PHYSIOLOGY OF THE ORGANS OF VISION.
917. To comprehend the theory of vision, it is not sufficient to know
the structure of the eye. We must be familiar with some of the
properties of a subtile fluid, which is constantly emanating from all
luminous bodies, called _light_.
918. It is the province of natural philosophy, rather than physiology,
to enter minutely upon the properties of light. It may be observed,
however, that, when light passes through any medium of the same
density, the rays are in straight lines; but, when it passes from one
medium into another of different density, it is refracted, or
turned from a straight course, unless it strikes the medium in a
perpendicular direction--then light passes through without a change
of direction.
919. When a ray of light meets with a body, it either passes through
it, or is reflected by it, or it may be absorbed. Again, in proportion
as the rays of light become distant from the body from which they
emanate, they diverge one from the other. In accordance with the laws
of optics, the rays of light, in passing through an optical instrument
like the eye, must cross each other, and thus produce an inverted
image of the object from which the rays proceed. With the general
view of the structure of the eye, we will now examine the use of each
part in the function of vision.
-=-=-=-=-=-=-=-=-=-=-=-=
917-933. _Give the physiology of the organs of vision._ 917. What is
necessary in order to understand the theory of vision? 918. When light
passes through a medium of the same density, in what direction will be
its rays? Of a different density? What exception? 919. When light
meets with a body, what takes place? What is said in reference to rays
of light in passing through the eye?
-=-=-=-=-=-=-=-=-=-=-=-=
920. The sclerotic coat not only gives form to the body of the eye,
but protection to the interior and more delicate parts. The choroid
coat seems to be chiefly composed of a tissue of nerves and minute
blood-vessels; the latter give nourishment to the different parts of
the eye. One of the uses of this coat is, to absorb the rays of light
immediately after they have passed through the retina. This is
effected by the black pigment that lines its inner surface. Were it
not for this provision, light would be too intense, and vision
indistinct.
_Observation._ In albinos, where there is an absence of the black
pigment, the rays of light traverse the iris, and even the choroid
coat, and so overwhelm the eye with light, that their vision is quite
imperfect, except in the dimness of evening, or at night. In the
manufacture of optical instruments, care is taken to color their
interior black, for the same object, namely, the absorption of
scattered rays.
921. The iris, by means of its powers of expansion and contraction,
regulates the quantity of light admitted through the pupil. If the
iris is thin, and the rays of light pass through its substance, they
are immediately absorbed by the uvea, and, if that layer be
insufficient, they are taken up by the black pigment of the choroid
coat.
_Observation._ When we look toward the bottom of the eye, the pupil
appears like a black spot, instead of an opening. This is caused by
seeing the black pigment through the retina and humors of the eye.
-=-=-=-=-=-=-=-=-=-=-=-=
920. What is the use of the sclerotic coat? Of what is the choroid
coat chiefly composed? What is the use of this coat? How is it
effected? What is said of albinos? What care is taken in the
manufacture of optical instruments? 921. What is the use of the iris?
When we look toward the bottom of the eye, why does the pupil look
like a black spot, instead of an opening?
-=-=-=-=-=-=-=-=-=-=-=-=
922. The cornea, and the aqueous, crystalline, and vitreous humors,
are transparent; so that rays of light traverse these parts of the
eye, and fall upon the retina. The office of these humors and the
cornea is to refract the rays of light in such proportion as to direct
the image in the most favorable manner upon the retina.
923. The office of the retina is to receive the impression of the rays
of light which leave an object at which we look, and it is upon it
that a small but very clear image of that object is formed. The
impression thus produced by the reflected light is transmitted by the
optic nerve to the brain, which receives the sensation. This
constitutes vision.
924. The optic nerve has but one function, that of sight. Sensibility
is conferred on this organ by a large branch from the fifth pair of
nerves, which ramifies upon the different parts of the eye and its
appendages. These parts, however, receive some nervous filaments from
the seventh pair.
_Observations._ 1st. The large number of sensitive nervous filaments
renders the visual organ very impressible to bodies that cause
irritation, as dust, or intense light. This compels us to use due care
to shield the eye from the influence of agents that would impair or
destroy vision.
2d. Although particles of dust, when in contact with the delicate
parts of the eye, induce severe pain, yet these parts may be cut in
surgical operations, and the patient's sufferings are not as great as
when an incision is made in the skin to remove a small tumor.
925. Different degrees of density, as already mentioned, modify the
refractory power of any transparent medium. It is found, on
examination, that the cornea, the vitreous, the crystalline, and the
aqueous humors, have each, severally, various degrees of density: and
that the crystalline lens, at its circumference, is less dense than at
its centre. These circumstances modify the direction of the refraction
of the rays of light, in their passage from the cornea to the retina.
-=-=-=-=-=-=-=-=-=-=-=-=
922. What is the use of the cornea, aqueous, crystalline, and
vitreous humors? 923. What is the office of the retina? 924. What is
the function of the optic nerve? How is sensibility conferred on
this organ? Give the 1st observation in this connection. The 2d
observation.
-=-=-=-=-=-=-=-=-=-=-=-=
926. The refracting powers of the plane, convex, concave, plano-convex,
plano-concave, and concavo-convex lenses,[22] are different. The
cornea and aqueous humors are convexo-concave, the vitreous humor is
concavo-convex, while the crystalline humor is a convexo-convex medium.
(Fig. 139.)
[22] The refracting character of differently-formed lenses is
illustrated in the works on Natural Philosophy, to which the
pupil is referred.
[Illustration: Fig. 142. The forms of the different lenses. 1, A plane
lens. 2, A globe lens. 3, A convexo-convex lens. 4, A plano-convex lens.
5, A concavo-concave lens. 6, A plano-concave lens. 7, Meniscus. 8, A
concavo-convex lens.]
-=-=-=-=-=-=-=-=-=-=-=-=
925. Have the cornea and the humors of the eye different degrees of
density? What is said of the crystalline lens? What effect has the
different density of the parts of the eye upon the light admitted to
this organ? 926. What kind of lenses do the humors exhibit? 927. What
modifies the refracting powers of transparent mediums? How does this
principle apply to the humors of the eye?
-=-=-=-=-=-=-=-=-=-=-=-=
927. The different degrees of convexity or concavity also modify the
refracting character of transparent mediums. The crystalline lens is
of different degrees of convexity on its two sides. The convex
surfaces of the aqueous and vitreous humors are segments of circles,
of different diameters from their concave surfaces. (Fig. 139.) All
these circumstances still further influence the refracting character
of the visual organ. The achromatic arrangement of the transparent
refracting mediums of the eye, remedies the aberration of refraction
in the different portions of the eye.
928. Again, the refracting power of lenses is modified by their
convexity or concavity. The more convex a lens is, the shorter the
distance from the refracting medium, where the different refracted
rays converge to a focus. To adapt the eye to view objects at
different distances, requires a change in the refracting power of some
of the transparent mediums of the eye.
929. Both surfaces of the crystalline lens are oval, not spherical,
and the refraction of the rays of light is mainly effected in this
portion of the eye. Change the inclination of this lens, so that
different portions of its anterior surface shall be directly behind
the pupil, and its refracting power is increased or diminished, as the
surface presented is more or less convex.
930. To view objects at a distance, a less convex lens is needed than
in examining articles very near the eye; and this organ, from its
structure, has the power of adaptation to different distances. It is
supposed that the muscular substance of the ciliary body and processes
changes, by its contraction, the inclination of the crystalline lens.
Without this, or some other adapting power, a picture of objects at
different distances would not be formed upon the retina, and the
vision of every person would be defective, except in reference to
objects at certain definite distances from the eye.
-=-=-=-=-=-=-=-=-=-=-=-=
928. What modifies the refracting power of lenses? What is necessary
to adapt the eye to view objects at different distances? 929. Where is
the refraction of the rays of light mainly effected? 930. When we view
objects at a distance, what kind of lens is required? Has the eye the
power of adapting itself to different distances? How is it effected?
-=-=-=-=-=-=-=-=-=-=-=-=
_Observation._ It is well known that a separate image is formed on
each eye, and, if they are not in the same direction, the objects will
appear double. This is proved by pressing one eye, so that the rays of
light cannot enter it in the same direction as they do in the other;
consequently, the vision is double.
931. By the action of the muscles of the eye, it is turned in
different directions, so that objects can be examined upon each side,
as well as in front, without turning the body. By the slight or
intense action of the straight muscles, the eye is more or less
compressed, and the form of the globe is changed, together with the
relative positions of the different humors. This modification also
adapts the eye to view objects at different distances.
[Illustration: Fig. 143. 1, A pen, an inverted image of which is painted
on the retina of the eye, at 2. The image of all objects upon the
expansion of the optic nerve, is inverted by the crossing of the rays of
light from objects as they traverse the pupil.]
_Observation._ If the eye is fixed for a time on some object which is
distinguished with difficulty, there is a painful sensation, similar
to that experienced by other muscles of the body when used too long.
This is called "straining the eye."
932. When the refraction of the rays of light is too great, as in
over-convexity of the cornea, or the crystalline lens, or the
vitreous humor, or all of them, the image is formed a little in
front of the retina. Persons thus affected cannot see distinctly,
except at a very short distance. This infirmity is called _near_, or
_short-sightedness_. This defect is in a great measure obviated by
the use of concave glasses, which scatter the luminous rays, and thus
counterbalance the too strong refracting force of the eye.
-=-=-=-=-=-=-=-=-=-=-=-=
What does fig. 143 represent? 931. Why can we see objects at the side
as well as in front of the eye, without turning the body? What is the
effect when the eye is fixed on an object that is indistinctly seen?
-=-=-=-=-=-=-=-=-=-=-=-=
933. When the different parts of the eye are not sufficiently convex,
the image is formed beyond the retina, and thus only distant objects
are distinctly seen. This defect is called _long-sightedness_. The
feebleness in the refracting power of the eye may be caused by
disease; but usually it is a consequence of old age, and is remedied
by wearing spectacles with convex glasses.
HYGIENE OF THE ORGANS OF VISION.
934. _The eye, like other organs of the body, should be used, and then
rested._ If we look intently at an object for a long time, the eye
becomes wearied, and the power of vision diminished. The observance of
this rule is particularly needful to those whose eyes are weak, and
predisposed to inflammation. On the contrary, if the eye is not called
into action, its functions are enfeebled.
935. _Sudden transitions of light should be avoided._ The iris
enlarges or contracts, as the light that falls upon the eye is faint
or strong; but the change is not instantaneous. Hence the imperfect
vision in passing from a strong to a dim light, and the overwhelming
sensation experienced on emerging from a dimly-lighted apartment to
one brilliantly illuminated. A common cause of _am-aur-o'sis_, or
paralysis of the retina, is, using the eye for a long time in a very
intense light.
-=-=-=-=-=-=-=-=-=-=-=-=
932. What is short-sightedness? How is the defect remedied? 933. What
is long-sightedness? How is the defect remedied? 934-942. _Give the
hygiene of the organs of vision._ 934. Do the same principles apply to
the use of the eye as to other organs? What is the effect if the eye
is fixed intently on an object for a long time? What results if the
eye is not called into action? 935. Why should sudden transitions of
light be avoided?
-=-=-=-=-=-=-=-=-=-=-=-=
_Note._ Let the anatomy and physiology of the eye be reviewed from
figs. 139 and 143, or from anatomical outline plate No. 10.
936. _Long-continued oblique positions of the eye should be avoided,
when viewing objects._ If the eye is turned obliquely for a long time
in viewing objects, it may produce an unnatural contraction of the
muscle called into action. This contraction of the muscle is termed
_stra-bis'mus_, or cross-eye. The practice of imitating the appearance
of a person thus affected, is injudicious, as the imitation, designed
to be temporary, may become permanent.
_Observation._ The vision of a "cross-eye" is always defective. In
general, only one eye is called into action, in viewing the object to
which the mind is directed. This defect can be remedied by a surgical
operation, which also corrects the position of the eye.
937. _Children should be trained to use the eye upon objects at
different distances._ This is necessary, in order that the vision may
be correct when objects at various distances are viewed. Any action
unnatural to the muscles, if frequently repeated, may and will modify
the character and action of the parts so operated upon. If a limb, as
the arm, be kept flexed for a long time, one set of muscles will be
relaxed and elongated, and another will be shortened, and its
contractile power will be increased. The same principle is true of the
eye.
-=-=-=-=-=-=-=-=-=-=-=-=
What causes palsy of the retina? 936. Why should we avoid oblique
positions of the eye in viewing objects? What is said of the practice
of imitating persons thus affected? What is said in reference to the
vision of a "cross-eye"? 937. Why should children be trained to use
the eye upon objects at different distances? What is the effect if an
unnatural action of the muscles is frequently repeated? Does the same
principle apply to the eye?
-=-=-=-=-=-=-=-=-=-=-=-=
938. In viewing objects very near the eye, the ciliary processes are
called into action to produce a proper inclination of the crystalline
lens, so that the rays of light may be properly refracted to form a
perfect image on the retina. In looking at objects at a great
distance, the ciliary processes are called into a different action, to
produce a different inclination of the lens. Let either of these
actions be repeated, again and again, for weeks and months, and they
will become natural, and the acquired inclination will be permanent.
939. From the preceding principle, a person becomes short or long
sighted, as the objects to which the eye is usually directed are near
or remote. This is one reason why scholars, watchmakers, and
artisans, who bring minute objects near the eye to examine them,
are short-sighted, and why hunters and sailors, who are habituated
to view objects at a distance, are long-sighted.
_Observation._ In the management of children, whether in the nursery
or school-room, it is very important that their books, or articles
upon which they may labor, should be held at an appropriate distance
from the eye. Were this attended to by the parent or instructor, we
should not see so many persons with defective vision.
940. Cleanliness, as well as the health of the eye, require that it be
bathed every morning with pure water, either cold or tepid,
accompanied with as little rubbing or friction as possible. In all
instances, the secretion from the lachrymal glands, that sometimes
collects at the angle of the eye, should be removed, as it contains
saline matter.
941. When small particles, or dust, get upon the eye, they produce
much inconvenience, which is often increased by harsh attempts to
remove them. The individual should be placed before a strong
light, the lids held open with one hand, or by another person, and
the particles removed with the corner of a fine linen or silk
handkerchief.
-=-=-=-=-=-=-=-=-=-=-=-=
938. What is the effect of repeatedly using the eye in one direction?
939. Why are artisans and scholars generally short-sighted? Why are
sailors and hunters long-sighted? How can defective vision in a great
degree be prevented? 940. What reasons are there for bathing the eye?
941. How can dust and other small particles be removed from the eye?
-=-=-=-=-=-=-=-=-=-=-=-=
942. Sometimes the substance is concealed under the upper eyelid, and
it may then be exposed by turning back the lid in the following
manner: Take a knitting-needle, or small, slender piece of stick,
which is perfectly smooth, and place it over the upper lid, in contact
with, and just under the edge of the orbit; then, holding it firmly,
seize the eyelashes with the fingers of the disengaged hand, and
gently turn the lid back over the stick or needle. The inner side of
the lid can then be examined, and any substance removed that may have
been there concealed. Too many trials ought not to be made, if
unsuccessful, as much inflammation may be induced; but a surgeon
should be consulted as soon as possible.
_Observation._ Eyestones ought never to be placed in the eye, as they
often cause more pain and irritation than the evil which they are
intended to remedy.
-=-=-=-=-=-=-=-=-=-=-=-=
942. How removed from the upper eyelid? Why should not eyestones be
used?
-=-=-=-=-=-=-=-=-=-=-=-=