sinus, cavity or hollow space in the body, usually filled with air or blood. In humans the paranasal sinuses, mucus-lined cavities in the bones of the face, are connected by passageways to the nose and probably help to warm and moisten inhaled air. When drainage from them is blocked, as after a cold, these sinuses often become infected, a condition called sinusitis. The accumulation of pus results in pressure, headaches, pain, and general discomfort. In invertebrates one of the spaces among the muscles and viscera through which blood returns to the heart is also known as a sinus.