An abscess is a pus-filled cavity, the pus consisting of tissue fluid, white blood cells, cell debris, and bacteria. An abscess in a horse's hoof is particularly troublesome, because the sole and hoof wall cannot expand with the buildup of pus. This causes a great deal of pain, and can lead to separation of the sole or hoof wall and the pedal bone, if not treated appropriately. A sole abscess is often the result of a puncture wound of the sole of the hoof, or a sole bruise that becomes infected. A hoof wall abscess, also called "gravel", develops when a tiny particle of dirt or debris works its way into the tissue beneath the hoof wall through the less dense "white line". As the infection develops, it travels up the hoof wall, between the laminae tissue, following a path of least resistance, often breaking out at the coronary band.