Part 60

In the =emmetropic= (normal) eye, the rays leave the surface of the cornea parallel to each other and it may be possible for the observer to secure a good image on his retina, without the aid of lenses. In the =myopic= (short sighted) eye they assume a convergent course on leaving the cornea, and to secure a satisfactory image a biconcave or plano-concave lens must be interposed between the cornea of the patient and the eye of the observer.

In the =hypermetropic= (long sighted) eye, the rays diverge in leaving the cornea of the patient, and a convex lens must be interposed between this and the eye of the observer, in order that the rays may be focused on the eye of the observer.

To adapt the vision to the different eyes the modern ophthalmoscope is furnished with a series of lenses concave and convex, any one of which can be moved behind the hole in the mirror to suit the demands of the particular case.

To make a satisfactory examination the pupil should be dilated as for oblique focal illumination. A 1:200 solution of apomorphia may be instilled into the eye (a drop or two) and in 20 to 25 minutes a satisfactory dilatation will have been secured. The effect of the homatropin will usually have disappeared in twenty-four hours.

Determination of Static Refraction.

This can be best done in the lower animals by determining the strength of the lens required to render clear the image of its fundus. By knowing the refracting power of the lens, we may ascertain what deviation from the normal refraction there is in the eye under observation.

In making this test the mirror of the ophthalmoscope must be brought closely to the eye of the patient—1 to 2 inches.

If in such a case and without the use of any lens a distinct image of the fundus is obtained, and if this is rendered less distinct by interposing the lowest convex lens in front of the eye of the observer, the eye is =emmetropic=.

If the ophthalmoscopic mirror without a lens gives an indistinct vision of the fundus, and if the image is rendered clear by interposing one of the convex lenses, the eye is =hypermetropic=. The strength of the convex lens, +1, +2 or +3, dioptrics will give the measure of the hypermetropia.

If, on the contrary, the ophthalmoscopic mirror gives an indistinct image of the fundus, which is rendered even more indistinct by the interposition of a convex lens, but is cleared up and rendered definite by a concave lens, the patient is myopic. The strength of the concave lens used will give the degree of myopia, –1 dioptric, –2 dioptrics, etc.

The tendency in the horse is constantly to slight long-sightedness, but the deviation is rarely found to be serious either in this direction or in that of astigmatism.

Mydriatics.

Dilation of the pupil by mydriatics (mydriasis dilation of the pupil) is a most important means of diagnosis, and therefore a knowledge of the action of the different mydriatics is essential. The mydriatics in common use not only dilate the pupil, but also paralyze the ciliary body and the power of accommodation in ratio with the strength of the solution employed. This determines an adaptation of the eye to the farthest point of vision and holds it there until the action of the mydriatic passes off and normal power of accommodation is restored. In short it renders the subject long sighted, during its action.

=Atropine= the alkaloid of atropa belladonna is the most generally available and persistent of the mydriatics, and is in most common use. It is usually employed as sulphate of atropine, though some prefer the nitrate, the salicylate or the borate to obviate the danger of atropinism. This form of poisoning may show in the occurrence of conjunctivitis and in case of one attack the susceptibility to atropine is greatly to be dreaded, so that it should never again be used on the same subject. The real cause of atropinism is uncertain, it has been variously ascribed to too great acidity or alkalinity, or to micro-organisms growing in the solution. Hence the importance of using the antiseptic salts of atropine, and of testing the solution to see that it is exactly neutral before it is applied.

The strength of the solution of atropine is an important consideration. Donders found that 1:120 of water produced a full effect, while Jaarsma obtained the full effect in one hour from a drop of a solution of one to twelve hundred of water. The action on carnivora (dogs and cats) is equivalent to that on man, while on the herbivora (rabbit, horse, ox, sheep) it is somewhat less, and on birds very slight indeed. On diseased eyes a large amount may be required, and with synechia (adhesion of the iris to the capsule of the lens) dilatation may be impossible. The full effect may last 24 hours, and accommodation may remain very imperfect for 11 days.

The direct action of atropine on the eye is shown in dilatation of the pupil of the frog after the eye has been detached from all connection with heart or brain, by excision. It acts also in the normal system through reflex nervous action, since, after division of the sympathetic trunk going to the eye, that eye does not dilate so much under atropia as the opposite eye.

Atropine is usually employed by lodging a drop in the pouch of the conjunctiva (inside the lower lid), and from this it makes its way into the aqueous humor, for if that liquid is transferred to the conjunctiva of another animal it causes dilatation. Puncture of the cornea with evacuation of the aqueous humor lessens the action of the atropine. Atropine dilatation is increased by following it with cocaine which causes contraction of the iridian vessels, the antithesis of the dilatation of the vessels which occurs when the cornea is perforated and the pressure of the aqueous humor is removed.

Atropine is one of the most potent poisons and must be used with caution especially in the carnivora and omnivora. The danger lies not alone in the absorption from the conjunctiva, but also from the escape of the liquid through the lachrymo-nasal duct, to the nose and later to the actively absorbing mucosæ of the lungs and stomach.

The symptoms of general poisoning are: rapid pulse, vertigo, weakness of posterior limbs, general prostration and thirst or dryness of the throat.

=Homatropine= is an oily liquid produced by the action of muriatic acid on the cyanate of atropine. With hydrobromic acid it forms a readily crystallizable salt, the solution of which acts on the eye like atropine, but more promptly and transiently. One drop of a solution of one to one hundred and twenty, usually gives in twenty minutes, full pupillary dilation and complete paralysis of accommodation which lasts only for twenty-four hours. Add to this that there is little danger of constitutional disturbance and poisoning, and homatropine must be accepted as a more desirable agent than atropine. It is especially to be preferred in cases of senility with shallow anterior chambers, and in glaucoma, in which atropine tends to aggravate the lesion.

=Daturine=, the alkaloid of datura stramonium is a potent mydriatic, causing pupillary dilatation in a solution of one to one hundred and sixty thousand of water. It appears to be identical with atropine.

=Duboisine= the alkaloid of duboisia myoporoides is also a potent mydriatic. Jaarsma found that a solution of the sulphate, of one to three thousand, paralyzed accommodation for twenty-four hours. It acts more promptly than atropine but is more poisonous.

=Hyoscyamin=, the alkaloid of hyoscyamus niger, is also strongly mydriatic. One drop of an one to three hundred solution of the sulphate paralyzed accommodation for from seventy-five to one hundred hours. Risley found it to act more promptly than atropine, and to be less dangerous than duboisine.

WOUNDS OF THE EYELIDS.

Traumas: bites, lacerations, blows, penetrating wounds, gunshot, scratches, kicks. Upper lid or commissure. Reparatory power of eyelid. Danger of distortion. Treatment: sutures, plaster, shellac, collodion, gelatine, Frick’s gelatine, birdlime, sterilisation: Quilled and twisted suture. Position in stall. Metallic guard for eye.

Causes. Traumatic injuries of the eyelids are especially common in the horse mainly because of his exposure in connection with the services required of him. In a team he is liable to be bitten by one of his fellows, or the lid may be caught on nails, in turning, or on hooks upon harness, chains or wagons. It is sometimes injured by a blow from a club or whiplash, or by knocking the head against solid objects that he failed to see on account of the blinds. Again the injury will come from running against prongs of bushes or trees, or of stump fences. Occasionally a blow with the horn of an ox or cow is the cause, but this is much more frequent with the bovine races. Then again gunshot wounds are found in all animals. In sheep the eyelids sometimes suffer from bites of dogs, while in dogs and cats, the teeth and claws are the main causes of injury. These smaller animals also suffer from brutal blows and kicks.

Nature. Wounds of the eyelids almost invariably affect the upper lid, because of its extra size and prominence. Sometimes one commissure or the lower lid is the injured part.

Clean incised wounds are rare, while lacerations with or without contusions are the common experience. The laceration often extends through the free margin of the lid, and then to one side, mostly the outer, in a direction more or less parallel to the tarsus. The result is that the detached flap drops downward exposing a greater or less portion of the bulb covered with blood. The conjunctiva, the cornea, the sclerotic or iris may be implicated in the lesion in different cases, so that such wounds are of the most varied degree of gravity. If, however, the lesion is confined to the lid, and in the absence of absolute detachment of the flap, or severe contusion, a good repair may be confidently hoped for. The vascularity and reparatory powers of the eyelid are unusually great, and the looseness of the skin, connective tissue, mucosa, and even the muscles is such that they do not draw injuriously upon the edges of the wound to disturb the process of cicatrization. If the two opposing ends of the divided tarsal cartilage are kept in accurate opposition, the elasticity of his structure serves to preserve the even contour of the palpebral margin, and the adhesion or granulation process between the edges of the wound, soon becomes firm enough to prevent further displacement. Even when one-half of the eyelid is torn loose, remaining attached only by a narrow portion, reunion without any unsightly distortion is not to be despaired of. In case of a mere vertical laceration on the other hand, the case is very simple and hopeful. Even when a portion of a lid has been completely torn off and lost, the loose textures of the remaining part, often appear to stretch in the process of healing so that a fairly serviceable, though by no means an æsthetic covering for the eye may remain. This may serve for a common work horse, but the unsightliness would necessarily debar him from use in a carriage or as a saddle horse. The imperfect protection too, exposes the eye to rainstorms, hail and snow, as well as to dust, and greatly predisposes to conjunctivitis.