The middle bone in the pastern of the horse, forming the proximal interphalangeal joint(pastern joint) with the proximal phalanx (long pastern bone) and distal interphalangeal joint (coffin joint) with the distal phalanx (coffin bone) and the navicular bone. The superficial digital flexor tendon inserts on the proximal extremity on the palmar/plantar aspect of the bone.