SINUS (Let. a bend or hollow) has two significations in antdomy, and one in surgery. The cells or cavities contained in certain bones—as the frontal, ethmoid, sphenoid, and superior maxillary—receive this desiguation. The frontal sinuses are two irregular cavities extending upward and onward, from their openings on each side of the nasal spine, b•Aween 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 con sidera'Ay in size, being usually larger in men than in women and young persons, in con sequence of the greater prominence of the superciliary ridges in the former. When very much developed they give a receding appearance to the forehead. They are larger in Eurapeans than in negroes, and are very imperfectly developed in the Australians, whose peculiar want of vocal resonance is apparently due to this deficiency. They communi cate on each side with the upper part of the nostril by a funnel-shaped opening, which transmits a prolongation of mucus membrane to line their interior. These cells are much more highly developed in certain mammels and birds than in man. Prof. Owen observes that "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-cells in the mammalian class is pre sented by the elephant; the intellectual physiognomy of this huge quadruped being caused, as in the owl, not by the actual capacity of the brain-ease, 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 sphenoidbone. They communicate with the upper part of the nose, from which they receive a layer of mucus 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 etlunold bone. They
open into the cavities of the nose. Their main use is to diminish the weight of the fore part of the skult. The superior maxillary sinus commonly known as the antrum of 117yhmore (the anatomist who first accurately described it) is-the largest of the facial si ntliCA. Its uses are the same as those of the othcrs,'and, like them, it communicates with the nasal cavities.
The sinuses of the (lora materare quite distinct from the above described bony sinuses; they are irregular channels for the transmission of venous blood, and are formed in the following way: the dam mater consists of two layers—an outer, belonging to the skull; and an inner, belonging to the brain. They can be easily separated in infancy, hut in the adult they Arc blended together for the greater part of their extent. In some places. however, as beneath the sagittal suture (formed by the two parietal bones at the top of the head, and running from before backward), they are separated on either side of the mesial line, the outer layer being continued beneath the bone, and in contact with it; while the inner one dips inward, and meeting with the corresponding layer of the oppo-, site side. forms a triangular canal or sinus, which is strengthened at the sides and angles by interlacing bands of fibrous tissue. The sinus whose formation we have thus described is called the superior longitudinal sinus, and the other sinuses are formed in the same way. They are all lodged in the intervals between the great divisions of the brain, and they are so constructed " that their shape cannot easily be altered. by any external pressure; consequently, the flow of blood through them cannot be impeded by the pulsations or pressure of the brain, in the varying positions of the body. The tense, wsyielding character of their walls. moreover, does not admit of chimer collapse or dis tension; hence, they must be equally full at all times, and must exert a uniform pressure on the lmmin."—Humphrey On the .Uumun Skeletm, p. 200.
In surgery the term sinus is nearly equivalent to fistula (q.v.).