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The Collaborative International Dictionary of English v.0.48 (gcide)

Sinus \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.]
1. An opening; a hollow; a bending.
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2. A bay of the sea; a recess in the shore.
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3. (Anat. & Zool.) A cavity; a depression. Specifically:
(a) A cavity in a bone or other part, either closed or
with a narrow opening.
(b) A dilated vessel or canal.
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4. (Med.) A narrow, elongated cavity, in which pus is
collected; an elongated abscess with only a small orifice.
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5. (Bot.) A depression between adjoining lobes.
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Note: A sinus may be rounded, as in the leaf of the white
oak, or acute, as in that of the red maple.
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{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.
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