Sinus:1.An air-filled cavityina dense portion of askullbone. The sinuses decrease the weight of the skull. The sinuses are formed in four right-left pairs. Thefrontalsinuses are positioned behind the forehead, while the maxillary sinuses are behind the cheeks. Thesphenoidandethmoidsinuses are deeper in the skull behind the eyes and maxillary sinuses. The sinuses are lined bymucous-secreting cells. Air enters the sinuses through small opening in bone called ostia. If an ostium is blocked, air cannot pass into thesinusand likewise mucous cannotdrainout. See also: Sinusitis.
2.A channel permitting the passage ofbloodorlymphfluid that is not a blood orlymphaticvessel, such as the sinuses of theplacenta.
3.A tract orfistulaleading to a cavity which may be filled withpus.
The word was borrowed from the Latin noun "sinus," which means "curve, fold, or hollow." The same root gave rise to "sinuous".
