The hard, horny outer layer of the covering of the digit in ungulates is the hoof wall.

• A. skin
• B. hoof wall
• C. horn
• D. sole

Answer: (B)

In ungulates, the hard, horny outer layer that covers the digit is the hoof wall. It is a keratinized, protective sheath that forms the visible exterior of the hoof and bears most of the impact and wear during movement. Underneath the wall lies the sensitive tissue that supplies nutrients, while the sole covers the bottom part of the hoof and is more flexible. The frog is a distinct, elastic structure on the underside that aids shock absorption and traction. Skin refers to the general outer covering of the body, and horn is a keratinized projection on the head of some species, not part of the hoof. Because the description points to the hard, protective outer layer of the hoof, the hoof wall is the correct term.