The Maxillary Sinus (sinus maxillaris; antrum of Highmore), the largest of the accessory sinuses of the nose, is a pyramidal cavity in the body of the maxilla.
Its base is formed by the lateral wall of the nasal cavity, and its apex extends into the zygomatic process. Its roof or orbital wall is frequently ridged by the infra-orbital canal, while its floor is formed by the alveolar process and is usually 1/2 to 10 mm. below the level of the floor of the nose; projecting into the floor are several conical elevations corresponding with the roots of the first and second molar teeth, and in some cases the floor is perforated by one or more of these roots.
The size of the sinus varies in different skulls, and even on the two sides of the same skull. The adult capacity varies from 9.5 c.c. to 20 c.c., average about 14.75 c.c. The following measurements are those of an average-sized sinus: vertical height opposite the first molar tooth, 3.75 cm.; transverse breadth, 2.5 cm.; antero-posterior depth, 3 cm. In the antero-superior part of its base is an opening through which it communicates with the lower part of the hiatus semilunaris; a second orifice is frequently seen in, or immediately behind, the hiatus.
The maxillary sinus appears as a shallow groove on the medial surface of the bone about the fourth month of fetal life, but does not reach its full size until after the second dentition.
At birth it measures about 7 mm. in the dorso-ventral direction and at twenty months about 20 mm.