The Eye in its Relation to Health
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By Chalmer Prentice, M.D.
Chicago, A.C. McClurg & Company, 1895
Transcription (c) A. Wik, 2004

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        | Chapter III |  pages 24-39
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SIGHT is not passive, it is an active function;
and, although we see apparently without
effort and without volition, yet every moment of
vision is costing its adequate amount of vital
energy.  True, we cannot say just how much
energy is utilized in looking any given length of
time at any particular scene; but we do know that
many nervous persons are very much exhausted by
the use of the eyes for a short time in an art gal-
lery, where things of great interest are constantly
attracting their attention.  Some are more ex-
hausted by one hour of such effort with the eyes
than they would be by ten hours of manual labor;
consequently the same amount of vital force that
would be required for ten hours of labor, may be
disposed of in one hour through the medium of
the eyes.  Now, if there be some defect in the
construction of the eyes, the consumption of
nerve-force will be much greater; for example, a
short superior muscle in one eye requires many
times the amount of nerve-force to perform
the function that the centers were ever intended
to furnish.  The function of sight may be as suc-
cessfully performed as if the eyes were perfect,

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but only by an excessive call on the nerve-cen-
ters to maintain a balance between the two eyes;
in such cases the vision is sometimes of more
than average sharpness.  This extravagant waste
might find a good parallel in the following:  An
electric plant is scientifically constructed for the
purpose of running fifty arc lights; each light
utilizes a given amount of electric force that is
generated at the electric center, and the fifty
lights use up what force the dynamo can
easily generate.  Now, in the place of one
of these arc lights, if we place an electric motor
which uses twenty times as much force as the
light that it has displaced, the whole electric sys-
tem will be disturbed, from the fact that the elec-
tric motor is utilizing more than its share of the
motive-force; consequently the other forty-nine
lights in the circuit will not receive enough to
perform their intended functions; they will sput-
ter, burn irregularly and feebly.  We have here
a state of affairs in our arc light system which
may be likened to that form of nervous debility
where the prominent feature is constant exhaus-
tion, due to a lack of sufficient force of nerve-
impulse.  But in the animal economy, another
condition may follow when the "dynamo" is
over-taxed; the nerve-centers may be excited to
an irritable condition in which they generate an
excessive amount of nerve-force, and then we

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have an excited or agitated form of nervous
derangement; and, whether it be general or local-
ized, it soon wears out life.
  Strictly construed the terms neurasthenia and
nervous debility mean a weak or depressed
nerve-force supply.  They are sometimes care-
lessly applied (but are not properly applicable)
to all nervous derangements; for we often meet
with nervous derangements that consist of an
excessive supply of nerve-force, usually accompa-
nied with more or less irritability, all functions in
general being emphasized.  The enormous assim-
ilation in abnormally fat persons, and the exces-
sive muscular development and activity in a class
of phenomenal athletes, whose histories are short
and generally end in some rapidly wasting dis-
ease, are extreme examples of that form of ner-
vous derangement which consists of an excess of
nerve-force supply and which should be called
neurosthenia.  The term ABNORMAL-INNERVATION
embraces both of these conditions.
  Our fast ocean steamers can run nineteen or
twenty miles an hour without excessive con-
sumption of fuel; but when the speed is raised
to its highest possibilities, the last two, three or
four miles are attained at a very extravagant
increase in the fuel, the consumption being far
greater in proportion per mile as well as resulting
in a great strain throughout the ship.  A ship is

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sometimes unable to sustain the highest tension
that can be put upon her by her engines, in
which case the good engineer turns off the excess
or abnormal quantity of steam, slows her speed
to an easy pace, and thus her life in the waters
is lengthened many years.  Likewise when we
learn that some of our functions are being per-
formed at an abnormally high tension requiring
an extravagant amount of nerve-force to sustain
them, which is rapidly wearing us out, we should
suspend this high rate of function and conserve
as far as possible our life forces, by removing the
cause of the irritation and waste.
  The nerve-centers are capable of generating a
certain amount of motive-force or nerve-impulse
and no more.  Whenever the call upon the cen-
ters exceeds their ability to respond, the result is
a disturbance somewhere.  A large amount of
motive-force is utilized in the function of vision
even when it is performed under the easiest
possible circumstances; but when there are de-
fects in the eye and its appendages, there is a
still greater demand for nerve-force to bring
about good vision.
  Each nerve-center is intimately connected and
in delicate sympathy, with all other centers of
the nervous system.  Any radical change in one
center sends its characteristic influence through-
out all.

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  Harmonious and pleasant sounds, picturesque
scenes filled with things of beauty, loveliness
and goodness, have the most marked influence
over the nerve-centers in general and manifest
themselves in the form of the body, the grace of
movement and the sweetness of expression in
the features.  Such external influences have a
tendency to produced, through the medium of
vision, the perfect man and woman; whereas the
one who is reared amid scenes of crime and bru-
tality, coarseness and privation, is so influenced
in his whole physical being that, to the experi-
enced eye, their marks are everywhere to be seen.
  The reason husband and wife grow to look
alike after living years in each other's company
is, that they have looked so constantly upon each
other's features, have been made glad by the
same influences, have enjoyed the same sunshine,
have met with the same sorrows and have been
so often subjected to the same experiences, that
their functions have become similar.
  Now what particular portion of the nerve-
centers should we expect to have the greatest
influence in changing and controlling the general
nervous system?  Would it be one of the duller
centers, or would it be the most acute and sensi-
tive?  We would reasonably look to the most
sensitive as bearing the highest influence both for
good and evil.

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  Of all the centers in the nervous system,
which is the most sensitive, the most highly acute
and the most constantly in use?
  The answer is, that center in which the most
positive impression is made by the lightest and
most delicate touch or impact.  Various parts of
the body, for instance, the fingers, come in con-
tact with a substance; immediately there is an
afferent impulse communicated to the nerve-cen-
ters that says, 'this is velvet, fur, wood, metal,
hot or cold.'  Again, substances of different
flavors are taken into the mouth.  When they
come in contact with the nerves of taste (gusta-
tory nerves) the afferent impulses from this con-
tact convey to the nerve-centers the consciousness
of the presence of lemon, orange, vanilla, etc.,
and the reflex of swallowing or deglutition fol-
lows.  If the flavor be something which conveys
to the nerve-centers a disagreeable or nauseous
taste, the opposite reflex follows, and an effort is
made to reject the offensive material instead of
swallowing it.
  The auditory center discriminates in all the
delicate sounds known to music, in all the varieties
of harmony and discord.  The potent influence
of music on the feelings of man and beast is well
known.
  Another sense equally or more delicate, is that
of smell.  The perfumes emanating from helio-

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trope, violet and other flowers can neither be
weighed, measured nor analyzed, being so deli-
cate and imponderable; yet, the sense of smell
produced by them is of a very positive nature.
  As delicate as the above named sense are,
there is yet another of a much more highly acute
nature; in fact, by far the most delicate sense of
all, sight.
  Light is imponderable; its rays, reflected
from various objects, pass through the cornea,
the aqueous humor, the crystalline lens, the vit-
reous body, and there reach the retinal nerves.
The delicacy of this touch or impact on the
retinal nerves is beyond the conception of the
human mind; yet it establishes from this point
an impulse which is conveyed to the visual or
sight centers of the brain, which with no uncer-
tainty, determines form, color, motion, quantity,
and space.  Here we have the most positive
sense or feeling known to man; sight, produced
by an impact or touch of the most imponderable
agent or force known to science.  There is not
an impression, feeling or sense with which the
nerve-centers have ever been made familiar which
is not in some measure reawakened through the
visual apparatus.
  The musician runs his eye over the page of
written music, and he hears it.  One sees a person
across the street eating a lemon, and he tastes it.

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The salivary glands begin to act ten rods away
from the lemon.  We see a person's foot run
over or crushed, and we feel the wound.  It
produces changes throughout the observer's
nervous system, nausea results, and often so
forcible is the effect that unconsciousness follows.
Through this medium, mothers have communi-
cated deformities to their offspring by changes in
the nerve-impulses that control the function of
construction or building.  There are many sub-
stances which we cannot recognize by the sense
of touch, but which sight determines for us at
once, showing that the visual nerve-centers are
the most sensitive of all.
  It is through the feelings awakened by sight
that in reading a book and looking at its illustra-
tions, we are enabled to live in the very atmos-
phere of the scene that is depicted and to drink
and satiate ourselves to the fullest extent, feeling,
tasting, smelling and hearing the various things
and surroundings that the author suggests.
  If the light of a single candle is sufficient to
produce a perfect impression of a given object in
the visual centers, say a letter A, there is a feeling
produced in the nerve-centers that tells us of the
presence of a letter A.  If twice the light that is
necessary is reflected from this letter A, twice the
impact or impression that is necessary is conveyed
to the visual centers and they are taxed to a

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double capacity.  Should this light be increased
to twenty candle power, where one would be
sufficient, the visual centers will receive an
impulse twenty times as strong as necessary to
produce the function of seeing the letter A, or
whatever object the eye may be resting upon, and
the feeling of sight is over-taxed and disturbed;
for sight is nothing more or less than feeling.
This excessive amount of useless work even in a
normal eye wears on these delicate centers
beyond necessity, and they become centers of
irritation and convey disturbed conditions to
other parts or centers of the nervous system.  I
touch a person on the shoulder gently; this per-
forms the function of attracting his attention.
Again, if I were to use a club and strike a severe
blow, the attention would be attracted, but the
force of the impact would be more than necessary
and an injury to the part would be the result.
It is the same with the delicate sight centers.
Just a sufficient amount of light to perform the
function is all that is required, but a flood of
enough force can be admitted to practically
bruise these delicate parts of the nervous system.
Since the advent of electric lights which have
practically turned night into day, there has been
an increase in nervous disturbances, due to over-
taxation of the visual centers of the nervous
system.  We turn our eyes in the direction

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of hills, valleys, rivers and woods, and the
light reflected from the various objects
passes into the eye and touches the nerves of
sight.  From here an impulse is conveyed to the
sight centers of the brain and here we feel the
form, size, color and motion of the various
objects in the scene.
  The feeling of harmonious sounds in the audi-
tory centers is enjoyable for the space of two or
three hours, but a continuance beyond this length
of time becomes tiresome, and the same sounds
repeated for five or six hours would be annoying.
These centers become temporarily exhausted or
worn out.
  The feeling of taste when eating the choicest
delicacies, in a short time, perhaps an hour,
becomes wearied, temporarily perverted, and
beyond this time enjoyment ceases.  The gusta-
tory centers refuse to be further taxed.
  The feeling of smell through the olfactory
centers is very delightful for the first few inhala-
tions of some delicate perfume; the sense of
smell is at first prominent, but after a few
moments the odor seems to excite no feeling at
all.  Nothing but the pungent feeling of alcohol
at last is present.  From over-work the olfactory
centers have suspended their functions.
  The feeling in the visual centers commences
at seven o'clock in the morning and continues
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until ten o'clock at night.  For fifteen hours con-
tinuously, more or less, the feeling in the visual
centers has been constant.  No other sense in the
nerve-centers is capable of such continued endur-
ance.
  For good or for evil, according to their char-
acter, the senses of feeling, taste, smell, hearing
and sight have all a marked influence both on our
mental and physical lives.  Sounds of harmony,
delicacies of wholesome and pleasant taste, odors
that delight and please the sense of smell, and
scenes of beauty, all have an elevating influence
over our mental and physical lives tending to a
refinement of both mind and body; while the
opposites of these experiences produce an un-
favorable or opposite effect throughout.
  "Why was the Grecian a poet and philosopher,
while the Scythian was a wanderer and robber?
One was reared amid surroundings of beauty and
culture, the other had a barbarian land, rude as
his manners and wild as his heart.
  Again I repeat, THE VISUAL CENTERS ARE THE
MOST SENSITIVE AND MOST CONSTANTLY TAXED, BE-
CAUSE THE MOST CONTINUOUS AND POSITIVE FEELING
IS PRODUCED BY THE IMPACT OF AN IMPONDER-
ABLE AGENT.  The touch of only a subdued ray
of light gives rise to a feeling which is the
most certain and continuous of all functions of
feeling.

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  As to the anatomical location of the nerve-
centers generally, it will have to be a matter of
speculation, because an immense amount of re-
search prosecuted by able men has located only
a few of them.  Victor Horsley in his work
on "The Brain and Spinal Cord" says: "Per-
sonally, I believe that anatomical research will
do less than physiology toward settling these im-
portant and most difficult questions, and that as a
fact, we shall not advance very far towards their
solution until we are able to approach them from
the standpoint of their functions, which of course
is a totally different mode of comparative invest-
igation."  Also, "Anatomy is not able to adduce
absolute evidence either as to agreement or disa-
greement in the evolution of the structure, it is
likely that the physiological aspect of the case is
the more correct one."
  From a physiological standpoint, there would
appear to be a great number of visual centers in
the nervous system.  When rays of light are re-
flected from an object and focussed in each
eye, there is a strong effort of the two eyes to fix
themselves in such a position that the picture
made or reflected on the posterior part of each
eye shall be in corresponding localities.  A ray
of light falling exactly on the most sensitive cen-
ter or yellow spot of the right eye, stimulates
through the nerve-centers, a tendency to fix the

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left eye in such a position that a ray of light
emanating from the same point will also fall upon
the most sensitive center or yellow spot in that
eye.  The same tendency prevails throughout
the field of vision.  If an eye is fixed on an ob-
ject, it will be possible to see other objects for
quite a distance in various directions without
moving the eye; for instance, objects upward as
far as 45 deg. can be seen, outward over 90 deg.,
downward about 70 deg., inward 55 deg. to 60
deg., according to the prominence of the bridge
of the nose.  To define an object as having form,
it is necessary that several impressions be re-
ceived in the nerve-centers simultaneously or
nearly so.  The average acuity of vision is said
to be an angle of one minute.  The top of a letter
E falls on a portion of the retina that conveys an
impression of its existence, also the tongue of
the letter and the bottom of it; the upright
portion and the two spaces between the tongue
and the upper and lower parts, fall upon
separate portions of the retina, each of which
takes a distinct impression, and their relative
positions are noted by separate and distinct
sensory centers in the brain; consequently we
feel the form of the letter E.  If it were a single
sensory center that these distinct and separate
objects were conveyed to, it would be a single
confused impression.  So, from a physiological

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standpoint, we see that each minute point in the
posterior part of the eye or field of vision is sus-
ceptible of taking a separate impression, and has
somewhere a distinct sensory center in the nerv-
ous system.  In looking directly in front of me,
I see a red light off to the right, because rays
emanating from it have fallen on that portion of
the field of vision that conveys its impression to
that particular center in the brain that locates it,
and always does locate any object falling upon it,
to the right of the optic axes; and so it is with
each minute and separate locality throughout the
entire field of vision.  When we contemplate the
numerous objects that may be distinguished with-
out moving the eye, we may conceive how numer-
ous must be the sensory centers for the function
of vision.  At least one for each angle of vision.
  When the two eyes are fixed so that rays of
light falling from a given point are reflected on
corresponding localities in the field of vision, a
single impression of the object ensues, because
each of these corresponding points is supplied
with a nerve-filament that leads to a common or
single sensory center; but when rays coming
from the same point fall on different, or non-cor-
responding localities in the two eyes, they neces-
sarily meet with nerve filaments originating from
tow different sensory centers, and each eye con-
veys an impression of the same object to dis-

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tinctly separate centers, and two impressions of
the same object are the result.  Always, under
such conditions, there is a strong effort of the
two eyes to fix their optic axes parallel so that
the two apparent objects may be fused into one,
thus avoiding confusion, and increasing the
sense of vision.  In doing so, the various mus-
cles act as follows:  Let two imaginary lines
bisect each other at right angles, one horizon-
tally and the other vertically, and we shall
have a cross.  Now, suppose the eyes to be out
of visual line so that a dot will appear to the
right eye to be in the lower right hand corner
of the space occupied by the cross, and that to
the left eye the same object will appear in the
left upper portion of the cross.  These impressions
simultaneously seen with both eyes will appear
as two dots.  To merge or fuse these two im-
pressions of the dot into one, the external and
internal muscles move the eyes laterally until
they are in such a position that the two dots are
brought to the imaginary vertical line of the
cross.  One will now be above the other.  This
we call the tendency to VERTICALIZE exerted by
the internal and external rectus muscles; while
at the same time, the superior and inferior rectus
muscles move the eyes upward and downward to
such a position that the dots are brought to the
imaginary horizontal line.  This we call the ten-

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dency to HORIZONTALIZE.  The vertical and lat-
eral muscles in accomplishing the above move-
ments, receive some aid from other muscles.
This is explained at length elsewhere.  These
tendencies to horizontalize and verticalize be-
ing exerted simultaneously, bring two impres-
sions of a dot to the center of the cross,
merging them into one object.  In this posi-
tion, rays emanating from a single point
fall on corresponding localities in each eye,
and are conveyed to a single visual center
producing the impression of one.  The su-
perior and inferior oblique muscles also play
their part in the rotations of the eyes, so that
rays from a single point may fall upon corre-
sponding localities.  The above process is called
FUSION.





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        | End of Chapter III |  pages 24-39
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