Ulcerative Keratitis (corneal ulcer) 
The cornea is the transparent front layer of the eyeball.  It is the most sensitive part of the body and readily reacts to irritants or stimuli from both outside and inside the eye.

Ulerative keratitis is corneal inflammation caused by disruption of one or more layers of the cornea, starting from the outside and progressing inward.  The disruption (ulcer) may be very shallow, similar to a scrape or abrasion, or it may be very deep, nearly penetrating all of the corneal layers.  The deeper the ulcer, the more vision is threatened.

Corneal ulcers have many causes, including trauma, infections, inborn weaknesses and nutritional deficiencies.