sinus

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.

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sinus

The paranasal sinuses.

(credit: © Merriam-Webster Inc.)

Body cavity or hollow. The paranasal sinuses, which are known commonly simply as the sinuses, are any of four sets of cavities in the bones adjoining the nose: maxillary, the largest, between the eye socket and the palate and upper jaw; frontal, just above and between the eye sockets; ethmoid, consisting of 3–18 thin-walled cavities between the nasal cavities and the eye sockets; and sphenoid, behind the nasal cavity. All are absent or small at birth, enlarge gradually until puberty, and then grow rapidly. They affect the sound of the voice and may help to warm inhaled air. Their lining produces mucus, which drains into the nasal cavity. Blockage of their outlets by swelling (from allergy or infection; see sinusitis), polyps, or structural problems hampers breathing through the nose and can lead to serious infection. Severe obstruction may require surgery, which must be done with extreme care to avoid harm to nearby brain structures or the eyes.

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sinus

1. Anatomy

a. any bodily cavity or hollow space

b. a large channel for venous blood, esp between the brain and the skull

c. any of the air cavities in the cranial bones

2. Pathol a passage leading to a cavity containing pus

3. Botany a small rounded notch between two lobes of a leaf, petal, etc.

4. an irregularly shaped cavity