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 ir regular cavities extending upwar41 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 the nasal emi nences. They are not developed till after pu berty, and vary considerably in size, being usu ally larger in men than in women and young persons, in consequence of the greater promi nence of the supereiliary ridges in the former. They communicate on each side with the upper part of the nolitril by a funnel-shaped opening, which transmits a prolongation of mucous mem brane to line their interior. The sphenoidal sinus es are two large irregular cavities, formed, after the period of childhood. in the body of the sphe
noid bone. They communicate with the upper part of the nose, from which they receive a layer of mucous membrane. Like the frontal 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 (Dah mer(' (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 cavi ties. The sinuses of the dam water 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.).