The Collaborative International Dictionary of English v.0.48:

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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The Collaborative International Dictionary of English v.0.48:

Pallial \Pal"li*al\, a. [L. pallium a mantle. See Pall.]
   (Zool.)
   Of or pertaining to a mantle, especially to the mantle of
   mollusks; produced by the mantle; as, the pallial line, or
   impression, which marks the attachment of the mantle on the
   inner surface of a bivalve shell. See Illust. of Bivalve.
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   Pallial chamber (Zool.), the cavity inclosed by the mantle.


   Pallial sinus (Zool.), an inward bending of the pallial
      line, near the posterior end of certain bivalve shells, to
      receive the siphon. See Illust. of Bivalve.
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