It is perhaps impossible to overestimate the importance of perfect soundness of the eyes of a horse. The extreme inconvenience of a slight defect is very apparent to a rider or driver of an animal which is in the habit of shying when, so far as can be seen, there is nothing to attract particular attention.

Unfortunately, even important defects of vision may be attended with such obscure signs as to be absolutely undiscoverable by the ordinary methods of examination which are at the command of an amateur. The use of the ophthalmoscope by an experienced and highly accomplished observer is often necessary for their detection. In the horse, slight errors in refraction cannot be found out even by an expert, and if they could, there is no remedy, because the optical appliances, which are so valuable in compensating even the least departure from perfectly normal vision in man, are inapplicable to any of the lower animals.

Obviously the examiner of a horse's eyes will be quite unable to distinguish some of the marked departures from the healthy state unless he has some acquaintance with the eye in its natural condition. With this view the reader is referred to the section on the anatomy and physiology of the eye. Meanwhile he is cautioned that the most ludicrous mistakes are made by persons who, in looking for diseased conditions of the eye of the horse, have not taken the trouble in the first instance to master the normal appearances which can be readily seen without the use of any optical apparatus.