Si"nus (?), n.; pl. L. Sinus, E. Sinuses (#). [L., a bent surface, a curve, the folds or bosom of a garment, etc., a bay. Cf. Sine, n.]
An opening; a hollow; a bending.
A bay of the sea; a recess in the shore.
3. Anat. & Zool.
A cavity; a depression.
A cavity in a bone or other part, either closed or with a narrow opening
A dilated vessel or canal.
A narrow, elongated cavity, in which pus is collected; an elongated abscess with only a small orifice.
A depression between adjoining lobes.
⇒ A sinus may be rounded, as in the leaf of the white oak, or acute, as in that of the red maple.
Pallial sinus. Zool. See under Pallial. -- Sinus venosus (?). [L., venous dilatation.] Anat. (a) The main part of the cavity of the right auricle of the heart in the higher vertebrates. (b) In the lower vertebrates, a distinct chamber of the heart formed by the union of the large systematic veins and opening into the auricle.
© Webster 1913.