These bones form a portion of the inferior wall of the cranium, as well as that part of the face corresponding to the forehead. They are situated between the parietal bones above and the nasal and lachrymal bones below, and have union with several other bones of the cranium and face. Each of the frontal bones assists in forming an irregular cavity of considerable extent (frontal sinus), which contains air and communicates with the nostril. These frontal sinuses are lined by mucous membrane. They give lightness to the head, and in "nasal gleet" sometimes require to be opened in order to give exit to the pus which accumulates within them. They are very small in early life, but enlarge as age advances, and are separated one from the other by a bony partition.