The upper jaw - bones are situated on the side of the face, and join together by means of a flattened plate (palatine process) in the centre of the roof of the mouth, a large portion of which they form. They also form the floor and sides of the nostrils, and the sockets, or alveoli, into which the fangs of the upper grinders or "molar" teeth are implanted. Like the frontal bones, they are hollowed out into sinuses, which sometimes become diseased as the result of direct injury to the jaw, or to disease extending from the fangs of the teeth. In these cases they become filled with matter to which exit has to be given by an operation.