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 XIII |  pages 174-188
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								174

ESOPHORIA, or a tendency to a convergence
of the optic axes, is the most deceptive and
troublesome condition with which we have to
deal, and from its very nature we can see how
unreliable all diffusion tests are, as indicating the
true conditions.
  We will suppose a case where the lengths of
the various long muscles of the eye are such that,
without effort the axes of the two eyes are per-
fectly parallel at a distance of twenty feet or more.
The muscles, in performing their function at an
infinite distance make their movements in various
directions synchronously, the superior muscles
turning both eyes upward, and the inferior mus-
cles turning them downward.  Turning to the
right, the right external and the left internal act
together, and the necessary innervation is com-
pensated for when the two eyes are directed to
the left; for then the left external and the right
internal require an amount of innervation equiva-
lent to that required for the movement in the
other direction.
  Supposing the various inclinations of the head
to the right and to the left to be of nearly equal

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frequency, the oblique muscles will also be uni-
formly exercised.  Thus in the various movements
of the eye at a distance, there is a tendency to
establish more or less of an equilibrium in the
nerve-impulses that contract the various muscles;
but when the vision is fixed upon some object at
a near point, as is necessary in reading, writing
and in the pursuit of various mechanical arts, the
internal rectus muscles receive an amount of
nerve-force necessary to break the parallelism of
the infinite distance, and turn the eyes toward
each other.  The repeated exercise of the internal
muscles has a constant tendency to develop their
power, and there is no counter movement of the
eyes in which the opposing muscles have any
opportunity for a like development.  It is for this
reason that we always find the internal muscles
much stronger than the external; but the differ-
ence in strength between the internal and external
muscles has no tendency to cause the eyes to
deviate from perfect parallelism at a distance.
Under normal conditions there remains the ability
to suspend the nerve-impulse that gives greater
strength to the internal muscles.  When the eyes
are removed from the near to the remote point
the relaxation is a negative act yet a normal func-
tion.  There is no contention between the inter-
nal and the external muscles.  Under normal
conditions, the function of suspending nerve-im-

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pulse exists in all the ocular muscles; when cer-
tain muscles contract to perform some office their
opposites relax to assist them.  In the fullest
sense this suspension of nerve-impulse is probably
not absolute.  Just sufficient impulse continues to
give the eyes a steady position; but, where the
employment of the eyes has been such as to
necessitate their use at the near point constantly,
from morning until night, day after day (as in the
case of an inveterate reader or writer, or an arti-
san with his work constantly close to his eyes),
the nerve-impulses of the internal muscles are
almost constant, with little or quite infrequent per-
iods of relaxation; thus the impulse becomes such
a fixed an constant quantity that ultimately there
is an inability to suspend it entirely.  Such eyes,
when examined at a distance of twenty feet or
more, show a positive tendency toward conver-
gence, while at the near point we may find a bal-
ance, a convergence, or even a tendency to diver-
gence, as a result of the excessive abnormal
innervation.
  By diffusion tests a convergence may appear
where the external and internal muscles are of
proper length, when it is apparent esophoria; or it
may appear where the externals are short, in which
case it is a reverse manifestation; or it may appear
where the internals are short.  Of the three con-
ditions, the last is the least likely to be the fact.

								177

  ESOPHORIA is the most confusing and obstinate
condition with which we have to deal.  The
above demonstrates how dangerous it is to rely on
any diffusion test as an indication of the true
anatomical condition of the ocular muscles; and
it is not strange that, by so doing, many have been
thrown into doubt and confusion as to the impor-
tance of this field of work.  Repression, in which
symptoms are our chief guide, is the only method
of safely determining whether the diffusion tests
have given a true or false indication of the condi-
tion of the muscles.  The majority of cases of
manifest esophoria are reverse manifestations, or
due to spasm.  When esophoria exists at the far
point, and exophoria at the near, it is pretty safe
to conclude that it is not true esophoria.


			EXOPHORIA.

  Of all defects, this is undoubtedly the most
common.  The tendency of the optic axes to
diverge may, as in other muscular defects, be due
to shortness of the external muscles, or it may
be apparent exophoria, due to spasm, receiving
its initial source of irritation from a latent defect
in some one of the other muscles, more usually a
superior or inferior.  Or, it may possibly be a
reverse manifestation.  In short, strain in any one
of the ocular muscles may give rise to a spasm
and apparent defect in any of the others, and
	12

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repression is the only method that will determine
with any safety the actual state of affairs.  When-
ever exophoria exists at the near point, I always
endeavor by repression to develop an exophoria
and in most instances succeed.  Also in some
cases where esophoria exists at the near point,
repression will develop a true exophoria, the cer-
tainty of which is rendered positive by the physi-
cal relief which accompanies the development.


		HYPERPHORIA AND CATAPHORIA.

  A greater amount of defect is likely to be
latent in the superior and inferior than in the
external and internal ocular muscles, for the latter
in the performance of their various functions are
alternatively converging and diverging their optic
axes, while the superior and inferior never cause
any relative change in the optic axes, upward or
downward, during their work.  Whatever position
they assume, the optic axes never deviate rela-
tively from a horizontal plane; consequently the
impulses to the superior and inferior muscles
become obstinately fixed, and defects in
these muscles are less likely to manifest them-
selves.  If the superior muscle of one of the eyes
is short, and the inferior muscle of the same eye
is sufficiently innervated it will draw the eye down
into line with its fellow.  Day after day nerve-
impulse is sent to the inferior muscle, contract-

								179

ing it and holding the eye in place.  During
months and years this process goes on until the
nerve-centers become accustomed to manufactur-
ing and sending out this excessive nerve-impulse,
and perfect vision is performed.  At the age of
thirty, more or less, the short muscle, having
been constantly kept on the stretch, is naturally
weak and undeveloped.   Its length is greater than
normal conditions would have left it.  It has
always been stretched by the opposite muscle,
which has contracted to draw the eyes into line,
while the contracted or innervated muscle is
over-developed, having received for thirty years
an excessive amount of nerve-impulse to perform
its work, an amount of motive-force for which the
nerve-centers were not originally intended to be
drawn upon.  In time, through differentiation,
the nerve-centers become accustomed to generat-
ing and sending out this excessive amount of
motive-force for the purpose of maintaining per-
fect vision; thus the impulse becomes a fixed
one requiring no further stimulant to call it forth.
It becomes fixed in sufficient force to establish a
balance between the two eyes even when prisms
or other diffusion tests are resorted to for the pur-
pose of ascertaining if one eye is higher or lower
than the other.  Although we have created dip-
lopia, the eye does not turn toward the short
muscle, because the nerve-impulse that opposes it

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pulls just as hard as the short muscle does.  It
has become an absolutely fixed impulse requiring
no stimulus to generate it.  It is as fixed as the
impulse that carries on the action of the heart or
the functions of the liver.  Under these circum-
stances it is useless to expect any diffusion test to
reveal a muscular defect if it is present.
  From this excessive demand for motive-force
irritation frequently results in the centers that are
furnishing it; the impulse becomes stronger than is
necessary to hold the eye down in line; and, when
diffusion tests are made, it is pulled below or in a
direction opposite the short muscle (reverse man-
ifestation).  To attempt to correct such a defect
as manifested by diffusion would increase the ner-
vous disturbance.  Repression in connection with
symptom tests is the only safe procedure.
  During repression by crowding on all the prism
that the eyes will fuse under we cannot expect
this short muscle in a few weeks or even months
to assume its normal length.  Will it be as short
as it would have been had it never been stretched;
or would the opposite or excessively innervated
long muscle under this process become stretched
to a length that is in any way equivalent to the
stretching that has taken place in the short muscle
during a period of thirty years?  This question is
a sufficient answer in itself.
  When we have discovered that a defect exists

								181

in the superior or inferior rectus muscle, it is
important to determine whether it is hyperphoria
or cataphoria; for to select the superior muscle as
being invariably the defective one will prove
wrong.  Dr. Stevens has pointed out the fact that
the head is usually tilted toward the shoulder
opposite the eye which tends the higher; in other
words, it is tilted toward the shoulder on the side
next to the eye which tends the lower, or would
in the absence of innervation.  Now, it is very
important to determine whether this tendency is
due to a short inferior muscle in one eye or a
short superior in the other.  This I think can
safely be determined by noting another position
of the head.  If, in connection with the side incli-
nation, the head is constantly thrown backward
and the chin elevated, the inferior muscle will
prove to be short; but if it is inclined forward
with the chin resting well down toward the breast
bone, it is evident that the superior muscle is
short.  These positions should be carefully noted
during repression tests, and if the chin is thrown
up and the head back, the greater the amount of our
repression prism may be put on, base up, which
will have a tendency to lower the chin.  If the
head is ducked forward, we may put on the greater
amount of repression prism, base down, which
will have a tendency to raise the chin into a more
normal position.

								182

  Before operating on an eye the oculist should
determine several important things.  First, is the
patient's condition a serious one, and does it in a
great measure depend on eye defects?  The
result of our examination by repression will
determine this for us, for if prominent symptoms
can in this way be subdued, we have reason to
feel encouraged.  Next, have the tests developed
a sufficient defect in the ocular muscles to warrant
an operation?  If the defective muscle is a supe-
rior or inferior, and the repression has not dis-
closed more than 10 deg. or 12 deg., treatment
with prisms may be the more practicable.  This
in no way relates to defects as developed by diffu-
sion tests.  In a case in which no more than 10
deg. or 12. deg. have been developed by repres-
sion, partial tenotomy might possibly bring the
optic axes up or down into line; but it would not
leave as true normal conditions as prisms.  A
partial tenotomy in which three-fourths, more or
less, of a tendon is divided leaves the other one-
fourth of the tendon on the stretch.  It offers
less, yet a proportionate amount of resistance.
If it permits the eye to turn sufficiently far, the
cut fraction may assume in a small measure the
functions of a normal muscle, but this is forever
out of the question with the uncut portion.
  If the muscles are so attached to the ball that
one of the eyes deviates from its fellow 20 deg.,

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when the eye is in this position the muscles are of
proper length and all innervation will be sus-
pended; but when this eye is moved into line
with its mate one muscle is too short and the
other too long; consequently, if the deviation is
within the practical limits of the use of prisms,
the eye is rotated into that position in which both
muscles are of proper length, and from this posi-
tion the associated movements of the eye will call
upon each muscle for its proportionate share of
normal function.
  If primarily an eye had a tendency to deviate
in any direction--for illustration, outward one-
fourth or an inch--and innervation to the opposite
muscle has lined the eyes up and held them in
proper position for a space of years, during this
time the short muscle has been stretched to one-
fourth of an inch greater than its natural length,
while the long muscle, but contraction, has taken
up one-fourth of an inch of excess in length.
Under these conditions the short muscle is longer,
and the long muscle shorter one-fourth of an inch
than they would have been had their lengths been
normal primarily.  Therefore, under these circum-
stances, a lengthening of the short muscle by
tenotomy does not by any means fulfill all the
requirements of the case.  It is true that the short
muscle needs greater length; but, for a normal
position of the eye, we must remember, also, that

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the opposite muscle has always been too long;
and if we relieve the strain in the short muscle,
thus allowing the eye to maintain a normal posi-
tion with less work, the long muscle on the oppo-
site side must contract upon itself by excessive
innervation a sufficient amount to shorten itself
one-fourth of an inch before it can assume normal
functions; and the necessity of shortening this
muscle to avoid this innervation is oftentimes as
necessary as the lengthening of the short muscle.
When a short muscle has been stretched by its
vis-a-vis for many years, and an operation is per-
formed which gives it greater length and removes
the stress upon it, the short muscle that has been
stretched begins to shorten itself after the opera-
tion by assuming a normal activity that it never
before had an opportunity to assume.  After the
operation the short muscle has a chance to act for
the first time in its existence.  It continues to
shorten week after week, month after month, and
will usually take up several degrees of deviation
made in the opposite direction by the operation.
This process of shortening continues to such an
extent that in variable lengths of time it will
obliterate double vision to the amount of 10 deg.
to 20 deg., and often a deviation in the original
direction will ultimately present itself again, re-
quiring still further correction by a repetition of
the operation.  Often it is deemed necessary to

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lose a limb to save life, and the little temporary
discomforts and inconveniences resulting from re-
pression should be looked upon in the same light.
After external tenotomies, double vision will
sometimes continue to exist for several months, in
looking to the extreme right or left, a condition
that need not annoy the patient at all.  This sel-
dom follows tenotomy of the internal muscle.
With care in developing fully, and continuance
of the repression under prisms for several weeks,
complete tenotomy can be performed in the ma-
jority of cases without the occurrence of double
vision.  If the development has been high and
double vision does occur, it soon corrects itself
by the normal shortening of the cut muscle.
  Never should an operation be performed until
the physician is absolutely certain that he has
ascertained which muscle is short.  An oversight
of this kind might occasion very serious conse-
quences to the patient.
  From the above conditions we can see that the
results will be but temporary in an effort merely
to balance the optic axes where there has been a
manifest deviation.  If we balance them for the
time being, the process of shortening the already
short muscle will in time tend to reproduce devia-
tion in the same direction, and it will keep on
doing so from time to time, necessitating many
operations; indeed I have known as many

								186

as twenty.  Life is too short for such a
procedure, when by repression we can more rapidly
discover the latent defects.  In applying the
maximum of prism under repression, and turning
the eye toward the short muscle, we enable it to
assume some normal action.  It also begins to
shorten.  Before operating it is best to wait, if
time is at our disposal, until the muscles have
correlated themselves to this new position.  Then
a tenotomy has less tendency to turn the eye in
the opposite direction and create diplopia.  The
ultimate outcome of immediate tenotomy that
creates double vision, and of tenotomy after a
new correlation has been established, will be the
same; but in the latter case the inconvenience to
the patient would be less immediately after the
operation.  In the case of the lateral muscles,
complete tenotomy is not advisable if repression
development has not reached 15 deg. or upwards,
and in the case of the vertical muscles it should
have reached 12 deg. or more.  In any deviation
of the axes amounting to less than the above, my
advancement operation with a ligature plate will
bring about the small change of position needed
with much more precision than tenotomy.  Even
in higher amounts of deviation, advancement with
the ligature plate will fulfill the normal conditions
in some cases more perfectly than tenotomy,

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especially in eyes that are large, prominent, and
apparently very loose in their capsules.
  In developing a short muscle by repression, it
is safe to conclude that, if we force it to its
highest point for several months, we shall not be
able to develop the muscle even then to as short
or as near a normal condition as it would have been
in had it not been stretched for thirty years or
more.  Just how much the short muscle has been
lengthened and the long muscle shortened, during
this long period of stretching, it is not possible
to determine at once; but it is absolutely safe to
conclude that in keeping the eye turned toward
the short muscle as far as we can for a few months,
we shall not stretch the long muscle as much as
the short one has been stretched.
  When guided by symptoms and certain that
we are right, we need have no fear of going too
far with our repression, if it be continued for
many months.  Where from the time of birth the
short muscle has been constantly stretched for
thirty years, it will probably never be possible in
the lifetime of the patient to make the muscle as
perfect by repression as it would have been had
it not been too short at the beginning.  In the
course of a few weeks, I have often discovered
more than 50 deg. of latent defect, where none,
or at most 2 deg. or 3 deg. was at first manifest

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by all diffusion tests.  The method of merely
balancing such manifest defects with prisms and
operations can afford only temporary relief, and a
small amount even of that.  In a short time after
all such corrections, a little more of the latent
defect manifests itself, requiring still further cor-
rection, and it continues to develop itself after
each correction again and again through a long
period of time; on the other hand, repression
enables us to arrive at the desired end as quickly
as possible, and perhaps lengthen, by many years,
a life that would not have lasted through the
slower process.  In those cases where the most
marked relief has followed tenotomies that
brought about an immediate balance, I conceive
that the eye has moved several degrees farther
than the manifest deviation, but still just within
the broad limit of the verticalizing and horizontaliz-
ing function exerted under diffusion tests.






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        | End of Chapter XIII |  pages 174-188
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