harms the individual
through its effects upon society in general, one has only to take into
account his dependence upon society for intellectual and moral stimuli,
for industrial and economic opportunity, for protection, and for general
conditions that make for health and happiness. As we strive to improve our
physical environment, so should we also strive for the betterment of
social conditions.
*Industrial Use of Alcohol.*--Interesting and instructive in this
connection is the fact that alcohol is, after all, a substance capable of
rendering great service to humanity. The injury which it causes is the
result of its misuse. Though unfit for introduction into the human body,
except in the most guarded manner, it is adapted to a great variety of
uses outside of the body. A combustible substance which is readily
convertible into a gas, it may be substituted for gasoline in the cooking
of food, lighting of dwellings, and the running of machinery. As a solvent
for gums, resins, essential oils, etc., it is used in the preparation of
varnishes, extracts, perfumes, medicines, and numerous other substances of
everyday use. Through its chemical interactions, it is used in the
manufacture of ether, chloroform, explosives, collodion, celluloid,
dyestuffs, and artificial silk. In fact, alcohol is stated by one
authority to be, next to water, the most valuable liquid known.(136)
Opposed to an extensive use of alcohol for industrial purposes is the
guard which the government must keep over its manufacture on account of
its use in beverages. Though alcohol may be profitably manufactured and
sold at thirty cents per gallon, the government revenue stamp of $2.08 per
gallon practically prohibits its use for many purposes. A step toward a
wider application to industrial purposes has been taken by the law
permitting the sale of so-called "denatured"(137) alcohol without the tax
for revenue. This law has proved beneficial to some extent, though the
practical solution of the problem is still remote.
*Nicotine and Social Custom.*--The influences which brought about a general
use of tobacco are similar to, though not identical with, those that
engrafted alcohol upon society. The drug nicotine is a habit-forming
substance and the plant producing it is easily cultivated.(138) Its
immediate effect upon the user is generally agreeable, acting as a
stimulant to some, but having a soothing effect upon the nerves of others.
Moreover, a strong deterring factor in its use is lacking, since its
harmful effects are not readily discernible and by many are avoided
through moderation in its use.
As with alcohol, tobacco is conveniently used to promote sociability among
men, a fact which has much to do with its very general use. If it could be
limited to social purposes, it would likely do little harm, but the habit,
once started, is continued without reference to sociability--a matter of
selfish indulgence. In fact, one effect of tobacco is to cause the user to
become less sensitive to the rights of others, this being evidenced by
smokers who do not hesitate to make rooms and public halls almost
unbearable to those unaccustomed to tobacco.
*Counts against Nicotine.*--The physiological objections to the use of
tobacco, as already stated (pages 56, 92, 326, 333, 336), are the
following:--
1. The use of tobacco before one reaches maturity stunts the growth. The
boy who uses it cannot develop into so strong and capable a man as he
would by leaving it alone.
2. Tobacco injures the heart.
3. Tobacco injures the air passages, especially when inhalation is
practiced.
4. Tobacco injures the nervous system and by this means interferes in a
general way with the bodily processes. For the same reason it interferes
with mental and moral development, the cigarette being a chief cause of
criminal tendencies in boys.
5. In some cases tobacco injures the vision.
6. The tobacco habit is expensive and is productive of no good results.
*Tobacco and the Rising Generation.*--The problem of limiting the use of
tobacco to the point where it would do slight harm, in comparison to what
it now does, would be solved if those under twenty years of age could be
kept from using it. But few would then acquire the habit, and those who
did would not be so seriously injured. In our own country it lies within
the province of the home and the school to bring about this result. The
fact that parents use tobacco is no reason why the boys should also
indulge. The decided difference in effects upon the young and upon the
mature makes this point very clear. Laws protecting boys from the evil
effects of tobacco, not only cigarettes, but other forms as well, are both
just and necessary.
*Social Custom and the Caffeine Habit.*--By suitable processes a white,
crystalline solid, easily soluble in water, can be separated from the
leaves of tea, and from the berry of the coffee plant. This is the drug
caffeine, the substance which gives to tea and coffee their stimulating
properties, but not their agreeable flavors. Less injurious, on the whole,
than either alcohol or tobacco, caffeine has come into general use in much
the same way as these substances. In a sense, however, caffeine is more
deceptive than either alcohol or nicotine, because the usual mode of
preparing tea and coffee gives them the appearance of real foods. The
housewife who would feel condemned in purchasing caffeine put up as a drug
somehow feels justified when she extracts it from plant products in the
regular preparation of the meal.
*Counts against Caffeine.*--People of vigorous constitutions and of active
outdoor habits are injured but slightly, if at all, by either tea or
coffee when these are used in moderation. As already stated (pages 56,
167, 326, 329), they do harm when used to excess and, in special cases, in
very small amounts, in one of the following ways:--
1. By stimulating the nervous system, thereby causing nervousness and
insomnia and interfering with vital organs.
2. By introducing a waste which forms uric acid into the body, thereby
throwing an extra burden upon the organs of elimination.
In this connection it may also be stated that there appears to be little,
if any, real advantage to the healthy body from the use of either tea or
coffee, beyond that of temporary stimulation and the gratification of an
appetite artificially acquired. Hence the large sums of money expended for
these substances in this country yield no adequate returns.
*Caffeine Restrictions Necessary.*--Though with many the cup of tea or
coffee at breakfast does no harm, but gives an added pleasure to the meal,
there is no question but that the use of caffeine beverages should be
greatly curtailed. Children should not be permitted to drink either tea or
coffee. Brain workers and indoor dwellers generally should use these
substances very sparingly, and people having a tendency to indigestion,
nervousness, constipation, rheumatism, or diseases of the heart, kidneys,
or liver frequently find it best to omit them altogether.
*Caffeine and "Soft" Drinks.*--Recently the practice has sprung up of using
caffeine as a constituent of certain drinks supplied at the soda-water
fountains. Such drinks usually purport to be made from the kola nut, which
contains caffeine, or to consist of extracts from the plants which yield
cocoa and chocolate, when in reality they consist of artificial mixtures
to which caffeine has been added. Those using these beverages are
stimulated as they would be by tea or coffee and soon acquire the habit
which makes them regular customers. Chief harm comes to the children who
frequent the soda fountains and to those who, on account of constitutional
tendencies, should avoid caffeine in all of its forms. It is generally
understood that the so-called "soft" drinks are harmless. If this
reputation is to be maintained, those containing caffeine must be
excluded.
*Danger from Certain Medicinal Agents.*--Among the most valuable drugs used
by the physician in the treatment of disease are several, such as
morphine, chloral, and cocaine, which possess the habit-forming
characteristic. Sad indeed are the cases in which some pernicious drug
habit has been formed through the reckless administration of such
medicines. Even the taking of such a drug as quinine as a "tonic" tends to
develop a dependence upon stimulation which is equivalent to a habit. In
the same list come also the drugs that are taken to relieve a frequently
recurring indisposition, such as headache. The so-called headache powders
are most harmful in their effects upon the nervous system and should be
carefully avoided.(139)
*Stimulants in Health Unnecessary.*--Stimulants have been aptly styled "the
whips of the nervous system." The healthy nervous system, however, like
the well-disposed and well-fed horse, needs no whip, but is irritated and
harmed through its use. Even in periods of weakness and depression,
stimulants are usually not called for, but a more perfect provision for
hygienic needs. Rest, relaxation, sleep, proper food, and avoidance of
irritation, not stimulants, are the great restorers of the nervous system.
A surplus of nervous energy gained through natural means is more conducive
to health and effective work than any result that can possibly be secured
through drugs. Then withal comes the satisfaction of knowing that one has
the expression of his real self in the way in which he feels and in what
he accomplishes--not a "whipped-up" condition that must be paid for by
weakness or suffering later on.
*Summary.*--To solve the problem of keeping well, one must live the life
which is in closest harmony with the plan of the body. Such a life,
because of differences in physical organization, as well as differences in
environment and occupation, cannot be the same for all. All, however, may
observe the conditions under which the body can be used without injuring
it and the special hygienic laws relative to the care of different organs.
Causes of disease, whether they be in one's environment, vocation, in his
use of foods or drugs, or in his mode of recreation, must either be
avoided or counteracted.
While the problem is beset with such difficulties as lack of sufficient
knowledge, inherited weakness, and time and opportunity for doing what is
known to be best for the body, yet study and work that have for their aim
the preservation or improvement of the health are always worth while.
_Health is its own reward._ The expression of the poet,
"Each morn to feel a fresh delight to wake to life,
To rise with bounding pulse to meet whate'er of work, of care, of
strife,
day brings to me,"
suggests the _joy_ of being well. But the ultimate realization of one's
aims and ambitions in life and the actual prolongation of one's period of
usefulness are _higher and more enduring rewards_.
*Exercises.*--1. Summarize the different laws of hygiene. Upon what one
fundamental law are these based?
2. State the important differences between a condition of health and one
of disease.
3. In what general ways may disease originate in the body?
4. Describe a model sanitary home. With what special hygienic problems has
the housekeeper to deal?
5. Describe a method of collecting a wholesome supply of cistern water.
State possible objections to well and spring water.
6. What means may be employed in preventing the spread of contagious
diseases?
7. By what means are malaria, typhoid fever, diphtheria, and tuberculosis
spread from one individual to another?
8. Why are extra precautions necessary in the recovery from certain
diseases, as typhoid fever, diphtheria, and scarlet fever?
9. How may one's vocation become a cause of disease? What conditions in
the life of a student may, if uncounteracted, lead to poor health?
10. Of what special value are the parks and pleasure grounds in a city to
the health of its inhabitants?
11. Discuss the hygienic value of work.
12. What conditions lead to the continuance of habit-forming substances
after their use has become general?
13. How is it possible for one not using alcohol to be injured by this
substance?
14. Discuss the effect of alcoholic abuse upon social environment.
15. Summarize the rewards of hygienic living.
SUMMARY OF PART II
For the maintenance of life the needs of the cells must be supplied and
_the body as a whole must be brought into proper relations with its
surroundings_. The last-named condition requires that the body be moved
from place to place; that its parts be controlled and cooerdinated; and
that it be adjusted in its various activities to external physical
conditions. To accomplish these results there are employed:
1. The skeleton, or bony framework, which preserves the form of the body
and supplies a number of mechanical devices, or machines, for causing a
variety of special movements.
2. The muscular system, which supplies the energy necessary for executing
the movements of the body.
3. The nervous system, which (_a_) controls and cooerdinates the various
activities and (_b_) provides for the _intelligent_ adjustment of the body
to its environment. (Review Summary of Part I, page 215, and consult Fig.
92, page 214.)
APPENDIX
*Equipment.*--Nearly all of the apparatus and materials called for in this
book may be found in the physical, chemical, and biological laboratories
of the average high school. There should be ready, however, for frequent
and convenient use, the following: One or more compound microscopes with
two-thirds and one-fifth inch objectives; a set of prepared and mounted
slides of the various tissues of the body; a set of dissecting
instruments, including bone forceps; a mounted human skeleton and a
manikin or a set of physiological charts; a set of simple chemical
apparatus including bottles, flasks, test tubes, and evaporating dishes;
and a Bunsen burner or some other means of supplying heat.
The few chemicals required may be obtained from a drug store or from the
chemical laboratory. Access to a work bench having a set of carpenter's
tools will enable one to prepare many simple pieces of apparatus as they
are needed.
*Physiological Charts* are easily prepared by teachers or pupils by
carefully enlarging the more important illustrations found in text-books
or by working out original sketches and diagrams. These, if drawn on heavy
Manila paper, may be hung on the wall as needed and preserved
indefinitely. By the use of colors, necessary contrasts are drawn and
emphasis placed on parts as desired. The author has for a number of years
used such home-made charts in his teaching and has found them quite
satisfactory. His plan has been to draw on heavy Manila paper, cut in
sizes of two by three feet, the general outline in pencil and then to mark
over this with the desired colors. There is of course an opportunity for
producing results that are artistic as well as practical, and if one has
time and artistic skill, better results can be obtained. Many of the cuts
in this book are excellently suited to enlargement and, if properly
executed, will provide a good set for general class purposes.
*Models.*--The use of prepared models of the different bodily organs is
strongly urged. These may be so used in elementary courses as to obviate
much of the dissections upon lower animals. Although the actual tissues
cannot be so well portrayed, the general form and construction of organs
are much better shown. Models well adapted to class or laboratory work are
easily obtained through supply houses. Illustrations of several of these
are shown in connection with the "Practical Work."
INDEX
Abdomen; dissection of, 169.
Abdominal cavity, 7, 138, 152.
Absorption, 173-186.
Defined, 18, 173.
Accommodation, 379.
To illustrate, 391.
Acid reactions, 171.
Acquired reflexes, 314.
Adipose tissue, 5, 178.
Afferent neurons, 296.
Air, 76.
Changes it undergoes in lungs, 101.
Complemental, 89, 103.
Reserve, 89, 103.
Residual, 89, 103.
Tidal, 88, 103.
Air passages, 80.
Albuminoids, 119.
Purpose served by, 121.
Alcohol,
A cause of crime, 333.
Effects on circulation, 55, 56.
Effects on digestion, 167.
Effects on energy supply, 195.
Effects on respiratory organs, 98.
Effects on social environment, 413.
Effect on temperature regulation, 271.
Effects on waste elimination, 212.
General considerations, 412-415.
Alimentary canal, coats of, 138.
Alimentary muscles, work of, 159.
Alkaline reactions, 171.
Alveoli, 82.
Amylopsin, 155, 156.
Anatomy, defined, 1.
Animal heat, 192.
Anopheles, 401.
Antiseptic ointment, 275.