The maxillary sinus is a large pyramidal cavity, within the body of the maxilla:
- Its apex, directed lateralward, is formed by the zygomatic process;
- Its base, directed medialward, by the lateral wall of the nose.
- Its walls are everywhere exceedingly thin, and correspond to the nasal orbital, anterior, and infratemporal surfaces of the body of the bone.
- Its nasal wall, or base, presents, in the disarticulated bone, a large, irregular aperture, communicating with the nasal cavity. In the articulated skull this aperture is much reduced in size by the following bones: the uncinate process of the ethmoid above, the ethmoidal process of the inferior nasal concha below, the vertical part of the palatine behind, and a small part of the lacrimal above and in front ; the sinus communicates with the middle meatus of the nose, generally by two small apertures left between the above-mentioned bones. In the fresh state, usually only one small opening exists, near the upper part of the cavity; the other is closed by mucous membrane.
- On the posterior wall are the alveolar canals,transmitting the posterior superior alveolar vessels and nerves to the molar teeth.
- The floor is formed by the alveolar process of the maxilla, and, if the sinus be of an average size, is on a level with the floor of the nose; if the sinus be large it reaches below this level.
Projecting into the floor of the antrum are several conical processes, corresponding to the roots of the first and second molar teeth;in some cases the floor is perforated by the fangs of the teeth. The infraorbital canal usually projects into the cavity as a well-marked ridge extending from the roof to the anterior wall; additional ridges are sometimes seen in the posterior wall of the cavity, and are caused by the alveolar canals.
The size of the cavity varies in different skulls, and even on the two sides of the same skull.