more rubbery over the heels, and it merges with frog material. Not all horses have the same amount of periople. Dry feet tend to lack this substance, which can be substituted with a hoof dressing. Characters and functions of the external hoof structures The walls The walls are considered as a protective shield covering the sensitive internal hoof tissues (like the exoskeleton of arthropods), as a structure devoted to dissipating the energy of concussion, and as a surface to provide grip on different terrains. They are elastic and very tough, and vary in thickness from 6 to 12 mm. The walls are composed of three distinct layers: the pigmented layer, the water line, and the white line. The pigmented layer is generated by the coronet, and its color is just like that of the coronet skin from which it is derived. If the coronet skin has any dark patches, the walls show a corresponding pigmented line, from the coronet to the ground, showing the wall's growth direction. This layer has a predominately protective role and is not as resistant to ground contact, where it can break and flake away. The water line is built up by the coronet and by the wall's corium (the living tissue immediately beneath the walls). Its thickness increases proportionally to the distance from the coronet and, in the lower third of the walls, is thicker than the pigmented layer. It is very resistant to contact with the ground, and it serves mainly a support function. The white line is the inner layer of the wall. It is softer and fibrous in structure and light in color; white in a freshly trimmed hoof, yellowish or gray after exposure to air and dirt. From the underside of the healthy hoof, it is seen as a thin line joining the sole and the walls. The white line grows out from the laminar connections. Any visible derangement of the white line indicates some important derangement of laminar connections that fix the walls to the underlying P3 bone. Since the white line is softer than both the walls and the sole, it wears fast where it appears on the surface; it appears as a subtle groove between the sole and the walls, often with some debris or sand inside. The three layers of the wall merge into a single mass, and they grow downwards together. If the wall does not