A term applied to narrow cavities of various kinds, occurring naturally in the body, or resulting from disease. Thus it is applied to the air-containing cavities which are found in the frontal, ethmoidal, sphenoidal and maxillary bones of the SKULL, and which communicate with the NOSE. The function of these paranasal sinuses is doubtful, but they do lighten the skull and add resonance to the voice. They enlarge considerably around puberty and in this way are a factor in the alteration of the size and shape of the face. The term is also used in connection with the wide spaces through which the blood circulates in the membranes (MENINGES) of the BRAIN. Cavities which are produced when an ABSCESS has burst, but remain unhealed, are also known as sinuses (see also FISTULA).