Sinus, in Anatomy, a term for the air cavities contained in the interior of certain bones—as the frontal, ethmoid, sphenoid, temporal, and superior maxillary. The frontal sinuses are two irregular cavities extending upwards and outwards, from their openings on each side of the nasal spine, between the inner and outer tables 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 superciliary ridges. They are not fully developed till after puberty, and vary considerably in size, being usually larger in men than in women and young persons. When very much developed they give a receding appearance to the forehead. They are larger in Europeans than in Negroes, and are very imperfectly developed in the Australians, whose peculiar want of vocal resonance is apparently due to this deficiency. 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. Sir Richard Owen observes that 'they extend backwards 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 elephant; 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 cellular structure between the outer and inner plates of the skull.' 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 lie in the lateral masses of the ethmoid bone. They communicate with the cavities of the nose. Their main use is to diminish the weight of the forepart 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 communicate with the tympanum or middle ear, and through it with the nose. The superior maxillary sinus commonly known as the
Antrum of Highmore (anatomist, 1613-84, who first accurately described it) is the largest of the sinuses, and the only one present in the infantile skull. Its uses are the same as those of the others, and, like them, it communicates with the nasal cavities.
The term sinus is also applied to certain channels for the transmission of venous blood. These are merely dilated veins formed by the separation of the layers of the dura-mater and lodged in grooves on the inner surfaces of the cranial bones. In Surgery the term sinus is nearly equivalent to Fistula (q.v.).