SENSE OF VISION.
890. This sense contributes more to the enjoyment and happiness of man
than any other of the senses. By it we perceive the form, color,
volume, and position of objects that surround us. The eye is the organ
of sight, or vision, and its mechanism is so wonderful, that it not
only proves the existence of a great First Cause, but perhaps, more
than other organs, the design of the Creator to mingle pleasure with
our existence.
ANATOMY OF THE ORGANS OF VISION.
891. The apparatus of vision consists of the _Op'tic Nerve_, the
_Globe_ and _Muscles_ of the eye, and its _Protecting Organs_.
892. The OPTIC NERVE arises by two roots from the central portion of
the base of the brain. The two nerves approach each other, as they
proceed forward, and some of the fibres of each cross to the nerve of
the opposite side. They then diverge, and enter the globe of the eyes
at their back part, where they expand, and form a soft, whitish
membrane.
893. The GLOBE, or ball of the eye, is an optical instrument of the
most perfect construction. The sides of the globes are composed of
_Coats_, or membranes. The interior of the globe is filled with
refracting _Humors_, or _me'di-ums_.
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890. Which sense contributes most to the enjoyment of man? What do we
perceive by this sense? What is said of the mechanism of the eye?
891-916. _Give the anatomy of the organs of vision._ 891. Of what does
the apparatus of vision consist? 892. Describe the optic nerve. 893.
Describe the globe of the eye.
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894. The COATS are three in number: 1st. The _Scle-rot'ic_ and
_Corn'e-a_. 2d. The _Cho'roid_, _Iris_, and _Cil'ia-ry processes_. 3d.
The _Ret'i-na_.
895 The HUMORS are also three in number: 1st. The _A'que-ous_, or
watery. 2d. The _Crys'tal-line_, (lens.) 3d. The _Vit're-ous_, or
glassy.
[Illustration: Fig. 137. The second pair of nerves. 1, 1, Globe of the
eye: the one on the left is perfect, but that on the right has the
sclerotic and choroid coats removed, to show the retina. 2, The crossing
of the optic nerve. 5, The pons varolii. 6, The medulla oblongata. 7, 8,
9, 10, 11, 12, 13, The origin of several pairs of cranial nerves.]
896. The SCLEROTIC COAT is a dense, fibrous membrane and invests about
four fifths of the globe of the eye. It gives form to this organ, and
serves for the attachment of the muscles that move the eye in various
directions. This coat, from the brilliancy of its whiteness, is known
by the name of "the white of the eye." Anteriorly, the sclerotic coat
presents a bevelled edge, which receives the cornea in the same way
that a watch-glass is received by the groove in its case.
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894. Name the coats of the eye. 895. Name the humors of the eye.
Explain fig. 137. 896. Describe the sclerotic coat.
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897. The CORNEA is the transparent projecting layer, that forms the
anterior fifth of the globe of the eye. In form, it is circular,
convexo-concave, and resembles a watch-glass. It is received by its
edge, which is sharp and thin, within the bevelled border of the
sclerotic, to which it is firmly attached. The cornea is composed of
several different layers; its blood-vessels are so small that they
exclude the red particles altogether, and admit nothing but serum.
898. The CHOROID COAT is a vascular membrane, of a rich chocolate-brown
color upon its external surface, and of a deep black color within. It
is connected, externally, with the sclerotic, by an extremely fine
cellular tissue, and by the passage of nerves and vessels; internally,
it is in contact with the retina. The choroid membrane is composed of
three layers. It secretes upon its internal surface a dark substance,
called _pig-ment'um ni'grum_, which is of great importance in the function
of vision.
899. The IRIS is so called from its variety of color in different
persons. It forms a partition between the anterior and posterior
chambers of the eye, and is pierced by a circular opening, which is
called the _pu'pil_. It is composed of two layers. The radiating
fibres of the anterior layer converge from the circumference to the
centre. Through the action of these radiating fibres the pupil is
dilated. The circular fibres surround the pupil, and by their action
produce contraction of its area. The posterior layer is of a deep
purple tint, and is called _u-ve'a_, from its resemblance in color to
a ripe grape.
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How are this coat and the cornea united? 897. Describe the cornea.
898. What is the color of the external surface of the choroid coat? Of
the internal? How is it connected externally? How internally? What
does this membrane secrete upon its internal surface? 899. Describe
the iris. Of how many layers of fibres is the iris composed? What is
the function of the radiating fibres? Of the circular?
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900. The CILIARY PROCESSES consist of a number of triangular folds,
formed, apparently, by the plaiting of the internal layer of the
choroid coat. They are about sixty in number. Their external border is
continuous with the internal layer of the choroid coat. The central
border is free, and rests against the circumference of the crystalline
lens. These processes are covered by a layer of the pigmentum nigrum.
[Illustration: Fig. 138. A view of the anterior segment of a transverse
section of the globe of the eye, seen from within. 1, The divided edge of
the three coats--sclerotic, choroid, and retina. 2, The pupil. 3, The
iris: the surface presented to view in this section being the uvea. 4,
The ciliary processes. 5, The scalloped anterior border of the retina.]
901. The RETINA is composed of three layers: The external; middle, or
nervous; and internal, or vascular. The external membrane is extremely
thin, and is seen as a flocculent film, when the eye is suspended in
water. The nervous membrane is the expansion of the optic nerve, and
forms a thin, semi-transparent, bluish-white layer. The vascular
membrane consists of the ramifications of a minute artery and its
accompanying vein. This vascular layer forms distinct sheaths for the
nervous papillae, which constitute the inner surface of the retina.
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900. How are the ciliary processes formed? What does fig. 138 exhibit?
901. Of how many layers is the retina composed? Describe the external
layer. The nervous layer.
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902. The AQUEOUS HUMOR is situated in the anterior and posterior
chambers of the eye. It is an albuminous fluid, having an alkaline
reaction. Its specific gravity is a very little greater than distilled
water. The anterior chamber is the space intervening between the
cornea, in front, and the iris and pupil, behind. The posterior
chamber is the narrow space, less than half a line in depth, bounded
by the posterior surface of the iris and pupil, in front, and by the
ciliary processes and crystalline lens, behind. The two chambers are
lined by a thin layer, the secreting membrane of the aqueous humor.
903. The CRYSTALLINE HUMOR, or lens, is situated immediately behind
the pupil, and is surrounded by the ciliary processes. This humor is
more convex on the posterior than on the anterior surface, and, in
different portions of the surface of each, the convexity varies from
their oval character. It is imbedded in the anterior part of the
vitreous humor, from which it is separated by a thin membrane, and is
invested by a transparent elastic membrane, called the capsule of the
lens. The lens consists of concentric layers, disposed like the coats
of an onion. The external layer is soft, and each successive one
increases in firmness until the central layer forms a hardened
nucleus. These layers are best demonstrated by boiling, or by
immersion in alcohol, when they separate easily from each other.
_Observations._ 1st. The lens in the eye of a fish is round, like a
globe, and has the same appearance, when boiled, as the lens of the
human eye.
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The vascular layer. 902. Where is the aqueous humor situated? What
part of the eye is called the anterior chamber? The posterior chamber?
With what are the chambers lined? 903. Where is the crystalline humor
situated? With what is it surrounded? Of what does the lens consist?
How are these layers best demonstrated? What is produced when the
lens, or its investing membrane, is changed in structure?
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2d. When the crystalline lens, or its investing membrane, is changed
in structure, so as to prevent the rays of light passing to the
retina, the affection is called a _cataract_.
[Illustration: Fig. 139. A section of the globe of the eye. 1, The
sclerotic coat. 2, The cornea (This connects with the sclerotic coat by a
bevelled edge.) 3, The choroid coat. 6, 6, The iris. 7, The pupil. 8, The
retina. 10, 11, 11, Chambers of the eye that contain the aqueous humor.
12, The crystalline lens. 13, The vitreous humor. 15, The optic nerve.
16, The central artery of the eye.]
904. The VITREOUS HUMOR forms the principal bulk of the globe of the
eye. It is an albuminous fluid, resembling the aqueous humor, but is
more dense, and differs from the aqueous in this important particular,
that it has not the power of re-producing itself. If by accident it is
discharged, the eye is irrecoverably lost; while the aqueous humor may
be let out, and will be again restored. It is enclosed in a delicate
membrane, called the _hy'a-loid_, which sends processes into the
interior of the globe of the eye, forming the cells in which the humor
is retained.
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904. Describe the vitreous humor. How does this humor differ from the
aqueous? What membrane encloses the vitreous humor?
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_Observation._ The structure of this organ can be seen by first
freezing the eye of a sheep or an ox; it then can be cut in various
directions, and each part separately examined.
905. The MUSCLES of the eye are six in number. They are attached, at
one extremity, to the bones of the orbit behind the eye; at the other
extremity, they are inserted by broad, thin tendons, near the junction
of the cornea with the sclerotic coat. The white, pearly appearance of
the eye is caused by these tendons.
[Illustration: Fig. 140. A view of the eye and its muscles. _a_, _b_,
_c_, _d_, _e_, Five of these muscles. _f_, The optic nerve. G, The
trochlea, or pulley over which one of the muscles passes. The bone is
seen above and below the eye.]
_Observation._ If the external muscle is too short, the eye is turned
out, producing the "wall eye." If the internal muscle is contracted,
the eye is turned inward toward the nose. It is then called a "cross
eye."
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905. How many muscles has the eye? Give their attachments. What causes
the pearly appearance of the eye? What does fig. 140 represent? What
is the effect if the external muscle is contracted? The internal
muscle?
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906. The PROTECTING ORGANS are the _Or'bits_, _Eyebrows_, _Eyelids_,
and _Lach'ry-mal Apparatus_.
907. The ORBITS are deep, bony sockets, in which the globes of the
eyes are situated. They have the form of a cone, the base of which is
open and directed forward. The bottom of the orbits is pierced by a
large hole which gives passage to the optic nerve. These cavities are
lined with a thick cushion of fat, in order that the eyes may move in
all directions, with perfect freedom and without friction.
908. The EYEBROWS are two projecting arches of integument, covered
with short, thick hairs, which form the upper boundary of the orbits.
The eyebrows are so arranged that they prevent the moisture that
accumulates on the forehead, in free perspiration, from flowing into
the eye, and also shade these organs from too vivid light.
909. The EYELIDS are two movable curtains placed in front of the eye.
They have a delicate skin on the outside, muscular fibres beneath, and
a narrow cartilage on their edges, which tends to preserve the shape
of the lid. Internally, they are lined by a smooth membrane, which is
reflected over the front of the eye upon the sclerotica. This membrane
is called the _con-junc-ti'va_. It secretes the fluid that moistens
and lubricates the eye, and which causes the eyelids to open and shut
without friction.
_Observation._ When the portion of this membrane that is reflected
over the globe of the eye, is inflamed, there is frequently a
deposition of whitish material, called lymph. This accounts for the
films, opacities, and white spots seen upon the eye after the
inflammation has subsided.
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906. Name the protecting organs of the eye. 907. Describe the orbits.
How are the movements of the eye facilitated? 908. Describe the
eyebrows. What does this arrangement prevent? 909. Describe the
eyelids. What is the use of the conjunctiva? How are the white spots
frequently seen upon the eye accounted for?
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910. There are found several small glands on the internal surface of
the cartilage, which have the appearance of parallel strings of
pearls. They open by minute apertures upon the edges of the lids. The
secretion from these glands prevents the edges of the eyelids from
being united during sleep.
911. The edges of the eyelids are furnished with a triple row of long,
thick hairs, called _eyelashes_, which curve upward from the upper
lid, and downward from the lower, so that they may not interlace with
each other in the closure of the eyelids. These appendages of the eye,
by closing, not only protect it from moisture, but from dust,
particularly during sleep. They likewise, by their movements in
opening and shutting, spread the lubricating fluid equally over the
eye.
912. The LACHRYMAL APPARATUS, which secretes the tears, consists of
the _Lachrymal Gland_ with its ducts, _Lachrymal Canals_, and the
_Nasal Duct_.
913. The LACHRYMAL GLAND is situated at the upper and outer angle of
the orbit. It is about three quarters of an inch in length, flattened
and oval in shape, and occupies a depression in the orbital plate of
the frontal bone. Ten or twelve small ducts pass from this gland, and
open upon the upper eyelid, where they pour upon the conjunctiva the
lachrymal fluid, or tears. This secretion is maintained while we are
asleep, as well as when we are awake. The eye from this cause is kept
constantly moist.
914. The LACHRYMAL CANALS commence at minute openings upon the free
borders of each eyelid, near the internal angle of the eye, by two
small orifices, called _punc'ta lach-ry-ma'li-a_, (tear points.) Each
of these points communicate with the sac at the upper part of the
nasal duct.
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910. What are found on the internal surface of the cartilage of the
eyelids? Where do they open, and what is their use? 911. With what are
the edges of the eyelids furnished? What are their uses? 912. Of what
does the lachrymal apparatus consist? 913. Describe the lachrymal
gland. How many ducts pass from this gland, and what do they convey to
the eye? Why is the eye constantly moist? 914. Where do the lachrymal
canals commence?
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915. The NASAL DUCT is a short canal, about three quarters of an inch
in length, directed downward and backward to the inferior channel of
the nose, where it terminates by an expanded orifice.
[Illustration: Fig. 141. 1, The lachrymal gland. 2, Ducts leading from
the lachrymal gland to the upper eyelid. 3, 3, The puncta lachrymalia. 4,
The nasal sac. 5, The termination of the nasal duct.]
916. The fluid (tears) secreted by the lachrymal gland, is conveyed to
the eye by the small ducts before described. It is then imbibed by the
puncta lachrymalia, and carried by the lachrymal canals into the
lachrymal sac, from which it is passed to the nasal cavities by the
nasal ducts.
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What are they called? With what do they communicate? 915. Describe the
nasal duct. 916. How are the tears conveyed from the lachrymal gland
to the nose?
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