A tear within the wall of any of the arteries of the neck (carotid artery or vertebral artery) and that allows blood to separate the wall layers. By separating a portion of the wall of the artery (a layer of the Tunica intima or tunica media), a dissection creates two lumens or passages within the vessel, the native or true lumen, and the "false lumen" created by the new space within the wall of the artery.