sinus
sinus From the Latin, meaning a cavity, channel, or hollow. In the body there are several types of sinus, matching these definitions. The air-containing cavities so-named are within the skull bones, enclosed except for openings into the nasal cavity; these are the frontal sinuses (in each side of the forehead), and the maxillary sinuses (in the cheek bones). Their lining can become acutely or chronically infected (sinusitis), causing pain and muco-purulent nasal discharge (catarrh). The channels known as sinuses are blood-containing spaces such as those in the spleen, liver, and bone marrow; they are not shut off from the circulating blood, but their width allows greater stagnation than in capillary blood vessels, for specific functions such as the addition and the removal of blood cells and other constituents. The venous drainage channels of the brain, inside the skull, and that of the heart muscle, also go by this name; likewise dilated regions of some blood vessels, notably the carotid sinuses in the neck, which have stretch receptors important in the regulation of the arterial blood pressure; also dilated parts of lymphatic channels, and of milk channels behind the nipples. The hollows so named are more often like dead-end channels or pits—abnormal connections to the surface of the skin from deeper areas of infection.
Stuart Judge
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sinus