Having proceeded so far, a snaffle-bridle may now be put on and the horse's head brought round to the door, where a careful examination of his eyes and their appendages should be made. Before, however, proceeding with this branch of the inspection, the examiner should assure himself as to the suitability of the light. A door facing an open space is the most suited to the purpose, so long as it is not exposed to bright sunlight. Too much light falling upon the eyes causes the pupils to contract, and the crystalline lens, which is of special importance to the enquiry, to be hidden from view. Having provided a suitable light, the form and disposition of the eyelids should first be noted. When paralysed they droop and give the eye a closed appearance, but another condition affecting them is that resulting from repeated attacks of specific ophthalmia, when the upper eyelid, instead of describing a graceful arch over the globe, is drawn up into an angular condition, as shown in Fig. 593. Of course the haw, or third eyelid - a thin triangular piece of cartilage in the inner angle of the eye, - should be present and free from any abnormal growth or thickening of its investing mucous membrane. A good view of it may be obtained by pressing the eyelid backward under the orbit, as shown in Fig. 594.
A general examination of the eye itself must now be made, first by comparing the one with the other as to size. Wasting of the eye-ball is a common result of constitutional ophthalmia, and is attended with that angular condition of the upper eyelid referred to above, as well as a more or less sunken state of the globe and textural alterations within it. As to matter of form, the normal condition of the eyes should be full and bold, and describe a regular convexity in front without there being any observable difference between them.
Eyes too prominent, although perfectly clear, sometimes result from paralysis of the optic nerve, in which case vision is more or less defective or altogether lost. Undue flatness of the cornea or front of the eye is also a condition in which sight is impaired, and would, like the other defects referred to, constitute unsoundness.
In order that objects may be clearly visible, the passage of light to the optic nerve should be uninterrupted by any cloudiness or opacity of the ordinarily transparent structures.
In this connection it will be necessary to examine not only the surface, but also the interior of the eye - the cornea, or surface, for opacities of various forms and densities, and the interior for these and other defects. Opacities on the surface are much more serious when in the centre than when near the circumference. They not uncommonly assume the form of pale milky streaks across the eye, such as are inflicted by the lash of a whip, and sometimes so faint as to be of no importance. On the other hand, they may be very dense and interfere with the function of vision. Sometimes these opacities are presented as white spots, small or large according to the nature and extent of the disease which produced them. Such examples are serious, and unquestionably constitute unsoundness.
Fig. 593. - Recurrent Ophthalmia.
Eye showing an angular condition of the upper lid the result of a succession of attacks of Specific Ophthalmia.
Fig. 594. - Examination of the Eye (a, the Haw).
While these observations are being-made as to the condition of the cornea or superficial coat of the eye, it will be necessary to note whether the pupil contracts when the eye is exposed to light, and dilates when its influence is withdrawn. The latter condition may be produced by covering the eye with the hat for three or four minutes, when the pupil should dilate and contract again when exposed to light. If it is found to remain widely dilated, with no disposition to contract, or to contract feebly and imperfectly, either the light is not reaching the optic nerve, or the nerve or the iris itself is diseased. In such a case the animal is clearly unsound.
To those unaccustomed to the examination of eyes the appearance of the corpora nigra (fig. 595) may cause some confusion. These are ordinarily small black bodies attached to the margin of the iris, but in some instances which have come to the writer's notice they have become so far developed as to hang over the pupil as large black masses, and while doing so obstructing the ingress of light and thus impairing vision.
The crystalline lens (j, fig. 596), enclosed in its capsule and placed behind the pupil, will now come under observation. In a normal condition both these structures are perfectly clear and transparent, but under circumstances of disease dense opaque spots, varying from the size of a pin's point to that of a pea, or a diffused opacity, appear, and these constitute the disease termed cataract (Vol. II, p. 119).
Fig. 595. - Interior View of the Eye.
A, Pupil. B, Iris, c, Ciliary Processes. D, Corpora Nigra.
Fig. 596.-Section of Eye.
A, Lachrymal Gland. B, Levator Palpehrae Superioris. C, Levator Oculi. D, Sclerotic Coat. E, Choroid Coat. F. Retina. G, Optic Nerve. H, Vitreous Humour. I, Capsule of the Lens. J, Crystalline Lens. K, Aqueous Humour. L, Cornea. M, Iris. N, Upper Eyelid.
The larger and denser of such developments are readily detected, but the smaller formations only become visible when carefully sought for. To detect these more minute opacities the eye requires to be viewed in a slanting direction while a dark shadow is thrown over it.
Standing at the right side of the horse's head, while still at the door, the examiner seizes the cheek of the bridle with the left hand, and with the right brings his hat, or other black surface, opposite the eye, and within a few inches of it. He then pushes the nose slightly away from him, when, by looking into the eye in an oblique direction from the right forward towards the left, he may see the lens and critically examine it. Anything in the form of a cataract will then be noticed either as a sharply circumscribed white spot (fig. 256, Vol. II, p. 119) or as a diffused cloudiness (fig. 597) of the lens, or its capsule, or both. The right eye having been examined, the left is then submitted to the same line of inspection.
Where deep-seated mischief is suspected, i.e. disease of structures behind the lens, the use of the ophthalmoscope may become necessary to bring it under observation.