The New International Encyclopædia/Sinus

SINUS (Lat., bend, hollow). The cells or cavities contained in certain bones, as the frontal, ethmoid, sphenoid, and superior maxillary, are called sinuses. The frontal sinuses are two irregular cavities extending upward and outward, from their openings on each side of the nasal spine, between the inner and outer layers of the skull, and separated from one another by a thin bony septum. They give rise to the prominences above the root of the nose called the nasal eminences. They are not developed till after puberty, and vary considerably in size, being usually larger in men than in women and young persons, in consequence of the greater prominence of the superciliary ridges in the former. They comnuinicate on each side with the upper part of the nostril by a tunnel-shaped opening, which transmits a prolongation of mucous membrane to line their interior. The sphenoidal sinuses are two large irregular cavities, formed, after the period of childhood, in the body of the sphenoid bone. They communicate with the upper part of the nose, from which they receive a layer of mucous membrane. Like the frontal sinuses, they serve to lessen the weight of the skull and to add to the resonance of the voice. The ethmoid sinuses or cells lie in the lateral masses of the ethmoid bone. They open into the cavities of the nose. The superior maxillary sinus, commonly known as the antrum of Highmore (after the anatomist who first accurately described it), is the largest of the facial sinuses. Its uses are the same as those of the others, and, like them, it communicates with the nasal cavities. The sinuses of the dura mater are quite distinct from the above described bony sinuses; they are irregular channels for the transmission of venous blood. In surgery the term sinus is nearly equivalent to fistula (q.v.).