If you were asked to perform a prepurchase examination on a horse with an eye like this one would you pass it and if not why not?
Although it looks like nothing to be worried about, the little vertical black line in the pupil is an iris rest – an adhesion from one of the corpora iridica (the black blobs on the dorsal border of the pupil) onto the anterior lens capsule, showng that in the past there has been some inflammation in the eye.
In fact that was a bit of a trick question, as the other eye looked like this, with more adhesions on the ventral border of the pupil, another central iris rest and some lens opacification secondary to the inflammation. The horse has had at least one bout of equine recurrent uveitis and may go on to have another.
At present the horse has good vision, and the current owner is brilliant at noting when anything is slightly amiss, and gets the eye treated as soon as possible, hence the relatively mild signs seen at present.