LAW OF PROJECTION
The movements of the eye are designed primarily to effect fixation—that
is, to bring upon the macula the image of the object that we wish to
look at. When this has been accomplished, we know as a result of long
experience, the direction of the object looked at and also direction of
other neighboring objects. This knowledge is doubtless afforded us, in
part, by our muscle sense. Thus we know that an object, A, is straight
in front of us because we can see it sharply without moving either the
head or the eyes from the position of rest or equilibrium; and we know
that an object, B, is on the right of us because to see it sharply we
have to move either the head or the eyes to the right, thus altering
the muscular condition from one of rest to one of tension. But without
moving either head or eye, we also know, while still looking at A, that
B is to the right, for the image of B is then formed on a portion of
the retina situated to the left of the macula. From long experience
we also know that an image so situated means an object placed on our
right. Moreover, the farther to the left of the macula the image B is,
the farther to the right do we judge B itself to be.
Similarly, if B is so placed that its image falls below the macula,
we then know B itself is really above A, which forms its image on the
macula; and if the image of B is above the macula, we know that B
itself is below A.
The table on page 116 is suggested as a guide in cases of muscular
imbalance:
SUPPRESSION OF IMAGE
All deviations should be and probably are primarily associated with
diplopia. Yet in the great majority of cases of established squint,
especially convergent squint, there is no double vision. This is due to
the mental suppression of the image by the squinting eye. In such cases
all attempts to evoke diplopia by our tests may be futile, the patient
not appreciating the presence of double images even when they are
widely separated by prisms. Moreover, this suppression usually persists
after the squint is cured, so that even though there are two retinal
images of the same object, the mind perceives but _one_ and ignores the
_other_, just as though it were not present. In this case there is no
true stereoscopic, or solid, vision.
MONOCULAR DIPLOPIA
Binocular diplopia, due to deviation of the eyes or to prisms, must
be distinguished from monocular diplopia, or the condition in which
the patient sees double with one eye alone. This occurs as the result
of astigmatism, plus spherical aberration and other conditions found
occasionally in squint. It can readily be differentiated by the fact
that binocular diplopia disappears when the patient shuts either eye;
while monocular diplopia, of course, does not.
TABLE OF DIPLOPIA
==================+===========+=====================================+
| Image of | |
| right eye | CAUSED BY |
|as compared+-------------------+-----------------+
Name of |with that | |(2) Artificially |
diplopia | of the | (1) By a natural | by a prism |
| left is | deviation of | placed, base |
------------------+-----------+-------------------+-----------------+
{Homonymous | On the |Either eye inward |In before either |
{ | right | (esophoria, | eye. |
Lat-{ | | esotropia.) | |
eral{ | | | |
{Heteronymous| On the |Either eye outward |Out before either|
{(or crossed)| left | (exophoria, | eye. |
| | exotropia.) | |
------------------+-----------+-------------------+-----------------+
{Right | Below |Right eye up |Up before right |
{ | | or left eye down | eye, down before|
Vert-{ | |(right hyperphoria,| left eye. |
ical{ | | right hypertropia,| |
{ | | left hypotropia.) | |
{ | | | |
{Left | Above |Right eye down |Down before right|
| | or left up |eye, up before |
| |(left hyperphoria, | left |
| | left hypertropia, | |
| | right hypotropia.)| |
==================+===========+===================+=================+
| Image of | |
| right eye | CORRECTED BY |
|as compared+-------------------+-----------------+
Name of |with that | | |
diplopia | of the | (1) Turning |(2) Prism placed |
| left is | | with base |
------------------+-----------+-------------------+-----------------+
{Homonymous | On the |Both eyes outward |Out before either|
{ | right | (divergence.) | eye. |
Lat-{ | | | |
eral{Heteronymous| On the |Both eyes inward |In before either |
{(or crossed)| left | (convergence.) | eye. |
| | | |
------------------+-----------+-------------------+-----------------+
{Right | Below |Right eye down |Down before right|
Vert-{ | | and left eye up | eye or up before|
ical{ | | (left | left eye. |
{ | | supravergence.) | |
{ | | | |
{Left | Above |Right eye up and |Up before right |
| | left eye down | eye or down |
| | (right | before left. |
| | supravergence.) | |
==================+===========+===================+=================+
MOVEMENT OF EACH EYE SINGLY
The movements of each eye individually are effected as follows:
The external rectus moves the eye directly outward; the internal
rectus, directly inward.
The primary action of the superior rectus is to raise the eye. Because
of the way in which the muscles run, obliquely from within outward, its
lifting action increases when the eye is abducted and diminishes to
little or nothing when the eye is adducted.
The inferior rectus carries the eye down. Owing to the oblique
direction of the muscle, its depressing action increases as the eye is
abducted and decreases to zero as the eye is adducted.
The inferior oblique is inserted back of the equator of the eye. Hence
it pulls the back part of the eye down and consequently throws the
front part up. It is thus an elevator of the eye, reinforcing the
action of the superior rectus. Owing to the way in which it runs, from
the front backward and outward, its elevating action is greatest when
the eye is adducted, and diminishes to little or nothing when the eye
is abducted.
The superior oblique, so far as its action on the eyeball is concerned,
may be regarded as arising from the trochlea. From this point it runs
backward and outward and is inserted back of the equator of the eye.
It there pulls up the back part of the eye and consequently throws the
front part down. It is thus a depressor, reinforcing the action of
the inferior rectus. Owing to the oblique way in which it runs, its
depressing action is greatest when the eye is adducted, and diminishes
to little or nothing when the eye is abducted.
SUBSIDIARY ACTIONS
Besides these actions, rightly regarded as the main action of the
ocular muscles, there are various subsidiary actions, due to the
oblique way in which the superior and inferior recti and the two
obliques run. Thus, both the superior and inferior recti adduct the
eye, their action being most pronounced when the eye is already
adducted. The two obliques, on the other hand, abduct the eye and do so
most effectively when the eye is already abducted.
The superior rectus and superior oblique rotate the top of the vertical
meridian of the eye inward (intorsion); while the inferior oblique
and inferior rectus rotate it outward (extorsion). The superior and
inferior recti act thus on the vertical meridian mainly when the eye is
adducted; the oblique, on the other hand, when the eye is abducted.
Hence the eye is adducted by the internal rectus, assisted toward the
end of its course by the superior and inferior recti. It is abducted by
the external rectus, assisted toward the end of its course by the two
obliques. It is carried straight up by the superior rectus and inferior
oblique, up and out by the superior rectus and external rectus (the
inferior oblique helping to carry it out, but not up; and in, mainly by
the inferior oblique and internal rectus). The superior rectus assists
in carrying it in, but hardly up at all.
The eye is likewise carried straight down by the inferior rectus and
the superior oblique; down and out by the inferior and external recti,
and down and in by the superior oblique and internal recti.
FIELD OF ACTION OF MUSCLES
As will be seen, each muscle acts most energetically in some special
direction of the gaze, termed field of action of that particular
muscle; thus the external rectus acts most powerfully when the eye
is directed outward, and acts little or not at all when the eye is
directed inward, except by purely passive traction. Likewise the
superior rectus acts mainly when the eye is directed down. Furthermore,
its action is limited to the upper and outer field; for in the upper
and inner field elevation is performed chiefly by the inferior oblique.
This is also true of all the other muscles.
DIRECTION OF THE GAZE
There are six cardinal directions of the gaze, each corresponding to
the field of action of one of the six ocular muscles as follows:
CARDINAL DIRECTION: MUSCLES SPECIALLY ACTIVE:
Straight out External rectus
Straight in Internal rectus
Up and out Superior rectus (as an elevator)
Up and in Inferior oblique (as an elevator)
Down and out Inferior rectus (as a depressor)
Down and in Superior oblique (as a depressor)
It is to be noted that the action of each muscle does not absolutely
_stop_ at the middle line, but extends somewhat beyond it. Thus the
action of the right externus extends not only throughout the whole
right half of the field of vision, but also some fifteen to twenty
degrees to the left of the median line; and that of the superior rectus
extends not only above the horizontal plane but also somewhat below.
PRIMARY POSITION—FIELD OF FIXATION
Under normal conditions, when the head is erect and the eye is directed
straight forward—that is, when its line of sight is perpendicular
to the line joining the centres of rotation of the two eyes in the
horizontal plane—the muscles are all balanced. This is called “the
position of equilibrium” or the primary position. It is this position
which must be assumed by the patient in conducting tests for balance of
the muscles.
From the primary position, the eye may make excursions in every
direction so that the patient can look at a whole series of objects in
succession without moving the head. This portion of space, occupied
by all the objects that may thus be seen directly by moving the eye
without moving the head, is called “the field of fixation.”
BINOCULAR MOVEMENTS
While either eye alone may move in all possible directions, one cannot
move _independently_ of the other eye. Under ordinary circumstances,
those movements only are possible which are regularly required to
subserve binocular vision, hence, binocular single vision, as well.
These movements are as follows:
PARALLEL MOVEMENTS
When one eye looks at a distant object the other is also directed to
it, so that the lines of sight of the two eyes are parallel; if the
distant object is moved about, the lines remain parallel, one moving as
fast and as far as the other. These parallel movements of the two eyes
are executed with considerable freedom in all directions, either eye
being able to move readily to the right, left, up, down, or obliquely,
provided the other eye moves precisely with it.
In executing any parallel movement, each eye is acted upon by at least
three and sometimes by as many as five muscles. At times, but one of
these muscles is required to produce any _great_ movement of the eye,
the others simply serving to steady it in its course. Thus when we look
up to the right, although there are five muscles really acting upon
each eye, the right eye is moved mainly by the _external_ rectus and
the left eye by the _internal_ rectus.
Similarly, when we look up and to the right, although other muscles
take part, the superior rectus is the chief muscle that moves the
right eye up, and the external rectus the chief one that moves it to
the right; while for the left eye the inferior oblique and the internal
rectus are the efficient muscles.
A careful study of the action of the individual muscles will make it
clear that these facts hold good for each of the cardinal directions of
the gaze.
Furthermore, if we attentively consider the action of the twelve
muscles moving the two eyes, we see that they may be divided into three
groups, _viz._; four _lateral rotators_, four _elevators_ and four
_depressors_.
LATERAL ROTATORS
Right rotators } { Left rotators
L. Internal rectus } to { R. Internal rectus
R. External rectus } { L. External rectus
ELEVATORS
Right-handed elevators } { Left-handed elevators
(acting mainly when the } { (acting mainly when the
eyes are directed to the } to { eyes are directed to the
right) } { left)
R. Superior rectus } { R. Inferior oblique
L. Inferior oblique } { L. Superior rectus
DEPRESSORS
Right-handed depressors } { Left-handed depressors
(acting mainly when the } { (acting mainly when the
eyes are directed to the } { eyes are directed to the
right) } to { left)
R. Inferior oblique } { R. Superior oblique
L. Superior oblique } { L. Inferior rectus.
Each group, it will be seen, comprises _two_ pairs of muscles; one
pair acting solely when the eyes are directed to the right, the other
when they are directed to the left. It will further be noted that of
the two muscles constituting any one pair, one is situated in the right
eye, the other in the left.
EYE ASSOCIATES
The muscles forming any one pair are called associates. Any two
associates acting together will move their respective eyes in precisely
the same direction and to the same extent. Thus the right superior
rectus moves the eye up to the left and rotates its vertical meridian
to the left; and its associate, the left inferior oblique, moves its
eye up to the left and rotates its vertical meridian to the left. This
likewise applies to each of the other five groups of associates.
If one eye fails to keep pace with the other in executing parallel
movements, diplopia ensues. If the eyes are moved in all directions
and the point noted where the patient just begins to see double, we
_delimit_ the field of binocular single vision.
Normally, however, the two eyes maintain parallelism to the very
limit of their excursion, so that diplopia occurs only at the extreme
periphery of the field of vision, if at all. In fact, the field of
binocular single vision usually extends not less than 40 degrees from
the primary position in every direction.
Each of the various parallel movements of the eye appear to be governed
by a distinct nerve mechanism, there being one centre for movements to
the right, one for movements to the left, one for movements up, etc.
MOVEMENTS OF CONVERGENCE
In order to see an object at a nearby point, the eyes have to
converge—a movement affected by a simultaneous and equal contraction
of both internal recti. This movement may be combined with a vertical,
lateral or oblique parallel movement. Thus, when we wish to look at a
near object situated twenty degrees to our right, we first turn both
eyes twenty degrees to the right, then converge both equally, turning
the left a little more to the right and the right a little back toward
the left.
Convergence is governed by a distinct mechanism of the nerves, the
source of which has not been determined.
MOVEMENTS OF DIVERGENCE
In passing from a position of convergence to a position of parallelism,
the lines of sight separate or diverge. This movement of divergence
is a simultaneous, equal contraction of both externi; or, probably,
of both actions combined. The eyes may even diverge somewhat beyond
parallelism, as in overcoming prisms, base in, when looking at a
distant object.
VERTICAL DIVERGENCE
The amount by which the lines of sight can separate in a vertical
direction is very limited—at most but one or two degrees.
ORTHOPHORIA
The term orthophoria is used to denote an absolutely normal balance of
the extrinsic muscles, just as the term emmetropia denotes a normal
refractive condition. They are equally rare.
HETEROPHORIA
The term heterophoria includes all those conditions in which there is
a tendency to depart from normal balance, but which nature is able to
compensate for; while the term also includes the conditions in which
nature has been unequal to the task and an actual turning or squint has
occurred.
SUBDIVISIONS
The subdivisions of these terms at first reading appear complicated,
but prove simple enough on closer study, indicating only the direction
of the turning or tendency to turn. For instance:
Esophoria signifies _inward_ tendency.
Exophoria signifies _outward_ tendency.
Hyperphoria signifies _upward_ tendency.
Hypophoria signifies _downward_ tendency.
Cyclophoria signifies _tendency_ to torsion.
Esotropia signifies _inward_ turning.
Exotropia signifies _outward_ turning.
Hypertropia signifies _upward_ turning.
Hypotropia signifies _downward_ turning.
Cyclotropia signifies _actual_ torsion.
Combinations are describable in similar terms. A tendency of the right
eye to turn up and inward, is a “right hyperesophoria”; the left eye
to turn down and out, a “left hyperexophoria,” etc. Tendencies of both
eyes together are denoted by the terms which follow:
Anaphoria signifies an _upward_ tendency.
Kataphoria signifies a _downward_ tendency.
Dextrophoria signifies a _right_ tendency.
Laevophoria signifies a _left_ tendency.