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

si·nus / ˈsīnəs/ • n. 1. (often sinuses) Anat. & Zool. a cavity within a bone or other tissue, esp. one in the bones of the face or skull connecting with the nasal cavities. ∎  an irregular venous or lymphatic cavity, reservoir, or dilated vessel. ∎  Med. an infected tract leading from a deep-seated infection and discharging pus to the surface. ∎  Bot. a rounded notch between two lobes on the margin of a leaf or petal.2. [as adj.] Physiol. relating to or denoting the sinoatrial node of the heart or its function as a pacemaker: sinus rhythm sinus tachycardia.

sinus (sy-nŭs) n.
1. an air cavity within a bone, especially any of the cavities within the bones of the face or skull (see paranasal sinuses).

2. any wide channel containing blood, usually venous blood. s. arrhythmia see arrhythmia. s. rhythm a normal heart rhythm, usually as recorded on an electrocardiogram. s. venosus a chamber in the embryonic heart that becomes part of the right atrium after birth.

3. a pocket or bulge in a tubular organ, especially a blood vessel.

4. (sinus tract) a blind-ending infected tract leading from a focus of infection to the surface of the skin or a hollow organ. See pilonidal sinus.

sinus A Latin word meaning ‘curve’ or ‘bay’.
1. The word ‘sinus’ was used by Giovanni B. Riccioli in 1651 to designate bay-like features on the lunar maria (see MARE). The best known example is Sinus Iridum, the ‘Bay of Rainbows’, on the north-western margin of the ‘Sea of Rains’ (Mare Imbrium).

2. In certain bivalves (Bivalvia), a recess or embayment in the pallial line; most bivalves with a pallial sinus are burrowers.

sinus
1. A space, found e.g. in certain bones of the face of a mammal where they are filled with air and connect with the nasal cavity.

2. In certain Bivalvia, a recess or embayment in the pallial line; forms with a sinus are generally burrowers.