Fig. 4.—Transverse Section of Wall showing variation in thickness.
The structure of the wall is fibrous—the fibres running parallel to each other, and with the same obliquity as that presented by the front of the wall. Although the wall varies in thickness from before backwards, it does not from above downwards. It maintains the same thickness from the coronet to its lower circumference.
The layers of the wall are hardest externally, becoming softer as they approach the inner surface—a condition due to the outer layers being exposed to friction and evaporation. This is a simple and valuable provision of nature which should not be interfered with. The hard outer layer is best adapted to withstand wear, and its density protects the deeper layers from evaporation. This maintains the whole wall at the degree of softness and toughness which best preserves elasticity and strength of horn.
The Sole is that division of the hoof which forms the floor of the foot. It is situated within the lower border of the wall, and is slightly arched so that on a hard level surface its central part takes no bearing. ([Fig. 5]). Posteriorly the sole is divided by a triangular space into which the frog fits, and thus its continuation to the heels consists of two angular portions embraced between the bars and the wall. The unmutilated sole is throughout of nearly equal thickness, but a slight excess round the circumference gives firmer attachment to the wall.
Fig. 5.—The Sole with Frog removed.