Total blindness may be the consequence of disease of the optic nerve, or its expanded filaments which form the retina within the globe, without the exhibition of any symptoms which would be apparent in the eye to the ordinary observer. In the disease which is known as amaurosis the retina or expansion of the optic nerve is insusceptible to the action of light, which therefore ceases to be a stimulant to the nerve. Consequently the circular fibres of the iris which close the pupil do not contract, and the pupil remains widely dilated even in the strongest light. The eye is perfectly transparent, and to the inexperienced observer affords no indication of unsoundness. If the disease is limited to one eye it may exist for some time without being detected, but the tendency always is for it to extend to the optic nerve of both eyes, and in that event the horse shows that the sight is defective even in the earlier stages of the malady in a manner sufficiently well defined to attract the attention of the attendant, who will probably examine the eye without finding anything wrong.

An inspection by a professional man will lead to the discovery of the inactive conditions of the muscular fibres of the iris, but until the disease has advanced so far as to cause total blindness, the iris will still to some extent contract and lessen the size of the pupil in a strong light. The expert will, however, observe that the contraction is very sluggish, but he may hesitate to diagnose the affection as amaurosis unless he is familiar with the use of the ophthalmoscope. With this instrument there wall be no difficulty in getting a good view of the optic disc, as the deficient sensibility of the nerve structures will render the horse indifferent to the light which is thrown into the interior. On getting a view of the optic disc in "an amaurotic eye it will at once be evident that the normal pink tint has been replaced by a white or grayish colour, and the vessels also will be indistinct in consequence of the small quantity of blood circulating in them.

When amaurosis has advanced so far that the animal is totally blind it will generally be comparatively easy to form a correct diagnosis. The dilated and immovable condition of the pupil, which is in no way affected by a strong light, is in itself almost sufficient, but it also may be ascertained in various other ways that the sight is lost.

The manner in which the animal progresses is one of the most striking indications of the existence of blindness. The fore-limbs are lifted unusually high, the head is elevated, the ears are constantly at attention, and the gait is of the careful, dwelling kind.

A common but not infallible test is that of riding the horse, quietly of course, on to a wall, which he will touch with his nose before becoming aware of its existence. This is a better method than that of making a feint to strike the animal with a stick, because the motion which is given to the air by the movement of the stick and the hand which holds it is often sufficient to induce the horse to jerk the head away under the impression that some foreign body is approaching him, thus leading the lookers-on to suspect that a certain amount of visual power still exists.

The great importance of the use of the ophthalmoscope for the examination of eyes which present any abnormal character, however slight, will be understood when it is remembered that an animal with partial or even complete amaurosis may be passed sound by a careless examiner. No treatment is ever attempted for the cure of amaurosis in the lower animals. Small doses of strychnia have been employed in the treatment of the disease in the human subject with slight benefit, which has, however, generally proved to be temporary. Galvanism has also been used, with result of improving the sight for a short time, but neither of these remedies has produced sufficiently good results to justify any recommendation of them in the treatment of amaurosis in the horse.