Fig. III, V, 1-3. At the junction of the inner and middle thirds of the supra-orbital margin, the supra-orbital notch or foramen may be felt, and a line drawn downwards from this foramen through the interval between the two lower bicuspid teeth will pass through both infraorbital and mental foramina. The former foramen lies 1/4 to 1/2 inch below the orbital margin, whilst the latter (in the adult) lies midway between the alveolar and inferior borders of the lower jaw.

The frontal sinuses are very variable in extent. They occupy the space between the inner and outer tables of the frontal bone, above the base of the nose and above the inner half of the supra-orbital margin. The sinus communicates with the nasal cavity by means of a narrow channel, the infundibulum, which opens into the middle meatus of the nose under cover of the middle turbinated bone, on a level with the inner margin of the palpebral fissure.

The antrum of Highmore usually occupies the greater part of the interior of the superior maxilla, and opens into the hiatus semilunaris, a depression which lies under cover of the middle turbinated bone. The opening is, however, situated at so high a level that pus only escapes into the nose when the antrum is practically full. Two teeth are, on the other hand, closely related to the antrum—namely, the second bicuspid and the first molar — the sinus usually extending downwards in the interval between the two labial and single palatal fangs of the latter tooth. The removal of either of these teeth, followed up by any necessary upward boring, will efficiently drain the sinus.

The sinus may also be drained by everting the upper lip and puncturing the outer wall through the canine fossa, a depression lying above the canine tooth.

The sphenoidal sinus occupies the greater part of the body of the sphenoid, and opens into the spheno-ethmoidal recess, a space lying above and behind the superior turbinated bone.