DISEASES OF THE EYE
GENERAL DISCUSSION.--_The eye_ is situated in the orbital cavity, to which
it is attached by muscles that rotate it in different directions. The
_orbit_ is lined by fibro-fatty tissues that form a cushion for the eye.
Anteriorly it is protected by the _eyelids_, and in birds by a third eyelid
that corresponds to the membrana nictitans of quadrupeds. The _lachrymal
gland_ which secretes the tears keeps the above parts moist.
_The eye_ is the essential organ of vision. It is formed by a spherical
shell which encloses fluid or semisolid parts. The shell is anteriorly made
up of a transparent convex membrane, the cornea, while the remainder of its
wall is formed by three opaque layers or tunics.
The external tunic is the _sclerotic_. It is a white, solid membrane,
forming about four-fifths of the external shell. Its external face is
related to the muscles and fatty cushion. It receives posteriorly, a little
lower than its middle portion, the insertion of the _optic nerve_, which
passes through the shell and spreads out to form a very thin membrane, the
retina or internal coat.
_The retina_ lines about two-thirds of the posterior portion of the shell
of the eye. It is made up of seven layers. The essential layer is named
from its appearance, rods and cones.
The middle coat is the _choroid_. This is a dark, pigmented, vascular and
muscular membrane. The posterior portion is in contact with the retina.
Anteriorly it forms the ciliary processes and the iris.
_The media_ of the eye are the crystalline lens, vitreous and aqueous
humors. The _crystalline lens_ is a transparent, biconvex body sustained by
the ciliary processes. The _vitreous humor_ is a transparent jelly-like
substance that fills all the cavity of the eye posterior to the lens. The
_aqueous humor_ is a liquid, contained in the anterior and posterior
chambers of the eye in front of the lens. This fluid separates the iris
from the front of the lens.
EXAMINATION OF THE EYE.--In examining and treating the eye we should avoid
rough and hasty manipulation. The animal should be approached slowly. It is
best for the attendant who is familiar with the animal to hold it for the
examiner. It is advisable on approaching the animal to stroke its face, and
in the horse to brush its foretop away. The hand should be carried slowly
to the front of the eye, and the lids separated with the fingers and thumb
if we wish to obtain a better view of the cornea. In cattle the best view
of these parts can be obtained by taking hold of the nose and lifting the
head. It is impossible to make a satisfactory examination of the eye
outside of the stable where the light is coming from all directions. The
most satisfactory conditions under which a general examination can be made
is to stand the animal facing a transom, window or open door. We may then
look directly into the eye and note the condition of the different
refracting media.
The lens should appear transparent and free from scars. The aqueous humor
free from any cloudiness or precipitate. Both pupillary openings should be
the same size, and not too small or too large in the bright light. As we
look through the pupillary openings, both the lens and the vitreous humor
should refract the light properly and not appear white or greenish-white in
color. The color of the iris should be noted. If it lacks lustre or appears
dull, this may indicate an inflammation. In periodic ophthalmia in horses
the iris loses its lustre and becomes a rusty-brown color. It is very
important to note this change in the appearance of the iris. We should
note, in addition, the expression of the animal's face, the position of the
ears and eyelids and manner of the walk. Horses that have defective sight
may show a deep wrinkle in the upper eyelid when startled or looking
directly at an object. Animals that are blind hold the ears in a
characteristic position, and may stumble and walk over, or run into objects
unless stopped. The ophthalmoscope is a very useful instrument for
determining the condition of the different structures of the eyes, when in
the hands of persons who are trained in its use.
CONJUNCTIVITIS.--This is an inflammation of the mucous membrane lining the
eyelids and covering the eyeball. The two forms of conjunctivitis common in
domestic animals are the _catarrhal_ and _purulent_.
_The symptoms differ_ in the two forms of conjunctivitis. They may be
distinguished from each other by the difference in the character of the
inflammatory discharge. In the catarrhal form, there is a discharge of
tears and the lids are held more or less closed. The mucous membrane is
usually brick red in color and swollen. A little later the discharge
becomes heavier and adheres more to the margins of the lids. The lids
continue tender and the inflammation painful. The surface of the cornea may
appear white and the blood-vessels prominent, but it is only in the severe
cases that inflammation of this portion of the eye occurs. In such cases an
elevation in body temperature may occur. This is especially true of
purulent conjunctivitis when primarily caused by an infectious agent. In
the purulent form the discharge is heavy and pus-like.
_The treatment_ is both preventive and curative. The first object must be
to remove the cause. Irritating gases resulting from stable filth should be
remedied by correcting the unsanitary conditions in the stable. Conditions
favoring injury to the eye from foreign bodies, such as chaff and a
careless attendant, should be corrected. Animals suffering from the
infectious or purulent form of inflammation should be separated from the
other animals. Foreign bodies should be removed promptly before they have
had an opportunity to set up a serious inflammation. It is necessary to
confine the animal in some way before attempting to do this. Horses should
be twitched, cattle held by the nose, and the head of a small animal held
firmly with the hands. It may be necessary to cocainize the eye before the
operator can remove the foreign object with absorbent cotton or with
forceps.
In case of injuries and irritation to the lids by foreign bodies, the eye
may be flooded with a three per cent water solution of boric acid twice
daily, or as often as necessary. Such washes or lotions may be applied with
a small piece of absorbent cotton, using a fresh piece each time the eye is
dressed. A medicine dropper may also be used. A lotion containing silver
nitrate two to four grains and distilled water one ounce, is useful in
combating the inflammation. This may be applied twice daily. Irritating
lotions should be avoided, if possible, in the treatment of eye diseases of
horses, because of the danger of making the animal disagreeable to handle.
Boric acid may be dusted over the ball of the eye of cattle with a powder
blower.
PERIODIC OPHTHALMIA, "MOONBLINDNESS."--This is a periodic inflammation of
one or both eyes of the horse. The internal structures of the eye are
involved by the inflammation, but it may appear as a conjunctivitis.
_The cause_ of this disease is not well understood. Certain local
conditions seem to favor its development. Undrained land, a humid climate,
the feeding of a one-sided ration or one that does not maintain the
vitality of the animal, and severe work seem to produce it. Heredity must
be accepted as a prominent accessory cause. A number of different bacteria
have been mentioned as causative factors for this disease.
_The symptoms_ at the very beginning indicate a general inflammation of the
eye. The eyelids are swollen, there is an abundant secretion of tears, the
eyeball is retracted and the lids are held more or less closed. As the
inflammation progresses, the cornea becomes milky in appearance and the
aqueous humor may show a precipitate toward the bottom of the anterior
chamber. The pupil is usually contracted and dilates slowly when the animal
is moved into the light. The acute inflammation gradually subsides, and
about the tenth to the fourteenth day the lids and cornea may appear
normal.
The periods between these acute attacks of ophthalmia may vary from a few
weeks to several months. Severe work, debility and the character of the
ration influence their frequency. It is not uncommon for animals that have
been given a rest to suffer from a second attack on being put to work. The
attendant may observe a hazy or whitish condition of the margin of the
cornea. The upper lid may show an abrupt bend of its margin and a deep
wrinkle. The color of the iris appears to have lost its lustre, and the
aqueous humor and lens may be cloudy. After a variable number of attacks
glaucoma or cataract develops.
_The history_ of the case will enable the attendant to recognize this form
of ophthalmia.
_Treatment_ is unsatisfactory. Preventive measures consist in avoiding
conditions favorable to the production of the disease. This should be
practised so far as possible. At the time the attack occurs, the animal
should be given a cathartic. One pound of Glauber's salts in a drench is to
be preferred. Rest in a darkened stall is indicated. An eye lotion
containing three grains of silver nitrate in one ounce of distilled water
should be applied to the eye three times daily. A water solution of
atropine or eserine should be used for the purpose of relieving the
symptoms of iritis or glaucoma. A very light diet should be fed.
INFECTIOUS OPHTHALMIA OF RUMINANTS.--This occurs as an acute inflammation
of the eyelids and cornea. The disease is highly infectious, affecting all
of the susceptible animals in the herd. It commonly occurs during the late
summer and fall.
_The symptoms appear_ suddenly. The animal is feverish, the eyes closed and
the cheeks are wet with tears. The cornea becomes clouded, white and
opaque. In severe cases, the blood-vessels around the margin of the cornea
become prominent, and ulcers form on its surface. The animal's appetite is
impaired or lost. There is loss of flesh and temporary blindness. The
blindness in one or both eyes may persist for a period of from two weeks to
several months. Permanent blindness is comparatively rare.
_The preventive treatment_ consists in practising the necessary precautions
against the introduction of the disease into the herd, and in carefully
quarantining the first cases of the disease that appear. The affected
animal should be given a darkened stall, and fed a very light ration until
the acute inflammation has subsided. From one to one and one-half pounds of
Glauber's salts should be given. The _local treatment_ consists in the
application of antiseptic lotions or powders to the eye. Equal parts of
boric acid and calomel, dusted into the eye twice daily with a powder
blower, is a very effective treatment.
QUESTIONS
1. Name the different structures that form the shell of the eye; name and
describe the different media of the eye.
2. Give the general method of examining the eyes of horses.
3. What is conjunctivitis? Give causes and treatment.
4. What is "moonblindness"? Give the symptoms.
5. Describe the symptoms of infectious ophthalmia of ruminants and the
treatment.