SINUS, in anatomy, a cavity of bone or other tissue, especially an air cavity of some size, such as those in the interior of certain bones, the frontal, ethmoid, sphenoid, temporal, superior maxillary, etc. 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 tables of the skull, and separated from one an other by a thin bony septum. They give rise to the prominences above the root of the nose called the superciliary ridges. They are not fully developed till after puberty, and vary con siderably in size, being usually larger in men than in women and young persons. When very much developed they give a receding ap pearance to the forehead. They communicate on each side of the upper part of the nostril by a funnel-shaped opening, which transmits a prolongation of mucous membrane to line their interior. These sinuses are much more highly developed in certain mammals and birds than in man: They extend backward over the top of the skull in the ruminant and some other quadrupeds, and penetrate the cores of the horns'in oxen, sheep and a few antelopes. The most remarkable development of air sinuses in the mammalian class is presented by the ele phant; the intellectual physiognomy of this huge quadruped being caused, as in the owl, not by the actual capacity of the brain case, but by the enormous extent of the pneumatic cellu lar structure between the outer and inner plates of the skull. The sphenoidal sinuses are two
large irregular cavities, formed, after the pe riod 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 eth moid sinuses lie in the lateral masses of the ethmoid bone. They communicate with the cavities of the nose. Their main use is to di minish the weight of the fore part of the skull. That part of the temporal bone which forms the projection behind the ear is termed the mastoid process. The interior of this process is hollowed out with air sinuses which com municate with the tympanum or middle ear, and through it with the nose. The superior maxillary sinus is the largest of the sinuses, and the only one present in the infantile skull. The term sinus is also applied to irregular channels for the transmission of venous blood.