Part 71

The cornea is the anterior transparent portion of the outer coat of the eyeball. It may be viewed as a part of the sclerotic specially modified to permit the passage of light into the interior of the eye. Its outline is elliptical, nearly circular, and its greatest diameter is transverse. At its periphery it joins the sclerotic by continuity of tissue, and as the edge of the cornea is slightly beveled and has the fibrous sclerotic carried for a little distance forward on its outward surface, the cornea is generally said to be fitted into the sclerotic like a watch glass into its rim. The venous canal of Schlemm runs circularly around the eyeball at the line of junction of the sclerotic and cornea. The anterior surface of the cornea is exquisitely smooth, and is kept moist by the lacrimal secretions. Its posterior surface forms the anterior boundary of the chamber in which the aqueous humor is contained. The cornea is of uniform thickness and is of a dense, almost horny, consistence. Save a few capillary loops of blood vessels at its margin the cornea is without vessels. Its structure is comprised of five distinct layers.

The aqueous humor occupies a chamber which is bounded in front by the posterior surface of the cornea and behind by the capsule and suspensory ligament of the lens and by the ends of the ciliary processes. It is across this chamber that the iris extends. The aqueous humor is composed of water, with a small proportion of common salt in solution.

The iris is a muscular, pigmented curtain extending across the interior of the eye and having about its center an aperture termed the pupil. By variations in the size of this aperture the amount of light transmitted to the retina is regulated. It varies somewhat in color, but is most frequently of a yellowish-brown tint. Its anterior face is bathed by the aqueous humor. The greater part of the posterior surface is in contact with the capsule of the lens and glides on it during the movements of the curtain. The circumferential border is attached within the junction of the sclerotic cornea. The inner border circumscribes the pupil, which varies in outline according to its size. When much contracted the pupil is a very elongated ellipse, the long axis of which is in the line joining the nasal and temporal angles of the eyelids. It contains muscular tissue, which, by contracting or relaxing, lessens or dilates the pupillary opening.

The choroid coat is a bell-shaped, dark membrane which lines the sclerotic. Its outer surface has a shaggy appearance, caused by the tunica fusca, which unites the two coats. Between the two the ciliary vessels and nerves pass forward. Behind it is pierced by the optic nerve; in front it is continued as the ciliary processes, which form, as it were, the rim of the bell. The ciliary processes form a fringe around the slightly inverted rim of the choroid.

The retina is the most delicate of the coats of the eyeball. It is formed by the expansion of the optic nerve on the inner surface of the choroid, and, like that coat, it is bell-shaped. Its inner surface is molded on the vitreous humor. The nervous structures of the retina terminate at a wavy line, the ora serrata, behind the ciliary processes. Ten distinct layers are described as composing the thickness of the retina.

The lens is situated behind the pupil and is contained within a capsule of its own.

The capsule is a close-fitting, firm, transparent membrane. The anterior surface forms the posterior boundary of the cavity containing the aqueous humor, and the iris in its movement glides on it. The posterior surface is in contact with the vitreous humor.

The vitreous humor occupies four-fifths of the interior of the eyeball. It is globular in form, with a depression in front for the lodgment of the lens. It is colorless, transparent, and of a consistency like thin jelly. It is enveloped by a delicate capsule--the hyaloid membrane--which is connected in front with the suspensory ligament of the lens, and ends by joining the capsule behind the lens.

The orbital cavity, at the side of the head, is circumscribed by a bony margin; posteriorly, however, there are no bony walls, and the cavity is often confounded with the depression above and behind the orbit--the temporal fossa. A fibrous membrane completes this cavity and keeps it distinct from the temporal fossa. This membrane--the ocular sheath or periorbita--is attached posteriorly around the opening in the back part of the orbital cavity (the orbital hiatus) and anteriorly to its inner face; then it becomes prolonged beyond the margin to form the fibrous membrane of the eyelids. When complete the orbital cavity has the form of a regular hollow cone, open at its base and closed at the apex. The opening of this cone is directed forward, downward, and outward. Independently of the globe of the eye, this cavity lodges the muscles that move it, the membrana nictitans, and the lacrimal gland.

The muscles of the eye are seven in number--one retractor, four straight, and two oblique. The retractor oculi envelops the optic nerve between the brain and the ball of the eye and becomes attached upon the external face of the sclerotic tunic. When this muscle contracts, it draws the globe back into the orbit, away from the light. The superior, inferior, external, and internal recti or straight muscles are attached to the back part of the orbital sheath and spread forward in four bundles over the globe of the eye, where they are inserted by a fibrous expansion into the sclerotic at the margin of the cornea. When they act singly, they turn the globe either upward, downward, inward, or outward. The great oblique, by its action, pivots the eye inward and upward in the orbit. The small oblique turns the eye outward and downward.

The eyelids are two movable curtains, superior and inferior, which cover and protect the eye in front. They are attached to the circumference of the orbit and have a convex external face formed by the skin and a concave internal face molded on the anterior surface of the eye and are lined by the conjunctiva, which is reflected above and below on the eyeball. The border of each lid is slightly beveled on the inner side and shows the openings of the Meibomian glands. These glands secrete an unctuous fluid, which is thrown out on the border of the lids, the function of which is to facilitate their movements and enable them to retain the tears in the ocular cavity. The eyelid is composed of a fibrous inner membrane ending in a stiff arch near the border, a muscle to close the lid, another to open it, the skin externally, and the conjunctival mucous membrane internally. The border of each lid is covered and protected by long hairs to prevent floating particles of matter in the atmosphere from gaining entrance to the eye.

The membrana nictitans, which is also named the third eyelid, winking eyelid, haw, etc., is placed at the inner angle of the eye, whence it extends over the eyeball to relieve it from foreign bodies which may fall upon it. It has for its framework a fibro-cartilage, irregular in shape, thick, nearly prismatic at its base, and thin anteriorly where it is covered by the conjunctiva; behind, it is loosely attached to a fatty cushion.

The lacrimal gland is situated between the orbital process and the upper part of the eyeball. It secretes the tears destined to lubricate the anterior surface of the eye. This fluid escapes upon the organ at the outer angle of the lids and is carried between them and the eyeball toward the inner angle.

The caruncula lacrimalis is a small round body, frequently entirely or partially black, situated in the inner angle of the eye, and is designed to direct the tears toward the puncta lacrimalia.

The puncta lacrimalia are two little openings, situated one in each eyelid, a short distance from the inner corner, which admit the tears into the lacrimal ducts leading to the lacrimal canal, whence they are emptied into the nasal passages.

CONJUNCTIVITIS (SIMPLE OPHTHALMIA).

This is an inflammation of the conjunctival mucous membrane of the eyeball and lids; in severe cases the deeper coats of the eye are involved, seriously complicating the attack.

Causes.--It may result from a bruise of the eyelid; from the introduction of foreign matters into the eye, as chaff, hayseed, dust, gnats, etc.; from exposure to cold; poisonous or irritating vapors arising from filthiness of stable. Dust, cinders, or sand blown into the eyes during transportation frequently cause conjunctivitis.

Symptoms.--A profuse flow of tears, closure of the eyelids from intolerance of light, retraction of the eyeball and corresponding protrusion of the haw, disinclination to move, diminution of milk secretion, etc. On parting the lids the lining membrane is found injected with an excess of blood, giving it a red and swollen appearance; the sclerotic, or white of the eye, is bloodshot and the cornea may be cloudy. If the disease advances, keratitis results, with its train of unfavorable symptoms.

Treatment.--Careful examination should be made to discover particles of chaff, etc., which may have lodged in the eye, and upon the discovery of such a cause prompt removal is indicated. This may be accomplished by flushing the eye with warm water by means of a syringe, or, if the foreign substance is adherent to the eyeball or lid, it may be scooped out with the handle of a teaspoon or some other blunt instrument. To relieve the congestion and local irritation, a wash composed of boracic acid in freshly boiled water, 20 grains to the ounce, or acetate of zinc, 5 grains to the ounce of pure soft water, may be used, to which may be added 20 drops of laudanum. A few drops of this should be placed in the eye with a camel's-hair pencil or soft feather three or four times daily. The animal should be placed in a cool, darkened stable; then a cloth folded into several thicknesses should be fastened to the horns in such manner as to reach below the eyes. This should be kept wet with cold water during the day and removed at night. If there is much fever and constitutional disturbance, it becomes advisable to administer 1 pound of Epsom salt dissolved in 1 quart of water.

INFECTIOUS CATARRHAL CONJUNCTIVITIS (SPECIFIC OPHTHALMIA).

This generally appears in an enzootic or epizootic form and affects a considerable number in the herd. It is distinctly a contagious disease and may be brought into a previously healthy herd by one infected animal. It may continue in a herd for a season or for several years, affecting all newly purchased animals. It is seldom seen in the winter months. It affects old and young animals alike.

Symptoms.--This form of catarrh conjunctivitis is characterized chiefly by a mucopurulent discharge from the eyes, an intense degree of inflammation of the mucous membrane, accompanied with swelling of the eyelids and an early opacity of the cornea. The flow of tears is mixed with pus, sometimes streaked with blood, and the skin of the face is kept moist and soiled. The eyes are kept continually closed. The implication of the cornea in the disease frequently blinds the animal for a time, and occasionally suppurative keratitis, ulcers of the cornea, or staphyloma supervene. The attack is marked from the onset by fever, partial loss of appetite, partial loss of milk, suspended rumination, and separation from the herd.

Treatment.--The animal should be housed in a cool, dark stable, supplied with plenty of fresh water to drink and soft, succulent feed. Administer 1 pound of Epsom salt--if a very large animal, use 1-1/2 pounds--dissolved in 2 or 3 pints of water. For an eyewash, take boracic acid, 1 dram, and pour 4 ounces of boiling water over it. Use this as often as is convenient, applying it directly to the eye. In the majority of cases improvement becomes manifest in a few days, and the eye becomes clear and free from inflammation in 10 days or 2 weeks. Where the disease develops ulceration of the cornea, or well-marked, deep-seated keratitis, the treatment recommended for those conditions should be adopted.

Prevention.--Whenever this affection appears in a herd all the unaffected animals should be moved to fields which possess a different character of soil and feed. The water should also be changed, especially if they have been obtaining it from a stagnant pond.

KERATITIS (CORNEITIS).

This is an inflammation of the cornea proper, although the sclerotic at the corneal border becomes involved to some extent. It may be divided into diffuse and suppurative.

Causes.--The cornea constitutes the most prominent portion of the eyeball, hence it is subject to a variety of injuries--scratches, pricks, contusions, lacerations, etc. Inflammation of the cornea may also be due to the extension of catarrhal conjunctivitis or intraocular disease, and it may occasionally occur without any perceptible cause.