Cracked hoof. Artwork of a cracked hoof and the way that the microscopic structure of the hoof limits the damage. When a hoofed animal, like the horse at upper left, injures a hoof, a crack may appear. A hoof is made of keratin, the material found in nails and hair. The sheets of keratin are arranged at an angle running away from the tender interior of the hoof. The crack will run parallel to the sheets and soon encounter one of the many tubes seen running vertically through the hoof. These tubes consist of sleeves where the keratin fibres run at different angles. These changing angles cause the crack to repeatedly switch directions (lower right) and eventually die out.