Appaloosa occurs in several overlay patterns on one of several recognized base coat colors. There are three other distinctive, "core" characteristics: mottled skin, striped hooves, and eyes with a white sclera. Skin mottling is usually seen around the muzzle, eyes, anus, and genitalia. Striped hooves are a common trait, quite noticeable on Appaloosas, but not unique to the breed. The sclera is the part of the eye surrounding the iris; although all horses show white around the eye if the eye is rolled back, to have a readily visible white sclera with the eye in a normal position is a distinctive
Human eye layer, known as the fibrous tunic, is composed of the cornea and sclera. The middle layer, known as the vascular tunic or uvea, consists of the choroid, ciliary body, pigmented epithelium and iris. The innermost is the retina, which gets its oxygenation from the blood vessels of the choroid (posteriorly) as well as the retinal vessels (anteriorly). The spaces of the eye are filled with the aqueous humour anteriorly, between the cornea and lens, and the vitreous body, a jelly-like substance, behind the lens, filling the entire posterior cavity. The aqueous humour is a clear watery fluid that is contained